Relationships between the dynamics of Pinus halepensis Mill. and Pinus sylvestris L. plantations and environmental parameters: a basis for sustainable management of stands

Pinus halepensis Mill. and Pinus sylvestris L. have been widely used for reforestation of poor and degraded soils in Spain. Pinus halepensis is a Mediterranean species adapted to drought and to a wide range of substrates and able to colonize very poor soils and improve them, promoting the growth of native broadleaved species such as Quercus ilex L. or Quercus faginea Lam. Notwithstanding the above, it has been poorly studied because of the limited economic interest of its wood. Pinus sylvestris presents great ecological amplitude responsible for its great distribution. Pinus sylvestris plantations were established in former Quercus pyrenaica Willd. stands that were converted into crops or overexploited for firewood. The aim of this thesis is to widen the knowledge about the existing relationships between the environmental parameters and the dynamics of these plantations to serve as a guide for sustainable forest management of stands.
Forest productivity determination is crucial to accomplish the established managerial objectives through sustainable forest management. It can be estimated through the site index of the stand and, when dominant height is not available, can also be estimated by means of environmental parameters (soil, climatic and physiographic). In this thesis, a discriminant model was developed to predict the site index for Pinus sylvestris by using latitude, soil porosity, inorganic Al and microbial biomass C as predictors. Besides, another discriminant model was developed to predict the site index for Pinus halepensis plantations with soil porosity, the Annual Hydric Index, the slope and the soil microbial biomass N as predictors.
As seen in the previous models, soil biochemical parameters are determining factors for forest productivity. Organic matter decomposition and nutrient cycling are mainly driven by the activities of the enzymes produced by bacteria and fungi. Therefore, it is essential to know how environmental parameters affect microbial performance in forest soils to understand how ecosystems function. The activities of several enzymes (FDA, DHA, catalase, urease and phosphatases) were studied in soils under Pinus halepensis and Pinus sylvestris plantations to determine those environmental parameters that influence  microbial performance in these ecosystems. Hydric deficit seems to be the most limiting factor for enzyme activities in the calcareous soils under Pinus halepensis plantations. However, in soils under Pinus sylvestris plantations, the low pH and the high amount of soluble phenols seem to limit both the activity and the composition of the microbial communities.
Litterfall is the main source of nutrients for forest soils. The amount of litterfall shed by the stand and the dynamics of litter decomposition are driving factors for soil fertility. Decomposition processes are driven by the abundance and diversity of microorganisms, the quality of the substrate and the climate, since temperature, humidity and nutrient availability drive, in turn, the  decomposers performance. Usual silviculture often includes stand density management. These practices may alter the amount and chemical composition of litterfall and can also modify the microclimate altering the microbial performance and therefore, the litter decomposition processes. To shed light on these relationships, the effect of local basal area on litterfall, litter decomposition and soil temperature and humidity was studied in four Pinus sylvestris and four Pinus halepensis stands. The local basal area of the plot significantly affected the amount of litterfall in the stands of both Pinus species. The needle litter decomposition rate was significantly affected by the local basal area in Pinus halepensis stands, but not in Pinus sylvestris stands. Besides, a significant and negative correlation between the local basal area of the stand and the topsoil humidity was found in Pinus halepensis and Pinus sylvestris plantations. Therefore, the amount of litterfall is lower in plots with lower local basal area due to the lower aboveground tree biomass. The lower amount of tree biomass also intercept less amount of water from precipitations, and then, a higher amount of water reach the soil increasing soil humidity. The activity of decomposers is higher in plots with lower local basal area in Pinus halepensis plots (which are the ones limited by hydric deficit) because of the higher soil humidity and finally, the needle litter decomposition rate is also higher. The chemical composition of the litterfall in Pinus halepensis and  the nutrient release from decomposing needle litter of both Pinus species is also affected by the local basal area of the stand. Therefore, silvicultural practices involving  density management also have an impact on the nutrient cycling of the Pinus sylvestris and Pinus halepensis plantations studied.