We utilized a site in the eastern Washington Cascades subject to fire exclusion for > 100 years to examine relationships between ponderosa pine (Pinus ponderosa) above-ground forest structure and below-ground fine root mass. Much of the research conducted on P. ponderosa forests has focused on quantifying above-ground structural changes resulting from fire exclusion, and emphasizes the need to protect older trees. While protecting root systems by removing combustible organic material from around trees within the drip-line is a common method to protect trees, relatively little is known about fine root distribution in these forests. We address relationships between above-ground forest structure and below-ground fine root distribution at three scales: 1) at the level of individual trees; 2) the intra-plot level using GIS interpolation; and 3) at the plot level. Our study was conducted in the Sinlahekin Wildlife Area in North central Washington State, USA, from May to October, 2008. We measured standing P. ponderosa forest structure, and root distribution in 30, 0.02-ha plots. Plots were stem-mapped and spatially reconstructed using GIS. Predictive interpolation and density models were created based on standing above-ground biomass in order to examine intra-plot spatial relationships with fine root distribution.
Results/Conclusions
While, fine root biomass distribution and aboveground biomass were weakly related at the individual tree scale (r2= 0.09, P= 0.05), no relationship was found at the intra-plot scale (P> 0.05). Boundary line analysis (using the top 10% of points) suggested distance to nearest tree was an important variable bounding the maximum fine root mass at any given location. At the whole plot level, total above-ground biomass was significantly related to mean plot fine root mass (r2=0.21, P= 0.05). Results suggest that root distribution at fine scales (< 0.02-ha) is likely too heterogeneous to easily predict, while the plot scale (> 0.02 ha) presents a more realistic scale for the prediction of fine root mass. These results also suggest that restoration efforts that focus on raking duff from the drip line prior to fire do not necessarily protect the majority of older tree fine roots since fine roots may be distributed over larger spatial scales. Restoration efforts may improve efforts to protect fine roots by increasing buffers around large old trees to scales greater that 0.02 ha.
