Spatial patterns of total and available N and P at alpine treeline
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Background and aims
Vegetation can have direct and indirect effects on soil nutrients. To test the effects of trees on soils, we examined the patterns of soil nutrients and nutrient ratios at two spatial scales: at sites spanning the alpine tundra/subalpine forest ecotone (ecotone scale), and beneath and beyond individual tree canopies within the transitional krummholz zone (tree scale).
Soils were collected and analyzed for total carbon (C), nitrogen (N), and phosphorus (P) as well as available N and P on Niwot Ridge in the Colorado Rocky Mountains.
Total C, N, and P were higher in the krummholz zone than the forest or tundra. Available P was also greatest in the krummholz zone while available N increased from the forest to the tundra. Throughout the krummholz zone, total soil nutrients and available P were higher downwind compared to upwind of trees.
The krummholz zone in general, and downwind of krummholz trees in particular, are zones of nutrient accumulation. This pattern indicates that the indirect effects of trees on soils are more important than the direct effects. The higher N:P ratios in the tundra suggest nutrient dynamics differ from the lower elevation sites. We propose that evaluating soil N and P simultaneously in soils may provide a robust assay of ecosystem nutrient limitation.
KeywordsTreeline Nitrogen Phosphorus fractions Biosequence Nutrient ratios
Thanks to Nate Wojcik, Carrie Renaud, and Sheena Anderson for help with sampling and with laboratory analyses at the University of Denver. We appreciate the comments from Alan Townsend on an early version of this manuscript as well as providing unpublished data on phosphorus concentrations. Support was provided by National Science Foundation grants to support the Niwot Ridge Long-Term Ecological Research program. In addition, DL was supported by grants from the University of Colorado, the Colorado Mountain Club, and the Marr Foundation as well as a National Science Foundation Doctoral Dissertation grant.
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