Abstract:

Epiphytes are an abundant and diverse component of many wet temperate forests and have significant roles in ecosystem processes. Little is known about the processes and rates of their death and decomposition when they fall from the canopy, which limits our understanding of their role in forest carbon sequestration and nutrient cycling. In the temperate rainforest of the Quinault River Valley, Washington State, our aim was to test hypotheses regarding four elements of disturbance that might contribute to their decline.

We established set of experiments in which we placed samples of canopy epiphytes and their branch segments: (i) in the canopy versus forest floor microenvironment (stratum); (ii) attached to live versus dead branch substrates; (iii) subjected to physical disruption and ‘jarring’; and (iv) in direct versus indirect contact with the forest floor. Over the 2-year study, we assigned a non-destructive ‘vitality index’ (based on color and apparent mortality and dryness) to each sample every 2–3 months to compare effects of the experimental treatments and analyzed with a statistical model and post hoc pairwise comparisons of treatments.

The canopy versus ground stratum and live/dead branch status significantly affected epiphyte vitality. Effects of physical disruption and ground contact were not significant. There were seasonal effects (low vitality during the sampling times in the summer, revitalization upon sampling times in the winter) for all treatments except samples in contact with the ground. One implication of these results relates to effects of climate change, which is predicted to shift to hotter, drier summers and wetter winters. Climate change may affect forest dynamics and nutrient cycling in unpredictable ways. Results also point to future experiments to understand biotic and abiotic effects on epiphyte disturbance and dynamics.

Key words: disturbance, epiphytes, forest canopy, forest dynamics, moss, Olympic rainforest