Structural and physiological plasticity in response to light and nutrients in five temperate deciduous woody species of contrasting shade tolerance

1. Plants encounter a variety of light and nutrient availabilities during succession. However, there is an ongoing debate to which extent light-dependent structural and physiological plasticity is associated with species shade tolerance. 2. Seedlings of five species, Betula pubescens Ehrh., B. pendula Roth, Populus tremula L., Quercus robur L. and Acer platanoides L. (from most intolerant to most shade-tolerant), were grown at four different light and nutrient availabilities to test the hypotheses that intolerant species have higher physiological and tolerant species higher structural plasticity to light and also that there is an overall increase in plasticity with increasing nutrient availability. Two replicate experiments in different years were conducted. Plasticity was characterized by four estimates: (1) the range of variation of the components of relative growth rate (RGR), leaf area ratio (LAR) and net assimilation rate (NAR) (RGR = LAR.NAR) at common RGR; (2) average standardized slopes of physiological (RGR, NAR, i.e. physiological plasticity, Pi(P)) and structural (LAR, leaf dry mass per unit area, biomass allocation traits, i.e. structural plasticity, Pi(S)) traits vs. irradiance relationships; (3) standardized difference of plant traits measured at low to medium irradiance; (4) coefficient of variation across different irradiance treatments. 3. Plant growth was more strongly associated with NAR than with structural traits, but shade-intolerant species had a greater range of variation in both NAR and LAR at a common RGR. RGR, NAR and structural characteristics also responded more strongly to increases in irradiance in shade-intolerant species, but at low irradiance RGR and NAR were similar among all species. Owing to higher biomass fraction in leaves, the intolerant species produced less woody biomass. In nonfertilized plants, both Pi(P) and Pi(S) were negatively associated with shade tolerance. The plasticity was enhanced by nutrient addition, but the nutrient-dependent enhancement in plasticity was greater in more tolerant species. Therefore, differences in plasticity among species of varying tolerance were lower at higher nutrient availability. 4. Our study does not support the hypothesis of a trade-off between structural and physiological plasticity. Shade-tolerant species are generally less plastic than intolerant species, but increases in nutrient availability during succession reduce the differences in plasticity. Despite similar RGR in low light, first-year seedlings of shade-tolerant species produce more woody biomass, favouring survival and growth in subsequent years.