Patrones espaciales y procesos condicionantes en la diversificación específica de pinares mediterráneos de repoblaciónfactores ecológicos y selvícolas

Resumo

Large areas in Spain are occupied by pine plantations established during the second half of the 20th century. Many of these stands are vulnerable to disturbances and stressors and often show successional stagnation due to their developmental stage and high stocking. This causes low effective regeneration of pines and other mid-and late- successional forest trees. Enhancement of the ecological resistance and resilience of these stands involves increasing their structural and specific diversity, which is a scientific and socio-economic challenge both in the current conditions and the forecasted climate change scenario. Transformation of these plantations into mixed forests needs new silvicultural strategies for manipulating the canopy and understory of pine plantations. These strategies should be aimed at catalyzing the ecological processes that drive the dynamics of these stands and reducing the incidence of the environmental factors that constrain vegetation establishment. This PhD Thesis contributes to develop the scientific bases for designing such strategies. The general objective was to study the spatial patterns and processes that drive species diversification, either by planting seedlings or by natural regeneration, of Mediterranean pine plantations in response to various ecological and silvicultural factors. The general hypothesis was that the colonization of mid- and late-successional woody species in the understory of Mediterranean pine plantations depends on the interaction between the environmental conditions and the functional attributes of plant species, such as drought tolerance, rooting capacity and dispersal strategy. Consequently, we think that by silvicultural practices it is possible to manipulate the conditions that enhance understory diversification, either by planting or by promoting natural regeneration. The PhD Thesis has six chapters. Chapter 1 establishes the conceptual framework and presents the background of the main factors that can influence active and passive restoration of Mediterranean pine plantations. The following three chapters (Chapters 2-4) contain research results. Chapter 5 integrates and discusses in a transversal approach the main findings in the context of the diversification of Mediterranean pine plantations. Some results not presented in the research chapters are included in this chapter and new future lines of research are highlighted. Finally, Chapter 6 summarizes the main conclusions of the Thesis. In Chapter 2, we quantify the effect of the microtopography legated from the soil preparation done to plant the pine stand, with a succession of mounds, furrows and undisturbed areas, the type of shrub in the understory and the adult pine trees on the abundance and spatial structure of the mid- and late-successional trees and shrubs. We show that the areas undisturbed by soil preparation had higher regeneration than mounds and furrows. Recruitment was generally higher under shrubs than in the understory gaps, suggesting facilitation by the shrubs. This effect was especially noticeable under the large Juniperus oxycedrus that were nucleation points, mainly of oak juveniles. Mature pine trees also had an aggregation effect, especially of synzoochorous (Quercus ilex and Quercus faginea), and eventually some endozoochory species (Rosa sp.). In contrast, pines did not affect the spatial distribution of other endozoochorous plants. Despite the apparently homogeneity of pine plantations, several structural elements drive the spatial distribution and abundance of mid- and late-successional woody species. Chapter 3 is an experiment in which we measured the rooting capacity in rhizotrons of four late and intermediate successional broadleaf species (Acer monspessulanum, Quercus pyrenaica, Sorbus torminalis and Q. ilex) in response to moderate and low light levels. These light levels mimicked the light in the understory in two thinning treatments tested in a 50-year-old Pinus pinaster stand. Root dynamics and structure depended more on species functional characteristics than on light increase. Acer monspessulanum and Q. pyrenaica showed the highest root elongation rate and rooting depth. The results of this chapter can help to select species for underplanting pine plantations, based on root development in edaphic environments presumably highly competitive due to the pine presence. In Chapter 4, we assessed survival, growth and physiological performance of the same tree species studied in Chapter 3 in response to the reduction in pine canopy density after applying different thinning treatments: control (unthinned); moderate thinning from bellow with a 20% reduction in basal area (BA); heavy thinning from bellow with a 35% reduction in BA; and crown thinning of crop (candidate) trees with a 35% in BA. Reduction of canopy density increased precipitation percolation through the canopy, soil moisture and light in the understory, as well as the survival of all species except of A. monspessulanum, which was unaffected, and growth, especially of S. torminalis. Variations in survival and growth were related to low water stress during the driest periods of summer, especially in moderately thinned stands. In summary, thinning reduced the negative effects of dry shade as a main constraint for the establishment in the understory of Mediterranean pine plantations of either planted or naturally regenerated late-successional tree species. Moderate thinning had a slightly higher improvement effect as it maintained a lower pine cover, which reduces competition and soil evaporation. In conclusion, this Thesis demonstrates the importance of the structure of the canopy, the understory and the microtopography of P. pinaster plantations in the natural and artificial regeneration of mid- and late- successional trees and shrubs. The influence of these factors depends largely on the functional characteristics of plant species. The legacy microtopography from soil preparation and the facilitative capacity of some pioneer shrubs could be considered in the design of active diversification and restoration strategies as suitable microsites for planting species in the understory. Likewise, given the facilitation ability of some pioneer shrubs, their conservation in clearing treatments is important. Finally, the results related to thinning treatments allow establishing guidelines for the adaptive management of artificial plantations in the context of climate change.