OOS 20-1 - High and dry: How do plants that generally exist as terrestrial shrubs and trees live as facultative epiphytes in the Coast Redwood forest canopy?

Epiphytes have long captured the imagination of botanists and naturalists. This life form has evolved multiple times and comprises a substantial proportion of the biodiversity and biomass in many habitats, worldwide. There has been an increase in research focusing on the epiphyte community in recent years and this work is improving our understanding of patterns of diversity, ecophysiological responses to climate change and the importance that this community will have on larger ecosystem processes. In general, there is substantial species turnover within the epiphyte community along environmental gradients including elevation, vertical height, and exposure in the tree crown. Counter to this trend, we present one spectacular example of a group of plants that occupies a huge range of microclimatic conditions- from the dark and cool forest floor under a closed canopy of Coast Redwood (Sequoia sempervirens), the world’s tallest trees, to the bright, dry and exposed treetops, 100 meters up. An understanding of the existence of this growth pattern, begs the question: How are these species able to live in such different habitats? In this study, we measured a number of ecophysiological traits on four plant species that live along a gradient in height in the Coast Redwood forests of N. California. These species (Gautheria shallon, Vaccinium ovatum, Vaccinium parviflora and Tsuga heterophylla) have individuals that live terrestrially as either shrubs or even tall trees in the case of Tsuga, but also live epiphytically in redwood tree crowns. In addition to these facultative epiphytes, we also studied one obligate epiphytic fern, Polypodium schouleri, and one terrestrial fern, Polystichum munitum for comparison. As expected, there were clear increases in vapor pressure deficit throughout the vertical height profile, which ranged from near zero on the forest floor to 3kPa in the upper crown. The species studied responded strongly to these changes in microclimate. We found evidence of phenotypic plasticity in a number of leaf traits relating to gas exchange and water relations. These trait shifts, including reductions in specific leaf area and leaf water content, and increases in stomatal density and leaf thickness, allowed individuals to maintain water balance under increasingly desiccating conditions. Together, these trait shifts led to a reduction in the minimum leaf conductance and turgor loss point for epiphytes relative to terrestrial individuals and permitted the colonization of a wide range of microclimates and substrates.