Abrupt fragmentation thresholds of eight zonal forest types in mainland Spain

Abstract

This study quantifies patchiness of eight types of zonal forests in three biogeographic regions of mainland Spain (Atlantic, Alpine and Mediterranean) which together occupy 1,726,578 ha. Their dominant species and European Habitat Type codes (EU Directive 92/43 EEC) are: Fagus sylvatica (9120, 9130 and 9150), Quercus robur and Q. pyrenaica (9230), Q. suber (9330), Pinus uncinata (9430), P. nigra ssp. salzmannii (9530) and P. pinea (subset of 9540). We applied the Korcak’s exponent B, which describes a hyperbolic relationship between the cumulative frequency of the number of patches and their sizes. The objectives were: 1) detect possible patch size intervals in which B varies significantly, explicitly identifying area thresholds, and 2) contribute to development of a robust forest mass fragmentation indicator. Exponent B was found by segmented regression analysis. The vector data were extracted from a filtered version of the Spanish Forest Map 1:50,000 (1997–2006). After validating the procedure by applying it to a previously published dataset, we found that in all cases the patch size range could be split into two significant intervals around relatively small threshold areas of 27–101 ha. In the one on the left, the rate at which relatively large patches become less abundant was always very slow (B = 0.017–0.094). After this threshold had been passed, the rate increased abruptly (B = 1.100–2.590). Both this high fragmentation and its lack of parsimony were unexpected in zonal forest types. General interpretations converge to the coexistence of forest patches of different ages due to human pressure events.