Effects of defoliation on growth and N fixation in Alnus tenuifolia: Consequences for changing disturbance regimes at high latitudes

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 (P-max) were similar among treatments between defoliation events, increasing levels of defoliation led to progressive short-term declines in P-max immediately following treatments. Plants with 40% leaf removal had N fixation rates (48.3 +/- 2.4 mu mol N.g(-1).h(-1)) that were 67% less than undefoliated plants (147.6 +/- 8.9 mu mol 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 mu mol N.g(-1).h(-1)) were 40% less than control values (264.1 +/- 18.3 mu mol 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 mu mol N.g(-1).h(-1)) remained 73% less than that of control plants, suggesting a strong competition between symbiont and host sinks for photosynthate.