Diverse responses of maple saplings to forest light regimes

Abstract

Seedlings of 11 species of forest maples (Acer L.) were grown outdoors from budburst to senescence under three light regimes: 'gap centre under clear skies' (approx. 20% open sky irradiance; red:far-red ratio = 1.12); 'gap centre under cloudy skies' (1.5 %, ratio = 1.03); and 'gap edge' (2.5 %, ratio = 0.6). Seedlings grown under the gap centre (clear sky) regime had significantly greater height growth, greater specific leaf mass, higher root:shoot ratio, greater investment in roots, higher leaf nitrogen concentrations, greater chlorophyll a:b ratio, lower photosynthetic rates under dim light, higher maximum photosynthetic rate, higher stomatal conductance, and lower leaf internal CO2 concentrations compared with those grown in either gap edge or gap centre (cloudy) regimes. Responses to the gap edge vs. gap centre (cloudy) treatments differ little, suggesting that shade acclimation in forest maple seedlings is mainly a response to light intensity rather than spectral quality. The ubiquitous and, except for leaf internal CO2 concentration, highly significant interspecific variation in traits was broad-ranging and continuous. These results suggest that (1) the responses to light quality found in shade intolerant herbaceous and woody species growing in more open habitats may not have a selective advantage in seedlings of shade tolerant forest trees, and (2) the adaptive plastic response to understorey vs. gap environments in forest maples, which is qualitatively consistent across species, is founded on co-ordinated, small shifts in sets of functionally inter, related traits.