Geology topics

Thomas A. Bytnerowicz

Publications and source records attributed to Thomas A. Bytnerowicz.

2 recordsLinked to original sources

USGS2023

Leaf nitrogen affects photosynthesis and water use efficiency similarly in nitrogen-fixing and non-fixing trees

Nitrogen (N)-fixing trees are thought to break a basic rule of leaf economics: higher leaf N concentrations do not translate into higher rates of carbon assimilation. Understanding how leaf N affects photosynthesis and water use efficiency (WUE) in this ecologically important group is critical. We grew six N-fixing and four non-fixing tree species for 4–5 years at four fertilization treatments in field experiments in temperate and tropical regions to assess how functional type (N fixer vs. non-fixer) and N limitation affected leaf N and how leaf N affected light-saturated photosynthesis ( A sat ), stomatal conductance ( g sw ) and WUE (WUE i and δ 13 C). A sat , WUE i and δ 13 C, but not g sw , increased with higher leaf N. Surprisingly, N-fixing and non-fixing trees displayed similar scaling between leaf N and these physiological variables, and this finding was supported by reanalysis of a global dataset. N fixers generally had higher leaf N than non-fixers, even when non-fixers were not N-limited at the leaf level. Leaf-level N limitation did not alter the relationship of A sat , g sw , WUE i and δ 13 C with leaf N, although it did affect the photosynthetic N use efficiency. Higher WUE was associated with higher productivity, whereas higher A sat was not. Synthesis : The ecological success of N-fixing trees depends on the effect of leaf N on carbon gain and water loss. Using a field fertilization experiment and reanalysis of a global dataset, we show that high leaf-level photosynthesis and WUE in N fixers stems from their higher average leaf N, rather than a difference between N fixers and non-fixers in the scaling of photosynthesis and WUE with leaf N. By clarifying the mechanism by which N fixers achieve and benefit from high WUE, our results further the understanding of global N fixer distributions.

Journal of Ecology↗

USGS2023

Tree symbioses sustain nitrogen fixation despite excess nitrogen supply

Symbiotic nitrogen fixation (SNF) is a key ecological process whose impact depends on the strategy of SNF regulation—the degree to which rates of SNF change in response to limitation by N versus other resources. SNF that is obligate or exhibits incomplete downregulation can result in excess N fixation, whereas a facultative SNF strategy does not. We hypothesized that tree-based SNF strategies differed by latitude (tropical vs. temperate) and symbiotic type (actinorhizal vs. rhizobial). Specifically, we expected tropical rhizobial symbioses to display strongly facultative SNF as an explanation of their success in low-latitude forests. In this study we used 15 N isotope dilution field experiments in New York, Oregon, and Hawaii to determine SNF strategies in six N-fixing tree symbioses. Nitrogen fertilization with +10 and +15 g N m −2 year −1 for 4–5 years alleviated N limitation in all taxa, paving the way to determine SNF strategies. Contrary to our hypothesis, all six of the symbioses we studied sustained SNF even at high N. Robinia pseudoacacia (temperate rhizobial) fixed 91% of its N (%N dfa ) in controls, compared to 64% and 59% in the +10 and +15 g N m −2 year −1 treatments. For Alnus rubra (temperate actinorhizal), %N dfa was 95%, 70%, and 60%. For the tropical species, %N dfa was 86%, 80%, and 82% for Gliricidia sepium (rhizobial); 79%, 69%, and 67% for Casuarina equisetifolia (actinorhizal); 91%, 42%, and 67% for Acacia koa (rhizobial); and 60%, 51%, and 19% for Morella faya (actinorhizal). Fertilization with phosphorus did not stimulate tree growth or SNF. These results suggest that the latitudinal abundance distribution of N-fixing trees is not caused by a shift in SNF strategy. They also help explain the excess N in many forests where N fixers are common.

Hawai'i, New York, Oregon↗