Field sites and climate manipulations
Four sites along a natural rainfall gradient from Northern to Southern
Israel were chosen because they share most abiotic characteristics
except for mean and variance in rainfall (detailed description:
Supporting Information Fig. S1, and Tielbörger et al. 2014). They
have Mediterranean-type climate and similar mean annual temperatures
(17.7–19.1°C), yet spanned a more than eight-fold difference in mean
annual rainfall (MAP). They represent mesic-Mediterranean (MM, 780 mm
MAP), Mediterranean (M, 540 mm), semi-arid (SA, 300 mm) and arid
conditions (A, 90 mm), respectively. The average length of the rainy
season increases from the driest (Dec–March) to the wettest site
(Oct–May). Rainfall variability among years decreases from the driest
(40% Coefficient of Variation) to the wettest site (25% CV) (Metzet al. 2018). All sites are
semi-natural shrublands with mostly annual species in the inter-shrub
matrix, located at south exposures on limestone bedrock with similar
elevation (470–620 m a.s.l.).
Three rainfall manipulation treatments (dry, control, wet) were imposed
on entire resident plant communities for ten years (2002-2012) in the
two intermediate sites, M and SA. Each treatment was replicated by five
randomized 10m × 25m plots per site. The two extreme sites, A and MM,
contained only five control plots each and extended the range of the
natural rainfall gradient. For dry manipulation, rainout shelters
intercepted 30% of each rainfall event; for wet manipulation,
additional sprinkler irrigation after every other rain event added in
total 30% of the site’s MAP; control plots received ambient rainfall
(details in Tielbörger et al. 2014). The magnitude of the dry
manipulations realistically approximated climate change scenarios for
our study region, predicting 20% less rainfall until 2050
(Smiatek et al. 2011;
Samuels et al . 2013), while the wet manipulations extended the
general range of scenarios that were uncertain at the onset of our
study.