Long-term forest and fire dynamics in the central Pyrenees: a proxy-model comparison for the Lateglacial-Holocene transition



Fire plays a very relevant role in Earth processes at both abiotic and biotic domains as fire presents feedback with climate modulating biogeochemical processes linked to carbon cycle, albedo and aerosol emissions (Li et al., 2012). Most importantly, fire is a strong disturbance factor in ecosystem dynamics, reshaping landscapes, acting on ecological patterns and processes fostering new community assembling, mediating on species diversity, and rejuvenating ecosystems (Levin et al, Pausas, Lloret). While fire is an inherent process to the Earth system, fire has become over the last decades an important hazard for humans especially as population, fire severity and intensity increase, resulting in wildfires with fatal consequences in human communities. Understanding fire effects on vegetation and ecosystem post-fire response become thus essential, especially in non-fire prone regions, where wildfires may have a more lasting imprint on vegetation dynamics.

Palaeofire science has recently provided multiple examples of charcoal records insightfully contributing on defining the climate-fire-vegetation relationships (Colombaroli, Tinner, Montoya, Bush, O'Connor, Daniau, Marlon, Powell, Feurdean), identifying whether fire regimes changes are either climatically or anthropogenically driven (Colombaroli, Tinner, Vannière) and quantifying the strong feedbacks amongst these three elements at local, regional and global scales (see GPFW). Even considering the relevant role of fire in the Mediterranean biome (Keely et al., Pausas), these quantifications have been traditionally scarce and very locally addressed in the Iberian Peninsula. However in recent years new studies have expanded our knowledge on past fire activity in mountain areas (Morales del Molino, Jiménez-Moreno, Pérez-Obiol, Carrión, Perez-Sanz, Gil-Romera) and lowlands (Morales del Molino, Carrion, Gil-Romera, Muñoz-Sobrino, Ramil-Rego, Burjachs). Generally all these studies have aimed to reconstruct either fire history regionally or fire regimes locally using charcoal records but yet little stress has been made on quantifying the impact that fire may have on forest dynamics at centennial to millennial time scales (Gil-Romera et al., 2010, Gil-Romera et al., 2014). This knowledge is still lacking as identifying the driving factor for a vegetation change is often challenging since inference is based on fossil proxies, as an indirect measure of past environmental change, and the effect of different disturbances as climate, fire or human activities.

Recently, Gil-Romera and colleagues (2014) produced a high resolution Lateglacial-Holocene fossil pollen, macro and microcharcoal record from the exhaustively studied lacustrine sequence of El Portalet in the central Pyrenees (González-Sampériz et al., 2006). This research tested biomass as fire driver in an alpine region and explored the post-fire vegetation response, concluding that, under fire-climate conditions, biomass triggered fire frequencies resulting in long-lasting effects in these fire-sensitive regions where ecosystem resilience may had been affected by increasing disturbance levels. However it remained unclear to what extent fire was shaping the vegetation dynamics and how biotic interactions amongst tree taxa were determining the observed fossil spectra.

In the current study we used the Laglacial-Holocene dataset produced by Gil-Romera et al (2014) to focus in the next objectives: 1) testing the relative role of fire activity in the forest dynamics at El Portalet, by comparing the sedimentary fossil pollen and charcoal records with a spatial-explicit simulation of the population dynamics of five tree taxa (Pinus uncinata, Pinus sylvestris, Betula pendula, Corylus avellana, Quercus petraea), 2) identifying fire intensity thresholds that may explain to what potential abrupt vegetation responses [(and 3) defining how species interaction may play an important role in understanding fossil sequences) ]

The Lateglacial-Holocene transition is an interesting phase from the forest evolution perspective as mesophyte taxa expanded their presence almost linearly with temperature, especially in alpine areas, where vegetation is particularly sensitive to temperature changes (Villar, nature paper). Wildfires at high altitudes are often scarce as climate is rarely fire-conducive and fuel flammability low so at this period, both climate conditions and fuel availability, probably enhanced fire activity (Rius et al., Doyen et al., Gil-Romera et al). As this phase would include the Bølling/Allerød warming phase, the Younger Dryas cooling and the Holocene onset warming (Rasmussen et al), this transition provides ideal conditions to test vegetation response to temperature.

Our simulation is mechanistic and has been performed with an agent based model which illustrates the species population dynamics including a surrogate evidence of temperature and the effect of fire occurrence on a radius of 10km around El Portalet (EP) bog. Our output is the pollen production of each taxa in the area, that can be compared with the observed fossil pollen spectra obtained at EP (Gil-Romera et al., 2014). Despite there exist some simulation studies focused on the long-term relationships of fire with burnt biomass, emissions and human impact (Ali et al 2012, Feurdean et al., 2013 ; Brücher et al., 2014; Vannière et al., 2016), few have ever addressed the post-fire vegetation response, simulating biotic interactions of tree forests and the effect of fire disturbance (Colombaroli et al., 2010), comparing the expected trend with the actual fossil sequence.

This model and its results could be used for testing hypotheses on tree-line shifts, forest resilience to disturbance and long-term biotic interactions, especially in palaeoecological studies using fossil pollen, where the variability observed is often due to a number of drivers interacting and these are normally difficult to assess.