Pellegrini Adam F A
Ecology. 2016 Feb;97(2):313-24. doi: 10.1890/15-0869.1.
Nutrients have been hypothesized to influence the distribution of the savanna biome through two possible mechanisms. Low nutrient availability may restrict growth rates of trees, thereby allowing for intermittent fires to maintain low tree cover; alternatively, nutrient deficiency may even place an absolute constraint on the ability of forests to form, independent of fire. However, we have little understanding of the scales at which nutrient limitation operates, what nutrients are limiting, and the mechanisms that influence how nutrient limitation regulates savanna-forest transitions. Here, I review literature, synthesize existing data, and present a simple calculation of nutrient demand to evaluate how nutrient limitation may regulate the distribution of the savanna biome. The literature primarily supports the hypothesis that nutrients may interact dynamically with fire to restrict the transition of savanna into forest. A compilation of indirect metrics of nutrient limitation suggest that nitrogen and phosphorus are both in short supply and may limit plants. Nutrient demand calculations provided a number of insights. First, trees required high rates of nitrogen and phosphorus supply relative to empirically determined inputs. Second, nutrient demand increased as landscapes approached the transition point between savanna and forest. Third, the potential for fire-driven nutrient losses remained high throughout transitions, which may exaggerate limitation and could be a key feedback stabilizing the savanna biome. Fourth, nutrient limitation varied between functional groups, with fast-growing forest species having substantially greater nutrient demand and a higher susceptibility to fire-driven nutrient losses. Finally, African savanna trees required substantially larger amounts of nutrients supplied at greater rates, although this varied across plant functional groups. In summary, the ability of nutrients to control transitions emerges at individual and landscape scales, and is regulated through different mechanisms based on spatial (differences in underlying geology), temporal (stage in biome transition) and biological (species traits and community composition) variability.
营养物质被认为可能通过两种机制影响热带稀树草原生物群落的分布。低养分有效性可能会限制树木的生长速度，从而使间歇性火灾得以维持较低的树木覆盖率；或者，养分缺乏甚至可能对森林形成能力施加绝对限制，而与火灾无关。然而，我们对养分限制起作用的尺度、哪些养分受到限制以及影响养分限制如何调节热带稀树草原 - 森林过渡的机制了解甚少。在此，我回顾文献、综合现有数据，并进行简单的养分需求计算，以评估养分限制如何调节热带稀树草原生物群落的分布。文献主要支持这样一种假说，即养分可能与火灾动态相互作用，以限制热带稀树草原向森林的转变。一系列养分限制的间接指标表明，氮和磷都供应不足，可能会限制植物生长。养分需求计算提供了一些见解。首先，相对于经验确定的输入量，树木需要高比例的氮和磷供应。其次，随着景观接近热带稀树草原和森林的过渡点，养分需求增加。第三，在整个过渡过程中，火灾导致的养分损失潜力仍然很高，这可能会加剧限制，并且可能是稳定热带稀树草原生物群落的关键反馈。第四，功能组之间的养分限制有所不同，快速生长的森林物种养分需求大得多，且对火灾导致的养分损失更敏感。最后，非洲热带稀树草原树木需要以更高的速率供应大量养分，尽管这因植物功能组而异。总之，养分控制过渡的能力在个体和景观尺度上显现出来，并通过基于空间（底层地质差异）、时间（生物群落过渡阶段）和生物（物种特征和群落组成）变异性的不同机制进行调节。
