植物种群更新限制——从种子生产到幼树建成

更新限制是指种子由于各种原因,不能够萌发并生长成幼树。它作为解释生物多样性的理论,一直受到国内外群落生态学家关注。从种源限制、传播限制和建成限制3个角度,对更新限制机制研究进展进行了综述。从种源限制而言,时空因素是影响植物种群更新限制的重要因素,因为植物结实量存在明显时空变化,造成植物更新个体出现明显的时空规律。从传播限制而言,传播数量、距离和食果动物行为均限制植物种群更新。数量上,缺乏有限传播者势必减少传播数量,但如果种子拥有较高质量,则能逃脱数量限制;距离上,植物更新个体显示出明显的Janzen-Connell格局,但传播距离趋向稳定,形成植物种群的进化稳定对策;食果动物行为上,不同传播者对更新贡献存在差异,捕食者直接降低更新,融入两类动物行为的模型更能反映食果动物对更新的限制。从建成限制而言,环境因子制约植物生长。小尺度下,微生境的好坏对于植物幼苗建成至关重要;大尺度下,植物提供较好的广告效应则能摆脱生境限制。将传播者行为、捕食者行为与幼苗的空间分布格局、种子传播机理模型等结合,建立植物更新限制机理模型应是更新限制未来的研究热点。选择稀有种和古老种为主题的长期更新限制研究,为种群恢复提供指导,也是未来重要研究方向。

Recruitment limitation of plant population: from seed production to sapling establishment

Recruitment limitation, which occurs when seeds do not germinate and successfully develop into saplings, can result from three broadly defined mechanisms: (i) source limitation, which occurs when recruitment is limited by low population-level seed availability; (ii) dissemination limitation, which occurs when recruitment is limited by a failure of dispersing seeds to reach potential recruitment sites; and (iii) establishment limitation, which occurs when recruitment is limited by unsuitable biotic or abiotic environmental conditions. Recruitment limitation is an active research area for community ecologists as it potentially explains biodiversity patterns. In this article, we review research on recruitment limitation including source limitation, dissemination limitation and establishment limitation. Source limitation may occur in the early stages of plant regeneration and it is affected by small population sizes, temporal changes of plant productivity and habitat patch quality. Spatiotemporal changes in the condition are the main factors influencing plant recruitment. Mast seeding and high habitat patch quality may attract frugivores, helping plants avoid source limitation. Dispersal rate and distance, as well as frugivore behavior, could affect plant dissemination limitation which is the most important cycle in recruitment limitation. A lack of effective dispersers will reduce the number of seeds dispersed, which restricts recruitment. But high quality of seeds may help it escape limitation. For distance-restricted recruitment, recruitment patterns of plant species conform to those predicted by the Janzen-Connell hypothesis. Species with seeds capable of long distance dispersal are less impacted by spatially non-random mortality than those with seeds moving shorter distances. Through coevolution, the specific dispersal kernel of a plant population should be an evolutionary stable strategy that optimizes plant recruitment. For plants restricted by the behavior of animal dispersal vectors, non-random movement of frugivores would lead to seed aggregation, causing high density-dependent mortality. Different dispersers can make different contributions to recruitment. However, predators always decrease the density of seeds and seedlings. Few studies have incorporated animal behavior into seed dispersal models; a modified model incorporating animal behavior can better reflect the pattern of recruitment. Environmental factors play important roles in establishment limitation. At small scales, environmental conditions can affect seedling recruitment. For instance, soil nutrients are important for seedling growth and poor or excessive soil nutrients may limit growth. Seedling growth is also influenced by water availability. Shade-tolerant species can adapt to low light conditions, but late-succession species need better light conditions. Seedlings show greater survival and growth in canopy gaps than beneath the canopy. Living with heterospecific neighborhood can enhance seedling growth. At large scales, different vegetation types vary in their suitability for sapling establishment. However, plant species can avoid habitat limitation by attracting animal dispersal vectors. Future research should focus on developing a mechanistic model of plant recruitment limitation by incorporating behavior of dispersers and predators, and the spatial pattern of seedlings. Forest and wildlife managers also need to work together to account for spatial mechanisms of recruitment limitation for rare species over the long-term.