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Young trees 0.03-1.7 m high of three coexisting Betula species were investigated in four sites of varying soil fertility, but all in full daylight, to separate nutrient and plant size controls on leaf dry mass per unit area (MA), light-saturated foliar photosynthetic electron transport rate (J) and the fraction of plant biomass in foliage (F(L)). Because the site effect was generally non-significant in the analyses of variance with foliar nitrogen content per unit dry mass (N(M)) as a covariate, N(M) was used as an explaining variable of leaf structural and physiological characteristics. Average leaf area (S) and dry mass per leaf scaled positively with N(M) and total tree height (H) in all species. Leaf dry mass per unit area also increased with increasing H, but decreased with increasing N(M), whereas the effects were species-specific. Increases in plant size led to a lower and increases in N(M) to a greater FL and total plant foliar area per unit plant biomass (LAR). Thus, the self-shading probably increased with increasing N(M) and decreased with increasing H. Nevertheless, the whole-plant average M(A), as well as M(A) values of topmost fully exposed leaves, correlated with N(M) and H in a similar manner, indicating that scaling of MA with N(M) and H did not necessarily result from the modified degree of within-plant shading. The rate of photosynthetic electron transport per unit dry mass (J(M)) scaled positively with N(M), but decreased with increasing H and M(A). Thus, increases in M(A) with tree height and decreasing nitrogen content not only resulted in a lower plant foliar area (LAR = F(L)/M(A)), but also led to lower physiological activity of unit foliar biomass. The leaf parameters (J(M), N(M) and M(A)) varied threefold, but the whole-plant characteristic FL varied 20-fold and LAR 30-fold, indicating that the biomass allocation was more plastically adjusted to different plant internal nitrogen contents and to tree height than the foliar variables. Our results demonstrate that: (1) tree height and N(M) may independently control foliar structure and physiology, and have an even greater impact on biomass allocation; and (2) the modified within-plant light availabilities alone do not explain the observed patterns. Although there were interspecific differences with respect to the statistical significance of the relationships, all species generally fit common regressions. However, these differences were consistent, and suggested that more competitive species with inherently larger growth rates also more plastically respond to N and H.

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Biomass allocation and growth of Scots pine, Pinus sylvestris L., of various sizes (height 0.03–20 m) and ages (1–151 years) were investigated in two infertile sites (raised bog and sand dunes) to determine relative nitrogen and phosphorus limitations on productivity and their interactions and size-dependent controls. Dry mass weighted average nitrogen (NW) and phosphorus (PW) contents were higher in P. sylvestris in sand dunes than in those in the raised bog, but PW/NW ratios overlapped between the sites. Leaf dry mass ratio (FL) and leaf-area ratio (LAR) increased with NW, and FL increased with PW. The relative growth rate (RG) was more strongly associated with PW than with NW. The net assimilation rate per leaf dry mass (NARM) scaled positively with PW but not with NW, demonstrating that the stronger effect of PW on growth was due to modified biomass allocation and physiology (RG = NARM × FL), while NW affected growth via biomass allocation. Partitioning and growth characteristics were poorly related to the PW/NW ratio. The overall decrease of growth in larger trees resulted from their lower LAR and FL. Increases in size further led to a lower NW but higher PW. We conclude that optimum productivity at a given NW requires a certain minimum PW, not a specific "non-limiting" PW/NW ratio. While nutrients affect growth by changing biomass allocation and physiological activity, size primarily modifies biomass allocation.