|Paul A. Schwarz
Department of Natural Resources
Ithaca, New York 14853-3001
My research is focused on characterizing and explaining the spatial variation in abundance of individual tree species in the Hubbard Brook Experimental Forest (HBEF), a mesoscale landscape of approximately 31 km2. Spatial patterns are scale dependent, and a forested valley the size of the HBEF presents a complex combination of climatic gradients, hydrologic and edaphic variability, and previous disturbance, all of which are influenced by topography. My research seeks to clarify the roles of these environmental factors and conditions in regulating the distribution and abundance of the dominant tree species in the HBEF. Moreover the spatially explicit sampling approach I've employed enables me to also examine the variation in species abundance in terms of spatial factors, some of which may include species-specific life history traits such as dispersal and growth rate.
My research approach is centered around the sampling of 0.05 ha (12.6 m radius) circular plots. During the past three years, 371 permanent plots have been positioned at pre-established locations along 15 transects which span the Hubbard Brook valley from the southern border of the HBEF to the northern border. The overall layout of the plots is based on a systematic-cluster design to facilitate the detection of spatial patterns (Fortin et al. 1989). The plots were positioned using high-precision global positioning satellite (GPS) receivers that were capable of real-time navigation within the HBEF. The plots are positioned along each transect at 100 m or 200 m intervals. The transects, themselves, are 500 m apart. The spatial coordinates of each plot enable the field data to be integrated with a high-resolution (10 m) digital elevation model (DEM) of the HBEF.
Within each plot, every tree (>= 10 cm dbh) has been identified to species, marked with an aluminum identification tag, and its stem diameter measured. In addition, each tree has been assessed visually and assigned to a canopy class (dominant, codominant, intermediate, or suppressed) and crown foliage rating (80-100%, 50-80%, 10-50%, < 10%). Standing dead trees were also measured and recorded. Current growth was measured by using an increment hammer to extract a 3 cm long core of recent wood from each tree. Growth rate is being analyzed in the laboratory using an automated tree ring reader.
Along a 1 m wide strip transect across the east-west diameter of each plot, saplings (> 1.3 m tall but < 10 cm dbh) were identified and recorded in one of five size classes, and established seedlings (0.5-1.37 m) were tallied by species. Soil depth to bedrock or till layer was measured at meter intervals along the strip transect. Finally, slope in the four cardinal directions, aspect, and elevation was measured (for comparison with the DEM), and the presence of shrub and herb species has been recorded for each plot.
In addition to the vegetation data, quantitative soil samples of the both the forest floor and upper mineral soil horizons have been collected from a subset of plots and are in the process of being analyzed in the laboratory.