Alder plays an important role in the nitrogen (N) economy of boreal forests because of its high capacity for atmospheric N fixation. Range expansion and increased insect and/or pathogen attack are two potential consequences of climate change that may result in significant, albeit opposite, effects on these systems through influences on alder-mediated N inputs. This study contrasted the effects of weekly defoliation at different intensities on growth and N fixation in Alnus tenuifolia seedlings with recovery of these traits following a single but intensive defoliation event. Weekly removal of 15, 25, or 40% leaf area for 9 weeks reduced total plant weight by 7, 13, and 29%, respectively, and led to progressive increases in leaf weight ratio at the expense of shoot growth. Although maximum photosynthetic rates (Pmax) were similar among treatments between defoliation events, increasing levels of defoliation led to progressive short-term declines in Pmax immediately following treatments. Plants with 40% leaf removal had N fixation rates (48.3 ± 2.4 μmol N·g−1·h−1) that were 67% less than undefoliated plants (147.6 ± 8.9 μmol N·g−1·h−1), and even the lowest level of leaf removal led to significant reductions in fixation rates relative to controls. In the recovery experiment, N fixation rates in defoliated plants (158.4± 12.1 μmol N·g−1·h−1) were 40% less than control values (264.1 ± 18.3 μmol N·g −1·h−1) 24 h following defoliation. After 28 d of regrowth, the total biomasses of defoliated and control plants were indistinguishable; however, N fixation rate in defoliated plants (39.2 ± 2.0 μmol N·g−1·h−1) remained 73% less than that of control plants, suggesting a strong competition between symbiont and host sinks for photosynthate.
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