Tropical forest mortality patterns, mechanisms and impacts.

Tree mortality within intact tropical forests is rising in some regions. Forecasts of increasing temperature and drought have led to concern of greater future mortality. Increased turnover of trees in tropical forests constrain the carbon sink, thus the increasing mortality has implications for the global carbon cycle. Similarly, changes in forest transpiration due to mortality appear large. However, not all locations are witnessing increasing mortality, and regional climate projections vary across the tropics, i.e., Southeast Asia and the Amazon and Congo basins. Here we review the evidence for changing mortality patterns, their underlying possible mechanisms, and preview future efforts to understand and predict mortality impacts within intact, moist tropical forests.

Forest mortality rates have increased in the Amazon while they have been stable in the Congo, with too few observations in Southeast Asia to determine a trend. The underlying candidate drivers include rising CO₂, rising temperature, rising frequency and/or intensity of droughts, and rising liana competition. Evidence for direct impacts of CO₂ on mortality is weak, while rising temperature and drought emerge as strong potential drivers, with increasing liana abundance remaining a logical but poorly tested driver. The underlying mechanisms of mortality have been rarely tested but appear consistent with hydraulic failure and perhaps also with carbon starvation playing significant roles. The role of biotic attack (aside from the influence of lianas) is untested. An apparent result of shifting climate is the increasing abundance of dry-affiliated taxa across the Amazon basin. The hydraulic safety margins of tropical trees appear smaller than the global average, yet this does not appear to promote elevated mortality. The mechanisms underlying such surprising survival may relate to rooting depths, water table depths, and the length of droughts in the moist tropics, which tend to be shorter than those in the extra-tropics. Models of tropical tree mortality can potentially capture observed patterns in turnover through representation of biotic interactions and the hydraulic and landscape-scale processes that define tree mortality. Resolving the mechanisms of mortality will greatly aid in prediction of future tropical carbon stores.