Primary Production in Terrestrial Ecosystems: Patterns and Controls in a Changing World

Abstract

Primary production is the process by which solar energy is converted to chemical energy by autotrophic organisms, primarily green plants on land, providing the energy available to power earth’s ecosystems. In this process atmospheric CO₂ is incorporated into organic matter, thereby playing a dominant role in the global carbon cycle with crucial implications for global climate change. Net primary production (NPP) is the amount of fixed energy or organic matter left over after the plants have met their own respiratory needs and represents the amount of energy available to the consumers, including humans. Across the earth’s terrestrial biomes, a large range of NPP is observed with the highest values in tropical forests and wetlands, intermediate values in temperate forests and grasslands, and lowest in extremely cold or dry deserts.

Accurate measurement of NPP is challenging despite the simple concept that it represents the amount of new biomass added to the plants in a given time period. This is because a significant and highly variable proportion of NPP is lost from the plants by processes such as herbivory, volatilization, and carbon flux to the soil. Methods of measuring NPP are diverse, being dependent on the structure and dynamics of the vegetation. For example, harvest methods in which the aboveground tissues are periodically clipped from quadrats of known area can be effective for quantifying aboveground NPP in herbaceous vegetation (e.g., grasslands), whereas in woody vegetation, the growth of woody tissues must also be measured. Moreover, measurements of total NPP in terrestrial ecosystems must account for root growth which can be very challenging. As a result, reliable estimates of total NPP are few.

Plants allocate a large proportion of their fixed energy to their root systems to fuel additional root growth and to meet their respiratory needs. The proportion of total NPP that goes to belowground NPP ranges from about 25 % to over 50 % and is higher in ecosystems where the degree of limitation by soil resources is greater, i.e., dry or nutrient-poor sites. Surprisingly, over 10 % of NPP is contributed by plants to the soil in the form of rhizosphere carbon flux including exudation, rhizodeposition, and allocation to mycorrhizal fungi and other symbionts.

Variation in NPP results from differences among ecosystems in the amount of photosynthetically active radiation (PAR) reaching the plant canopy, the amount of that PAR absorbed by the foliage (APAR), the biochemical efficiency of the plants under optimal environmental conditions, and the degree to which actual conditions are less than optimal. The APAR depends in part on the amount of foliage surface area per unit ground area (leaf area index – LAI) which ranges from less than 1 in dry or infertile sites to over 10 in some resource-rich forests. Large-scale monitoring of estimated NPP is possible using satellite imagery of reflected solar radiation that can be converted into vegetation LAI and combined with environmental measurements that indicate the degree of stress reduction to photosynthetic activity.

Four principal abiotic factors usually limit the amount of NPP on land – light, water, temperature, and mineral nutrients – and all these abiotic factors are changing rapidly as a result of human activity, with highly uncertain implications for global and local NPP. Commonly, two or more of these abiotic factors concurrently or sequentially limit NPP, but water deficit is arguably the most widespread single factor constraining global NPP. The effect of temperature on NPP is most closely related to subfreezing conditions that limit the length of the growing season in temperate and high-latitude environments. Nitrogen is the most important limiting mineral nutrient in most ecosystems, although in highly weathered tropical soils where nitrogen-fixing organisms are abundant, phosphorus may be the most limiting nutrient.

Biotic factors can play a key role in regulating NPP so that human activities such as vegetation management and introduction of exotic species will exert a major influence on future patterns of NPP. The effects of biodiversity on NPP have proven difficult to establish, but experimental tests suggest that loss of species can reduce NPP particularly if a dominant species is lost or when species numbers become very low, diminishing complementarity in resource use by coexisting species. Dramatic shifts in plant community structure, for example, the ongoing invasion of grassland vegetation by woody plants, can cause changes in NPP that appear to depend in part on climate. Consumption of plant tissues by herbivores often can have a negative effect on NPP, but in many grasslands, compensatory growth responses to herbivory can result in no reduction in NPP or in some cases even stimulation of NPP by herbivory.

Temporal variation in NPP results from interannual variation in both environmental and biotic factors as well as pulse disturbance events that can reset the successional clock. The response of NPP to interannual variation in rainfall seems to be greatest in semiarid and subhumid environments where average precipitation is sufficient to sustain highly productive communities (vs. true deserts). Following natural or human disturbances, forests exhibit a recurring pattern in which NPP peaks after a few decades of stand development, followed by a decline with age in older stands.

Global environmental changes – climate, atmospheric CO₂, nitrogen deposition, exotic species introductions, etc. – are certain to exert a major influence on global NPP in the future, but the outcomes are highly uncertain because of the complex ways in which all these changes interact with one another to influence the vegetation and NPP. For example, CO₂ enrichment experiments indicate that increasing atmospheric CO₂ concentration can significantly stimulate NPP in young forests, but the effect may be transient because of progressively greater stress by mineral nutrients – unless high N deposition overcomes this limitation.