The importance and challenges of detecting changes in forest mortality rates

Tree mortality rates determine forest health, community dynamics, and terrestrial carbon stocks. Climate change is predicted to increase global forest mortality, and some studies have observed recent changes in mortality rates. Assessing shifts in mortality and identifying their causes is challenged by the low mortality rates of trees and the limited scale at which tree mortality is typically assessed. Detecting changes in mortality and assigning mechanisms to those changes may not be feasible unless there are exceptionally large and persistent shifts in forest mortality rates. Here, we combine simulations and life-history theory to explore how a doubling in mortality might result in population decline, mean life expectancy, and carbon carrying capacity of forests. Using a simulated forest, we also identify that the sampling strategy and effort required to detect a doubling in stem mortality is extensive. We recommend increased efforts at monitoring mortality and a better incorporation of life-history theory, community composition, and application of changepoint detection statistics in mortality analyses.