Changes in forest structure and tree recruitment in Argentinean Chaco: effects of fragment size and landscape forest cover.

摘要

Subtropical Argentinean Chaco forests have been severely deforested and fragmented due to agriculture during the last six decades. The most affected forests are located in areas that are most favorable for crops. This is the case of the semi-deciduous "bosque de tres quebrachos" ("three quebrachos forest") in southwest of Chaco Province (Argentina), where this study was carried out. The aim of this work was to analyze the fragmentation process, considering spatial (fragments and landscape levels) and temporal (land use changes) scales, studying the effects on the forest structure (density and basal area) and tree recruitment. All trees (adults and saplings) were recorded and measured in 112 plots (400 m2) distributed in 28 forest fragments (0.9-160 ha) located at four different landscape samples (4500 ha) in one environmentally homogeneous zone (72,804 ha). The four landscape samples have different current forest cover (low/high) and different land use history (low/high). Using linear mixed models, we evaluated the effects of current and historic fragment size, landscape forest cover and land use changes on the density and basal area at stand level, and considering the two different strata (upper and middle). Results showed that fragmentation did not severely affect the structure of the older age classes of trees in the remaining fragments but affect the tree recruitment, which could influence over the ability of the forest to perpetuate itself. We found positive relationships between sapling density and fragment size for the tree species of middle stratum, and between sapling density and forest cover at landscape level for the species of upper stratum. Regarding adults, we found a negative relationship between density and historic fragment size, probably related to the past timber harvesting. We concluded that even small fragments of three quebrachos forest and those located in highly deforested landscapes have high conservation value since their structure does not differ from that of the larger fragments or landscapes with higher forest cover. Our results reveal the importance of considering landscape and fragment scales simultaneously to better understand the fragmentation process and improve the recommendations for the management of fragmented landscapes.