Growth of young trees of Acer platanoides and Quercus robur along a gap-understory continuum:: Interrelationships between allometry, biomass partitioning, nitrogen, and shade tolerance

Growth and biomass partitioning across a natural light gradient were examined in saplings (0.3-2.6 m in height) of relatively shade-tolerant Acer platanoides L, and relatively intolerant Quercus robur L. for dependence on relative irradiance, calculated from hemispherical photographs (RI), and on total tree height (H). H and RI were independent. All allometric relationships between H and the parameters of plant biomass and dimensions were positively affected by RI, signifying that at a common H, saplings increased horizontal growth relative to vertical growth in environments with greater irradiance. Root mass ratio (coarse plus fine roots per total plant biomass) increased with increasing irradiance in both species and was not different between the species. However, Q. robur had a larger stem mass ratio, while A. platanoides had a greater leaf mass ratio (LMR). At a common irradiance, leaf mass per area (LMA) was lower in A. platanoides. As a result of these interspecific differences in fractional allocation of sapling biomass in foliage and biomass requirement for construction of foliar surface area, leaf area ratio (LAR = LMR/LMA) was larger in A. platanoides. Relative growth rate of standing biomass (RGR) was significantly correlated with RI in both species and was not different between the species. In contrast, net assimilation rate per unit leaf area (NAR) was larger in Q. robur. Thus, in these species a similar RGR (NAR X LAR) was achieved in a completely different way: A. platanoides increased the intercepting foliar area, while Q. robur increased the productivity per unit area. Growth analysis repeated on a nitrogen basis demonstrated that nitrogen use efficiency (biomass increment per unit nitrogen) was similar between the species, and that the species differences in NAR resulted from different foliar N concentrations; fractional nitrogen investment in foliage was independent of species. but lower nitrogen cost of leaves allowed A. platanoides to construct a more extensive foliar display with the same nitrogen investment in foliage. Biomass allocation between different compartments was also dependent on sapling size. With increasing H, A. platanoides maintained a constant proportion of biomass in foliage, but the relative amount of foliage decreased in Q. robur. Therefore, due to a decreasing investment of resources in foliage construction with advancing sapling ontogeny, saplings of similar height require more light to survive in Q. robur than in A. platanoides. Greater investment of biomass in leaves vs. standing biomass may result in lower volume pain and reduced competitive ability in more open habitats in A. platanoides. These results indicate that interspecific differences in allometry and nitrogen allocation patterns significantly alter species competitive relations during sapling development across the gap-understory continuum.