Quantifying and Restoring Stand-Level Spatial Pattern in Dry Forests of the Eastern Washington Cascades

Abstract

There is increasing evidence that spatial heterogeneity at multiple scales is a critical component of ecosystem resilience and adaptive capacity. In frequent-fire pine and mixed conifer forests in the western US, pre-settlement era forests were complex mosaics of individual trees, tree clumps, and openings. There is a broad scientific consensus that restoration treatments should seek to restore these mosaic patterns as these reference forests were adapted to frequent-fire and shifting climatic conditions. Yet, methods to quantify and incorporate spatial reference information into restoration treatments are not widely used. In addition, targets from reference conditions must be critically evaluated in light of climate change. In this dissertation, I develop a new set of spatial metrics to quantify within-stand pattern in terms of widely spaced individual trees, tree clumps, and openings (ICO). Within 0.5 ha tree neighborhoods, I found evidence that a definable range and distribution, or envelope, of pattern and structure was present. This envelope ranged from low density patterns with few clumps and high opening levels, to patterns with a mid-range of density and varying levels of clumping, to high density, highly clumped patterns. The envelope was constrained by an upper limit of clump size, maximum density levels well below site potential, and the presence of at least some clumping in all plots. Across 3 x 6ha plots, tree neighborhood patterns of clumps and openings were spatially dependent. Aggregations of large clumps formed sub-patches that occupied 7-16% of plot area. A gradient of low to moderate density with low levels of clumping was found on the remainder. A silvicultural approach to translating reference patterns into restoration prescriptions and monitoring protocols was also developed and applied in a case study. Treatments using this ICO approach resulted in a distribution of tree clumps and openings within the range of reference envelopes. I also developed a method based on climatic water balance parameters, downscaled climate projections, and plant associations to assess historical reference sites in the context of projected future climate and identify climate analogue reference conditions.