Department of Biology, Miami University, Oxford, Ohio, 45056, USA.
Earth Systems Research Center, University of New Hampshire, Durham, New Hampshire, 03824, USA.
Ecology. 2018 Feb;99(2):438-449. doi: 10.1002/ecy.2100. Epub 2018 Jan 16.
Forest productivity on glacially derived soils with weatherable phosphorus (P) is expected to be limited by nitrogen (N), according to theories of long-term ecosystem development. However, recent studies and model simulations based on resource optimization theory indicate that productivity can be co-limited by N and P. We conducted a full factorial N × P fertilization experiment in 13 northern hardwood forest stands of three age classes in central New Hampshire, USA, to test the hypothesis that forest productivity is co-limited by N and P. We also asked whether the response of productivity to N and P addition differs among species and whether differential species responses contribute to community-level co-limitation. Plots in each stand were fertilized with 30 kg N·ha ·yr , 10 kg P·ha ·yr , N + P, or neither nutrient (control) for four growing seasons. The productivity response to treatments was assessed using per-tree annual relative basal area increment (RBAI) as an index of growth. RBAI responded significantly to P (P = 0.02) but not to N (P = 0.73). However, evidence for P limitation was not uniform among stands. RBAI responded to P fertilization in mid-age (P = 0.02) and mature (P = 0.07) stands, each taken as a group, but was greatest in N-fertilized plots of two stands in these age classes, and there was no significant effect of P in the young stands. Both white birch (Betula papyrifera Marsh.) and beech (Fagus grandifolia Ehrh.) responded significantly to P; no species responded significantly to N. We did not find evidence for N and P co-limitation of tree growth. The response to N + P did not differ from that to P alone, and there was no significant N × P interaction (P = 0.68). Our P limitation results support neither the N limitation prediction of ecosystem theory nor the N and P co-limitation prediction of resource optimization theory, but could be a consequence of long-term anthropogenic N deposition in these forests. Inconsistencies in response to P suggest that successional status and variation in site conditions influence patterns of nutrient limitation and recycling across the northern hardwood forest landscape.
根据长期生态系统发展理论，在具有可风化磷 (P) 的冰川衍生土壤上，森林生产力预计将受到氮 (N) 的限制。然而，根据资源优化理论的最新研究和模型模拟表明，生产力可能会受到 N 和 P 的共同限制。我们在美国新罕布什尔州中部的 13 个不同年龄阶段的北方硬木林分中进行了一项全因子 N × P 施肥实验，以检验森林生产力受到 N 和 P 共同限制的假设。我们还询问了生产力对 N 和 P 添加的反应是否因物种而异，以及不同物种的反应是否有助于群落水平的共同限制。每个林分的地块都用 30 kg N·ha·yr、10 kg P·ha·yr、N + P 或两者都不添加（对照）进行了为期四个生长季节的施肥。使用每棵树的年相对基面积增量 (RBAI) 作为生长指标来评估处理对生产力的响应。RBAI 对处理的响应显著（P=0.02），但对 N 的响应不显著（P=0.73）。然而，在各林分之间，P 限制的证据并不一致。RBAI 对中龄（P=0.02）和成熟（P=0.07）林分的 P 施肥有响应，这两个年龄组作为一个整体，但在这两个年龄组的两个林分的 N 施肥地块中最大，而在幼龄林分中 P 施肥没有显著效果。白桦 (Betula papyrifera Marsh.) 和山毛榉 (Fagus grandifolia Ehrh.) 对白桦的响应均显著，而对白桦的响应均不显著。没有证据表明树木生长存在 N 和 P 的共同限制。N + P 的响应与单独的 P 响应没有差异，也没有显著的 N × P 相互作用（P=0.68）。我们的 P 限制结果既不支持生态系统理论的 N 限制预测，也不支持资源优化理论的 N 和 P 共同限制预测，但可能是这些森林中长期人为 N 沉积的结果。对 P 的反应不一致表明，演替状态和地点条件的变化会影响北方硬木林景观中养分限制和再循环的模式。
