Biomass allocation and growth rates in Pinus sylvestris are interactively modified by nitrogen and phosphorus availabilities and by tree size and age

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 (N-W) and phosphorus (P-W) contents were higher in P. sylvestris in sand dunes than in those in the raised bog, but P-W/N-W ratios overlapped between the sites. Leaf dry mass ratio (F-L) and leaf-area ratio (LAR) increased with N-W, and F-L increased with P-W. The relative growth rate (R-G) was more strongly associated with P-W than with N-W. The net assimilation rate per leaf dry mass (NAR(M)) scaled positively with P-W but not with N-W, demonstrating that the stronger effect of P-W on growth was due to modified biomass allocation and physiology (R-G = NAR(M) x F-L), while N-W affected growth via biomass allocation. Partitioning and growth characteristics were poorly related to the P-W/N-W ratio. The overall decrease of growth in larger trees resulted from their lower LAR and F-L. Increases in size further led to a lower N-W but higher P-W. We conclude that optimum productivity at a given N-W requires a certain minimum P-W, not a specific non-limiting P-W/N-W ratio. While nutrients affect growth by changing biomass allocation and physiological activity, size primarily modifies biomass allocation.