Foliar morphological and physiological plasticity in Picea abies and Abiesalba saplings along a natural light gradient

The role of morphological versus physiological foliar plasticity in the cap acity for, and mechanisms of, photosynthetic acclimation was assessed in Pi cea abies (L.) Karst. and Abies alba Mill. saplings in a forest gap-underst ory light gradient (relative irradiance, RI, ranging from 0.02 to 0.32). Th e species investigated showed a similar foliar morphological plasticity alo ng the light gradient, at both the needle level (through alteration in leaf dry mass per area) and the shoot level (through alteration in the silhouet te area ratio, e.g., shoot silhouette to projected needle area ratio). In b oth species chlorophyll (Chl) concentration on a mass basis decreased at in creasing RI, but was independent of RI when expressed on an area basis. In contrast, leaf N concentration on a mass basis was independent of RI, but w as positively influenced by RI when expressed on an area basis. The paramet ers describing photosynthetic performance at low light (dark respiration ra te, apparent quantum yield and light compensation point) suggest that Abies alba was better suited to maintain a positive carbon balance in shaded con ditions. By contrast, parameters describing biochemical capacity at high li ght (maximum electron transport rate, J(max). and maximum ribulose-1,5-biph osphate carboxylation capacity, V-cmax) indicate that only Picea abies was capable of acclimating physiologically to high photosynthetic photon flux d ensities (PPFDs) by increasing nitrogen partitioning to rubisco and V-cmax/ mass, by increasing RI. These results support the hypothesis that interspec ific differences in nitrogen partitioning within the photosynthetic apparat us may provide a mechanistic basis for species separation along a light gra dient. The differences in photosynthetic plasticity observed are likely to influence regeneration patterns and habitat breadth of the species investig ated. The limited ability of Abies alba saplings to exploit high-light cond itions may be a competitive disadvantage in large canopy gaps and thus limi t recruitment of this species to small gaps.