Canopy Microclimates and Epiphytes: Examining Dynamic Patterns and Influences.

Old-growth forests are structurally complex in both vertical and horizontal dimensions. This complexity arises from biological characteristics, typically branch and leaf structure, but also includes epiphytes with varying form and function. I characterized the biomass and species composition of epiphytes along the vertical axis of an old-growth Douglas-fir (Pseudotsuga menziesii) tree called the Discovery Tree (~60 m tall, estimated age 300 years). Epiphyte biomass was estimated approximately every 10m on both the trunk and the branches attached in a given zone, using calibrated visual estimates. The highest total epiphyte biomass and species richness was found at 30 m; the upper canopy had reduced bryophyte biomass, but an increased presence of lichens dominated, while the lower canopy had much less epiphytic biomass. The common genera (Dicranum, Neckera, Porella, and Isothecium) were used in a greenhouse watering and drying experiment to assess water retention of each species. Taxa that tend to be associated with old-growth forests (Dicranum and Neckera) had greater water absorption and longer durations of water retention by up to 2 days in a simulated 60 mm rain event. By retaining and storing more water in the canopy, these bryophytes may alter rates of evaporation, heating, and cooling, potentially buffering microclimates from extremes. The bryophyte drying experiment suggests that bryophyte species will affect its role in canopy water retention and evaporation.
To characterize microclimate gradients associated with these epiphytes, I installed microclimate sensors along the vertical axis of the same Douglas-fir tree at six microclimate stations, located 1.5, 10, 20, 30, 40 and 56 m above ground. These sensors measured air temperature, relative humidity, wind speed and direction, and leaf surface wetness from (August 2, 2016 – July 31, 2017), and I analyzed the data at the daily and quarter-monthly time scales. Using leaf wetness and temperature data, I used times-series cluster analysis and generalized additive models to assess how the microclimate stations differed and to establish zones within the canopy that had relatively consistent values among the microclimate stations. I classified canopy zones (low, middle and upper canopy) for four representative months (August, November, February and May). Canopy zones changed in size and relative values across seasons. The upper canopy experienced a greater range of microclimate variability than the lower canopy. In each of the four months, 56 m was identified as the only microclimate station in the upper canopy, while the middle and lower canopy zones expanded and contracted depending on the time of year. Relative humidity variation was greater at the top of the canopy than the lower canopy. Wind speed was much greater and consistently from the east at the top of the canopy, while the lower canopy has much lower wind speeds which came at different directions.
Monitoring microclimates in conjunction with the bryophyte assessment allows for conjecture on feedbacks between the microclimate and vegetation. Because bryophytes that tend to be associated with old-growth forests held water longer, we can speculate that this increased water retention feeds back into the microclimate by buffering temperatures. This study also shows that the seasonality of microclimate partitioning may be an important factor in understanding vertical bryophyte distribution and potential climate feedbacks.