Interacting controls by light availability and nutrient supply on biomass allocation and growth of Betula pendula and B. pubescens seedlings

Interactions among environmental drivers commonly occur in natural environments, but their influence on plant performance is not fully understood. We tested for significant interactive effects of light and nutrient availabilities on foliage architecture, chemistry, biomass allocation and growth in shade intolerant temperate deciduous trees Betula pendula Roth. and B. pubescens Ehrh. First-year seedlings were grown at four irradiance and four N and P availability combinations, and the experiment was repeated in two consecutive years. In both species, increases in light availability resulted in larger net assimilation rate (NAR, total plant biomass production rate per unit leaf area), root mass fraction (FR) and leaf mass per unit area (MA), but reductions in leaf area ratio (LAR, foliage area per total plant dry mass), stem mass fraction (FS) and leaf mass fraction (FL). The strong decrease in LAR (LAR = FL/MA) with light mainly resulted from increases in MA. The light-dependent increase in NAR was larger than the decline in LAR such that plant relative growth rate (RGR = NAR × LAR) scaled positively with light in both species. Nutrient availability mainly enhanced LAR, and moderately NAR, further improving RGR. The increase in LAR was primarily associated with increases in FL and decreases in FR. Significant interactions between light and nutrients were found for most of the studied variables, overall indicating that plant allocation and growth were more responsive to nutrients at higher light availability. Several significant species and year-to-year differences were observed among the allocation and growth characteristics that were driven by plant size, climate and seedling source. Greater RGR of B. pubescens that is a species colonizing habitats with lower nutrient availability was linked to its greater foliage nitrogen concentrations. Overall, the study highlights a significant light × nutrient interaction that can modify seedling performance along natural gap-understory gradients.