Rainfall reliability, a neglected factor in explaining convergence and divergence of plant traits in fire‐prone mediterranean‐climate ecosystems

Abstract

Aim Rainfall reliability has been neglected as a determinant of plant trait convergence and divergence in mediterranean-climate ecosystems. This paper reports on patterns of rainfall reliability -quantified as interannual variation in monthly and seasonal rainfall, and as the frequency of individual events in terms of their size, duration and intensityfor four fire-prone mediterranean-climate ecosystems. LocationThe four mediterranean-climate regions of the world with fire-prone ecosystems, namely SW Cape (South Africa), SW Australia, California and the Mediterranean Basin (Andalusia, Spain).Methods Using long-term monthly rainfall data from stations dispersed across the four regions, we computed monthly means and interannual variation for each month of the year -the latter quantified as the coefficient of variation (CV) -and divided these into winter and summer seasons. We also computed the mean number of rainfall events, the mean frequency in various categories of event duration (days), the amount of rainfall per event (mm) and the rainfall intensity per event (mm/day) per year for winter and summer seasons for a subset of the rainfall stations. ResultsThe fraction of rain falling in summer was lowest in California (5%) and similarly low ( c . 25%) in the other three regions. The hierarchy of values of coefficient of variation (CV) of monthly rainfall during the winter period was as follows: California > Andalusia >> SW Cape > SW Australia; results for summer were: California > > Andalusia >> SW Australia ∼ SW Cape. SW Australian sites experienced the greatest frequency of short, small and low-intensity rainfall events in both seasons; patterns in the SW Cape were intermediate between Australia and the two northern hemisphere sites which both received fewer, larger and more intense events. Overall, the two southern hemisphere regions (SW Australia and the SW Cape) had significantly more reliable regimes than the two northern hemisphere ones (Mediterranean Basin and California).Main conclusions These differences in rainfall reliability regimes may provide a novel perspective on the distribution of certain plant life-history traits in mediterranean-climate ecosystems. Less reliable regimes would select for germination and seedling survival traits that enable persistence of genets in the face of uncertain moisture conditions during the winter and spring establishment phase. Study systems that accommodate for phylogenetic constraints, namely invasive species derived from mediterranean-climate ecosystems, as well as shared lineages, provide good opportunities to develop and test hypotheses on the implication of different rainfall reliability regimes. One of the novel implications of this study is that the distinctive trait of assemblages in the southern hemisphere regions may be a consequence not so much of their shared nutrient-poor soils as of their similarly reliable rainfall regimes.

Altered water and nitrogen input shifts succession in a southern California coastal sage community

“…We hypothesize that type conversion in CSS is primarily caused by N addition in combination with fire, drought, or other disturbance that provide European grasses with open space to colonize (Wood et al 2006). The strong effect of drought on shrub cover is somewhat surprising given the drought-tolerating and drought-avoiding adaptations of CSS shrubs (Harrison et al 1971, Miller et al 1983, Desouza et al 1986, Cowling et al 2005, Rundel 2007). Increased establishment and growth of grasses in postfire CSS is presumably due to the transient creation of open space and increased availability of light and other resources (Zedler et al 1983, Keeley et al 2005a.…”

Seeds as a Source of Carbon, Nitrogen, and Phosphorus for Seedling Establishment in Temperate Regions: A Synthesis

“…Resource costs (essentially C) of producing a woody fruit/infructescence to protect the stored seeds have to be taken into account but they appear small compared with the fitness advantages of only releasing seeds postfire in nutrient-impoverished environments [15,39,45]. The greatest incidence of serotiny in SWA among the mediterranean regions (in fact, world) adds further support to the notion that SWA soils are the most nutrient-impoverished, tempered by its reliably wet winter [46].…”

Characterizing Precipitation Variability and Trends in the World’s Mediterranean-Climate Areas

“…Low variability in Australia and South Africa suggests that these areas are not subject to the climate patterns that create occasional very wet years in other Mediterranean-climate regions. Lower variability among Mediterranean biomes in the southern hemisphere may lead to a different composition of flora with respect to species traits compared to areas with higher variability [59].…”