Epiphytic lichen growth in Mediterranean forests: Effects of proximity to the ground and reproductive stage

Reasonable lichen growth rates are required to maintain the important ecosystem functions provided by epiphytic lichen populations. Previously, lichen growth has mainly been quantified in boreal forests. Thus, there is a need for comparable studies from Mediterranean forests. We used reciprocal transplantation to compare relative biomass (RGR) and thallus area (RTAGR) growth rates of the cephalolichen Lobaria pulmonaria and the cyanolichen Lobarina scrobiculata (n = 720 thalli) in two Mediterranean oak forests. Both juvenile and reproductive thalli were transplanted for 334 days at the base and at breast height of random trees in both forests. We measured functional traits, specific thallus mass (STM) and water holding capacity (WHC), to search for adaptation/acclimation patterns between regions and between local stands. Linear Mixed Models were used to assess the effects of (1) forest, (2) height on the trunk and (3) reproductive stage on RGR, RTAGR, and functional traits. Lobaria pulmonaria grew faster than L. scrobiculata, consistent with its more flexible hydration traits. Growth rates in both species were fastest in juveniles at the trunk bases, consistent with trade-offs between (1) growth and reproduction and (2) humidity availability and distance from the ground. In L. pulmonaria, STM increased with increasing evaporative demands, consistent with acclimation mechanisms. Fundamental and realized niches differed in L. scrobiculata, indicating that high abundance does not necessarily coincide with optimal growth habitat. Both species grew as fast in drier and warmer Mediterranean forests as reported from boreal forests, presumably because local conditions during hydration periods differ less between macroclimate regions than average climatic parameters, and/or because dew formation is frequent, particularly near the ground. Furthermore, STM and WHC of lichens in Mediterranean forests were high, thus partly compensating for the drier conditions. This study improves our understanding of mechanisms underlying epiphytic lichen growth in Mediterranean climates.