Controls on the occurrence and activity of a nitrogen-fixing epiphytic lichen in tropical forests of varying nutrient status.
Benner, Jon*,1, Lunch, Claire1, Conroy, Shannon 1, Vitousek, Peter1, 1 Stanford University, Stanford, CA
ABSTRACT- The role of nutrient availability in structuring epiphyte communities is not well understood. If epiphytes obtain their nutrients from atmospheric deposition, the nutrient status of host trees should not influence the productivity and composition of epiphyte communities, but this might not be the case if epiphytes rely on nutrients leached from host tree foliage. We investigated changes in the abundance of the nitrogen-fixing epiphytic lichen Pseudocyphellaria crocata in response to nitrogen and phosphorus fertilization of Metrosideros polymorpha forests growing on 136 year old and 4.1 million year old substrates in the Hawaiian islands, and we used natural abundance of 15N in tissues of P. crocata and other epiphytes to compare sources of nitrogen nutrition to epiphytes. Pseudocyphellaria crocata abundance in N-fertilized and control plots was very low in both forests, but increased dramatically (136 year, p=0.005; 4.1 million year, p<0.0001) in response to P fertilization. In an unfertilized forest growing on a 3000 year old lava flow, 15N of P. crocata (-3.7 ± 0.1 ‰) did not differ (p=0.39) from 15N of non-N-fixing bryophytes and lichens (-3.5 ± 0.1 ‰), suggesting that P. crocata relies on the same N sources as other epiphytes. All epiphytes were significantly enriched in 15N relative to the leaves of host M. polymorpha trees (-7.1 ± 0.5 ‰, p<0.0001). The 15N of epiphytes as a whole was consistent with atmospheric deposition, rather than canopy leachate, as a primary N source. Phosphorus availability to host trees, however, appears to be a critical determinant of P. crocata abundance across Hawaiian forests.
Key words: epiphytes, nitrogen fixation, natural abundance of nitrogen isotopes, phosphorus
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