Forests are structurally complex, temporally and spatially heterogeneous ecosystems. We know that heterogeneity has important effects on ecological processes yet we have few tools to compare resource availability and regeneration processes across patches that differ strongly in structural complexity and spatial heterogeneity. A traditional experimental design framework (i.e. means comparison) may mask ecologically important differences in the distribution of the process of interest. We used a neighborhood approach to scale resource availability (light and nutrients) and tree seedling growth in stands that differ in spatial heterogeneity of canopy trees. Our study was implemented as a randomized block design replicated four times in red pine forest in Minnesota, USA. Each block was ~65 ha with ~16 ha treatment stands. It included three overstory treatments and an unharvested control. Treatments included a thinning with residual trees dispersed evenly throughout the stand (dispersed) and two patch cuts that left 0.1 ha gaps (small gap) or 0.3 ha gaps (large gap) in a forest matrix. Residual basal area was held near constant in the three harvest treatments, such that treatments differ only in spatial pattern of canopy trees. We planted seedlings of three common pines (Pinus banksiana, P. strobus and P. resinosa); measured light, soil nutrients and growth over 7 growing seasons; and mapped trees in ~ 15 m diameter plots around each seedling.
Results/Conclusions
There were significant differences in the distribution of seedling neighborhoods, resource availability and seedling growth among treatments. The more homogeneous dispersed treatment had narrow distributions compared to gap treatments. Neighborhood models predicted non-linear declines in light, nutrients and growth as basal area in the neighborhood increased. Stand averages of resources and seedling growth depended on where on the non-linear surface the range of values in that stand fell. Our results reveal the importance of neighborhood models to understand and scale ecological processes in heterogeneous ecosystems.
