Relationships of leaf dark respiration with light environment and tissue nitrogen content in juveniles of 11 cold-temperate tree species

It has been argued that plants adapted to low light should have lower carbo n losses via dark respiration (Rd) than those not so adapted, and similarly , all species would be expected to down-regulate Rd in deep shade, because the associated advantages of high metabolic potential cannot be realized in such habitats. In order to test these hypotheses, and to explore the deter minants of intraspecific variation in respiration rates, we measured Rd, le af mass per unit area (LMA), and nitrogen content of mature foliage in juve niles of 11 cold-temperate tree species (angiosperms and conifers), growing in diverse light environments in forest understories in northern Minnesota . Among the seven angiosperm species, respiration on mass, area, and nitrog en bases showed significant negative overall relationships with shade toler ance level. Mass-based respiration rates (Rd(mass)) of angiosperms as a gro up showed a significant positive overall relationship with an index of ligh t availability (percentage canopy openness, %CO). Rd(mass) of most conifers also showed evidence of acclimation of Rd(mass) to light availability. LMA of all species also increased with increasing %CO, but this response was g enerally much stronger in angiosperms than in conifers. As a result, the re sponse of area-based respiration (Rd(mass)) to %CO was dominated by Delta R d(mass) for conifers, and by Delta LMA for most angiosperms, i.e., function al types differed in the components of acclimation of Rd(area) to light ava ilability. Among the seven angiosperm species, the relationships of leaf N on a mass basis (N-mass) with %CO were modulated by shade tolerance: negati ve slopes in shade-tolerant species may be related to the steep increases i n LMA of these taxa along gradients of increasing light intensity, and asso ciated dilution of N-rich, metabolically active tissue by increasing invest ment in leaf structural components. Although N-mass was therefore an unreli able predictor of variation in Rd(mass) along light gradients, respiration per unit leaf N (Rd/N) was significantly positively correlated with %CO for most species. This probably reflects variation in the proportion of leaf N allocated to protein and/or the influence of leaf carbohydrate status on R d. Species shade tolerance differences were not significantly correlated wi th the magnitude of either Delta Rd(mass) or Delta Rd(area), indicating tha t variation in acclimation potential of Xd is much less important than inhe rent differences in this trait. Acclimation of Rd(mass) to light availabili ty appears to be a generalized feature of juvenile trees, and the important ecological trade-off is likely between high metabolic capacity in high lig ht and low respiratory losses in low light.