Clearcutting is defined as anthropogenic disturbance that converts a forest stand to an early successional stage and is known to have potentially important environmental impacts (Smith et al., 1997). Most experimental or comparative studies of the effects of such disturbance have shown that the highest levels of species diversity were maintained at an “intermediate” frequency of intensity of disturbance (Huston, 1994).

The structure and dynamics of laurel forest in the Canary Islands have not been widely studied, and research carried out to date has focused on spatial patterns, seed banks, regeneration and gap dynamics (Ohsawa et al., 1999; Arévalo & Fernández-Palacios, 1998). Today, only 10% of the original laurel forest on the island of Tenerife remains unaltered (Santos, 1990). Study of a laurel forest unmanaged since clearcutting provides a valuable opportunity to learn more about the long-term dynamics of laurel forest, as succession following this type of disturbance may be similar to succession following other types of stand replacement or gap openings.

Because of the lack of information about the impacts of clearcutting in laurel forest, we used a long-term data set to test the following hypotheses: i) Erica arborea and Morella faya act as pioneer species and their dominance in terms of both density and biomass decrease throughout succession, and ii) 50 years after clearcutting, the stand has recovered the status of laurel forest as indicated by the species composition and richness. Because the present study was designed to evaluate the effects of clearcutting in a laurel forest, we restricted the study to the only available plot with this site history and with characteristics representative of laurel forest.

Six tree species have been identified in the plot in the past 20 years. The AGB increased twofold, while the BA increased by 61% (Table 1). After 1993 (40 years after clearcutting), the dominant species, in terms of tree density, was L. novocanariensis, followed by E. arborea. In the same period, the dominant species based on AGB was M. faya, followed by L. novocanariensis, and the same pattern was observed for basal area (Table 1). At present (i.e. 50 years after clear-cutting) the dominant species are P. indica in terms of DBH, M. faya in terms of AGB, and L. novocanariensis in terms of density. During the last 20 years, 56% of the trees have died and there has been no new recruitment (DBH>4cm) of the tree species growing in the plot. The Shannon index (expressed as e^H’) has decreased greatly over the years, especially in the last sampling period, when calculated on the basis of both density (8.13, 8.15, 8.14, 5.98 for 1993, 1995, 1997 and 2013 respectively) and biomass (6.81, 6.86, 6.60, 5.36 for 1993, 1995, 1997 and 2013 respectively). This decrease in biological diversity was mainly caused by the disappearance of some species (i.e., I. canariensis and E. arborea) and the increasing dominance of other species (i.e., P. indica and M. faya). Morella faya has remained the most abundant species throughout all the sampling periods (based on basal area, AGB and density; Table 1) and is one of the tallest trees. Thus, in a period of only 20 years the pioneer species have largely been replaced by late successional species, thus indicating that the stand has continued to change in species composition.

Throughout the last 20 years, the DBH of the late successional species (L. novocanariensis, P. indica) and some intermediate successional specis (M. faya and I. perado) has increased, although the dominant species has always been M. faya, followed by P. indica. The same species always yielded the tallest trees over the years. Crown height has been found to be more variable, and although the dominant species based on top height were initially I. canariensis and I.perado, these have been replaced by L. novocanariensis and P. indica (Table 1). Therefore, M. faya should be considered a relatively long-lived pioneer species, because it was present during the early stages of the sampling, but still remains in this later successional stage after 50 years, and was not replaced during the intermediate phases of the succession. Also, we did not find regeneration for these species in the last sampled periods.

The probability of survival of the trees during the period considered also varied among species: I. canariensis 0%; E. arborea 4%; I. perado 43%; L. novocanariensis 50%; P. indica 75% and M. faya 76%., The number of trees dying during the period 1993-95 was very low (Figure 1, probability of survival ranged between 98 and 100%), while during the period 1995-97 the mortality increased, especially in individuals of E. arborea (probability of survival, 72%, which corresponds to a mortality rate of 142 ‰) and those of the genus Ilex (probability of survival ca. 85%). The survival rates of other species were higher than 90% (Figure 1). Furthermore, the probability of survival is not the same for all individuals of the same species, but rather increases with the initial size of the tree (Figure 1). In addition to the structural characteristics of the species, we have revealed changes in species composition throughout the sampling period.

The change in species composition followed similar patterns based on the importance value (Figure 2). We identified E. arboreaand I. canariensis as dominant species after 40 years of natural stand dynamics (displaced in the bidimensional space of the DCA as an important species for 1993) and P. indica and L. novocanariensis as dominant at a later successional stage (50 years). The other species were more or less intermediate in terms of successional changes. Species turnover appears to have been governed by competition. The ability of some species to alter the allometric relationship between DBH and tree height enables them to obtain light in the upper layers of the canopy. On the other hand, some species (i.e. E.arborea and I.canariensis) reached the canopy level and then died and were replaced by characteristic late successional species such as P. indica and L. novocanariensis.

Our results reveal that an intermediate successional status of mature laurel forest has been reached (compared with natural laurel forest; Arévalo & Fernández-Palacios, 1998) as a result of some favorable conditions: the low intensity of disturbance (spatial and temporal); a well-conserved (although not adjacent) surrounding matrix of laurel forest (beyond the limits of the exotic plantions) that characteristically (Carvalho, 2002) provides a constant source of propagules from bird dispersion (Arévalo & Fernandez-Palacios, 2005); and maintenance of species richness due to vegetative regrowth, which is also characteristic of the tree species that comprise laurel forest.