Interactive effects between drought and warming in field manipulative experiments across grasslands globally: a systematic review and meta-analysis

James L King, Pieter Arnold, Zachary A Brown, Peter B. Reich, Oskar Scherer, Susanna E. Venn, Adrienne Nicotra 

1.       In grasslands, interactions between temperature and moisture are possible whereby increases in temperature can result in decreases in soil moisture, exacerbating drought conditions. Both uptake and emission of carbon from grassland ecosystems are, for the most part, governed by climate warming and changing precipitation regimes, yet it remains unclear how grasslands globally will respond to concurrent warming and drought conditions in the future (~2100) or what impact that will have on the terrestrial carbon cycle.

2.       We performed a systematic review and meta-analysis to synthesise the data from in situ field manipulative experiments that investigated the effects of warming and drought factorially, in natural grassland ecosystems around the world. We focussed on experiments that simulated a realistic future climate (>1 °C warming).

3.       Our review reveals that factorial warming and drought studies are limited to Asia, Europe, and North America. Our analyses indicate that warming and drought will decrease net primary productivity (-8.8%) and soil respiration (-18%), and that interactions resulting in greater than additive effects are likely to occur in belowground net primary productivity, soil respiration and net primary productivity. Duration was the most explanatory moderator variable.

4.       This meta-analysis highlights several knowledge gaps that hinder our capacity to fully understand the impacts of temperature and precipitation on grasslands: (1) There are no published experimental studies in tropical grassland ecosystems or the southern hemisphere, (2) Experiments that employ belowground warming to realistically simulate future climate change are few and (3) despite a large number of studies from factorial experiments, many of these did not include detailed meta-data including edaphic factors that allow deeper analysis of responses to climate change.

5.       Synthesis. Overall, emission of carbon fell more than uptake, suggesting that grassland net carbon flux may be more resilient to future climate change than previously thought. Interactive effects between warming and drought are likely, highlighting the risk associated with interpreting results from single factor experiments when making predictions of ecosystem carbon flux responses under future climate scenarios.