Influencia de factores ecológicos y selvícolas en el crecimiento diametral de la encina (Quercus ilex ssp. Ballota (Desf.) Samp.) en el suroeste de España

Abstract

Measurements of tree growth provide information about overall tree health and vigor of forests, phenological processes, effects of climate, physiology and water status, competition, resource allocation, and the influence of management practices. Holm oak (Quercus Hex ssp. ballota (Desf.) Samp.) is the most widespread species in the Iberian peninsula, being one of the most representative trees in forests and open woodland forests. In this dissertation the intra-annual stem growth and the daily stem radius variations of holm oak were analysed in 128 trees with band dendrometers, and in 9 trees with high-resolution electronic point dendrometers, located in three open woodland forests and one mixed Mediterranean forest of SW Spain. The study comprised a period of nine and a half years, from May 2003 to October 2012, including years with contrasting climatic conditions, especially in precipitations. Apart from the intra-annual patterns of stem growth and the daily cycles of stem radius variations, the influence of climatic variables and the effects of tree size, hydrological parameters and competence on the variability of stem growth between trees have been assessed in this work. Moreover, the existence of trade-offs in the allocation of resources between stem growth and acorn production at individual tree level has been studied. Finally, the effects of traditional management practices as soil treatments (ploughing, fertilization and sowing with a N2 fixation legume plant) and pruning on stem growth have been analyzed. Results showed that the Mediterranean climate is the main driving force of the stem growth of holm oak, provoking a characteristic intra-annual pattern with two main periods of growth. The first one occurred during late winter and spring, and the second one during late summer and autumn. Between these two growth periods there were two periods of low growth rates or growth cessation, that occurred in late spring and summer, when water stress increase, and in late autumn and winter, when temperatures decreased. Intra¬annual growth patterns varied significantly between years because the high variability of Mediterranean climate, especially in relation to precipitations. The main climate variables that affected stem growth were those related with tree water status. Precipitation, soil moisture and relative humidity were positively correlated to stem growth, but reference evapotranspiration and solar radiation had a significant negative effect on stem growth. Temperatures were negatively correlated to stem growth, probably because its negative influence in relation to increasing transpiration and water losses, despite low temperatures decreased growth rates in late autumn and winter. All trees generally followed the same climatic signal and were highly synchronized, but there was also a high individual variability in growth rates that was not explained by climate. Tree size, represented by circumference at breast height, and some competition indexes explained part of that individual variability, indicating a competition for the scarce resources in open woodlands and Mediterranean forests of holm oak. Acorn production had a significant negative effect on stem growth at individual level, during the time when acorns fattened, i.e., late summer and autumn, and occurred only in years of high acorn production or mast years. These results showed the existence of a trade-off between stem growth and reproduction in holm oak, that explained part of the individual variability in growth rates. The soil treatments did not influence significantly the stem growth, indicating a scarce response of holm oak in our study site to shrubs competition, soil compaction and water infiltration, and soil nutrient improvement. Intense pruning treatments had a significant negative effect on stem growth, especially in trees located on poor soils and under other stress factors, such as oak decline, increasing the effect of water stress on stem growth. This negative influence was significantly lesser in trees with good sanitary status located on better soils, despite intense pruning decreased slightly stem growth rates in spring. Therefore, intense pruning could affect the vigor and vegetative status of trees in areas where tree survival is already compromised.