Abstract:

Objective: In order to enhance understanding of the storage and allocation of carbon (C), nitrogen (N) and phosphorus (P) at the whole tree level, we quantified these inter-organ and inter-specific differences in C, N, and P contents and their stoichiometric relationship in the co-existing tree species, and explored the strategies of inter-organ nutrient allocation of coexisting tree species and inter-specific difference in nutrient uptake. Method: The C, N, P contents and their stoichiometry for different organs (leaf, branch, bark, sapwood, heartwood, stump, 5 mm < diameter≤ 30 mm large root, 2 mm < diameter≤ 5 mm coarse root, and diameter ≤ 2 mm fine root) were measured for 10 broadleaf tree species in a temperate mixed deciduous forest in Maoershan area, Heilongjiang Province. The coefficient of variation (CV) was used to express the differences among organs or tree species, and the slopes of standardized major axis regressions were used to test the allometric or isometric scaling relationships between elements. Result: The effects of tree species, organs and their interaction on C, N, P contents, C: N, C: P and N: P were all significant except the effect of tree species on C: P. For mean values, resource acquisition organs (leaf and fine root) had higher C, N, P contents and N: P, while the xylem (sapwood and heartwood) showed higher C: N, C: P than the other organs. For inter-specific variations, leaves had low inter-specific CVs of element contents and their stoichiometric ratios; fine roots had high inter-specific CVs of N, C: N, and N: P, but low inter-specific CVs for Pand C: P, suggesting that tree species in this area preferentially allocated the limited element N to the photosynthetic organ. For most organs, relationships between elements were not significant, indicating that the nutrient stoichiometric relationships for single organ could not directly represent the nutrient stoichiometries for other organs or at the whole tree level. The relationships between C and N or C and P across the nine organs varied with species, but the relationship between N and P was highly consistent in different species. Conclusion: The differences in C, N, P contents and their stoichiometric ratios were closely related to the functional differentiation among organs. Overall, the limited element N was preferentially allocated to leaves and constrained carbon sequestration in wood; whereas the utilization of C, N, and P for each organ diverged obviously among the coexisting species, which was likely to help reduce the inter-specific competition. No matter at the intra-specific or the inter-specific levels, the nutrients contents and their stoichiometric relationships of single organ could not directly reflect that of other organs. Therefore, only using one organ to assess the C, N, and P storage and allocation patterns at the whole tree level was inaccurate, thus future studies should fully consider the inter-organ and inter-specific differences of element contents.

Key words: ecological stoichiometry, inter-organ variation, inter-specific variation, local scale