6.6 Lessons for Forest Use and Conservation
The reconstructed history of growth releases in Pyrenean silver-fir forests suggests
that dieback is the product of both predisposing and inciting factors (sensu Manion
1981) such as historical logging and warming-induced drought stress, respectively.
Historical land-use changes have persistent effects on forest dynamics on decadal
and even millennial time scales (Dupouey et al. 2002). However, historical legacies
have not always been considered when explaining the causes of forest dieback;
although several authors have illustrated how changes in management cause
over-stocking and increase tree-to-tree competition for water making tree species,
including firs, more susceptible to drought-induced damage (Becker et al. 1989; see
some cases in the reviews by Allen et al. 2010, 2015).
The Pyrenean silver-fir dieback was triggered by pronounced late-summer water
deficit (inciting factor) due to the rapid temperature rise observed in the Aragón
Pyrenees since the 1980s. Historical logging is the most likely predisposing factor
of this dieback process. The geographic pattern of forest dieback was a response to
a regional climatic gradient with decreasing summer precipitation westwards,
whereas dieback and crown defoliation and growth decline were highest in marginal low-elevation sites. Declining stands were characterised by being mixed
forests with silver-fir trees of low size and growth rate. Silver-fir growth is
becoming more dependent on previous September climatic conditions which may
be connected with changing modes of atmospheric variability affecting the Iberian
climate. Finally, I partly concur with Auclair (2005) that some dieback cases might
be regarded as an additional disturbance factor of forest dynamics. However, both
historical management and warming-induced drought stress are altering these
dynamics. First, management might set the stage for forest dieback through the
selection of particular trees or cohorts highly vulnerable to climatic stress. Second,
inciting climatic stressors as warming-related droughts may become more frequent
in a warmer world. Thus, the recurrence and severity of forest dieback episodes may
be exacerbated leading to unprecedented growth drops outside the historical range
of growth variability (sensu Veblen 2003).
To manage mountain forests in a more sustainable way so as to preserve biodiversity and provide ecosystem services including timber production it must be
considered that their past use and history constrain their current structure and how
they will respond to climate warming. Mountain forests are erroneously perceived
as intact and wild ecosystems, but they have been shaped by centuries of
exploitation, at least in Europe. If this past use has lead to uniform stands of trees
vulnerable to climate warming, more dieback processes are expected across
southern European mountains holding the southernmost limits of several tree
species as is the case of silver fir in the Pyrenees (Gazol et al. 2015). A more
sustainable use and conservation of mountain forests will require managing their
structure to make them more resilient and less vulnerable to climate warming. Such
management should consider the enhancement of functional and structural diversities which can contribute to increasing forest resilience and promote the
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151
The reconstructed history of growth releases in Pyrenean silver-fir forests suggests
that dieback is the product of both predisposing and inciting factors (sensu Manion
1981) such as historical logging and warming-induced drought stress, respectively.
Historical land-use changes have persistent effects on forest dynamics on decadal
and even millennial time scales (Dupouey et al. 2002). However, historical legacies
have not always been considered when explaining the causes of forest dieback;
although several authors have illustrated how changes in management cause
over-stocking and increase tree-to-tree competition for water making tree species,
including firs, more susceptible to drought-induced damage (Becker et al. 1989; see
some cases in the reviews by Allen et al. 2010, 2015).
The Pyrenean silver-fir dieback was triggered by pronounced late-summer water
deficit (inciting factor) due to the rapid temperature rise observed in the Aragón
Pyrenees since the 1980s. Historical logging is the most likely predisposing factor
of this dieback process. The geographic pattern of forest dieback was a response to
a regional climatic gradient with decreasing summer precipitation westwards,
whereas dieback and crown defoliation and growth decline were highest in marginal low-elevation sites. Declining stands were characterised by being mixed
forests with silver-fir trees of low size and growth rate. Silver-fir growth is
becoming more dependent on previous September climatic conditions which may
be connected with changing modes of atmospheric variability affecting the Iberian
climate. Finally, I partly concur with Auclair (2005) that some dieback cases might
be regarded as an additional disturbance factor of forest dynamics. However, both
historical management and warming-induced drought stress are altering these
dynamics. First, management might set the stage for forest dieback through the
selection of particular trees or cohorts highly vulnerable to climatic stress. Second,
inciting climatic stressors as warming-related droughts may become more frequent
in a warmer world. Thus, the recurrence and severity of forest dieback episodes may
be exacerbated leading to unprecedented growth drops outside the historical range
of growth variability (sensu Veblen 2003).
To manage mountain forests in a more sustainable way so as to preserve biodiversity and provide ecosystem services including timber production it must be
considered that their past use and history constrain their current structure and how
they will respond to climate warming. Mountain forests are erroneously perceived
as intact and wild ecosystems, but they have been shaped by centuries of
exploitation, at least in Europe. If this past use has lead to uniform stands of trees
vulnerable to climate warming, more dieback processes are expected across
southern European mountains holding the southernmost limits of several tree
species as is the case of silver fir in the Pyrenees (Gazol et al. 2015). A more
sustainable use and conservation of mountain forests will require managing their
structure to make them more resilient and less vulnerable to climate warming. Such
management should consider the enhancement of functional and structural diversities which can contribute to increasing forest resilience and promote the
6 The Multiple Factors Explaining Decline in Mountain Forests …
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