Introduction

Disturbances can have significant consequences on many ecological processes including vegetation regeneration (Shive et al. 2018). Disturbances may negatively or positively affect the regeneration of plant species. The negative impacts of disturbance on the population dynamics and regeneration attributes of woody species have been widely investigated (de Winter et al. 2018; Swaim et al. 2018; Awoke and Mewded 2019). The physical damage of the species in different vegetation stages (seedling, sapling and mature individuals) and environment modifications (soil nutrient depletion, microclimate irregularity, canopy cover alteration) caused by anthropogenic disturbance negatively affect the species regeneration, population structure and recruitment. From the other paradigm little to intermediate disturbance has a positive influence on species prominence through instigating the important traits of regeneration such as dispersal, colonization, germination, emergence and re-sprouting of the species (Chapagain et al. 2021). However, the positive effect of disturbance on the tree/shrub species regeneration is usually less considered (Funk et al. 2016; Gillison 2019; Heilmeier 2019).

The comparative analysis of regeneration in disturbed and undisturbed sites is among the important methods to investigate the impact of anthropogenic disturbance on regeneration and population recruitment of a certain species. The result is also vital for further evaluation of the species’ conservation status and trends and vigorous for predicting the sustainability of the species (Wiehle et al. 2016). Predicting the ecological effects of anthropogenic disturbance relies on a solid understanding of the resistance and resilience nature of the species. From a practical point of view, such analysis could also be important in order to take proper actions towards the main drivers and design specific management plans for in-situ conservation and other schemes such as field gene banks and botanical gardens.

Vepris dainellii (Pichi-Serm.) Kokwaro is a shade-tolerant evergreen shrub or tree species under the Rutaceae family. The species is endemic to Ethiopia and primarily occurs in moist forests. Vepris dainellii is the least concerned species in IUCN red least (Vivero et al. 2005) and rarity (Magurran 2004) category while it had the least number of individuals in the country (< 100,000) according to local criteria (Bekele et al. 1999). Vepris dainellii species is one of the top ten ecological important species in Sirso moist evergreen Afromontane forest of southern Ethiopia (Mewded et al. 2019).

This species is traditionally utilized by the local community for different purposes. Its termite-resistant hardwood is highly preferred by the people for house construction and farm tools (i.e. Mofer, Kenber, Deger, Eref, Metrebia and Doma). Seed, fruit and bark of the species are also utilized for the treatment of intestinal worms, headaches, skin diseases and tooth pain. All mature, premature and immature seeds of V. dainellii are harvested for hot drinks while the preparation is different. Ruta chalepensis, Ocimum basilicum, Stellaria media, Coriandrum sativum, Allium sativum, Capsicum annuum, Zingiber officinale, Artemisia annua, Cyperus spp. and a leaf of Coffea arabica are additive medicinal and spice plant species often taken with V. dainellii seeds as a hot drink. Despite the species currently cultivated in home gardens due to its extraordinary use, it has overexploited from the wild for house use as well as commercial purposes in local markets.

Although V. dainellii have experienced several human influences, there is no comprehensive study that covers the stand and regeneration dynamics of V. dainellii in disturbed and undisturbed sites of moist forests. The observed overexploitation of the ecologically important species V. dainellii in Sirso moist evergreen forest has initiated great concern because significant reductions in the species’ abundance might likely have profound effects on the moist forest ecosystem. Based on the previous studies and our observation of anthropogenic disturbance particularly clear-cut of mature trees and harvest of the mature, premature and immature seeds from parent trees, we hypothesized that (1) regeneration and population recruitment of V. dainellii species in Sirso moist evergreen Afromontane forest is poor, (2) the regeneration of V. dainellii in the undisturbed site is better when compared to the disturbed site, and (3) the regeneration of V. dainellii is negatively correlated with the clear-cut intensity. Thus, the capacity of the species to maintain its wild population is significantly reduced. Therefore, our study aims to examine the comparative analysis of regeneration and population recruitment status of V. dainellii in disturbed sites versus undisturbed sites of the Sirso moist evergreen forest and analyze the correlation of the V. dainellii regeneration with its clear-cut intensity, canopy cover, altitude, slope and aspect of the forest.

Materials and methods

Description of study area

The study was conducted at Sirso natural forest of Melokoza district which is located between 6°18′–6°42′ N and 36°00′–37°00′ E in Gamo Gofa Zone of South Nation Nationality People’s Regional State, Ethiopia (Fig. 1). Sirso natural forest has 3501 ha land coverage with typically mountainous and some extent undulate and flat landscapes. Its elevation and slope range from 1547–2707 m and 5–75%, respectively (Mewded and Lemessa 2019; Mewded et al. 2019). Sirso natural forest is one of the moist evergreen Afromontane forests found in Melokoza district (Friis et al. 2010; Mewded et al. 2019) and comprises V. dainellii and other economically important plant species, for example, Coffea arabica, Aframomum corrorima, Ensete ventricosum, Prunus africana and Cordia africana. Syzygium guineense, Schefflera abyssinica and Galiniera saxifraga are the top ecologically important and dominant canopy tree species in Sirso natural forest (Mewded et al. 2019). In addition to the above-mentioned species, Hagenia abyssinica, Lippia adoensis, Vernonia spp., Acanthus spp., Dombeya torrida, Pentas schimperiana and Ficus spp. are among the honey bee floras. Rubiaceae is the dominant family followed by Euphorbiaceae and Asteraceae. The climate data from the nearest meteorology station indicated the average annual rainfall ranges 1200–1300 mm and temperature of 14–24 °C. The main soil type of the study area belongs to alisols which is the common soil type in tropics regions (Amajo 2018).

Fig. 1
figure 1

Map of the study area, Sirso moist evergreen Afromontane forest and sample plots; red marked sample plots are where Vepris dainellii (Pichi-Serm.) Kokwaro species recorded

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Sampling design

A transect sampling technique was employed to collect vegetation data of V. dainellii species (Mueller-Dombois and Ellenberg 1974; Kent 2012). Five transects were laid out across the altitudinal gradients from lower to upper altitudes. Transects were aligned parallel at 2 km intervals and 50 main plots (each, 20 × 20 m size) were systematically laid down at 100 m altitudinal intervals along the transect lines. According to Yirdaw et al. (2015), environmental factors such as temperature, rainfall and radiation and soil characteristics might vary within 100 m altitudinal variation and consequently species distribution. Within the main plots, five 2 m × 2 m size sub-plots were established, four in each corner and one in the middle, for seedling and sapling inventory. Totally 50 main plots and 250 sub-plots were used for exploring V. dainellii species in Sirso moist evergreen forest. Sample plots, which comprise V. dainellii were used to record, count and measure the vegetation data (mature individuals, seedlings and saplings) and stumps or timber pieces of V. dainellii species.

Data collection

In all sample plots, V. dainellii species were assessed in seedling, sapling and mature growth forms. Individuals of the species with a DBH ≥ 2.5 cm and height ≥ 1.5 m were measured and recorded as mature. Saplings and seedlings were only counted. Saplings were DBH < 2.5 cm and height ≥ 1.5 m; seedlings with root collar diameters < 2.5 cm and heights < 1.5 m. The stump individuals (refers to timber pieces resulting from the illegal cutting of trees by machete) of the species were recorded in each sample plot. The DBH (at 1.3 m) and tree height were measured using Diameter Tape and Hypsometer (Forestry Pro), respectively. Geographical location, altitude and aspect of sample plots were recorded using GPS (Garmin 72). The slope of the sample plots was also recorded using Suunto Clinometer.

The extent of anthropogenic activities in each plot was assessed following an arbitrary scale of disturbance with a particular focus on logging or clear-cut of individuals of species, non-timber forest product exploitation (i.e., A. corrorima and C. arabica), agricultural expansion, grazing (harvesting of ground vegetation cover including seedlings by cattle and equine), browsing (refers to the shots and leaf of saplings, shrubs and trees pollarding by Goats and Sheep), charcoal production, fire, debarking, honey production and tracking (roads for people and cattle inside the forest). Following the methods in Ethiopian Biodiversity Institute forest inventory and genetic resource conservation manual, the disturbance level was estimated and categorized under a zero to five ordinal scale, where zero is sample plots with no disturbance, 1 (negligible), 2 (very light), 3 (light), 4 (moderate) and 5 (intensive) disturbance. Moreover, the canopy cover of each sample plot was estimated.

Data analysis

First, sample plots in which V. dainellii were recorded were selected and grouped into two. These are: 1) plots with none and negligible disturbance were categorized as undisturbed sites and 2) plots with very light, light, moderate and intensive disturbance were categorized as disturbed sites. Then histograms were constructed to show the recruitment patterns of the species in disturbed sites, undisturbed sites and in the whole forest based on the ratios of seedling to sapling to mature individuals. The differences between the two categorized groups (disturbed and undisturbed) with regards to regeneration (sum of seedlings and saplings) were statistically tested using a one-way analysis of variance (ANOVA) with the R statistical program (version: 3.4. 3). Furthermore, the relation between the regeneration of the species with its clear-cut intensity, forest canopy cover, altitude, slope and aspect was tested using Pearson's correlation using the R statistical program.

Results

Vepris dainellii was recorded in 29 of 50 sample plots at least in one growth form (i.e., seedling, sapling, or mature). Mature individuals of the studied species were recorded in 24 plots, saplings in 16 plots and seedlings in 21 plots. The species were found in the altitude range of 1607–2507 masl in the studied forest. The disturbance level of 11 plots was negligible and categorized under undisturbed site while others were very light to intensively interrupted and categorized under disturbed site.

Recruitment and regeneration structure of Vepris dainellii

The recruitment structure of V. dainellii species shows a positively skewed recruitment pattern which is an indicator of healthier population status. The number of seedlings is greater than saplings and the number of saplings is greater than mature individuals in all disturbed, undisturbed and the whole forest ecosystem (Fig. 2). However, the average seedling, sapling and mature individuals per plot in disturbed sites (58.9, 42.2 and 7.5, respectively) were greater than in undisturbed sites (45.5, 21.8 and 2, respectively). One way analysis of variance revealed that the regeneration of V. dainellii was higher in disturbed sites compared to undisturbed sites while the variation was not significant at P < 0.05 (Fig. 3).

Fig. 2
figure 2

Recruitment structure of Vepris dainellii in the whole forest (a), in disturbed site (b) and undisturbed site (c)

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Fig. 3
figure 3

The regeneration of Vepris dainellii species per plot (400 m2) in disturbed and undisturbed sites

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Correlation between regeneration and clear-cut intensity

Totally 398 bowdlerized stems of 32 woody species were recorded (Fig. 4). The clear-cut variation within the species was significant (P < 0.01). Vepris dainellii is the first ranked woody species by accounted 56 bowdlerized stems. The clear-cut of V. dainellii species was recorded in 18 of 29 sample plots whereas there is no clear-cut recorded in the remaining 11 sample plots. The maximum clear-cut recorded per plot was nine. The regeneration of V. dainellii species and the number of clear-cut or timber pieces had a positive and strong correlation with an R-value of 0.735 (Fig. 5).

Fig. 4
figure 4

Clear-cut variations of woody species in the studied forest

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Fig. 5
figure 5

Correlation of clear-cut intensity and regeneration of Vepris dainelli per plot (400 m2) in disturbed site

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Correlation of regeneration with canopy cover, altitude, slope and aspect

Although the canopy covers of the sample plots ranged from 35–86%, the regeneration status of V. dainellii species was not correlated with the sample plots’ canopy cover. Often altitude is mentioned as the main factor for the spatial variation of species regeneration. Meanwhile, the studied species were recorded in a wide altitudinal range (900 m). However, the regeneration status had no significant correlation with the altitudinal gradients. Moreover, the regeneration of V. dainellii did not correlate with the forest’s slope and aspect (Fig. 6).

Fig. 6
figure 6

Correlation of Vepris dainelli regeneration per plot (400 m2) with forest canopy cover (%), altitude, slope (%) and aspect

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Discussion

The average clear-cut intensity of V. dainellii in Sirso moist evergreen forest is 3.1 per plot. The recruitment pattern of V. dainellii species is positively skewed by exhibiting more seedlings than saplings and more saplings than mature individuals which is an indicator of healthy regeneration. This infers rather extensive disturbance was observed; the species in the study forest have good potential to regenerate and ensure the perpetuation. Though, when we compare the recruitment and regeneration pattern of the species across the sample plots, it is healthier in disturbed plots than in undisturbed plots. Although several scholars reported the negative impact of disturbance on regeneration and population structure of species through alteration of seed source, soil characteristics and incoming radiation, comparable results were also reported for Magnolia schiedeana (Vasquez-Morales et al. 2017), Miombo woodland species (Syampungani et al. 2016), Acacia senegal (Omondi et al. 2017), Mallotus philippensis (Chapagain et al. 2021), Celtis mildbraedii and Nesogordonia papaverifera (Hammond et al. 2021) and Mallotus spp. (Gautam et al. 2016). According to the reports of these studies, species respond differently to different environmental and anthropogenic stress. For example, some species are known to have the capacity to resilience and resistance to the impact of disturbances through regeneration strategies (Johnstone et al. 2016). The higher regeneration evidence of V. dainellii is indicating it is among these which have regeneration strategies for anthropogenic disturbance. As stated by Vasquez-Morales et al. (2017) study, individuals of species in the presence of disturbance tend to produce a larger number of polyfollicles than those at undisturbed sites. Consequently, the higher seed production makes a difference in terms of regeneration and population growth. Species with multiple regeneration strategies are more likely to inhabit a variety of environments and insurance for surviving an unpredictable disturbance (Banihashemi et al. 2018). The pattern of seed production differs among populations of the same species in response to environmental and disturbance factors. Likewise, Gautam et al. (2016) report various types of disturbances can promote seed germination. The disturbance responses of V. dainellii species have also been enhanced by the productive nature of the forest (Seidl et al. 2015). Occasionally associated tree and shrub species could determine the regeneration patterns of the other species (Johnson et al. 2021). The dominant associated species of V. dainellii in disturbed site are Galiniera saxifraga, Dracaena fragrans, Croton macrostachyus and Millettia ferruginea. However, these species are also dominant in undisturbed sites as well as the whole forest and are among the top eleventh ecologically important tree species. Other resilient species have slow growth attributes, including long-life histories, delayed and intermittent reproduction (Vasques et al. 2022). Re-sprouting from vegetative tissue is also the other strategy of plants for the response to disturbance (Zeppel et al. 2015).

The significant variation of clear-cut intensity of woody species in Sirso moist evergreen forest shows selective logging is the major threat to the forest. Selective logging can significantly disturb the ecological succession, create opportunities for invasive species and reduce the biomass of a tropical forest (Cazzolla Gatti et al. 2015; Poudyal et al. 2018). The tolerant regeneration trend of V. dainellii species in the disturbed site and its positive correlation with clear-cut intensity (73.5%) is vital strategies and evolutionary implications for the responses of direct or indirect disturbance induced factors such as soil, light, moisture, nutrient and habitat succession (Vasquez-Morales et al. 2017). The species may develop functional traits such as plasticity of morphological, physiological and developmental traits in the response of disturbance and ecotype variation. A persistent seed bank is used as a predictor of the resilience of species following vegetation disturbance (Long et al. 2015; Vasques et al. 2022). The study by Hammond et al. (2021) examines the lower average regeneration density of trees at the undisturbed site than at intermediate and disturbed sites due to the multilayered, complex canopy structure and consequent litter biomass which prevents seeds from reaching the forest floor. Moreover, relationships between canopy cover and tree regeneration could determine for various species in forests (Johnson et al. 2021) through its openness creating the chance to expose the seeds in soil seed banks to sunlight and enhances the germination and growth. However, the regeneration prominence of V dainellii was not correlated with the forest canopy cover. This result is inconsistence with either the reports of Stevens et al. (2015); canopy disturbance has a negative effect on the number of seedlings or Johnson et al. (2021); species exhibited a positive regeneration response to the total size of the proximate opening. Mostly altitude and in some cases aspect and slope are the important factors for regeneration variation (negative or positive effect) of tree species. But, in Sirso moist evergreen forest, none of these factors affected the regeneration of V. dainellii. Reasonably this species is among “auto succession” or the “direct regenerated” species (Rodrigo et al. 2004). Accordingly, the ecological implication of direct regeneration of this plant species is highly resilience and adapted to disturbance.

The adaptive strategies of the established and regenerative phase in the life history of dominant plants are useful for the prediction of their sustainability. The present study investigates that V. dainellii species is resilient to disturbance and its regeneration and recruitment potential has positively influenced. Hence, patterns of regeneration following anthropogenic disturbances are determined by interactions between the disturbance regime such as intensity, frequency, scale and species physiology (Darrigo et al. 2016; de Carvalho et al. 2017; Dyderski and Jagodzinski 2018; Chapagain et al. 2021), this study recommends further detail researches of how this species is disturbance resilience or examine the adaptation mechanisms of disturbance regimes, the thresholds of clear-cut intensity for sustaining the positive influence or how regeneration and stasis would respond to larger disturbances like deforestation and fire. Moreover, it needs to explore its effects on the whole forest ecosystem functioning, plant diversity and structure. It may affect the species richness and diversity as a successor itself or/and trigger the succession of other competent species.

Conclusion

Our study examines the regeneration response of Vepris dainellii species to anthropogenic disturbance. Predicting the biological effects of anthropogenic disturbances and planning a proper conservation strategy relies on a solid understanding of these processes. The pattern of regeneration differs among populations of the V. dainellii species in response to environmental and disturbance factors. Nevertheless, the seed and hardwood of V. dainellii species is vastly exploited by the local community, it is a resilient species through regeneration strategies. Meanwhile, the species regeneration is promoted by the disturbance factors of specifically clear-cut and positively related to the clear-cut intensity in the presence of mother trees (seed sources). Though, our study suggested detailed studies on the threshold of clear-cut intensity and the functional traits of the species in response to disturbance in moist forest ecosystems.