Tree mortality is a critical determinant of forest stand dynamics, influencing forest structure, composition and capacity for ecosystem service provision. Results from number of recent studies indicate that tree mortality rates are increasing in forests that appear undisturbed. This phenomenon has been linked to warming temperatures and associated water deficits. We sought to determine whether long- term trends in tree mortality rates were increasing in the boreal forests of Alberta, Canada using data from permanent sample plots. We predicted risk of mortality as a function of stand age and calendar year and used maximum likelihood methods to estimate model parameters.

Results/Conclusions

We found large increases in background mortality rates across the study period from 1956 to 2007. During this same period, forest density declined, basal area increased, and recruitment rates decreased, indicating that competition and self-thinning may be contributing to the pattern of increasing mortality. Once stand age was included in the model, there was no association between tree mortality and calendar year, suggesting that global warming is not contributing to increasing tree mortality rates in our study area. 

Instead, trends in the data appear to be related to successional processes alone.