Sapling growth gradients interact with homogeneous disturbance regimes to explain savanna tree cover discontinuities

IF 7.1 1区 环境科学与生态学 Q1 ECOLOGY Ecological Monographs Pub Date : 2022-02-20 DOI:10.1002/ecm.1514
Ricardo M. Holdo, Jason E. Donaldson, Deusdedith M. Rugemalila, T. Michael Anderson
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引用次数: 3

Abstract

Savanna tree cover often exhibits sudden discontinuities across space. It has been proposed that local spatial processes imposed by variation in tree cover itself (as opposed to by external drivers such as edaphic variation) can reinforce such discontinuities. Despite this, we generally lack data on tree demography and the environmental drivers affecting the former as a function of tree neighborhoods in these systems. Given the importance of disturbance traps in savannas, spatial processes affecting the likelihood of escape from the seedling/sapling stage to the adult tree stage are likely to be critical. In a longitudinal survey of 800 saplings distributed along eight 1-km transects spanning woodland–grassland transitions in Serengeti National Park, we found a positive association between tree cover and sapling growth and survival, but no relationship with sapling abundance, maximum tree height, disturbance, or topkill. Based on microclimate and soil moisture dynamics data, we found no evidence to suggest that tree cover itself drives variation in growth. Based on a prior analysis of soil properties along these transects, we hypothesized that underlying edaphic conditions may be responsible for variation in growth. Regardless of the underlying mechanism, we used simulations to show that subtle growth rate gradients interacted with intense disturbance regimes to produce sharp discontinuities in tree cover, with strong demographic bottlenecks where growth is slowest, explaining the observed patterns of tree cover along the transects. Our results indicated that disturbance and herbivory are equally intense in areas of high and low tree cover, and that although trees have the potential to successfully establish and reach adulthood in open, grassy sites, they grow too slowly to escape disturbance traps there. Importantly, we showed that although herbivory and fire are fundamental for explaining savanna structural patterns, their effects are not necessarily reinforced by tree cover itself.

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树苗生长梯度与均匀扰动机制相互作用,解释稀树草原树木覆盖不连续性
稀树草原的树木覆盖常常在空间上表现出突然的不连续性。有人提出,由树木覆盖本身的变化所施加的局部空间过程(与外部驱动因素如土壤变化相反)可以加强这种不连续性。尽管如此,我们通常缺乏树木的人口统计数据和影响前者的环境驱动因素作为这些系统中树木社区的功能。考虑到热带稀树草原干扰陷阱的重要性,影响从幼苗期到成树期逃逸可能性的空间过程可能是至关重要的。通过对塞伦盖蒂国家公园8个1公里林地-草地过渡样带的800棵树苗进行纵向调查,我们发现树木覆盖与树苗生长和存活呈正相关,但与树苗丰度、最大树高、干扰或顶杀无关。基于小气候和土壤水分动态数据,我们发现没有证据表明树木覆盖本身驱动生长变化。根据先前对这些样带土壤性质的分析,我们假设潜在的土壤条件可能是导致生长变化的原因。不管潜在的机制是什么,我们使用模拟显示,细微的生长速率梯度与强烈的干扰机制相互作用,产生了树木覆盖的急剧不连续性,在生长最慢的地方存在强大的人口瓶颈,解释了沿样带观察到的树木覆盖模式。我们的研究结果表明,干扰和草食在树木覆盖度高和低的地区同样强烈,尽管树木有可能在开阔的草地上成功建立和长成成年,但它们的生长速度太慢,无法逃脱干扰陷阱。重要的是,我们表明,尽管草食和火是解释稀树草原结构模式的基础,但它们的影响并不一定被树木覆盖本身所加强。
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来源期刊
Ecological Monographs
Ecological Monographs 环境科学-生态学
CiteScore
12.20
自引率
0.00%
发文量
61
审稿时长
3 months
期刊介绍: The vision for Ecological Monographs is that it should be the place for publishing integrative, synthetic papers that elaborate new directions for the field of ecology. Original Research Papers published in Ecological Monographs will continue to document complex observational, experimental, or theoretical studies that by their very integrated nature defy dissolution into shorter publications focused on a single topic or message. Reviews will be comprehensive and synthetic papers that establish new benchmarks in the field, define directions for future research, contribute to fundamental understanding of ecological principles, and derive principles for ecological management in its broadest sense (including, but not limited to: conservation, mitigation, restoration, and pro-active protection of the environment). Reviews should reflect the full development of a topic and encompass relevant natural history, observational and experimental data, analyses, models, and theory. Reviews published in Ecological Monographs should further blur the boundaries between “basic” and “applied” ecology. Concepts and Synthesis papers will conceptually advance the field of ecology. These papers are expected to go well beyond works being reviewed and include discussion of new directions, new syntheses, and resolutions of old questions. In this world of rapid scientific advancement and never-ending environmental change, there needs to be room for the thoughtful integration of scientific ideas, data, and concepts that feeds the mind and guides the development of the maturing science of ecology. Ecological Monographs provides that room, with an expansive view to a sustainable future.
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