Tradeoffs are evolutionary conundrums where genetic change enhances fitness in one environmental condition but inescapably leads to loss of fitness in another. Such tradeoffs are thought to be a fundamental driver of species distributions across the planet, but we do not understand how fine root traits drive these distributions despite their well-known importance for water and nutrient uptake. It has recently been shown that root traits are coordinated along two axes, one for root tissue conservation ranging from low to high investments in root construction, and one for nutrient uptake ranging from a do-it-yourself strategy to reliance on collaboration with mycorrhizal fungi. We synthesized a new global root trait database (gRooT) with a new global vegetation database (sPlot) to ask how do root traits influence the probability of species occurrence along climatic gradients?
Results/Conclusions
Here we show that the position of plant species on these two root trait axes affect their occurrence along global gradients of temperature and water availability. Species that invest in root construction with high root tissue density and rely on mycorrhizal fungi with large root diameter have a higher chance of occurring in warm environments, while the opposite is true in cold climates. However, such a clear two-way tradeoff was not observed with respect to water availability. Investing in root construction and relying on mycorrhizal fungi did increase the probability of occurrence in dry environments, but the probability of occurring in wet environments was not related to these root traits. Clear two-way tradeoffs were not always observed and, in many cases, we only found evidence for what we call ‘one-way benefits’, thereby challenging our understanding of trait-based fitness tradeoffs and reducing our ability to predict how traits affect species occurrence in warmer and wetter conditions. The species pool that can tolerate dry and warm conditions is functionally more constrained than for wet and warm conditions. Research frameworks that allow for the possibility of one-way benefits may be necessary to discover which trait combinations drive species responses to changing environmental conditions.