Climatic and topographic drivers of stand structure and composition in old-growth mixed-conifer forests

Background/Question/Methods

Old-growth mixed-conifer forests (OMCF) are found in the transition between dry ponderosa pine, Pinus ponderosa, woodlands and moist grand fir, Abies grandis, forest types. A combination of steep ecological gradients and a mixed-severity fire regime historically created a mosaic of structurally and compositionally diverse forest within OMCF.  Logging and fire suppression during the late 1800s and 1900s have altered the historic structure, composition, and landscape pattern of OMCF.   As a result, remnant stands face increased risk of mortality from high-severity wildfire, insects, and drought.  Restoration of these forests and landscapes depends on understanding of how anthropogenic disturbance has altered OMCF.  However, to understand anthropogenic effects, climatic and topographic drivers of structure and composition must first be understood.  Our objective was to determine the current structure and composition of OMCF and determine how these attributes are related to major climatic and topographic gradients across the eastern Cascade and Ochoco mountains of Oregon. Structure and composition of OMCF was recorded at eighty-eight sites in the Deschutes and Ochoco National Forests.  We identified forest structure and composition types within OMCF using 2-way cluster analysis.  Nonmetric Multidimensional Scaling (NMS) was then used to determine how types of OMCF were related to climatic and topographic gradients.

Results/Conclusions

Cluster analysis identified six distinct stand structure and composition types in the eastern Cascades and five types in the Ochocos.  An NMS ordination for stand types in the eastern Cascades explained 86% of the variance.  The first axis was most strongly correlated with increasing precipitation (r=0.485) and decreasing maximum temperature (r=-0.599). An NMS ordination of the Ochoco National Forest accounted for 84% of the variance.  In contrast to the eastern Cascades ordination, the first axis showed comparatively weak correlations with maximum temperature (r=0.266) and precipitation (r=-0.317), while topographic gradients of slope (-0.493) and potential solar radiation (0.424) were most strongly correlated with axis 2.   Our results show that the mosaic of OMCF in the east Cascades is primarily driven by a broad climatic gradients associated with coarse-scale physiographic features.  In comparison, the mosaic of OMCF in the Ochoco mountains was associated with topographic complexity at relatively fine scales.  Our knowledge of the forest types within OMCF coupled with an understanding of climatic and topographic drivers of these types provides a framework to evaluate effects of logging and fire suppression, and will facilitate more effective restoration and management.