Functioning of Mediterranean ecosystems in response to forest fires and post-fire management activities

Abstract

In Valencia region (SE Spain), many post-fire communities are dominated by non-resprouting (seeder) species, because of the long history of land exploitation and subsequent abandonment during the last half of 20th century. These communities accumulate fine dry biomass and, therefore, can burn again easily. In fact, Mediterranean forests are suffering from an increase in wildfire frequency since the early 1970s. Wildfires shape the composition and functioning of Mediterranean ecosystems, but we do not know how these ecosystems respond to both the higher fire recurrence and shorter recovery times expected for future climatic scenarios. In this sense, Aleppo pine forest (Pinus halepensis) is one of the most fire affected vegetation of this type in the Mediterranean Basin and to know how it respond to fire is fundamental to design management plans. After fire, regeneration of this forest can be highly variable, and it can go from extremely dense tree stands (overstocked pine) to treeless shrublands dominated by seeder species. All these regenerated stands are fire prone with limited ability to deliver multiple ecosystem services. Although several management techniques are applied to redirect these post-fire ecosystems towards less vulnerable and more functional communities, we do not know yet which amongst them could serve to foster more diverse and multifunctional landscapes. Therefore, the general objective of this thesis is to investigate the functioning of these Mediterranean ecosystems as consequence of shifts in fire regime and forest management application, using different techniques, in different post-fire regenerated ecosystems (overstocked pine forests and dense shrublands). To do so, we calculate, within Mediterranean Pinus halepensis forests affected by wildfires, the supply of multiple ecosystem services (biodiversity conservation, carbon sequestration, disturbance regulation, food production, supporting services, and multifunctionality), through up to 25 aboveground and belowground attributes. Our main findings are (1) High fire recurrence and time since last fire interacted to determine ecosystem services but did not affect their synergies and trade-offs between them. Their combined effects reduced carbon sequestration and multifunctionality. Disturbance regulation diminished drastically with the first fire, with no effect of further fires. However, their effects dampened, and even became positive, for biodiversity conservation and food production services if provided enough time to recover. (2) Thinning in overstocked pine stands enhances ecosystem attributes associated with biodiversity conservation without compromising the provision of carbon sequestration. After 10 years, two levels of thinning, (600 and 1200 trees·ha-1), similarly affected ecosystem attributes, which suggest that 1200 trees·ha-1 suffice to enhance individual ecosystem attributes. (3) Clearing within dense shrubland dominated by seeder species enhances ecosystem attributes associated with biodiversity conservation without compromising the capacity of ecosystem to sequester carbon. (4) Plantation of resprouting species combined with thinning and clearing, in overstocked pine forests and dense shrublands respectively, can enhance the provision of ecosystem services of disturbance regulation, food production and ecosystem multifunctionality. (5) Prescribed burning reduces the amount of dead fuel, increases biodiversity conservation, and improves food production. However, these effects become negative, in addition to the decline in disturbance regulation and multifunctionality, if prescribed burning is applied frequently. (6) Combining different management activities can enhance the supply of multiple ecosystem services simultaneously by reducing the trade-offs in between them and therefore, establish multifunctional Mediterranean landscapes.