Ecological and hydroclimatic determinants of vegetation water-use strategies.

Vegetation responses to soil moisture limitation play a key role in land-atmosphere interactions and are a major source of uncertainty in future projections of the global water and carbon cycles. Vegetation water-use strategies-that is, how plants regulate transpiration rates as the soil dries-are highly dynamic across space and time, presenting a major challenge to inferring ecosystem responses to water limitation. Here we show that, when aggregated globally, water-use strategies derived from point-based soil moisture observations exhibit emergent patterns across and within climates and vegetation types along a spectrum of aggressive to conservative responses to water limitation. Water use becomes more conservative, declining more rapidly as the soil dries, as mean annual precipitation increases and as woody cover increases from grasslands to savannahs to forests. We embed this empirical synthesis within an ecohydrological framework to show that key ecological (leaf area) and hydroclimatic (aridity) factors driving demand for water explain up to 77% of the variance in water-use strategies within ecosystem types. All biomes respond to ecological and hydroclimatic demand by shifting towards more aggressive water-use strategies. However, woodlands reach a threshold beyond which water use becomes increasingly conservative, probably reflecting the greater hydraulic risk and cost of tissue damage associated with sustaining high transpiration rates under water limitation for trees than grasses. These findings highlight the importance of characterizing the dynamic nature of vegetation water-use strategies to improve predictions of ecosystem responses to climate change.