Competitive Displacement of Trees in Response to Environmental Change or Introduction of Exotics

Various global change factors such as natural and anthropogenic climate change, tropospheric ozone, CO₂, SO₂, and nitrogen deposition affect forest growth, but in species-specific ways. Since even small differences in growth rates between competing species can lead to eventual competitive exclusion, it is important to know the rate at which displacement might occur. Similarly, invasive species may displace native species and cause their extinction. A simulation study of displacement velocity was conducted. Competitive displacement between pairs of similar tree species in which one species has a growth advantage produced trajectories that fit an exponential decay model, leading to the use of the half-life as a useful summary statistic. At any given level of growth differential, the half-life for shade-tolerant species was found to be much longer than for shade-intolerant species due to the ability of shade-tolerant species to survive even when their growth is very slow. Trees with longer life-spans also persisted longer, but this effect was weaker than the shade-tolerance effect. Disturbances speeded up displacement by increasing turnover. For short-lived, intolerant species with a 20% disturbance rate and 20% growth suppression, the estimate of an 100-year half-life could be considered a precipitous rate of decline, with a risk of extinction at about 500 years. In the absence of disturbance, and with a 20% growth reduction or differential between competing species, half-lives for species replacement ranged from 100+ to nearly 800 years. With lesser growth differentials, half-lives are much longer. Such gradual competitive displacement processes will be very difficult to detect in the field over periods of even decades. Results of this study have implications for exotic species invasions. It is predicted that intact forest is not truly resistant to invasion, but that invasion of shade-tolerant tree species should be very slow. Invasion of shade-intolerant species is predicted to be accelerated by disturbance, as has been frequently observed. Results of the simulations were supported by data compiled from several parts of the world.