Abstract
Climate change and extreme drought and heat events impact the Mediterranean evergreen sclerophyllous vegetation in South Europe, especially in Iberian and Italian peninsula, where widespread crown defoliation and dieback have been observed since the 90s of the XX century. Field observations and long-term experiments showed different sensitivity of the various woody species, Quercus ilex and Arbutus unedo being prone to drought, whereas Phillyrea latifolia and Pistacia lentiscus appeared to be resistant. The present review aims at exploring the phylogenetic and evolutionary basis of the resistance (or susceptibility) to drought of Mediterranean vegetation and its possible mechanisms of resilience. The main findings are summarized as follows: (1) Mediterranean regions in the world are refuge areas for several plant evolutive lineages and migratory routes. Evergreen sclerophyllous species, currently presented in Mediterranean basin, evolved under different climatic conditions; (2) the evergreen habitus represents an adaptation to mild drought conditions. Deciduous (specially summer deciduous) species are better performing under severe drought and low air relative humidity than evergreen species; (3) severe drought events acts selectively by favouring the species evolved in the Quaternary era and those originated in drier regions; (4) the evergreen trees and shrubs are resilient to the severe drought events and can restore the pre-event condition by resprouting from dormant buds in the cambium tissue. This ability is related to the non-structural carbohydrate content in the parenchyma-rays in woody stems. The amount and availability of these strategic reserve can be compromised by frequent drought events; (5) plant seed regeneration can be affected by drought and seedling establishment may be limited by soil dryness and microenvironment conditions; (6) the role of phenotypic plasticity of the species and epigenetic responses in Mediterranean-type ecosystems, although discussed in few papers, is still poorly known. We hypothesize that instead of latitudinal (South to North) or altitudinal (lowland to upland) plant migrations, Mediterranean forest ecosystems may respond to climate change by modulating their species composition and community structure with genetic resources (i.e., taxonomic diversity) already present in loco. Changes in vegetation assemblages and community structure may lead changes in ecological and landscape ecosystem values, with changes in related ecosystem services. A redefinition of management criteria of natural resources and a pro-active silviculture to make forest ecosystems more resilient are required.
Highlights
Widespread forest tree mortality and dieback caused by drought and temperature stress, often followed by insect and pathogen attacks, is a worldwide problem connected to climate change [1,2,3,4,5,6,7]
Deciduous species are better performing under severe drought and low air relative humidity than evergreen species; (3) severe drought events acts selectively by favouring the species evolved in the Quaternary era and those originated in drier regions; (4) the evergreen trees and shrubs are resilient to the severe drought events and can restore the pre-event condition by resprouting from dormant buds in the cambium tissue
Phillyrea latifolia is better performing than Q. ilex in drier soils: whereas Q. ilex assimilation rates increased with soil humidity; in P. latifolia, assimilation rates did not increase above 17% of soil humidity, showing no water availability response above such threshold [59]
Summary
Widespread forest tree mortality and dieback caused by drought and temperature stress (i.e., warm winters and hot summers), often followed by insect and pathogen attacks, is a worldwide problem connected to climate change [1,2,3,4,5,6,7]. Extreme climate events, including severe drought and heat waves, as described in these papers, affected selectively evergreen sclerophyllous species, both trees and shrubs. Quercus ilex L. and Arbutus unedo L. were the most sensitive species with severe crown dieback and high mortality rates, whereas Phillyrea latifolia L. and Pistacia lentiscus L. resulted in being resistant, with no or little crown damage (i.e., defoliation, branch and leaf desiccation). Arbutus unedo L. were the most sensitive species with severe crown dieback and high mortality rates, whereas Phillyrea latifolia L. and Pistacia lentiscus L. resulted in being resistant, with no or little crown damage (i.e., defoliation, branch and leaf desiccation) These findings were confirmed in a long-term experiment of drought imposed in Catalunya [17]. We hypothesize that in ecosystems that are species-rich and have complex structures, such as the Mediterranean evergreen forests and shrubland, the worsening of climatic conditions, with increasing drought and extreme event frequency, will act as a factor triggering vegetational dynamics, modifying plant species composition, individual interactions in resource use and structure of the stands [18]
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