6.1 Introduction
Forests store almost half of terrestrial carbon, but the long-term net carbon uptake
by forests is a slow process mainly controlled by the growth rate of woody tissues
(Bonan 2008). Contrastingly, huge carbon emissions may occur rapidly from
sudden or widespread mortality episodes often preceded by forest dieback and
growth decline (Breshears and Allen 2002). In water-limited regions, climate
warming may intensify drought stress and lead to growth decline and forest dieback
(Allen et al. 2010, 2015). This dieback phenomenon is linked to rapid defoliation
and selective mortality of overstory trees (McDowell et al. 2008). However, in
mountain and temperate forests, the factors causing forest dieback under more
mesic conditions are not as well understood (van Mantgem and Stephenson 2007).
Mountains are characterised by: (i) a high heterogeneity in local climate conditions which change over short distances as a function of altitude and topography
and create steep ecological gradients (Barry 2008), and (ii) by preserving large
forested areas subjected to an extended historical management, particularly in
Europe (Kirby and Watkins 1998). It is predicted for the twenty-first century a
greater warming in the mountains than in the lowlands of the Northern Hemisphere
(Kohler et al. 2014). Mountain forests face climatic gradients which limit their
productivity, cold stress towards the uppermost treeline but also drought stress
downwards, and make them sensitive to climate warming but also to changes in
management. The management and use of these forests in Europe have changed
rapidly during the past century. Mountain forests provide many ecosystem services
(carbon uptake, regulation of water cycles, protection from snow avalanches, biodiversity conservation, etc.) but they also represent a source of timber, biomass for
energy production and non-woody goods (e.g. mushrooms, hunting). During the
twentieth century, timber extraction was still profitable in some European forests as
those formerly exploited at the Spanish Pyrenees (Cabrera 2001). However, currently, many of these mountain forests are not managed for timber production
because commercial forestry is no longer profitable. In this chapter, I explore how
changes in historical management interact with climate warming and intensified
drought stress to trigger dieback in mountain forests.
Here I argue that the historical use of forests strongly interacts with current
climatic trends as the rise in temperatures to determine the current fate of forests,
which in some case can jeopardise its future. I will illustrate how past historical
logging and recent warming-related drought stress contribute to silver-fir dieback in
Pyrenean mountain forests subjected to mesic conditions. The processes leading to
forest dieback are still poorly understood because of the interaction of several stress
factors acting at different spatiotemporal scales, which complicates the disentangling of lagged cause–effect relationships (Pedersen 1999). Many dieback episodes
have been studied following Manion’s (1981) conceptual model, which includes
predisposing, inciting and contributing stress factors causing a loss in tree vigour.
Predisposing factors such as site conditions reduce a tree’s vigour over the long
term (Suarez et al. 2004), whereas inciting factors such as drought lead to a sharp
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J.J. Camarero
Forests store almost half of terrestrial carbon, but the long-term net carbon uptake
by forests is a slow process mainly controlled by the growth rate of woody tissues
(Bonan 2008). Contrastingly, huge carbon emissions may occur rapidly from
sudden or widespread mortality episodes often preceded by forest dieback and
growth decline (Breshears and Allen 2002). In water-limited regions, climate
warming may intensify drought stress and lead to growth decline and forest dieback
(Allen et al. 2010, 2015). This dieback phenomenon is linked to rapid defoliation
and selective mortality of overstory trees (McDowell et al. 2008). However, in
mountain and temperate forests, the factors causing forest dieback under more
mesic conditions are not as well understood (van Mantgem and Stephenson 2007).
Mountains are characterised by: (i) a high heterogeneity in local climate conditions which change over short distances as a function of altitude and topography
and create steep ecological gradients (Barry 2008), and (ii) by preserving large
forested areas subjected to an extended historical management, particularly in
Europe (Kirby and Watkins 1998). It is predicted for the twenty-first century a
greater warming in the mountains than in the lowlands of the Northern Hemisphere
(Kohler et al. 2014). Mountain forests face climatic gradients which limit their
productivity, cold stress towards the uppermost treeline but also drought stress
downwards, and make them sensitive to climate warming but also to changes in
management. The management and use of these forests in Europe have changed
rapidly during the past century. Mountain forests provide many ecosystem services
(carbon uptake, regulation of water cycles, protection from snow avalanches, biodiversity conservation, etc.) but they also represent a source of timber, biomass for
energy production and non-woody goods (e.g. mushrooms, hunting). During the
twentieth century, timber extraction was still profitable in some European forests as
those formerly exploited at the Spanish Pyrenees (Cabrera 2001). However, currently, many of these mountain forests are not managed for timber production
because commercial forestry is no longer profitable. In this chapter, I explore how
changes in historical management interact with climate warming and intensified
drought stress to trigger dieback in mountain forests.
Here I argue that the historical use of forests strongly interacts with current
climatic trends as the rise in temperatures to determine the current fate of forests,
which in some case can jeopardise its future. I will illustrate how past historical
logging and recent warming-related drought stress contribute to silver-fir dieback in
Pyrenean mountain forests subjected to mesic conditions. The processes leading to
forest dieback are still poorly understood because of the interaction of several stress
factors acting at different spatiotemporal scales, which complicates the disentangling of lagged cause–effect relationships (Pedersen 1999). Many dieback episodes
have been studied following Manion’s (1981) conceptual model, which includes
predisposing, inciting and contributing stress factors causing a loss in tree vigour.
Predisposing factors such as site conditions reduce a tree’s vigour over the long
term (Suarez et al. 2004), whereas inciting factors such as drought lead to a sharp
132
J.J. Camarero