Effects of fire recurrence and burn severity in fire-prone pin ecosystems. Basis for forest management

Resumen

Fire prone ecosystems dominated by Pinus pinaster Ait. and Pinus halepensis Mill. are the forests most affected by fire in the Mediterranean Basin. Consequently, they are priority ecosystems to study the ecological consequences of changes in fire regimes, which are expected in the current context of global change. The objective of this Doctoral Thesis is to analyze the effects of different regimes of fire recurrence and burn severity in fire-prone ecosystems dominated by P. pinaster and P. halepensis. The starting point has been to study the ability of multispectral Landsat scenes to characterize fire recurrence (article I) and burn severity (article II) in pine forests affected by large forest fires. The spatio-temporal patterns of wildfires during the period 1978-2014 were determined by visual analysis of Landsat false-color composites, specifically using Landsat 2, 4, 5, 7 and 8 time series (MSS, TM, ETM + and OLI respectively). Then, fire scars were validated with the official wildfire reports. This method allowed mapping all fires documented in the official wildfire reports, demonstrating that the use of Landsat image time series is an effective tool to characterize fire recurrence. Similarly, to characterize burn severity we calculated different spectral indices (thermal, reflective and mixed metrics) using Landsat imagery. The ability of these metrics to determine the soil burn severity, the vegetation burn severity and the site burn severity (soil plus vegetation) along a climatic gradient was evaluated using linear regression models. We calculated the CBI (Composite Burn Index) in field plots to be used as ground truth in the evaluation of the performance of the spectral indices. Overall, bi-temporal indices based on the near-infrared and short-wave infrared regions of the spectrum (dNBR -differenced Normalized Burn Ratio- type indices) showed a high capacity to quantify the vegetation burn severity and site burn severity, whereas the capacity of Landsat spectral indices to determine the soil burn severity was relatively low. Additionally, we found large differences in indices performance along the climatic gradient, so we proposed a new spectral index, the dNBR-EVI, which shows greater capacity to determine burn severity and better transferability throughout of the different climatic conditions than the previously existing indices. To investigate the effects of burn severity on soil properties, we conducted a study immediately after the fire (1 week) in a P. pinaster forest with siliceous soils (article III) and another study over the medium term after the fire (3 years) in both, P. pinaster and P. halepensis ecosystems, with siliceous and calcareous soils respectively (article IV). Results showed significant effects of burn severity on most soil properties studied immediately after the fire (aggregate size, pH, organic carbon, available phosphorus, microbial biomass carbon, and enzymatic activities β-glucosidase and acid phosphatase), as well as in some ratios between these properties (C:N ratio, microbial quotient and the specific activity of β-glucosidase). Among these properties and quotients, those related to enzymatic activities showed the highest sensitivity to fire, as they start to decrease at low severities. Over the medium term after the fire, we found that some soil properties (available phosphorus, microbial biomass carbon and enzymatic activities) remained affected by burn severity. The microbial biomass carbon, the β-glucosidase activity and the urease activity decreased with burn severity in the two studied ecosystems, and therefore, they could be used as potential indicators to investigate the burn severity legacy over the medium term after the fire in Mediterranean fire-prone pine forests. The effects of fire recurrence and burn severity in the natural regeneration of vegetation were studied through remote sensing methods (article V) and through studies based on field information, focused on both the pine population (article VI) and the woody plant community (article VII). In the study based on remote sensing methods, we used the difference of the NDVI (Normalized Difference Vegetation Index) calculated with Landsat to estimate the vegetation greenness recovery after the fire. Results indicated that, following the fire (2 and 5 years), areas affected by high severity showed a low greenness recovery. On the contrary, the areas with high fire recurrence showed higher greenness recovery than low recurrence areas, because of the dominance of shrub species in high recurrence scenarios, which require less time than trees to reach the pre-fire situation. The field study revealed that in low recurrence and low severity areas the density and coverage of P. pinaster and P. halepensis seedlings was relatively high and enough to ensure the recovery of the tree canopy. As opposed, in the areas affected by high recurrence (3 fires in 34 years), as well as moderate recurrence with high severity (2 fires in 34 years), the natural regeneration of pines was endangered due to the lack of a viable seed bank. At the community level, we found that shifts in fire recurrence and burn severity modified the woody species diversity, as well as the species relative cover depending on their functional traits. The effects were particularly significant in the regions with summer drought such as Mediterranean areas. In general, our results indicated that woody vegetation with (i) non-arboreal life form, (ii) high specific leaf area, (iii) nitrogen fixing capacity, (iv) resprouting ability, (v) low seed mass or (vi) heat-stimulated germination may be more resilient to increases in fire recurrence and severity. The results obtained in this PhD Thesis contribute to the advancement of knowledge in the discipline of fire ecology, and provide the scientific basis for an efficient forest management in fire-prone pine forests of the Mediterranean Basin. Likewise, this work can be considered as a reference to design forest management strategies in the face of future changes in fire regimes.