Mean annual precipitation, as an important climate component, influences plant species distributions and trait attributes. These relationships are well-known in perennials, while annuals have received less attention. The mechanisms known for perennials are not transferable to annuals, since a short life cycle enables annuals to evade stress (escape strategy) rendering resistance mechanisms like tolerance (i.e. traits to withstand stress) or avoidance (i.e. traits to reduce stress exposure) less important than in perennials. Accordingly, to escape stress is considered to be the most pronounced strategy in annuals with increasing significance towards arid conditions.
Material and Methods
We investigated 31 annual species (grasses, forbs, legumes) from semi-arid rangelands, differing in their distribution along a steep rainfall gradient in the Eastern Mediterranean Basin. The plants grew under common conditions in a greenhouse and we measured 22 physiological, morphological and life-history traits.
The trait attributes varied species-specific, but a trend along the rainfall gradient was almost missing. However, based on their trait attributes the annuals can be categorized into three functional groups; (i) small forbs pursuing an escape strategy, (ii) large forbs characterized by drought avoidance traits, and (iii) grasses with drought tolerance traits.
Annual species from more arid regions were not necessarily characterized by more pronounced escape traits. Instead, annuals with different strategies coexist along the rainfall gradient.