Patterns of leaf mass, area and nitrogen in young northern hardwood forests

Patterns of leaf mass per unit area (LMA) and area-based foliar N concentra tion (N area) through the canopy were examined for seven tree species growi ng in reference and fertilized plots of two early successional northern har dwood stands in New Hampshire. Increases in LMA with height in the canopy w ere significantly related to increasing average daily photosynthetically ac tive radiation (PAR). As expected, the slopes of regressions between LMA an d PAR (i.e., plasticity) were higher for shade tolerant than intolerant spe cies in both stands. Fertilization increased leaf area index in both stands . For the shade intolerant species (Prunus pensylvanica, Betula papyrifera) , average canopy LMA increased in response to fertilization but no response was observed in the more tolerant species (Acer spp., Fagus grandifolia, B etula allegheniensis). The slopes of regressions between N area and PAR wer e highly significant, with higher plasticity of N area for shade tolerant t han intolerant species. Average canopy N area increased in response to fert ilization, and the response was particularly large for the intolerant speci es. The increase in N-area for these species was associated with the signif icant response of shaded foliage, as sun exposed foliage showed no response to fertilization.