Theoretical and empirical basis for height-dependence of tree mortality during drought

Background/Question/Methods

Increasing drought severity and rising vapor pressure deficit (VPD) threaten tree survival. Large trees have often been found to die more during drought than smaller trees, with height-associated hydraulic constraints implied as the potential mechanism driving increased drought mortality. Given that large trees play crucial roles in forest ecosystems, understanding the mechanisms behind this increased vulnerability is critical to forecast future forest dynamics and inform adaptive management. Darcy’s law predicts that, everything else being equal, rising VPD or drying soil will cause stronger declines in leaf water potentials (Ψleaf) in taller individuals than shorter ones, which would theoretically increase their likelihood of suffering drought-induced hydraulic failure or carbon starvation. Acclimation to height could occur, however, though shifts that compensate for the increased gravity and path length constraints, including decreasing crown-scale conductance to water vapor (Gs) or whole-tree leaf-to-sapwood area ratio (Al/As) or increasing specific hydraulic conductivity (Ks). We performed a quantitative synthesis of more than 4000 published observations from 19 traits and 153 species on how key traits that drive plant water and carbon economy change with height at the intra- and inter-specific levels and examined the implications of these shifts for tall-tree vulnerability to drought.

Results/Conclusions

While height-associated hydraulic constraints have often been implied as the potential mechanism driving either or both hydraulic failure and carbon starvation in taller trees, we show that the empirical support for this hypothesis is scarce. Ψleaf became more negative with height. However, this drop closely followed the gravitational gradient, suggesting that the observed decreasing Gs, increasing Ks and species-specific changes in Al/As are largely able to compensate for increased path resistance. Other structural and physiological shifts we observed, such as leaf xeromorphic properties and increasing cavitation resistance and water use efficiency, together with greater water and carbon reserves, may further allow tall trees to resist episodic water stress. Nonetheless, these adjustments may not be sufficient to prevent increasing mortality rates in taller trees under more severe climate change-induced droughts, unless further acclimation occurs. Finally, size-dependent drought-induced mortality has often been associated with pest attacks. Although vulnerability to pests and pathogens is determined by water and carbon responses to drought, it also reflects species-specific differences among the biotic agents involved. Further evidence is still needed, particularly at the intra-specific level, to elucidate the interactive roles that hydraulic failure, carbon starvation and insect attacks play on enhanced tall-tree vulnerability to drought-induced mortality.