Section of Plant Ecology, Institute of Botany, Academy of Sciences of the Czech Republic, Třeboň, Czech Republic.
PLoS One. 2013;8(1):e53514. doi: 10.1371/journal.pone.0053514. Epub 2013 Jan 10.
Many cushion plants ameliorate the harsh environment they inhabit in alpine ecosystems and act as nurse plants, with significantly more species growing within their canopy than outside. These facilitative interactions seem to increase with the abiotic stress, thus supporting the stress-gradient hypothesis. We tested this prediction by exploring the association pattern of vascular plants with the dominant cushion plant Thylacospermum caespitosum (Caryophyllaceae) in the arid Trans-Himalaya, where vascular plants occur at one of the highest worldwide elevational limits. We compared plant composition between 1112 pair-plots placed both inside cushions and in surrounding open areas, in communities from cold steppes to subnival zones along two elevational gradients (East Karakoram: 4850-5250 m and Little Tibet: 5350-5850 m). We used PERMANOVA to assess differences in species composition, Friedman-based permutation tests to determine individual species habitat preferences, species-area curves to assess whether interactions are size-dependent and competitive intensity and importance indices to evaluate plant-plant interactions. No indications for net facilitation were found along the elevation gradients. The open areas were not only richer in species, but not a single species preferred to grow exclusively inside cushions, while 39-60% of 56 species detected had a significant preference for the habitat outside cushions. Across the entire elevation range of T. caespitosum, the number and abundance of species were greater outside cushions, suggesting that competitive rather than facilitative interactions prevail. This was supported by lower soil nutrient contents inside cushions, indicating a resource preemption, and little thermal amelioration at the extreme end of the elevational gradient. We attribute the negative associations to competition for limited resources, a strong environmental filter in arid high-mountain environment selecting the stress-tolerant species that do not rely on help from other plants during their life cycle and to the fact the cushions do not provide a better microhabitat to grow in.
许多垫状植物改善了它们在高山生态系统中所栖息的恶劣环境，起到了抚育植物的作用，其树冠下的物种数量明显多于树冠外。这些促进作用似乎随着非生物胁迫的增加而增加，从而支持了胁迫梯度假说。为了验证这一预测，我们在干旱的横断山区（那里的维管束植物出现在世界上最高的海拔限制之一）探索了维管束植物与优势垫状植物穗状点地梅（石竹科）之间的关联模式。我们在 1112 个对生样方内进行了比较，这些样方分别位于垫状植物内和周围的开阔区域，样方所在的群落沿着两条海拔梯度（东喀喇昆仑：4850-5250 米和小西藏：5350-5850 米）从寒冷的草原到亚冰雪带。我们使用 PERMANOVA 评估物种组成的差异，使用基于 Friedman 的排列检验来确定单个物种的栖息地偏好，使用物种-面积曲线来评估相互作用是否取决于大小，以及使用竞争强度和重要性指数来评估植物-植物相互作用。在海拔梯度上没有发现净促进的迹象。开阔区域不仅物种更丰富，而且没有一种物种只喜欢在垫状植物内生长，而 56 种检测到的物种中有 39-60%对垫状植物外的栖息地有明显偏好。在穗状点地梅的整个海拔范围内，垫状植物外的物种数量和丰度都更高，这表明竞争而不是促进作用占主导地位。这得到了垫状植物内土壤养分含量较低的支持，表明存在资源抢占，并且在海拔梯度的极端端热量改善很小。我们将负相关归因于对有限资源的竞争，在干旱高山环境中，强烈的环境筛选选择了在其生命周期中不依赖其他植物帮助的耐受压力的物种，以及垫状植物不能提供更好的微生境供其生长的事实。
