Structural heterogeneity and tree spatial patterns in an old-growth deciduous lowland forest in Cantabria, northern Spain

Old-growth deciduous forests in western Europe, for the most part, consist of small tracts that often may be atypical due to human disturbance, poor soil productivity or inaccessibility. In addition, very little information on tree age distributions, structural heterogeneity and tree spatial patterns appears to be available for west-European forests. Characterization of the structural features of tree populations in these old-growth stands can provide the basis to design conservation plans and also inform on how present forests might look in the absence of human interference. Four old-growth stands in a deciduous forest in the Cantabrian lowlands, northern Spain, were surveyed to determine forest structure and spatial patterns. Live and dead trees were identified, measured and mapped, and live trees were cored for age estimation. Structural heterogeneity was analyzed by means of the spatial autocorrelation of tree diameter, height and age, and the uni- and bivariate spatial patterns of trees were analyzed. The dominant species, Fagus sylvatica and Quercus robur, showed reverse-J shaped size distributions but discontinuous age distributions, with maximum ages of 255-270 yr. Tree ages suggested that the forest was largely modified by past changes in forest-use, especially by temporal variation in grazing intensity. Spatial autocorrelation revealed that former parkland stands were heterogeneous with respect to tree height only, while high forest stands were composed of patches of even-aged and even-sized trees. Young trees were clumped at varying distances and establishment occurred preferentially in canopy gaps, except for Ilex aquifolium that mainly occurred beneath mature Quercus trees. Surviving trees became less intensely clumped in the dominant species, and more strongly clumped in understorey ones, which may have been due to the effects of intraspecific competition and of canopy trees on tree survival, respectively. The spatial associations between species varied within the forest, probably as a consequence of specific establishment preferences and competitive interactions.