Interannual variation in season length is linked to strong co-gradient plasticity of phenology in a montane annual plant

Species are commonly distributed along latitudinal and elevational gradients of growing season length to which they might respond via phenotypic plasticity and / or adaptive genetic differentiation. However, the relative contribution of these processes and whether plasticity, if it occurs, facilitates expansion along season length gradients remains unclear but are important for predicting species fates during anthropogenic change.

We quantified phenological trait variation in the montane annual Rhinanthus minor for three generations at 12 sites across 900 m of elevation in the Canadian Rocky Mountains and conducted a reciprocal transplant experiment for two generations among nine sites. We compared clines and interannual variation of phenological traits between natural and transplanted individuals.

Season length declined by ~ 37% along our elevational gradient and, as expected, plants emerged, reached first flower and made their first seed in ~ 41% fewer growing degree days under shorter growing seasons. Although reciprocal transplants revealed modest genetic differentiation across elevation, trait clines were primarily due to striking co-gradient plasticity that paralleled genetic differentiation.

Co-gradient plasticity likely evolved in response to considerable interannual variation in season length across our elevational transect, and should prepare R. minor to make adaptive changes to phenology in response to ongoing climate change predicted for montane environments.

物种通常沿生长季长度的纬度和海拔梯度分布，并可通过表型可塑性（phenotypic plasticity）和/或适应性遗传分化（adaptive genetic differentiation）对这些梯度做出响应。然而，这两种过程的相对贡献，以及若存在表型可塑性，其是否能促进物种沿生长季梯度扩张，目前仍不明确，但这对于预测人为活动影响下物种的生存命运至关重要。

本研究针对加拿大落基山脉内沿900米海拔梯度分布的12个样地中的山地一年生植物小鼻花（Rhinanthus minor）开展了三代物候性状变异量化研究，并在其中9个样地间开展了两代的互易移植实验（reciprocal transplant experiment）。我们对比了自然生长个体与移植个体的物候性状梯度及年际变异。

沿本次研究的海拔梯度，生长季长度下降了约37%；正如预期，在更短的生长季中，植物的出苗、始花期及首次结籽所需的生长度日（growing degree days）减少了约41%。尽管互易移植实验显示海拔梯度间仅存在微弱的遗传分化，但性状梯度的形成主要源于与遗传分化平行的显著协同梯度可塑性（co-gradient plasticity）。

协同梯度可塑性的演化，大概率是响应了本研究海拔样带（elevational transect）内生长季长度显著的年际变异；这一特性可帮助小鼻花针对山地环境已预测到的持续气候变化，做出物候学上的适应性调整。