RELATIVE GROWTH-RATE IN RELATION TO PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS FOR NORTHERN HARDWOOD TREE SEEDLINGS - SPECIES, LIGHT ENVIRONMENT AND ONTOGENIC CONSIDERATIONS

The influence of ontogeny, light environment and species on relationsh ips of relative growth rate (RGR) to physiological and morphological t raits were examined for first-year northern hardwood tree seedlings. T hree Betulaceae species (Betula papyrifera, Betula alleghaniensis and Ostrya virginiana) were grown in high and low light and Quercus rubra and Acer saccharum were grown only in high light. plant traits were de termined at four ages: 41, 62, 83 and 104 days after germination. In h igh light (610 mu mol m(-2) s(-1) PPFD), across species and ages, RGR was positively related to the proportion of the plant in leaves (leaf weight ratio, LWR; leaf area ratio, LAR), in situ rates of average can opy net photosynthesis (A) per unit mass (A(mass)) and per unit area ( A(area)), and rates of leaf, stem and root respiration. In low light ( 127 mu mol m(-2), s(-1) PPFD), RGR was not correlated with A(mass) and A(area) whereas RGR was positively correlated with LAR, LWR, and rate s of root and stem respiration. RGR was negatively correlated with lea f mass per area in both high and low light. Across light levels, relat ionships of CO2 exchange and morphological characteristics with RGR we re generally weaker than within light environments. Moreover, relation ships were weaker for plant parameters containing a leaf area componen t (leaf mass per area, LAR and A(area)), than those that were solely m ass-based (respiration rates, LWR and A(mass)). Across light environme nts, parameters incorporating the proportion of the plant in leaves an d rates of photosynthesis explained a greater amount of variation in R GR (e.g. LWRA(mass), R(2)=0.64) than did any single parameter related to whole-plant carbon gain. RGR generally declined with age and mass, which were used as scalars of ontogeny. LWR (and LAR) also declined f or seven of the eight species-light treatments and A declined in four of the five species in high light. Decreasing LWR and A with ontogeny may have been partially responsible for decreasing RGR. Declines in RG R were not due to increased respiration resulting from an increase in the proportion of solely respiring tissue (roots and stems). In genera l, although LWR declined with ontogeny, specific rates of leaf, stem, and root respiration also decreased. The net result was that whole-pla nt respiration rates per unit leaf mass decreased for all eight treatm ents. Identifying the major determinants of variation in-growth (e.g. LWRA(mass)) across fight environments, species and ontogeny contribut es to the establishment of a framework for exploring limits to product ivity and the nature of ecological success as measured by growth. The generality of these relationships both across the sources of variation we explored here and across other sources of variation in RGR needs f urther study.