Old trees contribute bio-available nitrogen through canopy bryophytes
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Symbiotic cyanobacteria—bryophyte associations on the forest floor are shown to contribute significantly to stand-level nitrogen budgets through the process of biological nitrogen fixation (BNF), but few studies have considered the role of canopy bryophytes. Given the high biomass of epiphytic bryophytes in many tree species of the North American temperate rain forest, we suggest that canopy bryophytes may contribute substantially to stand-level N dynamics. We confirm the presence of cyanobacteria and measure rates of BNF at three heights (0, 15 and 30 m) in Sitka spruce trees across three watershed estuaries of Clayoquot Sound, British Columbia, Canada. This study is the first to report BNF by cyanobacteria associated with epiphytic and forest floor bryophytes in the coastal temperate rain forest of North America. Cyanobacteria density was significantly greater in epiphytic bryophytes compared to mosses on the forest floor, and rates of BNF were highest at 30 m in the canopy. The majority of total stand-level BNF (0.76 kg N · ha-1 · yr-1) occurs in the canopy, rather than on the forest floor (0.26 kg N · ha-1 · yr-1). We suggest that BNF by cyanobacterial-bryophyte associations in the canopy of coastal temperate rain forests is a unique source of ecosystem N, which is dependent on large, old trees with high epiphytic bryophyte biomass.
KeywordsBryophyte Bryosphere Epiphytic Nitrogen fixation Old-growth forests Symbiotic cyanobacteria
This research was funded by a grant from the Natural Sciences and Engineering Research Council of Canada to Z. Lindo. A grant from Clayoquot Biosphere Trust to N. Winchester provided site access. We thank K. Jordan for logistical help in the field, A. MacKinnon, A. Gonzalez, T. DeLuca, J. Campbell, C. Prescott, M. Gundale, and anonymous reviewers for helpful comments on previous versions of this manuscript. A. Gonzalez provided laboratory space and equipment. Z. Lindo and J. Whiteley designed the experiment, Z. Lindo conducted the sampling, sample processing and data analyses. Z. Lindo and J. Whiteley wrote the manuscript.
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