Adaptation of Mediterranean forest species to climate: Lessons from common garden experiments

Abstract

1. Mediterranean ecosystems are biodiversity hotspots located between temperate mesic climates and semi-deserts and deserts. Mediterranean climates are characterized by a drought season but its length and severity can be highly variable across regions. In this review, we explored population genetic variation in functional traits and fitness in Mediterranean oak and pine species. We tested the hypothesis that increased drought tolerance has evolved as an adaptation to xeric and warm areas of the Mediterranean climate while increased competitive abilities have been favoured in more mesic regions, following a productivity-persistence trade-off within species.2. We first reviewed the literature and performed a meta-analysis of studies on Pinus sp. and Quercus sp. from Mediterranean climates, and then analysed a set of unpublished data from Spanish common garden experiments of species from the Mediterranean Basin, searching for evidence of trait genetic variation among populations, local adaptation and trait-environment associations.3. The meta-analysis showed that 82.9% (95% CI 75.7-89.4) of traits per study showed significant population genetic differentiation and that 52. 5% (35.6-69.2) of traits in studies with two or more Mediterranean environments exhibited population-by-environment interaction. Population differences in traits related to drought tolerance, cold hardiness and fitness were usually correlated with provenance precipitation and temperature gradients, suggesting that adaptation to different climatic regimes is an important process driving intraspecific genetic divergence. Within the Spanish trial network, however, predicted fitness functions based on climatic transfer distance failed to reveal significantly higher fitness of populations at experimental sites with climates closer to their provenances' ones.Neither the literature review nor the new analyses of the Spanish trial network provided evidence of a negative association between growth-and persistencerelated traits at the intraspecific level. 4. Synthesis. Population genetic differentiation in functional traits and fitness is common in Mediterranean species, driven, at least in part, by the adaptation to contrasting temperature and precipitation regimes. Because of experimental constraints or biological reality, our study failed to detect a trade-off between | 1023

What common‐garden experiments tell us about climate responses in plants

“…All four studies report significant trait correlations with climate provenance, equivalent to scenarios B and D developed here (Figure 1). This is also consistent with re- In the three studies from seasonally dry biomes, drought survival decreased, and growth potential increased with precipitation provenance, although not across all species examined and not in all common gardens (Custer et al, 2021;Johnson et al, 2020;Ramirez-Valiente & Robledo-Arnuncio, 2021). The studies from the Mediterranean Basin…”

“…Adding common‐gardens in more extreme locations on or beyond the trailing and leading edges of current ranges or manipulating climate factors by experimentation would also be very useful. This would capture plant responses to a wider range of climate conditions, perhaps more representative of future conditions (Huxman et al, 2021; Johnson et al, 2020; Ramirez‐Valiente & Robledo‐Arnuncio, 2021).…”

“…In common-garden experiments, regeneration traits are reported chiefly for annual plants and short-lived perennials, but long-lived, woody perennials also have highly specialized germination requirements (Bowers et al, 2004;Gutierrez & Meserve, 2003;Meyer & Pendleton, 2005). Unfortunately, the practice of transplanting greenhouse-grown yearlings into common gardens (to ensure uniform establishment) often eliminates early responses to climate (for further discussion, see Custer et al, 2021;Ramirez-Valiente & Robledo-Arnuncio, 2021).…”

“…These dynamics along stress gradients may be universal among members of similar niches, and even in long-lived species should be observable in juvenile stages(Kelly et al, 2008). To date, competitive arenas, such as those described byJohnson et al (2020) are not routinely implemented in common garden experiments, a lost opportunity for testing foundational theories of trait evolution and community assembly.At last, there is further unrealized potential for comparing and synthesizing the outcomes of many common-garden experiments, so that we can begin to develop a generalized conceptual framework for plant response to climate change(Huxman et al, 2021;Lortie & Hierro, 2021;Ramirez-Valiente & Robledo-Arnuncio, 2021).Publications based on common garden studies sharply increased inthe first decade of this century, and this trend will likely continue as scientists across the world prioritize climate change preparedness(Strassburg et al, 2020;Turney et al, 2020). Data integration will benefit from more standardized research techniques, including a set of priority traits, guidelines for garden selection, and reporting standards that facilitate quantitative synthesis.…”

Multi‐trait genetic variation in resource‐use strategies and phenotypic plasticity correlates with local climate across the range of a Mediterranean oak (Quercus faginea)

“…Higher drought intensity, longer dry periods and/or consecutive drought periods might have triggered population differences in survival (Mcdowell et al ., 2008; Forner et al ., 2018). In any case, failing to detect survival–growth trade‐offs is not uncommon in experiments on Mediterranean species (Michelaki et al ., 2019; Ramírez‐Valiente et al ., 2021), and further research is needed to establish whether the observed intraspecific genetic variation in drought responses does ultimately exist for survival and lifetime fitness as well (Griffith et al ., 2004; Heschel et al ., 2004; Carvajal et al ., 2017).…”