Data from: Effects of nitrogen deposition on reproduction in a masting tree: benefits of higher seed production are trumped by negative biotic interactions

Relatively little is known about the effects of anthropogenic environmental changes on reproductive ecology of trees. Yet, recruitment is a primary determinant of the long-term dynamics of plant populations in changing environments.
We used the Long-Term Ecological Research site at Harvard Forest to evaluate the effects of chronic (over 25 years) nitrogen fertilization on reproductive ecology of red oaks (Quercus rubra).
Oaks growing in fertilized plots produced 4–9 times more acorns than control trees. However, nitrogen deposition simultaneously affected oaks’ biotic interactions. It increased pre-dispersal seed predation by insects (primarily weevils, Curculio spp.) on fertilized plots, most likely as the result of the disruption of predator satiation. In addition, infestation by weevils was more likely to result in embryo destruction in fertilized than in control acorns. Furthermore, the proportion of acorns dispersed and cached by rodents decreased on fertilized plots. Finally, germination of fertilized acorns was lower than control acorns, even after controlling for the effects of weevils and rodents.
Inclusion of the altered biotic interactions reversed the final picture of the effects of long-term nitrogen fertilization on oak reproduction: the positive effects on acorn quantity were trumped by the nitrogen-mediated changes in biotic interactions.
Synthesis. Our results stress the importance of considering indirect effects and consumer interactions when evaluating the effects of environmental change on plant population dynamics. Long-term nitrogen fertilization has a strong potential to decrease the recruitment of masting trees. Given the ubiquitous increase in the anthropogenic nitrogen deposition, processes similar to those found in our system might operate in others, resulting in a widespread alteration in trees’ recruitment dynamics.

目前学界对人为环境变化如何影响树木繁殖生态学的认知仍相对有限。然而，种群补充是变化环境中植物种群长期动态的核心决定因子。
本研究依托哈佛森林的长期生态研究（Long-Term Ecological Research）站点，评估了持续25年以上的氮素施肥对北美红栎（Quercus rubra）繁殖生态学的影响。
施肥样地内的栎树所产出的橡子数量为对照样地树木的4至9倍。但氮素添加同时改变了栎树与其他生物的种间互作：施肥样地内昆虫（主要为象鼻虫属Curculio物种）的扩散前种子捕食率显著上升，这极有可能源于捕食者饱和效应被打破。此外，与对照橡子相比，施肥组橡子被象鼻虫侵染后更易发生胚损伤。进一步观测发现，啮齿动物对橡子的扩散与贮藏比例在施肥样地中有所降低。最后，即便排除象鼻虫与啮齿动物的干扰，施肥组橡子的萌发率仍低于对照组。
纳入这些改变的生物互作关系后，长期氮素施肥对栎树繁殖的最终影响图景发生逆转：氮素介导的生物互作改变抵消了其对橡子数量的正向增益效应。
【综合与结论】本研究结果强调，在评估环境变化对植物种群动态的影响时，需考虑间接效应与消费者互作关系。长期氮素施肥极有可能降低大年结实树种的种群补充量。鉴于人为氮沉降已呈全球普遍上升趋势，本研究揭示的过程或在其他生态系统中同样存在，进而导致树木种群补充动态的广泛改变。