Assessing Intraspecific Variation in Effective Dispersal Along an Altitudinal Gradient: A Test in Two Mediterranean High-Mountain Plants

Background
Plant recruitment depends among other factors on environmental conditions and their variation at different spatial scales. Characterizing dispersal in contrasting environments may thus be necessary to understand natural intraspecific variation in the processes underlying recruitment. Silene ciliata and Armeria caespitosa are two representative species of cryophilic pastures above the tree line in Mediterranean high mountains. No explicit estimations of dispersal kernels have been made so far for these or other high-mountain plants. Such data could help to predict their dispersal and recruitment patterns in a context of changing environments under ongoing global warming.
Methods
We used an inverse modelling approach to analyse effective seed dispersal patterns in five populations of both Silene ciliata and Armeria caespitosa along an altitudinal gradient in Sierra de Guadarrama (Madrid, Spain). We considered four commonly employed two-dimensional seedling dispersal kernels exponential-power, 2Dt, WALD and log-normal.
Key Results
No single kernel function provided the best fit across all populations, although estimated mean dispersal distances were short (<1 m) in all cases. S. ciliata did not exhibit significant among-population variation in mean dispersal distance, whereas significant differences in mean dispersal distance were found in A. caespitosa. Both S. ciliata and A. caespitosa exhibited among-population variation in the fecundity parameter and lacked significant variation in kernel shape.
Conclusions
This study illustrates the complexity of intraspecific variation in the processes underlying recruitment, showing that effective dispersal kernels can remain relatively invariant across populations within particular species, even if there are strong variations in demographic structure and/or physical environment among populations, while the invariant dispersal assumption may not hold for other species in the same environment. Our results call for a case-by-case analysis in a wider range of plant taxa and environments to assess the prevalence and magnitude of intraspecific dispersal variation.

**背景** 植物种群补充过程受多种因素调控，其中包括不同空间尺度下的环境条件及其动态变化。若要阐明支撑种群补充的核心过程中存在的自然种内变异，就必须对不同环境下的植物扩散特征进行系统刻画。Silene ciliata与Armeria caespitosa是地中海高海拔山区林线以上耐寒草甸的代表性物种。截至目前，针对这两个物种以及其他高山植物，尚未有关于扩散核的明确估算研究。此类数据可为在全球变暖持续、环境不断变化的背景下预测这些植物的扩散与种群补充模式提供支撑。

**方法** 本研究采用反演建模方法，对西班牙马德里瓜达拉马山（Sierra de Guadarrama）沿海拔梯度分布的5个Silene ciliata种群和5个Armeria caespitosa种群的有效种子扩散模式展开分析。本次研究选用了四种常用的二维幼苗扩散核模型，分别为指数幂模型（exponential-power）、2Dt模型、WALD模型与对数正态模型（log-normal）。

**主要结果** 尽管所有研究种群的估算平均扩散距离均较短（<1米），但并无单一核函数可适配所有种群的最优拟合模型。Silene ciliata的平均扩散距离在种群间无显著差异，而Armeria caespitosa的平均扩散距离则存在显著的种群间变异。两种植物的繁殖力参数均存在显著的种群间差异，但其扩散核的形状则无显著变化。

**结论** 本研究阐明了种群补充过程中种内变异的复杂性：即使同一物种的不同种群间存在显著的种群结构与/或物理环境差异，其有效扩散核仍可在种群间保持相对恒定；但在相同环境下，这一恒定扩散的假设并不适用于其他物种。本研究结果表明，需针对更广范围的植物类群与环境开展逐一分析，以评估种内扩散变异的普遍性与强度。