Size is a main driver for hydration traits in cyano- and cephalolichens of boreal rainforest canopies

We measured the water holding capacity per area (WHCA; mg H2O cm ), and thus the saturating rainfall (mm), for sympatric cyano(Lobaria scrobiculata and Pseudocyphellaria crocata) and cephalolichens (Lobaria pulmonaria) along canopy height gradients in boreal rainforests, to quantify the importance of specimen size, photobiont type and branch height as WHCA drivers. Size increased WHCA by a factor of four. Cyanolichens had z1.5 times higher WHCA than cephalolichens. Finally, branch height significantly increased WHCA for the Lobaria species. Reported responses are consistent with higher optimal rainfall requirements for reproducing versus juvenile specimens, and for cyanolichens versus those with green algal photobionts. Increased WHCA with height in the canopy is consistent with acclimation to stronger evaporative demands. The results highlight the link between rainfall patterns and maximal sizes realized for a given lichen species and habitat, and extend our understanding of why the cyanolichen/green algal lichen-ratio increases in forests with increasing rainfall. a 2013 Elsevier Ltd and The British Mycological Society. All rights reserved. Introduction green algae as the only or major photobiont, respectively (e.g. Poikilohydric organisms like lichens depend on hydration events to resume metabolic processes and grow (e.g. Palmqvist & Sundberg, 2000). While humid air alone can activate photosynthesis in chloroand cephalolichens with (Y. Gauslaa). ier Ltd and The British M Jonsson Cabrajic et al., 2010; Lid en et al., 2010), cyanolichens entirely depend on liquidwater for positive carbon gain (Lange et al., 1986). These two hydration traits associate with morphological adaptation and/or acclimation: Many lichens, chlorolichens in particular, are thin and rapidly resume ycological Society. All rights reserved. 60 S. Merinero et al. photosynthesis in humid air, whereas thick lichens need a longer time (Larson & Kershaw, 1976; Larson, 1981; Lange & Kilian, 1985), or remain inactive (cyanolichens). To compensate for the longer time needed to fill thewater storage of thick lichens with water from humid air, thick lichens retain water for longer periods after hydration events. Water holding capacity (WHCA; expressed as mass of water at saturation per thallus area) is an often ignored, but important, lichen trait that equates to mm rainfall needed for saturation. It determines the duration of hydration periods and influences the ecological niche (as reviewed by Gauslaa, 2014). Specific thallus mass (STM; dry mass per thallus area) drives the WHCA at intraand interspecific levels (Gauslaa & Coxson, 2011). STM is the inverse equivalent of the frequently used parameter specific leaf area, SLA, in plant science, connected to the resource use economy (e.g. Reich et al., 1998; Wright et al., 2004). In adult lichen life stages, STM increases with size (Dahlman et al., 2002; Gauslaa et al., 2009; MacDonald & Coxson, 2013). However, the size-dependency ofWHCA from early juvenile to late reproductive stages is not known. Because small juvenile thalli are likely particularly influenced by their ability to retain moisture, they may represent a more critical stage in the lichen life cycle than buffered mature thalli (Gauslaa & Solhaug, 1998). Epiphytic lichens representing a threatened biodiversity component show strong vertical gradients in tree canopies (McCune, 1993; McCune et al., 1997; Gauslaa, 1997; Coxson & Coyle, 2003; Gauslaa et al., 2008; Rambo, 2010; Marmor et al., 2013). However, regulating factors for such gradients are insufficiently known. We will search for vertical gradients in STM and WHCA for the cephalolichen Lobaria pulmonaria and for the cyanolichens Pseudocyphellaria crocata and Lobaria scrobiculata in boreal Picea abies-dominated rainforests in Norway recognized as hot spot areas for epiphyte diversity (Holien & Tønsberg, 1996; Rolstad et al., 2001). The canopy height gradient is complex because spruce drains the water away from the inner and lower canopy (see Stoutjesdijk & Barkman, 1987; Goward, 2003). Availability of liquid water may thus decline with increasing distance from tree tops, whereas air humidity tends to be highest in sheltered lower canopy, at least during the day (Geiger, 1950). Wind and light often increase with height, leading to increasing evaporative demands. In such canopy gradients, WHCA of cyanolichens Table 1 e Summary statistics of habitat characteristics in the s Parameter Lobaria pulmonaria No of thalli 223 No of trees 7 No of branches 8 Height range, m 2e9 Mean height of thalli, m 4.8 0.1 Height of trees, m 29.1 0.2 Tree circumference, cm 118 1 Tree age, yr 129 2 Branch length, cm 301 2 Branch circumference, cm 12.3 0.2 Mean values SE are given. that depends on liquidwatermay respond differently than the WHCA of lichens with green algae capable also of using humid air. By focusing on STM and WHCA, we add information to a recent trend in quantifying lichen traits (e.g. Asplund & Wardle, 2013; Lang et al. 2009). We will quantify these two traits across size classes for each of three sympatric study species sampled on branches from different height levels in Picea abies-dominated old rainforests. Our aim is to test the following hypotheses: (1) Thallus size drives WHCA in a similar way across size classes e consistent with the view that reproduction per se does not have major effects on hydration requirements. (2) WHCA is shaped by STM in species-specific ways e consistent with species-specific water storage traits; and/or (3) The photobiont influences the size-dependency of WHCA in lichens e consistent with a trade-off between rapid and frequent uptake of humidity in thin green algal lichens versus more water conserving strategies in cyanolichens that depend on liquid water. (4) WHCA increases with height in the canopy e consistent with acclimation tuning these traits to ambient hydration sources. Finally, we discuss howmeasured traits influence the ecology of our species. Material and methods