Ecosystem services of mixed stands of scots pine and maritime pinebiodiversity conservation and carbon sequestration

Abstract

ABSTRACT Many studies highlight the role of mixed vs monospecific forests to supply ecosystem services. Most reports of positive effects of the mixture of different trees, but little is known about the mixture of the same genus trees. This thesis assessed the overstory-understory-soil relationships of six triplets of Scots pine (Pinus sylvestris L.) and Maritime pine (Pinus pinaster Aiton.) of northern Spain. Each triplet has two plots dominated either by P. sylvestris or P. pinaster and one mixed plot of both species. In each plot, the soil was studied by one soil pit of at least 50 cm depth for organic and mineral horizons characterization. The understory was analyzed by ten inventories randomly located of one square meter, where the cover of every understory species and the number of individuals of the regeneration trees was estimated. The understory species data were codified according to Raunkiær´s life-forms, and the understory richness was also calculated. The overstory study was developed at two scales (small-scale: plots of 4 m radius, and stand-level: plots of 15 m radius). First of all, two trends were found in carbon storage (Cstock) and exchangeable cations in the soil profile: in the topsoil, these values were higher in P. sylvestris stands, lower in P. pinaster stands and intermediate in mixed stands; this pattern was related to the forest floor C:N ratio. In the intermediate soil layers, it tended to be higher in mixed stands and it was related to the higher percentage of fine roots. After, a water-stress gradient associated with the overstory composition was found: P. pinaster tolerates higher water-stress than P. sylvestris. Mixed stands maintained the same level of understory richness than P. sylvestris monospecific stands under greater water stress. In addition, a soil fertility gradient defined by Cstock and exchangeable-magnesium stocks was identified. Hemicryptophytes, more abundant in mixed stands, were the only understory life-form linked to soil fertility. Also, the percentage of the basal area of both Pinus species was found to be the only stand characteristic that influenced on understory composition and tree regeneration. In the Scots pine stands dominated Humid zones species (P. sylvestris regeneration) and in Maritime pine stands dominated well-drained area species (P. pinaster regeneration). In mixed stands, the highest native Pyrenean oak regeneration was accompanied by species that share the same niche. Finally, a small-scale overyielding was found in mixed stands related to the more efficiency in the use of space by both Pinus species, thanks to soil water and fertility niche complementarity. No negative effect on the understory richness was found that was maintained in mixed stands thanks to hemicryptophytes. Seems to exist a competitive advantage of the mixed forests of Scots pine and Maritime pine over the respective monospecific stands since, it helps to improve carbon accumulation, to increase the forest productivity, to conserve the endemic regeneration and to maintain understory richness under worse conditions.