Above ground biomass variability related to stand structure heterogeneity in agroforestry systems in Portugal

Abstract

Three forest species, cork oak (Quercus suber), holm oak (Quercus rotundifolia) and umbrella pine (for Pinus pinea), are the most representative species of the agroforestry systems in Portugal. These forest systems are characterised by low densities, large spatial variability, and have as main productions are bark and fruit for cork oak and fruit for holm oak and umbrella pine. The stand structure is diverse with pure, mixed, even aged and uneven aged stands, which in turn is also reflected in the above ground biomass. To tackle the variability in these stands, stand structure was analysed with absolute stand density measures and structure and diversity measures and indices for the number of trees and basal and above ground biomass. The results highlighted the differences between the different stand structures. Moreover, the analysis of variance revealed significant differences between the structure and diversity indices calculated with the number of trees or basal area and aboveground biomass. This could be related to above ground biomass incorporating both diameter at breast height and total height, thus integrating the horizontal and the vertical distribution, conversely to the number of trees and basal area that evaluate only the horizontal distribution. Overall, the evaluation of stand structure with above ground biomass can give further insights the variability of stand structure, including diversity, in a three dimensional frame, which can also encompass above ground biomass partitioning per component.