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The Mediterranean Region under Climate Change. A Scientific Update: Abridged English/French Version

 | 
Stéphanie Thiébault
, 
Jean-Paul Moatti

Part 2. Vulnerability and impacts

Chapter 4. Impacts on terrestrial biodiversity and ecosystems

Wolfgang Cramer

Résumé

This chapter explores four selected aspects of the influence of climate change on Mediterranean land ecosystems. First, it explores the interaction between climate change, changes in land use, and the functioning of Mediterranean forests from the human perspective (Gauquelin et al.). Mediterranean forests are characterized by exceptional biodiversity, with 290 woody species and numerous plants and animals depending on them. Most of the trees are evergreen, adapted to some level of drought stress, but deciduous trees are also present. Many forests are actually open woodlands. Mediterranean forested landscapes therefore display high spatial heterogeneity, offering multiple functions for associated wildlife, as well as for human recreation. Risks to the functioning of these forests originate not only from climate change but also from destruction and unsustainable use.
In some forested sites this situation can now be considered typical of many Mediterranean ecosystems, and raises the question of their stability, or resilience (Mouillot et al). Experimental treatments now make it possible to test the degree to which increased drought, for example, may have longer-lasting impacts on ecosystem function than simply a potentially temporary reduction in growth and productivity.
The Mediterranean is a large archipelago with approximately 10,000 small and large islands that alone explain a significant part of its biodiversity (Médail et al.). While these islands are subject to similar forcings i.e., warming, drought, land use change and sea-level rise, the local impacts differ strongly from place to place. One group of species with particular climate and local habitat requirements are orchids (Schatz et al.). Analysis of the few existing long observational series of these orchids makes it possible to distinguish between the most and the least sensitive groups, but more research is needed to develop predictive tools to assess future risks to them.

Dans ce chapitre, on présente quatre aspects spécifiques des effets du changement climatique sur les écosystèmes continentaux. Tout d’abord, les interactions entre le changement climatique, les changements d’usage des sols et le fonctionnement des forêts méditerranéennes selon une perspective sociétale (Gauquelin et al.). La biodiversité des forêts méditerranéennes est exceptionnellement riche, avec 250 espèces ligneuses et de nombreuses espèces animales ou végétales qui en dépendent. La plupart des arbres sont sempervirents, adaptés à un certain niveau de stress hydrique, mais il existe aussi des espèces décidues et de nombreuses forêts sont aujourd’hui des forêts ouvertes. Ainsi, les paysages forestiers méditerranéens montrent une grande hétérogénéité spatiale, ce qui profite à la faune sauvage et leur confère un aspect récréatif. En plus du changement climatique, leur destruction et leur utilisation de manière non durable pèsent également sur les forêts méditerranéennes.
Dans le cas de sites forestiers considérés comme caractéristiques de nombreux écosystèmes méditerranéens, les questions actuelles sont celles de leur stabilité ou de leur résilience (Mouillot et al.). Certaines expériences permettent de tester avec quel degré une sécheresse prolongée peut par exemple impacter le fonctionnement de l’écosystème sur le long terme, au-delà d’une simple réduction temporaire de la croissance et de la productivité.
La Méditerranée est un large archipel avec environ 10 000 îles grandes et petites, ce qui explique une part significative de la biodiversité méditerranéenne (Médail et al.). Alors que ces îles sont toutes confrontées au réchauffement, à la sécheresse, aux changements d’occupation du sol et à la remontée du niveau de la mer, les impacts locaux diffèrent grandement selon les régions. Des espèces comme les orchidées manifestent des besoins particuliers en termes de climat et d’habitat (Schatz et al.). L’analyse de quelques séries d’observation sur le long terme permet de distinguer différents groupes d’orchidées, plus ou moins sensibles. Cependant, des recherches supplémentaires sont nécessaires afin d’établir des outils prédictifs permettant d’estimer les risques futurs qu’ils encourent.

Texte intégral

Introduction

1Mediterranean land ecosystems are among the richest in the world. Located near one of the world’s regions of origin of human agriculture and civilization, they have evolved in close interaction with human society over several millennia. Direct human impacts have consequently been very strong and visible, in the form of deforestation, the development of agricultural landscapes, gardening, soil degradation, urbanization, and (during the last century or so) air and water pollution, the arrival of alien species and the protection of some areas for conservation. Some of these impacts have reduced biodiversity, while others have enhanced it.

2Despite these direct impacts of humans on biota, the highly specific Mediterranean climate remains one of the key determinants of the biodiversity and ecosystem structure in the region. Hot summers regularly lead to dry conditions in much of the basin, requiring little extra stress to cause biomass loss or dieback in a number of species. Mild humid winters provide the right conditions for high biological productivity. Warming is currently occurring faster in the Mediterranean than global averages, and affects land ecosystems in several different ways. First, direct heat stress may occur during heat waves in summer. Winter warming can be beneficial, but may also alter interactions with pests or alien species that find better conditions for survival and then have a negative impact on plant growth and survival. However, changes in water availability are more important than warming, and are the primary cause of productivity losses during the summer season, notably during dry spells which are tending to become longer. Reduced access to water is also an issue in the southern and eastern parts of the basin during winter, a tendency that will be exacerbated in the future. Remarkably, while global climate models still exhibit much regional uncertainty in terms of rainfall predictions, they all agree on the expected reduction in rainfall in the Mediterranean region. This enhanced drought has effects on all ecosystems, whether managed or unmanaged. In agro-ecosystems some potential for adaptation through irrigation exists, but is limited by the generally limited availability of water resources for irrigation, and also due to the limited economic potential in southern and eastern Mediterranean countries to pump and distribute more water to the fields. Finally, climate change also influences land ecosystems through the sea-level rise which can cause “coastal squeeze”, in which circumstances no space is available for coastal ecosystems to move to higher areas, and notably by affecting coastal wetlands such as the Camargue, the Nile Delta and other similar areas.

© IRD Éditions, 2016

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