Quantifying spatial patterns of tree groups and gaps in mixed-conifer forests: Reference conditions and long-term changes following fire suppression and logging

Fire suppression and past logging have dramatically altered forest conditions in many areas, but changes to within-stand tree spatial patterns over time are not as well understood. The few studies available suggest that variability in tree spatial patterns is an important structural feature of forests with intact frequent fire regimes that should be incorporated in restoration prescriptions. We used a rare dataset consisting of mapped locations for all trees >= 10 cm in three 4-ha plots in 1929 before logging and in 2007/2008, 78 and 79 years after logging, to assess changes in three spatial components of forest structure: individual trees, tree clusters and gaps. Comparing 1929 old growth to modern conditions, area in gaps decreased from 20% to zero, the percentage of stems that were single trees from 6% to 2% and in small or medium clumps (2-9 trees) from 28% to 9%, while trees in large clumps (>= 10 trees) increased from 66% to 89%. Concurrent with these changes, canopy cover increased from 45% to 62%, and the average number of trees in a clump increased from II to 26, resulting in much more homogenous conditions across the stand. These changes also altered tree size and species associations within different structural groups (i.e., single tree, small, medium and large clumps). In an effort to account for the alteration of the fire regime that had already taken place in 1929 (the last fire recorded in tree rings was in 1889) we also analyzed spatial patterns of 1929 conditions removing all trees <25 cm. In this analysis, 35% of the plot area was in gaps and canopy cover averaged 36%. Tree clusters had an average of 5.2 trees per clump, with 13% of trees being singles, 30% in small clumps, 24% in medium clumps and 33% in large clumps. Our results provide metrics that quantify spatial patterns and composition of individual trees, tree clumps, and gaps under the historical fire regime that may be useful to forest managers. Our study demonstrates that the contemporary forest is more homogeneous than it was historically, and variability that likely provided diverse microclimate and habitat conditions and fostered resilience to a variety of stressors and disturbances such as fire, insects and drought has been lost. Future management may benefit from restoring these structural components. Published by Elsevier B.V.