Spring plant phenology is especially sensitive to changes in local climate (microclimate), yet most climate models are at a regional scale, and are too coarse to predict changes in microclimate. Changes in the timing of plant phenology can result in trophic asynchrony and population declines of higher level consumers. Several studies have shown that microclimates within areas of high topographic relief can be decoupled from regional climate patterns and that elevation is not always the driving force behind microclimate variability. We conducted standardized weekly spring phenology surveys at the H.J. Andrews Experimental Forest from 2009-2015 to examine the effects of microclimate on the phenology of 17 native forest plants in western Oregon. The Andrews Forest consists of highly incised mountainous terrain with elevations ranging from 410 to 1630 meters. To capture microclimate variability, we surveyed permanently marked plants at 16 forested sites across a variety of aspects, elevations and slopes. Site elevation ranged from 490 to 1330 meters. Temperature data were collected at each site every 15 minutes using HOBO data loggers, and observational snow data were collected each survey. Long-term meteorological stations were used as quality control for site level temperature data. Regression analysis was used to relate phenology to microclimate variables.
Preliminary analyses showed a high degree of inter-annual variability of microclimate within and across sites, resulting in a changing pattern of microclimate diversity across the landscape from year to year. Overall plant phenology generally tracked this variability in microclimate, but individual species’ phenological responses to microclimate varied substantially. For example, elevation predicted between 65% (2012) and 90% (2015) of variation in the date of budbreak for Pseudotsuga menziesii between 2009 and 2015. However, for Linnea borealis elevation predicted 80% of budbreak variation in 2011 and was non-significant in 2015. Preliminary models suggest that cold air drainage leading to persistent temperature inversions mediated the effects of elevation during periods of clear winter weather; in years with low snowpack and relatively infrequent low pressure systems (e.g., 2015) this dynamic exerted a strong influence on understory plant phenology. This study highlights the importance of understanding the interactions between regional and local processes that determine microclimate conditions and how those conditions influence patterns of plant phenology within forest communities, across mountain landscapes and over time. Mountain microclimates are variable within and across years, and higher resolution climate models are needed to predict the local effects of climate change on spring phenology.