Growth Trends in Portuguese Forests: An Exploratory Analysis

Abstract

The objective of this chapter is to give a retrospective view of growth trends of Portuguese forests to identify possible causes for the observed trends, and, when possible, to quantify their effects. Growth trends were studied for two of the most important forest tree species in Portugal, maritime pine and eucalyptus, using growth data available from other research projects. Poplar (Populus sp.) is also included in the study. Different sources of data have been used: (1) individual tree stem analysis data (maritime pine and poplar); (2) permanent plots or long-term experiments (eucalypt); (3) forest inventories of small domains (maritime pine). Meteorological information from four climatic stations was also obtained. The permanent plots and continuous forest inventory data were restricted in order to minimise the heterogeneity in the silvicultural treatments present that could induce changes in growth and therefore lead to incorrect assumptions concerning changes in site productivity. Different methodologies were used for the plot growth data and for the stem analysis data. The first were analysed using multivariate linear regression analysis with the objective of testing the significance of adding the year at the start of the growing period andor precipitation during the growing season to a base model expressing stand growth as a function of stand variables such as age, site index or stems per ha. The analysis of the residuals from existing growth models was also used to confirm the trends that could be found in the previous analysis. The stem analysis data from maritime pine, representing a relatively long time series, were analysed by fitting a mean height growth curve and plotting the mean annual differences (residuals from the mean curve) over year. Climate-dominant height relationships were studied by modelling the mean annual differences in growth with the metereological data available, using a stepwise regression procedure. The growth of two generations of poplar trees available was compared by fitting the Richard’s function to the whole data set and then separately to the data corresponding to different generations. These analyses indicate that volume growth has been decreasing during the last two decades both for maritime pine and eucalyptus, although the trends in dominant height growth are positive for maritime pine and negative for eucalyptus. The negative trend seems to be related to the decreasing amounts of precipitation during the growing season (spring and early summer) within the last decades, although the variables used to account for growing season precipitation were not fully able to counter-balance the observed negative trends. The negative trend in dominant height growth of eucalyptus plantations may be related to the occurrence of free growth in eucalyptus. No reason was found for the positive trend in dominant height growth of maritime pine. No significant differences between the growth in the two generations of poplar trees was found, although the effect of spring precipitation in tree dbh growth could be seen graphically.