Increased Mortality

The Role of Individual-Tree Mortality

Human impacts often increase mortality of trees in the forest, either directly or indirectly. Many species of trees (notably chestnut and elm) have been virtually eliminated as a result of pests that were dispersed by human activity. In addition, environmental degradation has been implicated in the decline of some forests. We simulated forests under anthropogenic stress by increasing the mortality of individual trees. Instead of simulating one particular threat (say, beech blight), we deal with the more general case by increasing the random mortality for all trees. This factor allows more general insight into the role of tree mortality in controlling forest development in SORTIE.

In the base-line simulations, adult trees experience a low risk of mortality that is independent of species, age, and vigor. For this work, two regimes of increased tree mortality were compared with the base-line simulations. The increased individual mortality is contrasted with the disturbed forest base-line runs in which groups of neighboring trees are destroyed.

This increased mortality across all trees will clearly influence tree density. However, one might assume that it will not affect the qualitative forest dynamics because all species are equally impacted, or that the increased light availability resulting from increased mortality will benefit yellow birch in exactly the same ways observed with disturbance and clear-cut experiments. Interestingly, this is not the case. Indeed, the most striking feature of the increased mortality simulations is their lack of similarity to either base-line simulation.

Base line no disturbance	Intermediate mortality Animation
Base line with disturbance	High mortality Animation

Comparison of base line and increased mortality at the end of the 1000-year simulations.

The increased mortality rates are neither competitively neutral nor similar in effect to the disturbed runs. Increasing mortality from 1% year^-1 to 2% year^-1 allows hemlock to outcompete beech. Hemlock and beech are both shade tolerant, but hemlock has slightly lower survival in the dark, and slightly higher growth in modest light. Consequently, the modest increase in mortality shifts the balance of power between these two shade-tolerant species. Increasing mortality even further causes a more dramatic change in forest composition. Neither the shade-tolerant species (beech and hemlock) nor the disturbance specialist (yellow birch) is successful. Instead, the forest is dominated by the pioneer species black cherry, red oak, and white pine. Pioneer species are distinguished by their ability to grow quickly in open, high-light areas. The high disturbance rate maintains high light availability and puts a premium on fast growth, in order to reach reproductive size before being killed.

Increasing mortality equally across all species is not competitively neutral. The increased opening of the canopy alters the competitive environment in favor of the fast-growing pioneer species. Interestingly, increasing individual-tree mortality is not similar to disturbances that destroy whole neighborhoods. In the former case, growth rate alone controls which species will respond, whereas in the latter case, high dispersal is key.