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A. Gil-Tena et al. 2010. Disentangling recent changes in forest bird ranges in Mediterranean forests (NE Spain)
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perimeters. Fires were assessed by means of the accumulated amount of burnt forest area
(Burnt) during the first and second edition of the CBBA. Furthermore, to determine whether the
initial conditions of forest influence bird range changes, we also computed the initial state of
both basal and forest area (G and Forest, respectively).
2.3 Analysis
We performed Pearson’s correlations between the variation of forest bird species richness and
the initial forest conditions (Forest and G), the variation of basal area (ΔG) and forest area
(ΔForest), burnt forest (Burnt) and the removed basal area in improvement and regeneration
treatments (IMP and REG Removed G, respectively). Excluding the case of variation of forest
area that in Catalonia is mainly associated with afforestation in formerly cultivated areas, we
computed partial correlations controlling for the effect of initial forest conditions since changes
in forest structure due to forest maturation, fires and forest management may be strongly
influenced by them. In the case of partial correlations of the initial forest conditions, for forest
area we considered the influence that may have the initial basal area and vice versa. We also
calculated a multivariate general linear model for assessing forest bird species richness variation
as a function of the former independent variables. We used a hierarchical modeling approach
(two steps; see also Gil-Tena et al. (2009, 2010)) to progressively consider initial forest
conditions (Step 1) and forest dynamics (Step 2). Significant variables at Step 1 were introduced
in Step 2 and their contribution to the model evaluated by means of standardized partial
regression coefficients (β). In each step, we used a forward stepwise selection process (p-toenter=0.05).
3. Result
The variation of forest bird species richness during the period between Atlases was positive for
much of the study area covered by forests (59% of UTMs covered by forests increased species
richness) whereas the negative values mainly corresponded with poorly forested areas (Figure 1).
Agreeing with this, initial forest conditions were significantly related to the variation of forest
bird species richness (Table 2). In the case of initial forest area (Forest), Pearson’s correlation
was significantly different from partial correlation controlling by the effect of initial basal area
(G; p=0.005). This agrees with the positive correlations between variation of species richness
and initial basal area (Table 2) and with the fact that the difference between Pearson’s and
partial correlations was lower than for initial forest (p=0.026), indicating that the variation of
species richness was mainly associated with forests with a certain structural development (in
terms of G).
Variation of species richness was also associated with forest dynamics, particularly with
variation of basal area (ΔG). According to the difference between Pearson’s and partial
correlations when controlling for the effect of initial basal area (p=0.007), the maturation in
previous forest areas with a developed structure strongly influenced species richness variation.
Afforestation, in terms of increment of forest area, was also positively related with species
richness variation but to a lesser extent than maturation (Table 2). Burnt forest area showed a
non-significant positive correlation with species richness variation that only turned significant
when considering initial forest conditions (Table 2). Regarding forest management influence on
bird species variation, sylvicultural treatments seem not to preclude species richness variation,
which is particularly true for regeneration treatments (Table 2). These results were quite
independent of the initial forest conditions (in all the cases, p>0.05 when comparing Pearson’s
and partial correlations).
The results obtained in the multivariate model (R 2 = 0.24; Table 3) confirmed the importance, for
species richness variation, of the maturation produced in forests with a developed structure (in
terms of G). Afforestation also seems to be favoring species richness variation but changes in
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.