Nevins
Hoh Rainforest
Hoh Rainforest, Olympic National Park, WA
July & September 2017
I conducted my undergraduate thesis research at the University of Puget Sound in the Hoh Rainforest, under the advisement of Dr. Carrie Woods. The aim of my research was to identify the drivers of the distribution of non-vascular epiphytes in the canopy of Acer macrophyllum, the bigleaf maple. Due to their unique physiology, mosses and other epiphytes are known to grow in patterns driven by water and nutrient availability - but little work has been done to assess this in temperate forests.
I used quadrat surveys of epiphyte populations on host tree branches to assess species abundance and diversity in inner and outer canopy zones. These zones were largely delineated by the presence of a layer of canopy soil on the inner branches, and the absence of this soil on the outer branches. Tissue samples were collected from three target epiphyte species, listed below, which were abundant in the canopy but grew in distinct zones. Samples were prepared for stable isotopic analyses of δ13C and δ15N to assess water stress and N source, respectively. Lastly, dataloggers were placed in the canopies of the host trees to record temperate and humidity data for a full year.
Neckera douglasii
Outer Canopy
A small, slow-growing epiphytic bryophyte only observed in the outer canopy zone on bare branches.
Rhytidiadelphus loreus
Inner Canopy
A highly abundant bryophyte that grows primarily on the tops of branches in the inner canopy.
Selaginella oregana
Inner Canopy
An abundant lycophyte that grows pendulously below branches in the inner canopy.
To access the canopy, we used a single-rope climbing technique that lets you inch your way up into the canopy. While it uses a lot of the same equipment as rock climbing, the harness is much more comfortable and lets you work up in the tree for hours at a time.
We found that N source varied significantly between the target epiphyte species, showing that outer canopy species relied entirely on throughfall for N, while inner canopy species relied on a mix of N from throughfall and the canopy soil layer. Further, R. loreus and S. oregana appeared to utilize N from different sources, which could be a strategy to reduce competition for the limited resource when growing in close quarters.
I observed no significant differences in temperature or humidity between the canopy zones, suggesting that canopy structural heterogeneity, and the presence of canopy soil, could be the main drivers of epiphyte distributions in the canopy.
Read more of our results here:
