Forest fragmentation due to land-use change is a significant global environmental problem. Currently, more than one third of forest has been cleared, and remaining forest is highly fragmented with 20% within 100 m and 70% within 1km of an edge. Impacts to biodiversity and ecosystem function in the remaining forest fragments are largely driven by changes to the forest canopy. We studied forest dynamics within the Wog Wog fragmentation experiment in southeastern Australia. Surveys of almost 3000 Eucalyptus trees across continuous forest and experimental fragments were made in 1987, 1991, and 2013, creating a unique 26 year record of Eucalyptus growth and survivorship in response to forest fragmentation. In the short-term, fragmentation drove increases in tree biomass at fragment edges. However, as the pine plantation matrix matured, growth slowed at edges and was again similar in fragments and continuous forest. Further, a tree’s distance from edge and fragment size predicted tree growth and mortality in both the short- and long-term. From these field results, we predicted that changes in competition for light in forest fragments compared to continuous forest explain changes in tree mortality as well as the increase and then decline in tree growth detected at edges.
Results/Conclusions
We used an individual based model, SORTIE, to determine whether competition for light could produce the increases/declines in growth and changes in survival detected in our 26-year tree data set. The model was parameterized for continuous Eucalyptus forest and Eucalyptus fragments with Pinus radiata as the surrounding matrix. First, a continuous Eucalyptus forest was simulated for 1500 years until a relative steady-state was reached. Next, the simulated forest was removed and replaced with a pine seedling matrix surrounding a remnant small, medium, or large Eucalyptus patch, mimicking the Wog Wog experimental design. Replicate simulations were run for 30 years to simulate the time frame of the experiment. The model results showed that following fragmentation, given light as a limiting resource, tree growth at the edges rapidly increased and then decreased as the pine plantation matured, which is in line with data collected from the tree surveys. Furthermore, tree growth at the northern edge of a patch was greater than at the southern edge of the patch. We conclude that competition for light can lead to a dynamic response in forest biomass, growth and survival when the matrix surrounding fragments changes in structure through time.