Lichens

This study was conducted in two sites located in central (Aranjuez, 40°02′ N – 3° 32′W; 590 m a.s.l.), and south-eastern (Sorbas, 37° 05′N – 2° 04′W; 397 m a.s.l.) Spain (Fig. S1). Their climate is semiarid Mediterranean, with dry and hot summers and mean annual temperature values of 15°C (Aranjuez) and 17°C (Sorbas). Mean annual rainfall values are 349 mm (Aranjuez) and 274 mm (Sorbas), and precipitation events mostly occur in autumn/winter and spring. Soils are derived from gypsum, have pH values ~7 (Table S1), and are classified as Gypsiric Leptosols (IUSS Working Group WRB, 2006 ). Perennial plant cover is below 40%, and is dominated by grasses such as Stipa tenacissima and small shrubs such as Helianthemum squamatum and Gypsophila struthium. At both sites, the areas located between perennial plants are colonized by a well-developed biocrust community dominated by lichens such as Diploschistes diacapsis, Squamarina lentigera and Psora decipiens (see Table S2 for a species checklist).
At each site, we established a fully factorial experimental design with three factors, each with two levels: biocrust cover (poorly developed biocrust communities with cover < 20% vs. well-developed biocrust communities with cover > 50%), warming (control vs. temperature increase) and rainfall exclusion (RE, control vs. rainfall reduction). Ten and eight replicates per combination of treatments were established in Aranjuez and Sorbas, resulting in a total of 80 and 64 experimental plots, respectively. We kept a minimum separation distance of 1 m between plots to minimize the risk of sampling non-independent areas. In Aranjuez, the open top chambers and rainfall shelters were setup in July and November 2008, respectively. In Sorbas, the full experiment was set up in May 2010.
The warming treatment aimed to simulate the average of predictions derived from six Atmosphere-Ocean General Circulation Models for the second half of the 21^st century (2040-2070) in central and south-eastern Spain (De Castro et al., 2005 ). To achieve a temperature increase within this range, we used open top chambers (OTCs) of hexagonal design with sloping sides of 40 cm × 50 cm × 32 cm (see Fig. S2 for details). We used methacrylate to build our OTCs because this material does not substantially alter the characteristics of the light spectrum, and because it is commonly used in warming experiments (e.g., Hollister & Weber, 2000 ), including some conducted with biocrust-forming lichens (Maphanga et al., 2012). The methacrylate sheets used in our experiment transmit ~92% of visible light, have a reflection of incoming radiation of 4%, and pass on ~85% of incoming energy (information provided by the manufacturer; Decorplax S. L., Humanes, Spain). Direct measurements in our experiment revealed that these sheets filtered up to 15% of UV radiation (data not shown).
While predicted changes in rainfall for our study area are subject to a high degree of uncertainty, most climate models foresee important reductions in the total amount of rainfall received during spring and fall (between 10% and 50%; Escolar et al., 2012 (link)). To simulate these conditions, we set up passive rainfall shelters (described in Fig. S2). These shelters did not modify the frequency of rainfall events, which has been shown to strongly affect biocrust functioning and dynamics in other dryland regions (Reed et al., 2012 ), but effectively reduced the total amount of rainfall reaching the soil surface (average reduction of 33% and 36% in Aranjuez and Sorbas, respectively).
Air and surface soil (0-2 cm) temperatures, and soil moisture (0-5 cm depth) were continuously monitored in all treatments and sites using replicated automated sensors (HOBO® U23 Pro v2 Temp/RH and TMC20-HD sensors, Onset Corp., Pocasset, MA, USA, and EC-5 soil moisture sensors, Decagon Devices Inc., Pullman, WA, USA, respectively). Rainfall was also monitored using an on-site meteorological station (Onset Corp.).