Revegetación de campos agrícolas abandonados en ambientes mediterráneos continentales

Abstract

This PhD thesis investigates processes and techniques to understand and improve, respectively, the revegetation of abandoned croplands in Mediterranean continental environments. Specific aims were: a) to study the mechanisms that explain why large and high nitrogen (N) concentration seedlings (productive phenotypes) establish better than seedlings with the opposite traits (xeromorphic phenotypes) in Mediterranean reforestations; b) to analyse the interaction between seedling functional traits and environmental stress; c) to evaluate the use of Retama sphaerocarpa as nurse shrub in revegetation projects, analysing both direct and indirect facilitation of Quercus ilex seedlings; d) to examine how the interruption of the management applied during seedling establishment affects future performance of Q. ilex individuals; and e) to identify the effects of revegetation on soil chemical properties. We carried out four experiments and one observational study in the centre of Spain to examine: 1) survival, growth and physiological response of Q. ilex and Pinus halepensis Mill. seedlings of different size and N concentration planted in two contrasting water stressed environments; 2) effects of seedling size and N concentration on root growth and its interference with aerial elongation in P. halepensis seedlings; 3) R. sphaerocarpa and herb effects on survival, growth and physiological response of Q. ilex seedlings; 4) survival, growth and acorn production in a 14 year-old Q. ilex individuals, to compare direct effects of previous management (artificial shading and summer irrigation) and indirect effects through intraespecific competition and resource allocation to plant basic functions; and 5) soil variation in pH, macronutrient concentration, and nitrification and ammonification rates in contrasting scenarios of passive and active restoration. Large seedlings of P. halepensis with high N concentration had higher survival and growth than small seedlings, independently of tissue N concentration. This was attributed to higher root growth, gas exchange and N use in large seedlings compared to small seedlings. Size effect on seedling establishment was higher than N concentration effect. Root and shoot growth interfered in P. halepensis, but only in smaller seedlings. In Q. ilex, there were no establishment and physiological differences among seedling phenotypes or they were marginal. Retama sphaerocarpa facilitated Q. ilex, but the relative importance of the facilitation mechanism varied with the climatic conditions of the studied years. Indirect facilitation by release of herb competition under nurse shrubs occurred in the year with dry spring. By contrast, direct facilitation mediated by microclimate amelioration occurred in the year with humid spring and dry summer. Q. ilex seedlings responded differently to facilitation, since small seedlings but not large seedlings were benefited. Current growth and acorn production in 14 year-old Q. ilex individuals was, in part, determined by previous environment (initial management), due to both direct effects of management and indirect effects through intraespecific competition and acorn production. There were no trade-offs in resource allocation either between root and shoot growth or between growth and survival, but there was a trade-off between growth and acorn production that was only noticeable in individuals with canopy higher than 2 m3. Active restoration through Q. ilex plantation and passive restoration had low and similar effects on soil chemical properties after 13 years. Actively restored plots had higher ammonium concentration and mineral N availability than passively restored plots. The results of this PhD thesis improve our understanding of processes related to tree establishment in revegetation projects, which can be important for abandoned cropland restoration in Mediterranean environments and other dry systems of the world.