What drives European beech (Fagus sylvatica L.) mortality after forest fires of varying severity?
Introduction
Climate change and related predictions of a warmer and drier climate (IPCC, 2014) lead to increasing concerns about the future impact of wildfires on forest resistance and resilience in forest ecosystems (Bachelet et al., 2007, Fischer et al., 2010, Schumacher and Bugmann, 2006). In many regions, the size and intensity of wildfires have already increased in recent decades (e.g., Westerling et al., 2006, Sullivan et al., 2011, Sarris et al., 2014), raising questions about how to predict the rate of fire-injured tree mortality within the framework of planning post-fire silvicultural measures such as salvage logging and reforestation (Brown et al., 2003, Ledgard and Davis, 2004, Kobziar et al., 2006, Keyser et al., 2008, Moreira et al., 2012). Models that predict post-fire mortality, as a result of various driving factors, have been developed mainly for tree species in fire-prone ecosystems, which display fire resistance traits (e.g., McHugh and Kolb, 2003, Rigolot, 2004, Kobziar et al., 2006, Sieg et al., 2006, Hood et al., 2007, Fernandes et al., 2008, Stevens-Rumann et al., 2012). Comparatively little attention has been paid to fire-sensitive species that dominate temperate deciduous forests (e.g., Catry et al., 2010). From a forest management perspective, a major problem arises from the lack of data and experience regarding the vulnerability and resilience of such forest stands under increasing fire disturbance.
European beech (Fagus sylvatica L.), for example, represents a tree species with high economic and ecological value in Europe, and forests of beech are usually considered less fire-prone (Pezzatti et al., 2013). However, during the exceptional drought of 2003 (e.g. Beniston, 2004), beech stands in the southwestern Alps experienced numerous and atypical large forest fires. These fires may indicate a shift in fire regime driven by climate change (Valese et al., 2014).
To date, species survival strategies after fire are poorly understood, and post-fire silvicultural measures are usually limited to salvage logging followed by reforestation in very rare cases. Generally, beech is considered to be highly susceptible to fire due to its lack of fire-resistance (e.g. thick bark) and fire-adaptation (e.g. resprouting capability) traits (Peters, 1997, Packham et al., 2012). In fact, studies report that beech resprouts after fire, but the resulting shoots tend to dieback and hardly constitute a valuable new generation (van Gils et al., 2010, Conedera et al., 2010, Espelta et al., 2012, Maringer et al., 2012).
Furthermore, beech regeneration (from seeds) relies on seed dispersal by gravity and animals, and establishment is often close to the nearest seed-bearing tree (Wagner et al., 2010, van Couwenberghe et al., 2010). Consequently, natural beech regeneration becomes more limited within increasing burned area and greater distance from a seed source. Recent studies, however, suggest that beech stands exhibit surprisingly high resilience after single fire events (Ascoli et al., 2013, Maringer et al., in press). The fire-surviving strategy, in this case, is mainly based on rapid in situ seed production when mast years coincide with suitable germination conditions in the post-fire environment (e.g., improved light conditions and reduced litter cover on the soil, Ascoli et al., 2015). Thus, post-fire density and spatial distribution of mature surviving trees are critical for new cohort recruitment and rapid recovery of beech forests.
It is known that the timing of post-fire beech mortality depends on fire intensity. Beech mortality may occur immediately after very severe fires or be delayed by several years after low to moderate severe fire (Conedera et al., 2007, Ascoli et al., 2013). There is, however, a lack of knowledge regarding factors driving such delayed mortality, and especially the predictability of its timing. Such information would help forest managers in planning complex post-fire measures related to: (i) whether or not intervene with silvicultural measures, (ii) timing of the needed interventions, and (iii) the number of trees to salvage (Ascoli et al., 2013). Following the guiding principle that post-fire management decisions should be based on site- and species-specific ecological processes, we focus in this paper on the major drivers that influence post-fire beech mortality. In particular we ask:
- (1)
What are the mid-term temporal trends in fire-caused beech mortality?
- (2)
Which tree-specific traits (e.g., tree size) enhance the survivability of fire-injured beech trees?
- (3)
What are the main biotic and abiotic factors associated with beech mortality after fire disturbance?
Section snippets
Study area
The present study was conducted in the neighboring regions of Piedmont (Italy) and Ticino (Switzerland) located in the southwestern European Alps (Fig. 1). Both regions are characterized by a marked elevational gradient along which forest vegetation types are distributed. Beech-dominated forests occupy the intermediate elevation belt ranging from 600–1000 m a.s.l. to 1300–1700 m a.s.l. depending on the locality and aspect (Camerano et al., 2004, Ceschi, 2006). These forests are mostly in the
Forest structure
Most (61%) of the burned forest stands were classified as high-stand forests, a minority (16%) as coppices, and the remainder were intermediate in structure. In total, 3,504 mature trees (DBH >8 cm) were recorded, of which beech comprised 88% and 93% of the trees in burned and unburned forests, respectively. Other tree species rarely (<4%) grew within the pure beech stands (Appendix B).
Post-fire beech mortality
Half of the beech trees assessed in burned plots (N = 2,845) died whereas only 10% of the trees in unburned
Post-fire stand dynamics
The selected stands showed typical beech forest structural characteristics for the southwestern Alps, with overlapping transition stages from coppices to high forest stands (Nocentini, 2009, Ascoli et al., 2013). In these stands, fires of mixed severity caused changes in forest structure by triggering mortality in half of the pre-fire beech. In general, fire-induced beech mortality increased with time in the first two post-fire decades. Similar lags in mortality after fire have also been
Conclusions
In this study, we used a retrospective approach to examine post-fire dynamics and fire-related beech mortality in 36 sites in the southwestern Alps. Despite some methodological limits in our chronosequence approach, we provide important new insights on the fire ecology and post-fire mortality of European beech.
The major drivers of tree mortality in this study were related to a combination of factors: (i) the proportion of woody tissue damaged as a consequence of tree diameter in relation to
Acknowledgements
This study was partially supported by the Swiss Federal Office for the Environment (FOEN). Fieldwork assistance was carried out with the support of Franco Fibbioli, Simone Giavi, Marianne Steffen, Lisa Berghäuser, Jordi Murgadas from the Swiss Federal Institute for Forest, Snow and Landscape Research, and Sven Hofmann from the University of Karlsruhe (Germany). Thanks go to Curtis Gautschi who took care of revising the English version. Finally, we acknowledge the helpful comments of two
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2017, Forest Ecology and ManagementCitation Excerpt :It is, thus, important to study chrono-sequences of vegetation development after different fire intensities to quantify their protective capacity. In the case of beech stands after moderate severity burns, the gradual opening of the tree canopy leads to emerging beech regeneration and to an increase in the forest-protective effect after 20 years (Maringer et al., 2016b). It is important to monitor the evolution of natural regeneration in study areas to establish the timing of the recovery of the protective effect.
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2017, Forest Ecology and ManagementCitation Excerpt :Similarly, recent dieback of Scots pine in dry areas of the Rhone valley of the western central Alps is also related to extreme summer drought and will likely continue in the future (Bigler et al., 2006; Rebetez and Dobbertin, 2004; Rigling et al., 2013). Warming and related increases in drought frequency and severity (Rebetez, 1999) and associated fire risk (Reinhard et al., 2005; Wastl et al., 2013; Valese et al., 2014) will alter interactions among woody species (Moser et al., 2010; Maringer et al., 2016) and with pests and diseases (Battisti, 2008; Netherer and Schopf, 2010; Marini et al., 2012). In addition, newly introduced exotic species (e.g. Robinia pseudooacacia, Ailanthus altissima, Pawlonia spp.) have become invasive and highly competitive in low- to mid-elevation forest ecosystems and strongly interfere with fire regimes, silvicultural management practices (Grund et al., 2005; Maringer et al., 2012; Radtke et al., 2013; Knüsel et al., 2015), pests (Wermelinger, 2014; Roques et al., 2016) and disease (Kowalski and Holdenrieder, 2009; Pautasso et al., 2013; Sieber, 2014) as well as their possible interactions (e.g. Meyer et al., 2015).