Abstract
Purpose of Review
This review focuses on potential approaches to restoring vegetation across former agricultural land, mainly considering the relatively well-studied case of once-forested landscapes. It presents an ecological framework within which the potential consequences of different practical interventions are described and compared, and then identifies implications for restoration decision-making.
Recent Findings
There is a still-growing range of restoration interventions other than high-cost intensive tree-planting. These aim to accelerate vegetation recovery at different stages of forest redevelopment, by removing factors that would otherwise have an inhibitory influence. Potential interventions include adding seed, installing structures to attract seed dispersers, selectively protecting or removing different vegetation elements (trees or ground plants) in the regenerating communities, and managing fire, livestock grazing or wildlife.
Summary
Given the potential variety of approaches, at a landscape scale, the best solution is most likely a spatial mosaic that tailors specific restoration interventions to differing contexts and outcomes. However, the current evidence base is insufficient to adequately guide decisions about how to match method to site, landscape and cost. Research has typically been small-scale and often disconnected from restoration practice. Larger-scale investment in collaborative and innovative restoration trials and experiments is needed to enable better decision-making.
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Introduction
The advent of the Anthropocene epoch has seen the emergence of rapid and unprecedented industrial-scale destruction of native vegetation, and its conversion for human use. There has also been a growing realization of the widespread environmental impacts of large-scale land clearing and of the need to rapidly restore quasi-natural ecological communities at landscape and regional scales [1,2,3,4,5,6,7]. Large-scale return of forest to formerly cleared and cultivated land has several precedents in human history. For example, a recent review estimated that 56 million hectares of formerly cultivated land in the Americas regenerated to native forest in the 1500s, following decimation of indigenous peoples after European invasion [8]. Similarly, detailed historical reconstruction of changes after European settlement [9] showed that agricultural use in Massachusetts’ landscape peaked in the mid-1800s (with about 50% of land converted), but that much of this land had become reforested by the late twentieth century. Meanwhile, agriculture declined to about 7% of the land area due to technological and socioeconomic changes [8, 9]. In such cases, a diverse biotic community with broad similarity to communities present before clearing has typically re-emerged without active restoration.
During the 1900s, scientific observations of how forest ecosystems had progressively redeveloped on areas of abandoned farmland (“oldfields”) in North America and Europe underpinned the emergence of the foundational concept of ecological succession [10]. Ecological succession is most simply defined as “processes of vegetation change” [10]. Much early literature about the rates and patterns of vegetation change over time in oldfields was focused on the extent to which these successional processes were predictable and deterministic in the absence of much human intervention, albeit with largely unresolved outcomes [10, 11]. More recently, restoration ecologists have turned their attention to how the rate of vegetation recovery can be accelerated by human interventions (e.g. 1, 12,13,14,15,16). Many earlier concepts, even though somewhat unresolved, remain fundamental to ecological restoration [12, 13, 17, 18]. Additionally, contemporary thinking in successional processes and restoration ecology now also recognises the presence of high intrinsic variability, uncertainty and risk [17,18,19]. Moreover, the past 5 years have seen an upsurge in literature commentaries and reviews grappling with the theoretical and conceptual issues associated with ecological restoration, especially how these can be applied across large spatial scales [1,2,3,4,5,6,7, 14,15,16, 18,19,20,21,22,23,24]. The field is no doubt advancing in general terms, but there remains a persistent disconnect between some poorly resolved scientific concepts (such as “resilience”, and “degradation”) and the kinds of specific interventions that are typically involved in practical restoration [7, 11, 12, 25].
This paper synthesises current knowledge about the ecological processes and restoration practices involved in interventions to accelerate the recovery of forest vegetation on former agricultural land. Forest restoration in this context has been relatively well-studied, yielding approaches from which can be drawn analogues relevant to other ecosystems (e.g. 26, 27). First, I establish a summary framework within which the potential ecological consequences of different practical interventions can be compared, with reference to the speed and direction of vegetation redevelopment. Second, I describe the ecological interactions whereby each of these interventions operates. Finally, I outline some important emerging corollary issues that are faced by decision-makers who seek to achieve landscape-scale restoration.
An Ecological Framework for Post-Agricultural Interventions to Restore Forest
In once-forested landscapes, the plants that initially occupy disused agricultural land are typically pasture grasses, ferns, vines or herbaceous species that form a dense low-growing vegetation (henceforth termed “ruderal” vegetation), with few upright woody plants, and with a greatly depleted supply of soil-stored forest seeds [17, 28]. The subsequent pattern of vegetation development (i.e., the rate and trajectory of succession) then depends on the dispersal, survival, and growth of progressive generations of more diverse species and functional types of plants, and on the animals and microbes with which they interact (e.g. 19). These processes can all vary greatly, depending on many ecological factors, and can also be strongly influenced by human interventions (e.g. 17, 18), as shown in Fig. 1.
In optimal situations (Fig. 1, left-hand side), scattered small trees will rapidly grow from dispersed seeds, or regrow from scattered rootstock, initiating “Early Regeneration” of woody vegetation [17, 19]. Further recruitment and growth of trees results in canopy formation, along with the development of a somewhat forest-like structure, but with greatly different floristic composition (“Progressing Regeneration”, Fig. 1). This stage then transitions into several decades of ongoing further recruitment of a wider range of plants and animals, eventually producing a relatively high ecological similarity to never-cleared mature forest (“Advanced Regrowth”, Fig. 1) [17], albeit with some differences that slowly diminish over many subsequent decades.
However, this successional process can be greatly slowed, or even stalled [18, 19, 27]. Sometimes, especially where large areas were intensively used for decades, the transition from Early Regeneration to Progressing Regeneration is greatly inhibited by a series of ecological barriers which suppress early recruitment and growth of tree seedlings, creating a state of persistent ruderal vegetation which is often dominated by nonnative pasture grasses (Fig. 1, right-hand side) [29, 30]. These barriers include a lack of seeds of forest trees, physical stressors (such as exposure to sun and wind, fire and a scarcity or imbalance of soil nutrients), competition from ruderal vegetation, and predators or pathogens of seeds and seedlings [29]. In later successional stages (Early Regeneration, Progressing Regeneration), physical stressors are less extreme, but vegetation development may still be retarded by other factors, sometimes including competition from dominant tree species that inhibit the growth of many others [e.g. 31]. In highly modified landscapes, these early dominants can be nonnative trees, which are often assumed to be inhibitory, although this assumption has rarely been rigorously tested [28].
Most of the processes that inhibit the development of Early Regeneration can be bypassed by nursery-rearing trees from seed and then establishing them in plantations at a young sapling stage where they are better able to compete successfully with ruderal plants [30]. Tree planting is usually coupled with actions to suppress any redevelopment of ruderal vegetation. Methods for plantation establishment are derived from well-developed technologies used for timber and orchard production. If a high diversity of appropriate native species is planted at high density (henceforth “complex plantation”), the speed of transition from retired agricultural land to Advanced Regrowth can be greatly accelerated [32, 33]. However, complex plantations are limited to small areas because of their high cost, which can be most clearly justified in cases where ruderal vegetation is known or reliably predicted to be persisting over time [34,35,36]. At the other end of the cost spectrum lies unassisted successional regeneration over larger areas, although with more time needed to reach recovery goals [33, 37]. Indeed, some overviews of restoration have placed all approaches within such a binary framework of higher-cost “active” restoration (involving human intervention), compared with lower cost (“passive”) successional processes that proceed without human assistance (e.g. 2, 21, 23).
However, a dichotomous “passive-active” classification of approaches to landscape-scale restoration is incompatible with the real-world variety of pathways by which vegetation actually returns to disused agricultural land. This “passive-active” classification could also become misleading, because it ignores the following issues. First, even “passive” restoration requires often-unrecognised human interventions, which have direct or indirect costs (e.g., of livestock removal, fence installation and maintenance [37, 38]). Second, all types of approaches rely to varying degrees on the occurrence of “passive” successional processes after the completion of restoration interventions, because these actions are often restricted to the first few years. Third, there is a wide range of possible “active” tree planting designs, varying greatly in intensity (site preparation, tree density, species mix and variety, post-planting interventions), which produce a wide range of potential costs per hectare. Finally, there is also a range of interventional approaches other than conventional plantation technologies. These other forms of intervention also vary in intensity, duration and cost (e.g. 32, 39). The next part of this review is largely focused on the potential ecological roles and practical uses of these often-neglected “middle-ground” interventions, some of which may provide lower cost opportunities for large-scale restoration of forest ecosystems.
Expanding the Menu of Interventions to Accelerate or Redirect Forest Regeneration
An emerging variety of restoration interventions involve actions to accelerate forest succession by targeting one or more of three transitional stages: (A) persistent ruderal vegetation to Early Regeneration, (B) Early Regeneration to Progressing Regeneration and (C) Progressing Regeneration to Advanced Regrowth, as shown in Fig. 1 and Table 1. Here, I identify nine different approaches which act in various ways to potentially remove one or more factors that may otherwise inhibit the rate of vegetation development, describe the ecological logic of why and in what context they could be used, consider their limitations and flag selected examples. Many of the projects which have investigated or applied these approaches have employed a tailored combination of methods, both in small-scale trials by researchers [e.g. 40,41,42,43,44,45,46] or large-scale applications by practitioners [e.g. 47, 48]. Different methods may depend on similar ecological processes, and the available evidence suggests that many techniques’ effectiveness depends on their use in specific combinations with others [39]. The use of such combinations is flagged where relevant in the following descriptions.
Seeding
The first potential barrier to regeneration is a lack of tree seeds, due to depletion of soil seed banks, absence of nearby seed sources (mature trees) and a lack of seed-dispersing agents. Often all three are limiting, and actions to add seeds overcome them all. Seeding is mainly relevant to accelerating the transition from ruderal vegetation to Early Regeneration, but also has potential applications for enriching plant diversity in later phases. A range of techniques that were developed initially for agriculture and plantation forestry has been adapted for rapid and efficient delivery of a diverse mix of forest tree seeds over large land areas. Seed delivery mechanisms include sowing by aerial vehicles (plane or drone) and by ground-based agricultural machinery [47,48,49,50]. Because successful seedling establishment also requires overcoming further barriers (including competition from ruderal vegetation, seed predators and pathogens, Table 1), mechanised seeding techniques are most successful when they include additional treatments. For example, various projects have also incorporated seed burying or accompanying chemical agents (or pre- and post-seeding treatments) that selectively suppress competitors and repel or kill predators and pathogens [49•]. Other constraints include plant traits such as patterns of seed dormancy and the ability of seedlings to establish in exposed conditions [48, 49].
Seed-Disperser Attractants
A second type of intervention is to accelerate transitions from ruderal vegetation to Early Regeneration by adding devices to attract seed-dispersing fauna, which defecate or regurgitate seeds that were consumed elsewhere. This is most relevant to rainforests, where many trees are fleshy fruited and dispersed by vertebrates (mainly birds, but also bats or other mammals), and to a landscape context in which patches of remnant forest are not too remote. This technique has mainly been used in small-scale ecological trials, often with bird perches of varying construction [41, 51,52,53,54,55]. These trials have been very informative in revealing barriers to regeneration, and in demonstrating significant roles of fauna-mediated seed dispersal. However, their promise as cost-effective large-scale restoration tools seems limited because post-dispersal recruitment barriers may result in negligible tree seedling establishment even when many seeds have been deposited [51, 53, 55]. Studies reporting greatest recruitment of seedling trees have combined disperser-attractant devices with other actions, including suppression of ruderal vegetation and exclusion of seed and seedling predators (terrestrial granivores and herbivores) [52, 54].
Ruderal Suppression
Successful interventions to accelerate transitions from ruderal vegetation to Early Regeneration almost always include action to suppress the persistent ruderal vegetation using a wide range of methods (e.g. 32, 39,40,41,42,43,44, 46,47,48, 51, 52, 56). Over large areas, ruderal vegetation has been effectively removed by scalping (mechanical removal of topsoil) or by killing growing plants with herbicide application. Because broad-spectrum herbicides (such as glyphosate) also kill desired tree seedlings [43], grass-selective herbicides may be preferred in some situations [41]. Over smaller areas, ruderal plants have been killed with solarisation (light-absorbing ground sheets that intercept photosynthetic radiation and apply heat) and pulling (weeding). Depending on the situation and size of treated area, ruderal regrowth can be common (from seed, or partially killed plant parts) and requires repeat treatments over time. Additionally, above-ground parts of ruderal plants may be removed by cutting (slashing, mowing), although this does not prevent rapid regrowth from rootstock. Other treatments have included mulching, soil disturbance (ploughing), and addition of sugar to decrease soil N levels through microbial activity. Many of these treatments do not spare desirable emerging tree seedlings if applied repeatedly. Ruderal suppression alone may suffice to accelerate vegetation recovery in some situations (e.g. 41). However, it does not remove barriers of limited seed availability and dispersal (Table 1, see also [39]) and, therefore, often needs to be coupled with other interventions.
Seedling/Sapling Protection
The transition from Early Regeneration to Progressing Regeneration (Fig. 1) can be accelerated by interventions to protect pre-existing tree seedlings (or saplings or shoots from remnant rootstock) from ruderal competition, thereby increasing seedling survival and growth. Foundational applications of the idea were trialed in the Philippines in the 1980s [32], and the method was first described in the scientific literature by Shono and others in 2007 [57] as “assisted natural regeneration” (ANR). In that example, ANR actions suppressed dense tall nonnative grasses around rainforest tree seedlings that were otherwise struggling to persist; these actions resulted in the rapid growth of tree seedlings. More generally, suppression of any type of ruderal vegetation in a functionally relevant radius around the stems of tree seedlings or saplings may potentially be achieved with a range of different methods (including pulling, cutting, herbicide or mulching) as described above. A constraint to this approach is a requirement for pre-established regeneration of enough scattered tree recruits; Shono [57] recommended a minimum pre-existing density of 700 seedlings more than 15 cm tall per hectare. “ANR” has been used by some authors to encompass a very broad range of restoration interventions [1••], including installation of bird perches and selective removal of taller woody elements (e.g. 32, 56). Therefore, for clarity, I have avoided applying this term to define any method described here.
Subsequent vegetation development in the vicinity of these protected trees follows a pattern which has been frequently described in situations where scattered trees grow in persistent ruderal vegetation, with or without active interventions [41, 51, 57,58,59,60]. That is, once each tree has grown to form a local canopy that is sufficiently tall and dense to overtop the grasses, the competitive balance tends to reverse because the grasses are shade-intolerant and because the increased shade and leaf litter promote more favourable ground conditions for survival and growth of further tree seedlings. The trees may also attract seed-dispersing fauna, initiating a positive feedback cycle of further recovery. Similar processes have been described following initial tree establishment in large-scale direct seeding [48], and experiments have confirmed that seedlings beneath isolated remnant pasture trees grow faster than those in the open [61, 62]. These processes form the basis of two other types of restoration approach: nurse tree plantations, and nucleation and tree islands, as discussed below. The principle involved can be extended to different situations in which environmental factors substantially constrain the growth of existing seedlings, if their release would establish conditions that stimulate further recruitment of desired plant species. For example, shelters (using small plastic tubes to ameliorate extreme physical conditions) have been experimentally used to accelerate restoration of shrub cover in an arid ecosystem, with some short-term success [26].
Nurse Tree Plantations
The concept of nurse tree plantations [32, 63] involves an intervention that operates across two transitional stages (Fig. 1, Table 1). First, barriers to the transition from ruderal vegetation to Early Regeneration are bypassed by planting a simple tree cover (often of a single fast-growing species). Second, if planted trees establish successfully, they may accelerate the subsequent transition to Progressing Regeneration, through the same ecological feedback cycles described above under “seedling protection” provided that there are seed sources within reasonable distance, together with seed-dispersal processes. The costs of establishing nurse tree plantations include rearing seedlings, planting them, and follow-up protection until they are established. However, these costs could potentially be partly offset at a later time, by marketing timber from the initially planted trees, after sufficient naturally recruited seedlings have become established [64]. Accordingly, simple plantations that were initially established for purposes other than ecological restoration (such as carbon storage or timber production) may also have potential for later being repurposed as nurse plantations. Nevertheless, in all cases, the longer-term potential for seedling recruitment from ex-situ seeds is likely to be highly variable depending on complex interactions between nurse species selected, local conditions, landscape context and land-use history. Irrespective of the initial goal of the plantation, advanced tree suppression (see below) may become desirable decades later, if planted species also have inhibitory effects on later-stage regeneration.
Nucleation and Tree Islands
The related concepts of nucleation and tree islands also influence the same two transitional stages and harness the same positive ecological feedback cycles as for nurse tree plantations, but in a different manner. First, to bypass the initial barriers to Early Regeneration, interventions comprise either protecting isolated tree seedlings (or saplings or resprouts, as for “seedling protection” above (e.g. 57) or growing trees from seed and outplanting groups of several seedlings [46, 60, 65], with follow-up protection until they are established. Protection has typically involved at least ruderal suppression and livestock exclusion. An alternative variant of this approach is to use well-established larger trees as nuclei, but to mitigate against other regeneration barriers, such as ruderal competition [41] or livestock grazing [59] beneath them. Second, the transition from Early Regeneration to Progressing Regeneration is accelerated, because any resulting small trees or tree-clusters that grow amidst ruderal vegetation (forming a shady canopy a few metres above ground) constitute nuclei beneath and around which deposition of ex-situ tree seeds, and their subsequent germination, survival and growth are catalysed [41, 46, 59,60,61, 65, 66].
Robust evidence that the positive feedback for ongoing tree recruitment extends to areas beyond the canopy radius of the original trees has been provided by a relatively long-term (8 years to date) replicated restoration experiment using “tree islands” of 16, 64, and 144m2 [46, 66]. This trial has shown that separate tree nuclei tended to expand and ultimately coalesce over time, enabling tree-planting to be used more economically than is the case in conventional restoration plantations [66•]. As for nurse tree plantations, success must also depend on having seed sources within reasonable distance, suitable seed-dispersal processes, and overcoming other early-stage barriers through appropriate actions.
Advanced Tree Suppression
This approach accelerates the transition from Progressing Regeneration to Advanced Regeneration (Fig. 1, Table 1). It requires situations where there is a well-established regenerating forest (i.e., a canopy that shades the ground, reduced numbers of ruderal plants, and either a good supply of regenerating seedlings and saplings or a reinstated seed bank), but where the rate of development may be slower than desired because one or a few early-established tree species numerically dominate the canopy. Actions to remove or reduce the canopy trees (e.g., by cutting or poisoning them) cause a rapid increase in light penetration, which releases the growth of established seedlings and saplings and triggers further germination of soil-stored seed. The ecological changes triggered by this tree removal can ultimately lead to a more rapid increase in similarity to uncleared forest, for both ecosystem processes [56, 67] and composition of flora and fauna [68]. The newly released generation of seedlings and saplings can have a more forest-like diversity than the former low-diversity tree canopy because even this simple canopy provides the conditions for further seeds and seedlings to establish. However, achieving sufficient seedling recruitment is contingent on the previously discussed caveats regarding context-dependence (such as seed sources). Additionally, canopy removal may also stimulate establishment of ruderal plants or of undesired overstorey species [69], requiring additional actions to suppress them [68], and further barriers of seed and seedling predation may potentially limit outcomes.
Building on the opportunity provided by existing regenerating forest areas where an established tree canopy already exists, this approach bypasses the time that would otherwise be needed for early regeneration (Fig. 1). Some multi-use landscapes contain these regenerating forests as unexpected legacies of past land use decisions [28]. For example, understories of naturally recruited native seedlings and saplings may develop within industrial tree plantations that are not intensively managed [64, 70] and within tracts of “weedy” regrowth dominated by nonnative pioneer trees that establish readily in disused agricultural land [56, 68].
Management of Livestock and Fire
Regimes of livestock grazing and fire have the potential to strongly influence all stages of vegetation transition during forest redevelopment. Since both can often kill tree seedlings, their exclusion has frequently been a part of interventions to achieve Early Regeneration by increasing tree recruitment in retired agricultural land, typically accompanied by treatments to suppress ruderal vegetation whose growth would otherwise be released [e.g. 40, 41, 52]. In restoration case studies, exclusion of fire and grazing has often been mentioned within the description of the project, rather than having been manipulated as separate factors. Grazing can directly suppress survival and growth of tree seedlings [43, 71], but light grazing can also indirectly assist tree recruitment by suppressing ruderal vegetation [32, 72]. Indeed, grazing and fire interact in complex ways with each other, and with ruderal suppression, to either inhibit or promote tree regeneration [42], in a manner that is likely to be very context-dependent [32].
Moreover, the importance of managing regimes of livestock grazing and fire has been most extensively considered in the context of how suboptimal regimes can either degrade or restore remnant native forest (e.g. 45). In drier forest landscapes, naturally occurring forest-grassland mosaics are created and maintained dynamically by complex ecological interactions between fire regimes and grazing by large native herbivores [73, 74]; the latter having broad functional similarity to livestock. Grazing and fire are clearly important potential regulators of forest succession, and the ways in which they could potentially be managed to facilitate large-scale forest regeneration on former agricultural land deserve greater attention.
Wildlife Management
Wild animals, especially mobile vertebrates, can also significantly influence all stages of vegetation transition during forest redevelopment because they are key agents of seed dispersal and seed or seedling predation [75, 76]. These processes endow a strong potential to either enhance or suppress the recruitment, survival and growth of trees. However, the potential for targeting animals, rather than plants, as a primary intervention in forest restoration has rarely been explored. There is increasing discussion of wildlife management as a restoration tool in remnant native vegetation (e.g. 77). However, much less attention has been directed towards understanding how animal wildlife influence vegetation recovery in areas from which indigenous forest cover has been cleared [5, 75,76,77]. Harnessing wildlife management to accelerate revegetation requires improved knowledge of which animal species (or functionally similar groups of species) inhibit regeneration through their absence or their presence (depending on the interaction). Accordingly, interventions would aim to either exclude/reduce [55] or include/increase [78, 79] local populations of the target animals. These interventions could include approaches that alter the quantity or quality of local or landscape-scale habitat, in order to indirectly modify the animal populations.
Maintaining or reducing local densities of larger-bodied wild animals often involves management at the population level and landscape scale [76], either because the animals use resources in different parts of the landscape at different times or because they need to move across landscapes for other reasons. Consequently, consideration of their roles in vegetation regeneration requires greater attention to landscape-scale decision-making. In comparison, plant-centred approaches have been much more focused on manipulating individuals at the site scale [76], as described for the other approaches to revegetation.
Key Emergent Issues in Decision-Making for Large-Scale Restoration
The array of different potential interventions canvassed above, and the currently expanding state of this field of research and practice, indicates a promising outlook for achieving large-scale return of forest ecosystems across parts of former agricultural landscapes. However, there are also significant challenges and potential difficulties, especially in relation to two over-arching issues: the emergence of novel ecosystems and the landscape-scale decision processes.
Novel Ecosystems
During the past century, human actions have caused large local- and landscape-scale shifts in both abiotic conditions and species combinations, giving rise to novel ecosystems in which coexisting native and nonnative species are entwined within increasingly complex webs of interaction [5, 80, 81]. These changes have stimulated reconsiderations of the goals and methods of vegetation restoration, together with some vigorous and unresolved debate about many aspects of conservation and restoration [81,82,83]. The diverse ecological interactions during oldfield succession provide opportunities for species to have functional roles that either accelerate or retard regeneration trajectories, irrespective of their origin [28, 84]. For example, among the different interventions described above, the facilitative ecological role of nurse trees and regeneration nuclei in Early Regeneration is independent of whether they are formed from native or nonnative species, while both native and nonnative species can sometimes dominate the earlier stages of Progressing Regeneration, and retard further development [28, 41, 64].
From the perspective of biological conservation, a frequent goal of restoration actions has been to suppress or eradicate nonnative species, aiming to increase compositional purity towards a native-only species mix. However, in practice, efforts to remove nonnative species as a primary management goal have often been unsuccessful [85], or they have failed to produce ecosystem recovery [86]. Furthermore, in cases where nonnative species are nurse trees or nuclei, their successful removal at early stages of vegetation development is likely to inhibit existing regeneration trajectories [e.g. 28, 68]. Embracing some types of novel ecosystem is arguably a useful ecological and socioeconomic option for re-establishing diverse and well-functioning ecosystems in the Anthropocene [15, 80, 87], especially in the transitional stages towards recovery of forested landscapes [83].
Context, Decision Processes and Landscape Mosaics
In a field of research where reviews of empirical studies have tended to yield long laundry-lists of potential factors that influence the outcomes of forest regeneration, one theme has commonly emerged: there is large variation in outcomes among different sites [19, 88], even when they are subjected to the same treatments in the same landscapes [e.g. 34, 66, 89]. Factors underlying this variation include both those at the immediate site scale (such as soil condition, pre-existing vegetation and unpredictable disturbances) and those associated with large scales of space (landscape context) and time (land use history) (e.g. 1, 20, 66, 90). However, the available evidence is insufficiently firm to support conclusions about the relative importance of these factors. Moreover, high outcome variability for any given method means that rules of thumb for deciding which technique to implement may be elusive, and rigorous empirical research may yield outcomes that contradict expectations [66•].
Since the outcomes of any given restoration action potentially vary among different environments, what succeeds in one place or time may fail in another. Additionally, what is costly or difficult in one place or time may be more feasible in another. Unassisted regeneration has recently been advocated as a preferred first-line approach since it could be most cost-effective over large areas [1, 19, 21], but this logic would only apply where the risk of ruderal vegetation persisting is low [34], and the timeframe for recovery is not urgent [37]. Conversely, in situations where either unassisted regeneration or lower-intensity forms of intervention are likely to achieve acceptable outcomes in reasonable timeframes, it would be a waste of money and resources to invest in intensive actions such as complex tree planting. Among the various available and emerging approaches discussed in this paper, no single method can be generally preferred. The best solution for a given landscape is likely to be a spatial mosaic of different approaches, tailoring restoration interventions to differing contexts and likely outcomes.
However, this raises the complex question of how to design and implement a useful combination of interventions. Researchers and decision-makers have begun to consider this question [5], both within landscapes [30, 37, 80] and at regional [90], national [91], and global [20] scales. Landscape-scale prioritisation methods similar to those developed for selecting conservation reserves have been applied to setting restoration priorities using GIS layers of current land conditions [90, 91]. However, an important limitation to these case studies is the absence of an explicit time dimension. Restoration is intrinsically a time-dependent process, and a site’s vegetation characteristics at any given time depend on both the past (land use history, e.g. 66•) and the future (expected rates of change and their variability, e.g. 37•). The development of decision tools that can incorporate these variable temporal dynamics is a frontier of future landscape-scale restoration.
Many lines of information are needed if specific restoration interventions are to be effectively matched to sites and landscapes. This information includes the full menu of potential interventions, what outcomes might be expected over time from each type of intervention and with what feasibility and cost, in any given place. However, robust evidence relating to all these issues is generally unavailable. In particular, the evidence base for quantitative cost comparisons among different approaches is insufficient to support comparative analyses. Many of the interventions described previously have been limited to small-scale and short-term (often < 5 years) scientific trials that assessed the establishment of individual plants or early-stage woody vegetation, in case studies that focused on a single method. Their costs (and associated variability) have typically been either unreported or described in general terms only. Application of any technique over larger areas will bring economies of scale, which have been well developed for some methods only (e.g., seeding). Moreover, evaluating the ultimate success of different interventions depends on much longer-term assessments of physical structure, ecological functions and biological characteristics [12, 25]. Establishing comparative trials, coupled with monitoring of costs and outcomes, is an additional challenge for future research and practice.
Finally, in recent decades, both large-scale ecosystem destruction and large-scale revegetation have occurred in different regions globally, as consequences of decisions driven by socioeconomic and cultural factors (such as finances, competing land uses and new technologies) that are unrelated to environmental goals [92]. Conversely, the pressing contemporary environmental goal of restoration is likely to be achieved in some cases through actions that are social or economic rather than ecological in nature [93]. For example, these could include measures that encourage abandonment of agricultural activities in areas that have a high ecological potential for rapid regeneration with minimal intervention, and provision of alternative options for landholder livelihoods. Therefore, actions directed at economics and human behaviour are at least as fundamental to restoration as those involving ecological science and technology [1, 5, 15, 88].
Conclusions
Ecological restoration is needed over large aggregate land areas. Despite a range of strategic policy initiatives towards this end, maximising landscape-scale outcomes from targeted and cost-effective restoration interventions, while also conserving remaining natural ecosystems, is a current and future challenge [e.g. 3, 5, 20, 37, 94]. This review has described a growing array of options for less- or more-intensive interventions, but restoration science remains far from establishing the evidence base that is needed to make effective decisions about how, where, when and why to act.
If the goal of devising and implementing useful and economically feasible interventions for large landscape restoration is to be achieved, the disconnect between practice, evidence and policy will need to be bridged [4]. This means undertaking sound economic and ecological assessments of both the costs and outcomes of different methods, across multi-year sequences, in realistically scaled and well-replicated restoration projects. And that needs to be done multiple times, for a wide range of techniques, across many regions and ecosystems. To achieve this will require an order of magnitude increase in funding [3], establishment of processes and incentives for science-practice collaboration [7, 14] from design through to implementation and monitoring, and a much greater emphasis on innovation and risk-taking [14, 15]. Unpredictability of outcomes could be partially addressed by establishing both small-scale no-intervention plots and low-intervention trials and then monitoring to assess the extent of tree recruitment over a few years, before implementing large-scale or high-cost restoration projects [34, 36]. Within large-scale trials, there would also be considerable benefit from embedded small-scale research-driven experimental manipulations [7] to clarify ecological processes.
References
Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance
•• Chazdon RL. Landscape restoration, natural regeneration, and the forests of the future. Ann Mo Bot Gard. 2017;102(2):251–8 A comprehensive review of the potential of assisted and unassisted natural regeneration for restoring landscape forest cover.
Jones HP, Jones PC, Barbier EB, Blackburn RC, Rey Benayas JM, Holl KD, et al. Restoration and repair of Earth's damaged ecosystems. Proc R Soc B Biol Sci. 2018;285:20172577.
Menz MH, Dixon KW, Hobbs RJ. Hurdles and opportunities for landscape-scale restoration. Science. 2013;339:526–7.
Miller BP, Sinclair EA, Menz MH, Elliott CP, Bunn E, Commander LE, et al. A framework for the practical science necessary to restore sustainable, resilient, and biodiverse ecosystems. Restor Ecol. 2017;25(4):605–17.
Perring MP, Standish RJ, Price JN, Craig MD, Erickson TE, Ruthrof KX, et al. Advances in restoration ecology: rising to the challenges of the coming decades. Ecosphere. 2015;6(8):1–25.
Suding K, Higgs E, Palmer M, Callicott JB, Anderson CB, Baker M, et al. Committing to ecological restoration. Science. 2015;348:638–40.
Gellie NJ, Breed MF, Mortimer PE, Harrison RD, Xu J, Lowe AJ. Networked and embedded scientific experiments will improve restoration outcomes. Front Ecol Environ. 2018;16(5):288–94.
Koch A, Brierley C, Maslin MM, Lewis SL. Earth system impacts of the European arrival and Great Dying in the Americas after 1492. Quat Sci Rev. 2019;207:13–36.
Hall B, Motzkin G, Foster DR, Syfert M, Burk J. Three hundred years of forest and land-use change in Massachusetts, USA. J Biogeogr. 2002;29(10–11):1319–35.
Hobbs RJ, Walker LR. Old field succession: development of concepts. In: Cramer VA, Hobbs RJ, editors. Old fields: dynamics and restoration of abandoned farmland. Washington DC: Island Press; 2007. p. 15–30.
Young TP, Petersen DA, Clary JJ. The ecology of restoration: historical links, emerging issues and unexplored realms. Ecol Lett. 2005;8(6):662–73.
Hobbs RJ, Norton DA. Towards a conceptual framework for restoration ecology. Restor Ecol. 1996;4(2):93–110.
Nunez-Mir GC, Iannon BV, Curtis K, Fei S. Evaluating the evolution of forest restoration research in a changing world: a “big literature” review. New For. 2015;46(5–6):669–82.
•• Brancalion PH, van Melis J. On the need for innovation in ecological restoration. Ann Mo Bot Gard. 2017;102(2):227–37 Outlines how innovative approaches are critical for future success in landscape-scale restoration.
Stanturf JA. Future landscapes: opportunities and challenges. New For. 2015;46(5–6):615–44.
Stanturf JA, Palik BJ, Williams MI, Dumroese RK, Madsen P. Forest restoration paradigms. J Sustain For. 2014;33(sup1):S161–94.
Cramer VA, Hobbs RJ, Standish RJ. What's new about old fields? Land abandonment and ecosystem assembly. Trends Ecol Evol. 2008;23(2):104–12.
• Ghazoul J, Chazdon R. Degradation and recovery in changing forest landscapes: a multiscale conceptual framework. Annu Rev Environ Resour. 2017;42:161–88 A thorough review of the dynamic ecological processes involved in both degradation and restoration.
Chazdon RL, Guariguata MR. Natural regeneration as a tool for large-scale forest restoration in the tropics: prospects and challenges. Biotropica. 2016;48:716–30.
Brancalion PH, Niamir A, Broadben E, Crouzeilles RR, Barros FS, Zambrano AMA, et al. Global restoration opportunities in tropical rainforest landscapes. Sci Adv. 2019;5(7):eaav3223.
Crouzeilles R, Ferreira MS, Chazdon RL, Lindenmayer DB, Sansevero JB, Monteiro L, et al. Ecological restoration success is higher for natural regeneration than for active restoration in tropical forests. Sci Adv. 2017;3(11):e1701345.
Locatelli B, Catterall CP, Imbach P, Kumar C, Lasco R, Marín-Spiotta E, et al. Tropical reforestation and climate change: beyond carbon. Restor Ecol. 2015;23(4):337–43.
Meli P, Holl KD, JMR B, Jones HP, Jones PC, Montoya D, et al. A global review of past land use, climate, and active vs. passive restoration effects on forest recovery. PLoS One. 2017;12(2):e0171368.
Perring MP, Erickson TE, Brancalion PH. Rocketing restoration: enabling the upscaling of ecological restoration in the Anthropocene. Restor Ecol. 2018;26(6):1017–23.
Suding KN. Toward an era of restoration in ecology: successes, failures, and opportunities ahead. Annu Rev Ecol Evol Syst. 2011;42:465–87.
Abella SR, Chiquoine LP, Weigand JF. Developing methods of assisted natural regeneration for restoring foundational desert plants. Arid Land Res Manag. 2020;34(2):231–87.
Pérez DR, Farinaccio FM, Aronson J. Towards a dryland framework species approach. Research in progress in the Monte austral of Argentina. J Arid Environ. 2019;161:1–10.
Catterall CP. Roles of non-native species in large-scale regeneration of moist tropical forests on anthropogenic grassland. Biotropica. 2016;48:809–24.
Standish RJ, Cramer V, Yates CJ. A revised state-and-transition model for the restoration of woodlands in Western Australia. In: Hobbs RJ, Suding KN, editors. New models for ecosystem dynamics and restoration, vol. 169–88. Washington DC: Island Press; 2009. p. 30.
Holl KD, Aide TM. When and where to actively restore ecosystems? For Ecol Manag. 2011;261(10):1558–63. 31.
Ganade G. Processes affecting succession in old fields of Brazilian Amazonia. In: Cramer VA, Hobbs RJ, editors. Old fields: dynamics and restoration of abandoned farmland, vol. 75–92. Washington DC: Island Press; 2007. p. 29.
Elliot S, Blakesley D, Hardwick K. Restoring tropical forests: a practical guide. Kew: Royal Botanic Gardens; 2013.
Shoo LP, Freebody K, Kanowski J, Catterall CP. Slow recovery of tropical old field rainforest regrowth and the value and limitations of active restoration. Conserv Biol. 2016;30:121–32.
Reid JL, Fagan ME, Zahawi RA. Positive site selection bias in meta-analyses comparing natural regeneration to active forest restoration. Sci Adv. 2018;4(5):eaas9143.
Brancalion PH, Schweizer D, Gaudare U, Mangueira JR, Lamonato F, Farah FT, et al. Balancing economic costs and ecological outcomes of passive and active restoration in agricultural landscapes: the case of Brazil. Biotropica. 2016;48:856–67.
Holl KD, Zahawi RA. Factors explaining variability in woody above-ground biomass accumulation in restored tropical forest. For Ecol Manag. 2014;319:36–43.
• Shoo LP, Catterall CP, Nicol S, Christian R, Rhodes J, Atkinson P, et al. Navigating complex decisions in restoration investment. Conserv Lett. 2017;10(6):748–56 Illustrates the nature of method-cost-outcome interactions in restoration decision-making.
Zahawi RA, Reid JL, Holl KD. Hidden costs of passive restoration. Restor Ecol. 2014;22(3):284–7.
Shoo LP, Catterall CP. Stimulating natural regeneration of tropical forest on degraded land: approaches, outcomes, and information gaps. Restor Ecol. 2013;21(6):670–7.
Ammondt SA, Litton CM, Ellsworth LM, Leary JK. Restoration of native plant communities in a Hawaiian dry lowland ecosystem dominated by the invasive grass Megathyrsus maximus. Appl Veg Sci. 2013;16(1):29–39.
Elgar AT, Freebody K, Pohlman CL, Shoo LP, Catterall CP. Overcoming barriers to seedling regeneration during forest restoration on tropical pasture land and the potential value of woody weeds. Front Plant Sci. 2014;5:200. https://doi.org/10.3389/fpls.2014.00200.
Gunaratne AMTA, Gunatilleke CVS, Gunatilleke IAUN, Madawala Weerasinghe HMSP, Burslem DFRP. Barriers to tree seedling emergence on human-induced grasslands in Sri Lanka. J Appl Ecol. 2010;47(1):157–65.
Griscom HP, Griscom BW, Ashton MS. Forest regeneration from pasture in the dry tropics of Panama: effects of cattle, exotic grass, and forested riparia. Restor Ecol. 2009;17(1):117–26.
Méndez-Toribio M, Benítez-Malvido J, Zermeño-Hernández IE, Castillo-Mandujano J. Removal of climbing plants and soil plowing as a strategy to enhance forest recovery in tropical dry forests old fields. Ecol Restor. 2019;37(2):113–22.
Sapkota RP, Stahl PD. Effectiveness of passive restoration on density and diversity of regenerating tree species in mixed Dipterocarp forests of Nepal. Restor Ecol. 2019;27(3):581–91.
Zahawi RA, Holl KD, Cole RJ, Reid JL. Testing applied nucleation as a strategy to facilitate tropical forest recovery. J Appl Ecol. 2013;50(1):88–96.
Cuneo P, Gibson-Roy P, Fifiel G, Broadhurst L, Berryman T, Crawford A, et al. Restoring grassy woodland diversity through direct seeding: insights from six ‘best-practice’case studies in southern Australia. Ecol Manag Restor. 2018;19(2):124–35.
Rodrigues SB, Freitas MG, Campos-Filho EM, do Carmo GHP, da Veiga JM, Junqueira RGP, et al. Direct seeded and colonizing species guarantee successful early restoration of South Amazon forests. For Ecol Manag. 2019;451:117559.
• Grossnickle SC, Ivetić V. Direct seeding in reforestation–a field performance review. Reforesta. 2017;4:94–142 A comprehensive review of all aspects of direct seeding, very relevant to large-scale restoration.
Elliot S. The potential for automating assisted natural regeneration of tropical forest ecosystems. Biotropica. 2016;48(6):825–33.
Reid JL, Holl KD. Arrival ≠ Survival. Restor Ecol. 2013;2:153–5.
Carlo TA, Morales JM. Generalist birds promote tropical forest regeneration and increase plant diversity via rare-biased seed dispersal. Ecology. 2016;97:1819–31.
de Almeida A, Marques MCM, de Fatima Ceccon-Valente M, Vicente-Silva J, Mikich SB. Limited effectiveness of artificial bird perches for the establishment of seedlings and the restoration of Brazil’s Atlantic Forest. J Nat Conserv. 2016;34:24–32.
La Mantia T, Rühl J, Massa B, Pipitone S, Lo Verde G, Bueno RS. Vertebrate-mediated seed rain and artificial perches contribute to overcome seed dispersal limitation in a Mediterranean old field. Restor Ecol. 2019;27(6):1393–400.
Tomazi AL, Castellani TT. Artificial perches and solarization for forest restoration: assessment of their value. Trop Conserv Sci. 2016;9:809–31.
Stone MJ, Shoo L, Stork NE, Sheldon F, Catterall CP. Recovery of decomposition rates and decomposer invertebrates during rain forest restoration on disused pasture. Biotropica. 2019;52(2):230–41.
Shono K, Cadaweng EA, Durst PB. Application of assisted natural regeneration to restore degraded tropical forestlands. Restor Ecol. 2007;15(4):620–6.
Charle LS, Dwyer JM, Chapman HM, Yadok BG, Mayfield MM. Landscape structure mediates zoochorous-dispersed seed rain under isolated pasture trees across distinct tropical regions. Landsc Ecol. 2019;34(6):1347–62.
Laborde J, Guevara S, Sánchez-Ríos G. Tree and shrub seed dispersal in pastures: the importance of rainforest trees outside forest fragments. Ecoscience. 2008;15:6–16.
Zahawi RA, Augspurger CK. Tropical forest restoration: tree islands as recruitment foci in degraded lands of Honduras. Ecol Appl. 2006;16(2):464–78.
Hol KD, Loik ME, Lin EHV, Samuels IA. Tropical montane forest restoration in Costa Rica: overcoming barriers to dispersal and establishment. Restor Ecol. 2000;8:339–49.
García-Orth X, Martínez-Ramos M. Isolated trees and grass removal improve performance of transplanted Trema micrantha (L.) Blume (Ulmaceae) saplings in tropical pastures. Restor Ecol. 2011;19(1):24–34.
Lamb D, Erskine PD, Parrotta JA. Restoration of degraded tropical forest landscapes. Science. 2005;310:1628–32.
Brancalion PH, Amazonas NT, Chazdon RL, van Melis J, Rodrigues RR, Silva CC, et al. Exotic eucalypts: from demonized trees to allies of tropical forest restoration? J Appl Ecol. 2020;57(1):55–66.
Bechara FC, Dickens SJ, Farrer EC, Larios L, Spotswood EN, Mariotte P, et al. Neotropical rainforest restoration: comparing passive, plantation and nucleation approaches. Biodivers Conserv. 2016;25(11):2021–34.
• Holl KD, Reid JL, Chaves-Fallas JM, Oviedo-Brenes F, Zahawi RA. Local tropical forest restoration strategies affect tree recruitment more strongly than does landscape forest cover. J Appl Ecol. 2017;54(4):1091–9 Illustrates many aspects of uncertainty in outcomes of forest restoration, the associated processes, and alternative approaches.
Paul M, Catterall CP, Pollard PC, Kanowski J. Recovery of soil properties and functions in different rainforest restoration pathways. For Ecol Manag. 2010;259(10):2083–92.
Kanowski J, Catterall CP, Neilan W. The potential value of weedy regrowth for rainforest restoration: the case of camphor laurel in north-East New South Wales. Ecol Manag Restor. 2008;9:88–99.
Sample M, Aslan CE, Policelli N, Sanford RL, Nielsen E, Nuñez MA. Increase in nonnative understorey vegetation cover after nonnative conifer removal and passive restoration. Austral Ecol. 2019;44(8):1384–97.
César RG, Moreno VS, Coletta GD, Chazdon RL, Ferraz SF, de Almeida DR, et al. Early ecological outcomes of natural regeneration and tree plantations for restoring agricultural landscapes. Ecol Appl. 2018;28(2):373–84.
Lindenmayer DB, Blanchard W, Crane M, Michael D, Sato C. Biodiversity benefits of vegetation restoration are undermined by livestock grazing. Restor Ecol. 2018;26(6):1157–64.
Posada JM, Aide TM, Cavelier J. Cattle and weedy shrubs as restoration tools of tropical montane rainforest. Restor Ecol. 2000;8(4):370–9.
Olff H, Vera FWM, Bokdam J, Bakker ES, Gleichman JM, De Maeyer K, et al. Shifting mosaics in grazed woodlands driven by the alternation of plant facilitation and competition. Plant Biol. 1999;1(2):127–37.
Pausas JG, Bond WJ. Humboldt and the reinvention of nature. J Ecol. 2019;107(3):1031–7.
Catterall CP. Fauna as passengers and drivers in vegetation restoration: a synthesis of processes and evidence. Ecol Manag Restor. 2018;19:54–62.
McAlpine C, Catterall CP, Mac Nally RM, Lindenmayer D, Reid JL, Holl KD, et al. Integrating plant-and animal-based perspectives for more effective restoration of biodiversity. Front Ecol Environ. 2016;14(1):37–45.
Hayward MW, Scanlon RJ, Callen A, Howell LG, Klop-Toker KL, Di Blanco Y, et al. Reintroducing rewilding to restoration–rejecting the search for novelty. Biol Conserv. 2019;233:255–9.
Bellingham PJ, Kardol P, Bonner KI, Buxton RP, Morse CW, Wardle DA. Browsing by an invasive herbivore promotes development of plant and soil communities during primary succession. J Ecol. 2016;104(6):1505–17.
Griffiths CJ, Zuel N, Jones CG, Ahamud Z, Harris S. Assessing the potential to restore historic grazing ecosystems with tortoise ecological replacements. Conserv Biol. 2013;27(4):690–700.
Hobbs RJ, Higgs E, Hall CM, Bridgewater P, Chapin FS III, Ellis EC, et al. Managing the whole landscape: historical, hybrid, and novel ecosystems. Front Ecol Environ. 2014;12(10):557–64.
Radeloff VC, Williams JW, Bateman BL, Burke KD, Carte SK, Childress ES, et al. The rise of novelty in ecosystems. Ecol Appl. 2015;25:2051–68.
Harris JA, Hobbs RJ, Higg E, Aronson J. Ecological restoration and global climate change. Restor Ecol. 2006;14(2):170–6.
Lugo AE. Novel tropical forests: nature’s response to global change. Trop Conserv Sci. 2013;6:325–37.
Catterall CP. Values of weedy regrowth for rainforest restoration. Ecol Manag Restor. 2020;21(1):9–13.
Kettenring KM, Adams CR. Lessons learned from invasive plant control experiments: a systematic review and meta-analysis. J Appl Ecol. 2011;48(4):970–9.
Prior KM, Adams DC, Klepzig KD, Hulcr J. When does invasive species removal lead to ecological recovery? Implications for management success. Biol Invasions. 2018;20(2):267–83.
Evers CR, Wardropper CB, Branoff B, Granek EF, Hirsch SL, Link TE, et al. The ecosystem services and biodiversity of novel ecosystems: a literature review. Glob Ecol Conserv. 2018;13:e00362.
Guerrero AM, Shoo L, Iacona G, Standish RJ, Catterall CP, et al. Using structured decision-making to set restoration objectives when multiple values and preferences exist. Restor Ecol. 2017;25(6):858–65.
Catterall CP, Freeman AND, Kanowski J, Freebody K. Can active restoration of tropical rainforest rescue biodiversity? A case with bird community indicators. Biol Conserv. 2012;146(1):53–61.
Tambosi LR, Martensen AC, Ribeiro MC, Metzger JP. A framework to optimize biodiversity restoration efforts based on habitat amount and landscape connectivity. Restor Ecol. 2014;22(2):169–77.
Tobón W, Urquiza-Haas T, Koleff P, Schröter M, Ortega-Álvarez R, Campo J, et al. Restoration planning to guide Aichi targets in a megadiverse country. Conserv Biol. 2017;31(5):1086–97.
Meyfroidt P, Chowdhury RR, de Bremond A, Ellis EC, Erb KH, Filatova T, et al. Middle-range theories of land system change. Glob Environ Chang. 2018;53:52–67.
Latawiec AE, Strassburg BBN, Brancalion PHS, Rodrigues RR, Gardner T. Creating space for large-scale restoration in tropical agricultural landscapes. Front Ecol Environ. 2015;13:211–8.
Holl KD, Brancalion PH. Tree planting is not a simple solution. Science. 2020;368(6491):580–1.
Acknowledgments
I thank many past and present collaborators (especially Kylie Freebody, John Kanowski and Luke Shoo) for helping to stimulate the development of ideas around which this review is structured, and Griffith University for providing resources to support the work.
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Catterall, C.P. Influencing Landscape-Scale Revegetation Trajectories through Restoration Interventions. Curr Landscape Ecol Rep 5, 116–126 (2020). https://doi.org/10.1007/s40823-020-00058-5
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DOI: https://doi.org/10.1007/s40823-020-00058-5