Effects of defoliation on current-year stem growth and fine root dynamics in Fraxinus mandschurica and Larix gmelinii seedlings

Abstract

Aims Defoliation by insects leads to significant changes in tree growth and physiological metabolism. However, how fine root dynamics would respond to defoliation is still poorly understood. The objectives of this study were to: 1) compare the responses of height and stem collar diameter to defoliation in manchurian ash (Fraxinus mandschurica) and Dahurian larch (Larix gmelinii) seedlings; 2) quantify the effects of defoliation on the seasonal dynamics of fine root production and mortality in the two species. Methods Manchurian ash and Dahurian larch seedlings, with different biomass allocation and height growth strategy, were used to investigate the effects of different levels of artificial defoliation (0% (control), 40% and 80% leaf area removal) on the growth of aboveground (height and stem collar diameter) and belowground (production and mortality of fine roots (diameter ≤ 2 mm)). Minirhizotron approach was employed to determine the seasonal dynamics of fine roots, and seedling height and stem collar diameter were measured concurrently. Important findings The results showed that: 1) defoliation reduced the growth of seedling height (but statistically not significant) and stem collar diameter in both species, with a stronger effect on the stem collar diameter. Effects on aboveground growth by defoliation were enhanced by increasing defoliation intensity in both species. Compared with the control seedlings, the height and collar diameter of ash were reduced by 3.3% to 12.1% and 5.7% to 23.1%, respectively, while the height and collar diameter of larch were only slightly reduced (< 12%). 2) Defoliation significantly reduced live fine roots in both species (p < 0.001), with the reduction in relative growth rate of fine roots being enhanced with defoliation intensity. 3) Defoliation significantly reduced fine root production in both species (p < 0.05), had not apparent effect on fine root mortality. We conclude that defoliation imposes significant effects on aboveground growth (especially stem collar diameter) in ash and belowground growth 李俊楠等: 去叶对水曲柳和落叶松苗木当年生长及细根动态的影响 1083 doi: 10.3724/SP.J.1258.2014.00102 (particularly root production) in larch. Our findings provide some insights into the inter-specific difference in the response of fine root dynamics to canopy carbon supply.