and short-term reduction in tree growth and vitality (Bigler et al. 2006). Other
factors (mainly pathogens and insects) may contribute to dieback acting as secondary stress factors. According to this model, the most sensitive trees to short-term
inciting stress factors will be those that were previously most strongly weakened by
long-term predisposing factors. Here I use this conceptual model to assess the roles
played by long-term predisposing (historical management) and short-term inciting
(drought stress) factors as drivers of dieback in mountain forests.
Most studies on forest dieback have obviated the potential role of historical land
use (e.g. past logging) as a predisposing factor (but see Linares et al. 2009).
Furthermore, the different nature of interacting factors such as land-use legacies
(e.g. past logging) and climatic extremes (e.g. severe droughts) have precluded
considering the interactions between them. In Europe, historical effects have persisted for decades and centuries shaping the current structure of most mountain
conifer forests (Kirby and Watkins 1998). Therefore, past forest use should be
considered as an additional driver of dieback and its role should be assessed.
Here I focus on dieback episodes of silver-fir (Abies alba Mill.) forests reported
since the 1980s in the Western and Central Spanish Pyrenees (Navarra and Aragón
Pyrenees; see Fig. 6.1), near the south-western limit of the species’ distribution area
(Camarero et al. 2002). Silver-fir dieback has been systematically reported in the
1970s and 1980s across central Europe (Skelly and Innes 1994). In the Aragón
Pyrenees silver-fir dieback was more severe in western stands located at medium
elevation than in eastern high-elevation sites. In the most affected stands, up to
30–50% of trees showed severe defoliation, which was related to the occurrence of
severe summer droughts in the 1980s (Camarero et al. 2002). Therefore, it may be
hypothesised that drought stress has recently increased by climate warming and
precipitation regime shifts causing silver-fir dieback. In addition, most of these
forests were logged to extract timber up to the 1970s when their management
ceased due to rural migration to cities (Cabrera 2001). So, it remains to be answered
if those forests that were more intensively logged in the past were also more
predisposed to drought-triggered dieback during the 1980s and more recently. In
this study, I address the following questions: (1) How did silver-fir growth change
in the Aragón Pyrenees during the twentieth century and how was it affected by the
recent patterns of defoliation? (2) Did historical logging and warming-induced
drought cause the recent silver-fir dieback? To answer these questions, I will focus
on the retrospective analyses of tree-ring data.
6.1.1 Geographical and Climatic Backgrounds
The Pyrenees constitute a transitional mountainous area between more humid
conditions in their northern margin, where Eurosiberian vegetation is dominant, and
drier conditions southwards (Vigo and Ninot 1987). This gradient overlaps with a
similarly relevant longitudinal gradient caused by the location of the range between
the Atlantic Ocean and the Mediterranean Sea. According to meteorological data
6 The Multiple Factors Explaining Decline in Mountain Forests …
133
factors (mainly pathogens and insects) may contribute to dieback acting as secondary stress factors. According to this model, the most sensitive trees to short-term
inciting stress factors will be those that were previously most strongly weakened by
long-term predisposing factors. Here I use this conceptual model to assess the roles
played by long-term predisposing (historical management) and short-term inciting
(drought stress) factors as drivers of dieback in mountain forests.
Most studies on forest dieback have obviated the potential role of historical land
use (e.g. past logging) as a predisposing factor (but see Linares et al. 2009).
Furthermore, the different nature of interacting factors such as land-use legacies
(e.g. past logging) and climatic extremes (e.g. severe droughts) have precluded
considering the interactions between them. In Europe, historical effects have persisted for decades and centuries shaping the current structure of most mountain
conifer forests (Kirby and Watkins 1998). Therefore, past forest use should be
considered as an additional driver of dieback and its role should be assessed.
Here I focus on dieback episodes of silver-fir (Abies alba Mill.) forests reported
since the 1980s in the Western and Central Spanish Pyrenees (Navarra and Aragón
Pyrenees; see Fig. 6.1), near the south-western limit of the species’ distribution area
(Camarero et al. 2002). Silver-fir dieback has been systematically reported in the
1970s and 1980s across central Europe (Skelly and Innes 1994). In the Aragón
Pyrenees silver-fir dieback was more severe in western stands located at medium
elevation than in eastern high-elevation sites. In the most affected stands, up to
30–50% of trees showed severe defoliation, which was related to the occurrence of
severe summer droughts in the 1980s (Camarero et al. 2002). Therefore, it may be
hypothesised that drought stress has recently increased by climate warming and
precipitation regime shifts causing silver-fir dieback. In addition, most of these
forests were logged to extract timber up to the 1970s when their management
ceased due to rural migration to cities (Cabrera 2001). So, it remains to be answered
if those forests that were more intensively logged in the past were also more
predisposed to drought-triggered dieback during the 1980s and more recently. In
this study, I address the following questions: (1) How did silver-fir growth change
in the Aragón Pyrenees during the twentieth century and how was it affected by the
recent patterns of defoliation? (2) Did historical logging and warming-induced
drought cause the recent silver-fir dieback? To answer these questions, I will focus
on the retrospective analyses of tree-ring data.
6.1.1 Geographical and Climatic Backgrounds
The Pyrenees constitute a transitional mountainous area between more humid
conditions in their northern margin, where Eurosiberian vegetation is dominant, and
drier conditions southwards (Vigo and Ninot 1987). This gradient overlaps with a
similarly relevant longitudinal gradient caused by the location of the range between
the Atlantic Ocean and the Mediterranean Sea. According to meteorological data
6 The Multiple Factors Explaining Decline in Mountain Forests …
133