Patterns and challenges to understanding and predicting future forest disturbances

Abstract:

Forest dynamics arise from the interplay of chronic drivers and transient disturbances with the demographic processes of recruitment, growth, and mortality. The resulting trajectories of vegetation development drive the biomass and species composition of terrestrial ecosystems. Forest dynamics are changing due to anthropogenic-driven exacerbation of chronic drivers, such as rising temperature and CO₂, and increasing transient disturbances, including wildfire, drought, windthrow, biotic attack, and land-use change. There are widespread observations of increasing tree mortality due to changing climate and land use, accompanied by observations of growth stimulation of younger forests due to CO₂ fertilization. These antagonistic processes are co-occurring globally, leaving the fate of future forests uncertain.

A minimum of 30% of the Earth’s old-growth forests, defined as forests where the canopy tree age is >140 years, have been lost since 1900, due only to deforestation. Inclusion of other disturbances shows that environmental drivers and disturbance regimes are consistently increasing mortality, forcing forests towards shorter and younger stands and reducing potential carbon storage. Acclimation, adaptation, and migration may partially mitigate these effects. These increased forest impacts are due to natural disturbances (e.g. wildfire, drought, windthrow, insect/pathogen outbreaks) and land-use change, both of which are predicted to increase in magnitude in the future. Tree growth, and potentially recruitment, has likely increased globally in the 20^th century, but the growth of this carbon sink has slowed. Variability in growth stimulation due to CO₂ fertilization is evident globally, with observations and experiments suggesting that forests benefit from CO₂ primarily in early stages of secondary succession. Furthermore, increased tree growth typically requires sufficient water and nutrients to take advantage of rising CO₂. Collectively, the evidence reveals that it is highly likely that tree mortality rates will continue to increase while recruitment and growth will respond to changing drivers in a spatially and temporally variable manner. The net impact will be a reduction in forest canopy cover and biomass.