Old-growth forests are structurally complex, both vertically and horizontally. This complexity arises typically from branch and leaf structure, but also includes epiphytes of varying form and function. We installed microclimate sensors along the vertical axis of an old-growth Douglas-fir (Pseudotsuga menziesii) tree to characterize gradients in air temperature, relative humidity, wind speed, and surface wetness; we also characterized the biomass and species composition of epiphytes, focusing on four common genera (Dicranum, Isothecium, Neckera and Porella). The relationship between microclimate and epiphytes has been studied extensively from the perspective of microclimate influencing epiphytes. By reframing the question, we asked how epiphytes influence microclimates.
One old-growth Douglas-fir in the HJ Andrews Experimental Forest in Oregon was instrumented with temperature and leaf-wetness sensors every 10 meters from 1.5 to 50 meters. Relative humidity and wind speed and direction were measured at 1.5 and 50m. These measurements were complemented by thermal imaging of canopy surface temperatures using a thermal camera mounted at 56m. We calculated monthly correlations among the microclimate variables. Bryophyte biomass was estimated every 10m, and the four common genera were used in a greenhouse drying experiment to assess canopy water storage capacity and water retention of each species.
Air temperature had high correlations across the vertical gradient for all months examined (≥0.93). Leaf wetness had similarly high correlations between microclimate stations in August (0.92-0.97), but declined in December, particularly between 10m and 50m (0.69). The August correlations for surface wetness are due to a lack of both precipitation and dewfall events, with limited occurrences of dewfall at 50m. However, December is characterized by persistent wetness and relative humidity (>90%) in the lower canopy and understory, while the upper canopy frequently dries out. The most similar heights across seasons were at 30 and 40 meters for temperature and wetness; these heights correspond with high bryophyte biomass.
A buffering effect between microclimate and moss is highly likely, but as of yet unsubstantiated. However, storage capacity and retention are mediated by bryophyte life-history and growth-form; slower growing and structurally denser genera (Neckera, Dicranum), associated with old-growth forests, store precipitation from a 60mm rain event for 4.5-5 days, while faster growing, secondary-forest colonizers (Porella, Isothecium) hold the water for 3.5 days. By retaining and storing more water in the canopy system, the bryophytes alter rates of evaporation, heating and cooling, potentially buffering microclimates from extremes.