Evaluating grazing response strategies in winter annuals: a multi-trait approach

Plants minimize fitness losses through grazing by three fundamental
strategies: tolerance, avoidance and escape. Annual species have been
traditionally assumed to escape grazing through their short life cycle and
seed dormancy, however, their grazing response strategies remain almost
unexplored. How traits and their coordination affect species’ grazing
responses, and whether the generalized grazing model, which posits
convergent filtering by grazing and drought, is applicable to this
ecologically and economically important species group thus remain unclear.
We used a trait-based approach to evaluate grazing response strategies of
winter annuals from the Middle East. Across 23 species we examined the
coordination of 16 traits hypothesized to be relevant for grazing
responses, and linked them to species’ fecundity responses, as proxy for
fitness responses, to simulated grazing in controlled conditions, to
species’ abundance responses to grazing in the field, and to species’
distribution along a large-scale rainfall gradient. Winter annuals
exhibited both grazing escape and to a lesser extent tolerance indicated
by (i) independent coordination of escape and tolerance traits, and (ii)
maintenance of higher fecundity in species with more pronounced escape or
tolerance traits under simulated grazing. In the natural habitat, species
with a more pronounced escape but not tolerance strategy maintained higher
abundance under grazing in dry habitats, indicating convergent favouring
of escape by both grazing and drought. However, this finding at the local
scale was not mirrored by a strategy shift along a large-scale rainfall
gradient. Synthesis. The convergent favouring of escape traits by
grazing and drought in annuals is consistent with the generalized grazing
model. This model, which has been developed for perennials based on the
avoidance strategy, can thus be extended to annuals based on escape, a
finding that should facilitate projecting consequences of global change in
drylands dominated by annuals.

植物可通过三类核心策略降低牧食引发的适合度（fitness）损失：耐牧性（tolerance）、避牧性（avoidance）与逃牧性（escape）。传统观点认为，一年生植物可凭借较短的生活周期与种子休眠逃避免牧，但目前针对其牧食响应策略的研究仍近乎空白。性状及其协同关系如何影响物种的牧食响应，以及认为放牧与干旱存在趋同过滤作用的广义放牧模型（generalized grazing model）是否适用于这一兼具生态与经济重要性的类群，目前仍不明确。
本研究采用基于性状的研究方法（trait-based approach），评估了来自中东地区的冬季一年生植物的牧食响应策略。我们对23个物种的16个被推测与牧食响应相关的性状进行了协同关系分析，并将这些性状与以下指标进行关联：可控条件下模拟放牧实验中测得的物种繁殖力（fecundity）响应（作为适合度响应的替代指标）、野外条件下物种对放牧的多度（abundance）响应，以及物种沿大尺度降雨梯度的分布格局。
冬季一年生植物同时表现出逃牧策略，且在一定程度上表现出耐牧策略，相关证据包括：（1）逃牧性状与耐牧性状各自独立协同；（2）在模拟放牧条件下，具备更显著逃牧或耐牧性状的物种维持了更高的繁殖力。在自然生境中，具备显著逃牧策略（而非耐牧策略）的物种在干旱生境的放牧条件下维持了更高的种群多度，这表明放牧与干旱对逃牧策略存在趋同筛选作用。然而，局域尺度的这一发现并未在大尺度降雨梯度上的物种策略转变中得到印证。
综合分析表明，放牧与干旱对一年生植物逃牧性状的趋同筛选，与广义放牧模型的预测相符。该模型最初基于避牧策略针对多年生植物构建，如今可通过逃牧策略拓展至一年生植物类群，这一发现将有助于预测以一年生植物为主的干旱区的全球变化后果。