Elevated CO2 as a driver of global dryland greening
Abstract
While recent findings based on satellite records indicate a positive trend in vegetation greenness over global drylands, the reasons remain elusive. We hypothesize that enhanced levels of atmospheric CO2 play an important role in the observed greening through the CO2 effect on plant water savings and consequent available soil water increases.
Meta-analytic techniques were used to compare soil water content under ambient and elevated CO2 treatments across a range of climate regimes, vegetation types, soil textures and land management practices.
Based on 1705 field measurements from 21 distinct sites, a consistent and statistically significant increase in the availability of soil water (11{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117}) was observed under elevated CO2 treatments in both drylands and non-drylands, with a statistically stronger response over drylands (17{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} vs. 9{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117}). Given the inherent water limitation in drylands, it is suggested that the additional soil water availability is a likely driver of observed increases in vegetation greenness.
Introduction
Defined broadly as zones where mean annual precipitation is less than two-thirds of potential evaporation, drylands are critically important systems and represent the largest terrestrial biome on the planet. Climate change, increasing populations and resulting anthropogenic effects are all expected to impact dryland regions over the coming decades. Considering that approximately 90{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} of the more than 2 billion people living in drylands are geographically located within developing countries, improved understanding of these systems is an international imperative.
Recent regional scale analyses using satellite based vegetation indices such as the Normalized Difference Vegetation Index (NDVI), have found extensive areas of “greening” in dryland areas of the Mediterranean, the Sahel, the Middle East and Northern China, as well as greening trends in Mongolia and South America. More recently, a global synthesis over the period from 1982–2007 that used an integrated NDVI and annual rainfall, showed an overall “greening-up” trend over the Sahel belt, Mediterranean basin, China-Mongolia region and the drylands of South America.
To better predict system responses to possible climate changes, it is necessary to understand the drivers behind the observed greening response. Several mechanisms may contribute to the apparent trends in vegetation greenness. For example, increasing rainfall is one obvious driver of change, with a number of studies establishing a positive relationship between NDVI and precipitation. However, rainfall does not explain the observed trends at a global scale. Indeed, there are regions where greening occurs in the absence of any observed rainfall increases.
Likewise, there are areas where a significant rainfall increase occurs without a corresponding change in greening. In addition, even in those regions experiencing concurrent greening and rainfall increase (such as in the African Sahel), removing the effects of rainfall from the NDVI time series does not completely remove the NDVI residual, indicating that the vegetation greening in the Sahel may be attributable to other factors. Changes in land use or the implementation of improved management practices may also impact upon vegetation in certain areas, such as the observed agricultural expansions in Australia’s Murray-Darling basin, the Middle East and southwest United States, tree plantations in west China, as well as grazing practices triggering changes in plant community composition in South Africa. Greening can also result from variations in species composition (e.g., exotic species invasion in many drylands).
However, similar to rainfall changes, human-induced factors and species composition changes are more likely to be an important local driver impacting vegetation response. As vegetation greening has been observed across all drylands, discriminating the influence of a potential global driver that is enhanced or suppressed by local scale factors, is one of the goals of this work.
To this end, we hypothesize that higher levels of atmospheric CO2concentration are a key driver of the observed dryland greening, through an impact on plant water savings and consequent available soil water increase. A novel modeling framework introduced by Donohue et al., described higher vegetation water use efficiency (WUE) under CO2enrichment, with the authors using this mechanism to explain increases in maximum vegetation cover in warm and dry environments. The hypothesis developed in this study implies that the greening in global drylands is a response to higher CO2 levels increasing the available soil water.
The hypothesis is based on increasing atmospheric CO2 inducing decreases in plant stomatal conductance and enhancing vegetation WUE. Higher WUE encourages increased soil water under the same productivity levels. Since soil water is a limiting factor in dryland vegetation growth and function, any increase in available soil water is expected to enhance plant growth and greening.
Here we attempt to examine this hypothesis using a data driven meta-analytic approach. One of the key aims of this work is not just to identify the potential contribution of CO2 to observed changes in global greening, but also to identify different soil water responses that might be occurring within dryland and non-dryland systems. Understanding the varying interactions between soil water and vegetation under CO2enrichment between dryland and non-dryland systems would significantly increase our capacity to predict vegetation response to future climatic changes, as dynamic vegetation responses often pose large uncertainty in global models.
Conclusions
Dryland greening presents something of a paradox in our intuitive understanding of plant-water-CO2 interactions. Combining our meta-analysis results and early work, it illustrates that higher concentrations of atmospheric CO2 induce plant water saving and that consequent available soil water increases are a likely driver of the observed greening phenomena. Our results support recent modeling work showing higher vegetation WUE and higher maximum vegetation cover under CO2 enrichment in warm and dry environments.
The time scale of the CO2 enrichment effect on greening may have potential implications on global carbon budgets, as drylands have been found to be significant players in modulating the inter-annual variability of carbon cycling. By identifying the contributing mechanisms that result in vegetation greenness, our findings provide important insights into plant-water interactions. Predicting system level response to future climatic and/or anthropogenic perturbations in dryland systems remains a critically important but under-investigated area of inquiry.
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