Mites, tree senescence, disturbance dynamics, and the scientific rationale for temperate forest mega-plots

Background/Question/Methods Large mapped forest plots, from their Panamanian inception, have been controversial in that they constitute a large investment of resources in observational data collected at a single site.  Based on examples of ongoing projects at the Haliburton Forest plot, we make the general case for the importance of large plot data to scientific understanding and applied problems in temperate forest ecosystems.  Large mapped plots in general are important: (1) to quantify the spatial dynamics of forest processes in a manner that cannot be captured by current small plot networks; (2) to evaluate processes that show high spatial contagion at spatial scales of 1-100 meters, particularly processes that are highly dispersal limited; (3) to provide sufficient sample sizes for understanding the dynamics of rare organisms and events. These points are illustrated with ongoing work focused on canopy dynamics and biotic drivers of disturbance and community change in the forest at Haliburton. 

Trees within a 13.2-ha have been mapped, measured, and identified following CTFS procotols in an old-growth forest along a lake margin in Haliburton, Ontario, centrally located in the Great Lakes-St. Lawrence Forest of Canada. Within the plot area, we quantified the distribution of the maple spindle-gall mite (SGM: Vasates aceriscrumena) in the canopy of the dominant tree at the site (Acer saccharum) using a distributed sample of fallen leaf collections. Tree senescence was evaluated using a combination of site-specific relationships between LAI and tree size of dominant species, and a sparse sample of qualitatively scored crown thinning of larger trees in the plot. Geostatistical methods were used to describe spatial patterns of tree size, senescence, SGM abundance, and tree community composition, and as a constraint in hypothesis tests to examine covariation among these variables.

Results/Conclusions Gradual senescence of trees is marked at the site, supporting the hypothesis that age-related LAI declines are important in determining both understory light and the regeneration of relatively light-demanding tree species. SGM infection of Acer saccharum was high, but also highly spatially variable, at the site, with part of this variation attributable to tree size and apparent senescence.  Community composition of the sub-canopy stratum was also highly spatially variable, and co-varied with tree size and senescence.  Spatial analyses of tree senescence, spindle-gall mite infection, and community composition thus suggest strong biotic controls of forest dynamics that could not be quantified or modeled outside of the context of a large mapped forest area.