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The enigma of terrestrial primary productivity

Review by Irena Šímová and David Storch won the first (2015) Ecography E4 award (The Ecography Award for Excellence in Ecology and Evolution). Paper tackle an important but highly challenging topic, the measurement and, estimation, and biogeographical role of terrestrial primary productivity (NPP) on a global scale. The review is thorough and in-depth in its coverage and remarkably successful in explaining complicated problems.

Plants need water, sunlight and sufficient temperature to grow. It is thus no surprise that warm and wet tropical forests with year-long vegetation season are considered the most productive biome on the Earth. Tropical forests are also extremely diverse, productivity being often considered as a major factor limiting species richness. Nevertheless, scientists still disagree on the global productivity gradient, and there are actually many different models that predict the variation of productivity on Earth’s surface. We have reviewed different global productivity models and available sources of the field measurements of biomass production to reconcile different approaches to terrestrial productivity. 

Productivity, i.e. the amount of biomass (or organic carbon) produced by plants per unit area and time, cannot be directly measured at large spatial scales. Instead, we need to rely on various models. Some of them simply assume that productivity is a function of temperature, precipitation or solar radiation. Other models are more complicated, including remotely sensed data of vegetation greenness or the dynamic response of vegetation under the changing climate. The models differ in their assumptions and input variables, and they consequently differ also in their outputs comprising geographic trends in productivity. This is somehow frustrating, given that productivity is a key ecological variable, which is often used in macroecological and global change studies. It is even possible that so far there has been no consensus concerning the relationship between productivity and species richness simply due to our inability to properly measure and estimate productivity.

Which global productivity model is closest to the reality? The most obvious way of model validation would comprise using the data of ecosystem productivity directly measured in the field. However, although both the field data and the majority of productivity models agree on the general decrease of productivity from the equator towards the poles, there is high discrepancy between modelled and measured values within individual climatic zones. 

We outline three main reasons for this variability. First, field data are sampled at much finer scales than those used in models, so they do not represent random samples of the whole landscape. Second, besides water and sunlight, plants need nutrients in order to build their tissues. Whereas some models incorporate the effect of nitrogen availability on plant growth, the role of phosphorus, a key limiting nutrient in the tropics remains largely overlooked. Third, field data are rather heterogeneous, comprising a mix of natural vegetation and plantations.

We further show that the large-scale relationship between productivity and species diversity is not particularly sensitive to the exact way productivity is estimated. However, as the models assume that productivity is a function of climatic variables, it is difficult to distinguish the pure effect of productivity from the effect of other important variables such as temperature or environmental stability. To address this issue, it is necessary to develop a proper theory that would encompass the effect of productivity simultaneously with the other factors potentially affecting species richness. In order to do this, it is first necessary to refine the methods of measuring productivity.

We thus strongly recommend future studies to sample productivity in the field by a standardized protocol, so that these data could be used to parameterize global productivity models. Such models then should combine all the major variables that limit plant growth, including nutrients. The era of Anthropocene is characterized by fast rates of habitat degradation and human-induced climate changes. Productivity is a variable of primary interest as it is related both to resource availability, potentially limiting biological diversity, and to the dynamics of the carbon cycle. A clear consensus on global productivity patterns would enable us to understand the drivers of biodiversity variation, and to make better predictions of vegetation and biodiversity changes in the near future. 

Tropical rain forest
Monteverde, Costa Rica
(photoDavid Storch)

Temperate rain forest,
Washington, USA
(foto Irena Šímová)

Published: Jan 30, 2017 11:25 AM

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