The Multiple Factors Explaining Decline in Mountain Forests: Historical Logging and Warming-Related Drought Stress is Causing Silver-Fir Dieback in the Aragón Pyrenees

Abstract

The drivers and patterns of drought-related forest dieback are not as well understood in mountain conifer forests. Most studies have obviated the role of historical use as a predisposing factor of forest dieback. Here I focus on the recent silver-fir (Abies alba) dieback observed since the 1980s in the Aragón Pyrenees (NE Spain) as study case. I argue that such dieback was predisposed by past historical logging and incited by warming-induced drought. I analyzed environmental, structural and tree-ring data from 32 sites with contrasting degrees of dieback at the tree and stand levels. I found that a peak in late-summer water deficit observed in 1985 caused a severe growth reduction in 1986, resulting in subsequent crown defoliation, dieback and increased mortality. Dieback was more severe and widespread in western low-elevation mixed forests dominated by smaller trees with low growth rates. These marginal sites receive less late-summer rainfall, which is a key climatic variable controlling silver-fir growth, than eastern sites. Declining sites showed more frequent growth releases induced by historical logging than non-declining sites. Silver-fir growth is becoming more dependent on climatic conditions of previous September, which may be connected with changing modes of atmospheric variability affecting Iberian climate. Historical logging and warming-induced drought stress during late summer are the most likely predisposing and inciting factors driving silver-fir dieback, respectively. A sustainable management of mountain forests shaped by past historical use requires changing their current structure and composition to make them more resilient to climate warming.

Variación de los dominios forestal y herbáceo en el paisaje vegetal de la península Ibérica en los últimos 20.000 años. Importancia del efecto de los grandes herbívoros sobre la vegetación

“…Otro proceso que se observa en las últimas décadas en el Pirineo es el decaimiento de los abetales, cuyas causas no se han podido explicar satisfactoriamente (Oliva y Colinas, 2007). Se han formulado diversas hipótesis basadas en el parasitismo, la contaminación química, las sequías de las últimas décadas, algunos aspectos de explotación histórica (Camarero et al, 2002;Oliva y Colinas, 2007, 2010Linares y Camarero, 2012;Camarero, 2017), el cambio de los usos del territorio (P. Montserrat, com. oral), etc.…”

“…El paisaje vegetal ha experimentado grandes transformaciones en la península Ibérica durante los últimos 20 milenios (González-Sampériz et al, 2010, 2017. Entre los mayores cambios hay que destacar el de los dominios forestales respecto a las formaciones de pasto-matorral.…”

The High Mountain Conservation in a Changing World

“…The interaction between pests and climate change is a major source of uncertainty in conservation. Tree populations at their ecological limits may be more prompt to infection, which may accelerate and otherwise slow substitution of species (Camarero 2017a).…”

“…The present structure and functioning of woodlands are in many cases a delayed response to ancient logging. For instance, this included recent tree mortality in the Pyrenean Abies alba forests, a phenomenon predisposed by historical logging that enhanced dense tree populations and induced by recent climatic changes (Camarero 2017a). Similarly, shifts in alpine grazing have changed grassland structure and composition (Komac et al 2014).…”

Trade-offs in High Mountain Conservation

“…Moreover, bark beetle proliferation, a common driver of conifer mortality in association with drought, has been detected in damaged Scots pine stands in the Alps (Dobbertin et al 2007) and it is one of the main causes of forest dieback in Western North America (Hart et al 2014). A parallel die-off is occurring in silver fir forest in the Pyrenees, associated with logging in the past (Camarero et al 2011;Camarero 2017a). The primary or contributing role of pests and pathogens versus drought is also often hard to elucidate, as they can establish mutually reinforcing feedback (Hart et al 2014;Oliva et al 2014).…”