Post-fire regeneration of Pinus pinaster Ait. forests: environmental and biological constraints
Author(s)
Buscardo, Erika
Date Issued
March 10, 2011
Type
Doctoral Thesis
Abstract
The aim of this investigation was to identify the environmental and biological constraints for the natural regeneration of maritime pine after stand-replacing wildfires and to evaluate the effects of fire frequency (measured as fire return interval) on three components of the soil fungal communities associated with maritime pine (Pinus pinaster Ait.): the ectomycorrhizal (ECM) resistant propagules community (RPC), the total soil fungal community and the potential for the formation of common mycorrhizal networks between pine seedlings and shrub understorey species.
Firstly, the main environmental factors responsible for maritime pine regeneration variability in north-western Italy were identified. The diversity and structure of ECM fungal communities associated with maritime pine saplings naturally regenerated after wildfire were characterized. ECM taxa were identified by direct sequencing of fungal internal transcribed spacer (ITS) regions from individual ECM root tips. The environmental factors responsible for ECM fungal species distribution were also individuated and the relation between saplings performance and ECM fungal diversity indices assessed. Pine regeneration varied throughout the study areas and was enhanced at higher elevations, in correspondence with moderate slopes, shallower soils, and a reduced cover of ericaceous shrubs and bare ground. These conditions were found in close proximity to patches of pine trees that survived the disturbance event and were previously characterized by a higher pre-fire pine biomass. Even though no correlations were found between saplings performance and ECM fungal diversity indices, common environmental factors (i.e. ericaceous shrub cover, extent of erosion, slope and soil depth) were responsible for shaping the ECM fungal distribution and for describing most of the explained regeneration variability.
Secondly, the long-term effects of wildfire frequency on the structure and composition of the total soil fungal community in an open maritime pine forest of central Portugal were evaluated. Six years after the last fire event, the structure of the soil fungal communities in areas subjected to short and long fire return intervals were analysed using 454 pyrosequencing technology and denaturing gradient gel electrophoresis, and compared to the soil fungal community structure of an unburnt control stand. Although long-term effects of wildfire on fungal species richness were not detected, wildfire and wildfire frequency had a significant effect on the structure of the soil fungal community. Wildfire induced a general shift towards the Ascomycota in the stands characterized by the same shrub cover and favoured the abundance of fungal species that preferentially inhabit mineral soils and pioneer species in all the burnt stands, while differences in the abundance of specific ECM fungal species indicated a significant effect of the post-fire vegetation cover on the soil fungal community.
Thirdly, the effects of wildfire frequency on the RPCs of the above-mentioned open pine forest were compared. Soil samples were collected in four mountain sites with different fire return intervals and used to test ECM fungal development in two hosts, P. pinaster and Quercus suber L. RPCs were characterized by direct sequencing of fungal ITS regions from individual ECM root tips. Eighteen ECM fungal species were detected in the bioassay. The effect of the fire return interval length on individual species varied greatly: infrequent species found in the unburnt soil were completely eliminated in the short fire return interval sites (e.g. Laccaria spp. and Scleroderma spp.), some species were favoured by long fire return interval but suffered with short fire return interval (e.g. Inocybe jacobi), and other species positively responded to both fire regimes (e.g. Rhizopogon roseolus). Species richness of the ECM fungal community found on Q. suber was reduced by short fire return intervals. Overall changes in fire return interval length affected both RPC structure and composition.
Finally, the influence of fire frequency on the potential for common ECM networks between understorey shrub species and P. pinaster was investigated. ECM root tips were sampled from five shrub species belonging to the genera Arbutus, Cistus and Halimium and from P. pinaster at four stands with different fire return interval lengths. Fungal symbionts and relative hosts were identified using molecular techniques with direct sequencing of the nrDNA ITS region. Twenty eight ECM fungal species and seventeen non-ECM root inhabitants were identified. Six years after wildfire disturbance ECM species richness did not differ significantly between unburnt and burnt stands. Nine ECM fungal species were common to pine and shrubs and both their frequencies and abundances were significantly higher in the unburnt and in the low fire return interval stands when compared with the high fire return interval stands.
Fire is a major factor in the dynamics of Mediterranean forests and it is of vital importance in maintaining pine status in these ecosystems. However, changes in fire frequency can compromise the ecosystem functioning by leading to the regression of vegetation and modifying the structure and composition of soil fungal communities. This investigation revealed that highly recurrent fires have significant effects on the structure of the soil fungal communities. They also modify the structure and composition of the ECM RPCs and strongly reduced the potential for common mycorrhizal networks between understorey shrub species and pine seedlings. Therefore an increase of fire frequency could negatively affect Mediterranean pine forests by delaying the process of pine recolonization through direct and indirect effects on soil and on the above-ground plant community.
Additional information
Dottorato di ricerca in Ecologia forestale
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