Tradeoffs between functional strategies for resource-use and drought-survival in Mediterranean rangeland species

In environments where light is not a limiting resource such as rangelands and grasslands, there is much disagreement regarding the benefits provided by rapid light capture during the growing season and the species’ ability to withstand drought during the dry period. In this study, we selected four perennial herbaceous species with contrasting resource-use strategies (acquisitive versus conservative), which were transplanted as monocultures into PVC pots to evaluate their species-specific responses to drought. The two main strategies of drought-survival (avoidance versus tolerance) were driven by distinct underlying mechanisms that allow the plant to delay or tolerate water deficit in leaves. On the one hand, plants that produced reduced leaves with lower surface area:mass ratio (lower SLA) exhibited higher values of leaf water potential (LWP) and leaf relative water content (LRWC), which could be associated to a higher ability to delay tissue dehydration in enlarged leaves. Regarding the below-ground compartment, dehydration avoidance was promoted by prolonged elongation rates of thinner roots that allow the plant to increase water uptake and accessibility during the dry period. On the other hand, dehydration tolerance was positively related with progressive foliage senescence under water deficit, which probably favored a longer survival of meristematic basal tissues. The results presented in this study suggest the existence of a trade-off between the traits favoring rapid light-acquisition and those enhancing the ability to delay leaf dehydration. Thus, the species related most closely with a resource-acquisition strategy (Bromus erectus and Potentilla neumanniana) could be considered less efficient to delay leaf dehydration than the others (Carex humilis and Festuca christiani-bernardii), as indicated by their lower values of leaf water potential (LWP) and leaf relative water content (LRWC) under identical conditions of water deficit. Our findings support evidence that there is not a single strategy to effectively cope with drought and reveal the diversity of adaptive mechanisms among coexisting species.

Combined picture of the different experiments carried out in our glasshouse study: Experiment of drought tolerance (panel A); Potentilla neumanniana, a species with a predominant resource-acquisition strategy (panel B); Festuca christiani-bernardii, a species with a predominant resource-conservation strategy (panel C); experiment for quantifying root elongation rates (panel D); detail of a rhizotron with a monoculture of Bromus erectus (panel E); and example of a bi-dimensional sheet used to measure the trajectory of the root system (panel E).Figure optionsDownload as PowerPoint slideHighlights
► We evaluated plant responses to drought in species with contrasting resource-use strategies.
► Several morphological traits could be useful to predict plant responses to drought.
► Drought-avoidance and tolerance were driven by distinct underlying mechanisms.
► There is a trade-off between resource-acquisition and drought-avoidance strategies.
► Our results suggest the existence of a strong diversification of drought-survival strategies.