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 

Aim
Interactions between temperature and moisture are possible whereby increases in temperature can result in decreases in soil moisture, exacerbating drought conditions. Both uptake by and emission of carbon from grassland ecosystems are, in large part, governed by temperature and moisture. It is unclear how grasslands will respond to concurrent warming and drought conditions in the future (~2100) or what impact that will have on the terrestrial carbon cycle.
Location
Global.
Major taxa studied
Terrestrial carbon fluxes and pools.
Methods
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).
Results
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 biomass production and soil respiration, and that interactions resulting in greater than additive effects are likely to occur in belowground net primary productivity, soil respiration and net primary productivity. Overall, both inputs and outputs of carbon declined under simulated future conditions, suggesting no change to net ecosystem exchange and carbon stocks.
Main conclusions
This review highlights several knowledge gaps that currently hinder our capacity to fully understand the impacts of temperature and precipitation on grasslands: (1) experimental studies in tropical grassland ecosystems, particularly in the southern hemisphere, (2) experiments that employ belowground warming to realistically simulate future climate change and (3) experiments that collect a range of detailed meta-data including edaphic factors that allows deeper analysis of responses to climate change.