Drivers and mechanisms of tree mortality in moist tropical forests

Abstract

Summary Tree mortality rates appear to be increasing in moist tropical forests (MTFs) with significant carbon cycle consequences. Here, we review the state of knowledge regarding MTF tree mortality, create a conceptual framework with testable hypotheses regarding the drivers, mechanisms and interactions that may underlie increasing MTF mortality rates, and identify the next steps for improved understanding and reduced prediction. Increasing mortality rates are associated with rising temperature and vapor pressure deficit, liana abundance, drought, wind events, fire and, possibly, CO2 fertilization‐induced increases in stand thinning or acceleration of trees reaching larger, more vulnerable heights. The majority of these mortality drivers may kill trees in part through carbon starvation and hydraulic failure. The relative importance of each driver is unknown. High species diversity may buffer MTFs against large‐scale mortality events, but recent and expected trends in mortality drivers give reason for concern regarding increasing mortality within MTFs. Models of tropical tree mortality are advancing the representation of hydraulics, carbon and demography, but require more empirical knowledge regarding the most common drivers and their subsequent mechanisms. We outline critical datasets and model developments required to test hypotheses regarding the underlying causes of increasing MTF mortality rates, and improve prediction of future mortality under climate change.

Wood allocation trade‐offs between fiber wall, fiber lumen, and axial parenchyma drive drought resistance in neotropical trees

“…Enhanced drought‐induced mortality of high wood density embolism sensitive tree taxa might be driving observed compositional changes in neotropical forests (Esquivel‐Muelbert et al, ). Furthermore, increased drought‐induced mortality might underlie the observed trend of increased tree mortality observed in the Amazon Basin (McDowell et al, ) and the associated decline of the Amazon carbon sink strength (Brienen et al, ). Given the high drought sensitivity of many neotropical tree species of wet forests, future warming and drying could lead to the sudden collapse of standing biomass and the reversal of the carbon sink function of neotropical forests into a carbon source, enhancing global climate warming.…”

“…Episodic droughts, most notably in 2005, 2010, and 2015 have resulted in reduced tree growth and increased mortality in neotropical forests, specifically in the ever wet forest of the Amazon Basin (Feldpausch et al, ; Phillips et al, ; Rifai et al, ). Furthermore, increased drought and heat have been identified as possible drivers of a long‐term trend of increasing tree mortality in the Amazon Basin resulting in a reduction of the Amazon forest carbon sink strength (Brienen et al, ; McDowell et al, ). Despite the critical role of neotropical forests in driving future climate scenarios and their importance for biodiversity and human livelihoods, there are still large uncertainties surrounding the sensitivity of these forests to drought.…”

Canopy structure and topography jointly constrain the microclimate of human‐modified tropical landscapes

“…VPD is the difference between the saturation water vapour pressure ( e s ) and the actual water vapour pressure ( e )—in other words the difference between how much moisture the air can hold before becoming saturated and the amount of moisture actually present in the air. As such, VPD is intimately linked to water transport and transpiration in plants (Anderson, ; Motzer, Munz, Kuppers, Schmitt, & Anhuf, ; Will, Wilson, Zou, & Hennessey, ), with high VPD driving reduced growth and survival in both temperate and tropical trees (McDowell et al, ; Sanginés de Cárcer et al, ). Given that , VPD can be expressed as , where e s is derived from T using Bolton's () equation: .…”

Fire, fragmentation, and windstorms: A recipe for tropical forest degradation

“…In the Amazonia Basin alone, wind disturbances are estimated to cause temporary reductions in above‐ground forest biomass of ~1.7 Pg/year (Espírito‐Santo et al, ). While forests in this region have experienced droughts, fires, and windstorms for millennia (Cole, Bhagwat, & Willis, ; Ledru, ; Mayle et al, ), disturbance regimes are now changing in response to climate and land‐use change (Duffy, Brando, Asner, & Field, ; Emanuel, ; McDowell et al, ). There is growing evidence that a strong shift in disturbance frequency, intensity, and extent could push the Amazonia region into an alternate state (Brando et al, ; Emanuel, ; McDowell et al, ), potentially releasing to the atmosphere a large part of the ~120 Pg of above‐ground forest biomass that remain in the pan‐Amazonia intact forests (Mitchard et al, ).…”