title: Trait-based understanding of species’responses to drought and grazing in winter annuals from drylands
Creators: name: Kurze, Susanne
Publisher: name: University of Bayreuth
Year: 2023
Subjects: subject: annual species
subject: functional ecology
subject: drought
subject: drylands
subject: grazing
subject: plant ecology
subject: traits
subject: 570
subject: 580

description: A grand challenge in plant ecology is to identify the mechanisms underlying species’ responses to the environment. Especially trait-based approaches provide these insights since traits influence plants’ functioning. The knowledge of the underlying mechanisms is required to understand and project species’ performance, community composition, and ecosystem services, with increasing significance under global change. In winter annual species, characterized by a life cycle of less than 12 months (germination to plant death), the mechanisms underlying their responses to the environment remain understudied, although winter annuals exhibit a high ecological and economic importance in drylands. Drylands cover approximately 41 % of earth’s terrestrial land mass, harbour a large human population, and influence global nutrient, carbon, and water cycles. In addition, they are considered to be particularly vulnerable to global change due to an expected increase of water scarcity and livestock production. Here, I investigated the mechanisms underlying species’ responses to drought and grazing in winter annuals on the basis of two trait-based concepts: the stress response strategies and the plant economics spectrum, using 18 to 30 winter annuals from the southwestern Levant. According to the concept of stress response strategies, three main trait syndromes (i.e. suites of coordinated traits) minimize species’ fitness losses under drought and grazing: escape (to evade stress), avoidance (to minimize stress exposure), and tolerance (to maintain physiological functioning under stress). Each syndrome should be differently favoured by the environment, such as along rainfall gradients. I comparatively measured 26 traits and identified the main trait syndromes in the annuals. Species’ trait syndrome strengths were linked to (i) their fitness responses (based on fecundity) to drought or grazing within the growing season under common conditions in the greenhouse, (ii) to species’ abundance responses to grazing in dry and moist habitats in the field, and/or (iii) to their distribution along a large-scale rainfall gradient. The winter annuals unexpectedly showed four instead of three main drought response syndromes: avoidance, tolerance, and two escape syndromes (a tall and small escape syndrome). Only species’ strengths of the small escape syndrome were associated with their fitness responses to drought, with annuals exhibiting a pronounced small escape syndrome showing high fitness losses. Species with differing strengths of the avoidance, tolerance, and tall escape syndrome partly showed similar fitness responses to drought. This finding indicates that annuals with different trait syndromes can be similarly successful under drought. Accordingly, annual plant community composition may hardly change under (increasing) drought. In respect to grazing, the annuals exhibited an escape and tolerance syndrome, but no avoidance syndrome. Both annuals with a pronounced escape and tolerance syndrome showed low fitness losses under grazing, but only annuals with a pronounced grazing escape syndrome maintained high abundances under grazing in dry habitats. The favouring of the grazing escape syndrome in dry habitats should result from its convergence with the small drought escape syndrome, enabling the annuals to withstand grazing and to escape drought. Annuals’ strengths of the drought and grazing response syndromes were (almost) independent from their distribution along the rainfall gradient. On a large scale, annuals with different trait syndromes thus co-occur in a wide range from arid to moister conditions. The key hypothesis of the plant economics spectrum is an interspecific coordination of resource-use related traits along one main axis, resulting in a trade-off among acquisitive and conservative trait attributes at the whole plant level. Species’ trait attribute combinations along this axis are hypothesized to influence their growth rate, fitness responses to drought, and distribution along rainfall gradients. I tested this concept based on 12 resource-use related traits in winter annuals. Contrary to the hypothesis of the plant economics spectrum, structural and carbon gain traits were decoupled (i.e. independent) in the winter annuals. Annuals’ trait attribute combinations were (predominantly) independent from their growth rates, fitness responses to drought, and distribution along the rainfall gradient. Accordingly, the plant economics spectrum is not applicable to winter annuals, cautioning to use resource-use related traits to assess their drought responses. The explicit relations between annuals’ strengths of the drought and grazing response syndromes and their fitness responses reinforce a foundational assumption of trait-based ecology, i.e. the interspecific relation between traits and species’ fitness responses. The observed relations provide insight into the mechanisms that can be used to project species performance, community composition, and ecosystem functioning in winter annuals under (increasing) drought and grazing. The present findings suggest that increasing drought and grazing under global change will favour annuals with a pronounced escape syndrome (grazing escape and small drought escape syndrome, respectively) with likely pervasive consequences for ecosystem functioning. In contrast, under a hypothetical global change scenario with only rainfall decreases but no changes in grazing intensity, annuals with different trait syndromes should co-occur. Species richness and ecosystem services of annual plant communities under global change will thus strongly depend on human land-use.