Drivers of Forested Riparian Microclimate on the Olympic Peninsula of Washington State

Abstract

Riparian zones have unique microclimates that support distinct assemblages of aquatic and terrestrial species, which in recent decades have resulted in a regulatory emphasis on riparian protections. However, an understanding of the drivers of riparian microclimate is still lacking. This study examined drivers of variability in summer air temperature and vapor pressure deficit in the riparian zones of 10 drainage basins (31 to 789 ha) on the western Olympic Peninsula, Washington. Model selection analysis was used to explore hypotheses about the influences of 3 types of drivers on microclimate: regional climate gradients, proximity to stream, and solar radiation. Proximity to stream had the strongest influence on microclimate; the air became warmer and drier with increasing distance from the stream and with increasing steepness of the stream valley slope. Basins at lower elevations (range = 28 to 362 m) had cooler, more humid microclimates, a pattern attributed to marine influence. Variation in microclimate was also associated with variation in solar exposure, modeled as a function of topography. Testing of canopy closure influence on microclimate was hindered by uniformly high canopy closure across the study area (87 to 98%) as a result of unmanaged, primarily second-growth stream buffers. Each of the microclimate drivers identified in this study was a function of topography, across a range of scales. By understanding these relationships between topographic variation and riparian microclimate, managers and researchers will be able to more accurately and efficiently delineate the extent of riparian microclimate influence.