Nitrogen–phosphorous interactions in young northern hardwoods indicate P limitation: foliar concentrations and resorption in a factorial N by P addition experiment
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Resorption, the process of withdrawing foliar nutrients prior to leaf abscission, is one of the most important nutrient conservation mechanisms in trees. Along with foliar nutrient concentrations, foliar resorption can be used to infer nutrient limitation. We collected green and senesced leaves of five species in early successional stands in the White Mountains of New Hampshire. In unmanipulated controls, foliar N:P ratios ranged from 20 to 31 and litter N:P ratios ranged from 19 to 36. These values suggest P limitation, although this forest type has been assumed to be N-limited. Additionally, N:P resorption ratios in control plots were < 1, reflecting proportionately more conservation of P through resorption than N. Four years into a full-factorial N × P fertilization experiment, N and P additions had increased N and P concentrations in leaves; more importantly, P addition reduced N concentration, possibly indicating alleviation of growth limitation by P. Resorption of P was less proficient (indicated by the concentration of an element in leaf litter) with P addition, as expected. Resorption proficiency and efficiency (the proportion of leaf nutrients resorbed) of N increased with P addition, suggesting increased demand for N with alleviation of P limitation. Resorption of P was more proficient and efficient with N addition, consistent with exacerbated P limitation. Temperate forests on glaciated soils are generally thought to be N-limited, but long-term N × P manipulations in this biome are lacking. Our results suggest that decades of anthropogenic N deposition may have tipped the balance to P limitation in these forests.
KeywordsResorption proficiency Resorption efficiency Nutrient limitation Northern hardwoods Fertilization Foliar nutrients
This research was conducted on plots established by Matt Vadeboncoeur, Steve Hamburg, and others almost 15 years ago under funding to Yanai from the National Science Foundation (DEB 0235650). Beginning in 2010, Multiple Element Limitation in Northern Hardwood Ecosystems (http://www.esf.edu/melnhe) was funded (by DEB 0949324 to Yanai), guided by the early vision of Tim Fahey and Melany Fisk. Maintaining the sites and fertilizing them has involved 69 summer interns and 33 graduate students. We are especially grateful to Bali Quintero, Adam Wild, and Nora Yanai, who shot and processed green leaves in August 2014. Jerome Barner, Stephanie Suttenberg, and Alex Kuhn helped collect leaf litter in fall 2014. The paper was improved by comments from Keith Killingbeck, Craig See, and students in FOR 694 at SUNY-ESF. The Bartlett Experimental Forest is owned and operated by the USDA Forest Service. This paper is a product of the Hubbard Brook Long-Term Ecological Research network, funded by the National Science Foundation (DEB 1633026).
Author contribution statement
The idea for this study was part of a much larger project led by RDY. KEG joined the project in her final semester as an undergraduate student, collecting samples and processing them. As a graduate student, KEG analyzed the data. KEG and RDY interpreted the results, wrote the paper, and enhanced its value through extended revision.
This study was funded by the National Science Foundation (DEB 0235650 and 0949324).
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