Differences in species composition between calcareous and siliceous herbaceous communities are primarily explained by competition in favourable climates

Differences in vegetation composition between communities from calcareous
and siliceous soils might be due to high competition on siliceous soils
for species from calcareous origin, and high rock-induced drought stress
on calcareous soils for species from siliceous origin. We tested the
hypothesis that, with increasing climate stress, competition should
decrease on siliceous rock for species of calcareous origin and drought
stress increase on calcareous rock for species of siliceous origin because
of decreasing community biomass with increasing cold or drought stress.
This question is of high interest for predicting changes in species
distribution with climate change since bedrock type and climate change are
both complex factors that are likely to interact with climate. We set up a
transplant removal experiment in contrasting climate conditions of the
south of France, warm and wet temperate, warm and dry Mediterranean, and
cold temperate in the Alps and the Pyrenees. In each climate condition,
three targets (two species and one population from a third species) from
each origin were transplanted with and without neighbours on the two rock
types and for two years with different levels of drought stress. Variation
in the effects of neighbours and rock-induced drought stress on transplant
survival with and without neighbours were analysed with linear modelling
at each site, separately, and with ANCOVA in the whole design. The
competition was the highest on siliceous rock in the more favourable
climate conditions but for species of siliceous origin. The lower
competition found for species of calcareous origin was likely due to the
occurrence of allelopathic effects that decreased the negative effect of
neighbours as measured with the removal method. The rock-induced drought
stress was the highest on calcareous rock, in particular for species of
siliceous origin. Thus, the decrease in competition intensity on siliceous
rocks with increasing climate stress was more important for species of
siliceous than calcareous origin. Additionally, the importance of the
rock-induced drought stress was relatively low in the most stressful
climate conditions as compared to the overwhelming importance of climate
drought stress over the two years of our experiment, which likely induced
a collapse of the positive effect of growing on a siliceous rock.

石灰质土壤与硅质土壤群落间的植被组成差异，可能源于硅质土壤上石灰质起源物种面临的激烈种间竞争，以及石灰质土壤上硅质起源物种所承受的岩石诱导干旱胁迫。本研究验证了如下假说：随着气候胁迫加剧，石灰质起源物种在硅质岩石生境的种间竞争会减弱，而硅质起源物种在石灰质岩石生境的干旱胁迫会增强——这是因为低温或干旱胁迫加剧会导致群落生物量下降。鉴于基岩类型与气候变化均为可能与气候因子产生交互作用的复杂变量，该问题对于预测气候变化下的物种分布变化具有重要研究价值。本研究在法国南部不同气候梯度区域开展了邻体移除移栽实验，涵盖暖湿温带气候区、暖干地中海气候区，以及阿尔卑斯与比利牛斯山区的冷温带气候区。在每个气候区中，两类起源的物种各选取3份实验材料（2个物种，以及第三个物种的1个种群），分别在保留邻体与移除邻体的处理下，移栽至两种岩石类型生境中，并设置不同干旱胁迫梯度，实验周期为2年。针对不同邻体处理（有/无邻体）下移栽植株存活率受邻体与岩石诱导干旱胁迫的影响变异，本研究在每个试验站点分别采用线性模型（linear model）进行单独分析，并在整体实验设计中采用协方差分析（Analysis of Covariance, ANCOVA）进行整合分析。结果显示，在气候条件更适宜的区域，硅质岩石生境中的种间竞争强度最高，但该现象仅见于硅质起源物种。石灰质起源物种所受种间竞争强度较低，这可能源于化感作用（allelopathic effects）的存在——该作用削弱了邻体带来的负面效应，这一结论通过邻体移除实验方法得以验证。石灰质岩石生境中的岩石诱导干旱胁迫强度最高，该现象在硅质起源物种中尤为显著。因此，随着气候胁迫加剧，硅质岩石生境中的种间竞争强度下降幅度，在硅质起源物种中显著高于石灰质起源物种。此外，在本实验的两年观测期内，相较于气候干旱胁迫所展现出的压倒性影响，岩石诱导干旱胁迫在极端气候胁迫环境中的相对重要性较低——这可能导致了硅质岩石生境带来的生长增益效应完全丧失。