Foliar carbon isotope discrimination and related traits along light gradients in two different functional-type tree species

To understand how different plant functional types respond to light intensities, foliar carbon isotope discrimination (Delta C-13) and related traits, i.e., specific leaf area (SLA), mass- and area-based nitrogen concentrations (N-mass and N-area), leaf dry mass content (LDMC) of two evergreen coniferous and three deciduous broad-leaved species, were measured under four light intensities. Foliar Delta C-13 and SLA increased significantly from full- to low-light conditions for all species. These indicate that species studied could increase their light capture capacity under low-light conditions, leading to lower water-use efficiency (higher C-13 discrimination). There were significant differences in the responses of foliar N-mass or N-area to light variations in the two functional types, indicating that different functional-type tree species may have different N-use strategies to adapt to the light variations. It was found that there were large functional-type-dependent differences with regard to the relationships between foliar Delta C-13 and other leaf traits. Our findings suggest that all tree species could change foliar morphology to increase their light-harvesting ability under low-light conditions at the expense of decreasing their water-use efficiency. However, large differences in N-use strategy may exist between deciduous and evergreen species, which may be vital for the survival of these two functional-type tree species in a shaded understory. More important, our findings reveal that changes in Delta C-13 are not directly related to foliar N if N investment does not proportionally increase the photosynthetic capacity; this should be considered when exploring the relationships of nitrogen concentrations with Delta C-13.