Fragmentation as a key driver of tree community dynamics in mixed subtropical evergreen forests in Southern Brazil
Introduction
At present, human activity is a major force driving planetary change. For this reason, it has been proposed that the present era be named the Anthropocene (Smith and Zeder, 2013, Malhi et al., 2014). Human activity has resulted in the extensive loss of natural areas and the fragmentation of forest ecosystems. Forests are of global importance in that they maintain the climate, biogeochemical cycles, and biodiversity, among other environmental factors. Therefore, studies focusing on how human activities interact with environmental factors and their effects on forest function and dynamics are considered highly relevant (Wright, 2005, Malhi et al., 2015, Uriarte et al., 2016).
Permanent-plot monitoring is regarded as one of the best approaches to investigate spatiotemporal changes in the tree components of forest ecosystems. In tropical forests, studies of this nature have been conducted in areas exposed to different levels of disturbance. Such areas range from ancient sites (Phillips et al., 2004, Malhi et al., 2015) to sites at different stages of post-disturbance recovery (Chazdon et al., 2005, Aryal et al., 2014, Laurance et al., 2014). Research on the functional ecology of various tree species (Condit et al., 1996, Pessoa and Araujo, 2014, Visser et al., 2016) has indicated that, over time and space, floristic and structural changes of the tree components occur in response to relative differences in the performance of species within a given environment. Tree performance may be expressed in terms of demography (mortality, recruitment, growth, and other criteria).
Studies in tropical regions have shown that tree component dynamics and primary productivity are influenced by the interactions of climate, soil, disturbance, and landscape (Chazdon et al., 2005, Malhi et al., 2015, Uriarte et al., 2016, Wagner et al., 2016). Uriarte et al. (2016) reported that anthropogenic disturbances and fragmentation increase the vulnerability of forests to extreme climatic effects and delay succession by favoring pioneer species, which tend to have relatively high mortality rates (Van Breugel et al., 2007). Drought negatively influences tree species recruitment, survival, growth (Uriarte et al., 2016), and primary forest productivity (Malhi et al., 2015, Wagner et al., 2016). Drought also increases tree component mortality (Chazdon et al., 2005, Rolim et al., 2005). Thus, in non-stationary and heterogeneous environments, demographic variations in the tree component occur because of complex interactions among the regional species pool, the disturbance regime, the landscape, and climatic conditions (Uriarte et al., 2016).
There is extensive evidence for the impact of climate, soil, landscape, and anthropogenic disturbances on tropical areas. Nevertheless, little is known about the impact of these factors in subtropical regions (Zhou et al., 2013). Therefore, in the present study, we focused on the Brazilian Mixed Subtropical Evergreen Forest, also known as Araucaria Forest, which forms part of the southern portion of the Brazilian Atlantic Forest. This region is considered to be a global biodiversity conservation hotspot.
The canopy of Araucaria Forest is populated by Araucaria angustifolia. This forest is located in a region with no dry season and on landscapes with a strong altitudinal gradient (∼600–1600 m). According to Reis et al. (2014), Araucaria Forest has been subject to chronic disturbances since pre-Columbian colonization. Therefore, the authors considered it a cultural landscape. In the middle of the last century, the natural forest was severely affected by the predatory exploitation of timber species. After logging restrictions were imposed, Araucaria Forest was subject to chronic anthropogenic disturbances, such as the extensive cattle ranching, which is a common occurrence in the region (Sevegnani et al., 2012).
This study aimed to investigate regional variations in the demography of the tree component of Araucaria Forest in response to climate, soils, landscape, forest structure, and chronic anthropogenic disturbances. We tested the hypothesis that forest patch metrics (such as fragment area and proportion of interior area), the presence of cattle, and forest structure (basal area) have greater impacts on the observed tree component dynamics patterns than climate and soil conditions.
Section snippets
Study area
We obtained forest dynamics data for nine areas in Araucaria Forest from the Labdendro/UDESC database (Laboratory of Dendrology and Phytosociology at the State University of Santa Catarina, Brazil). This region is located in Santa Catarina State, Southern Brazil (Fig. 1, Table 1), and is classified as a Mixed Ombrophilous Forest according to the Brazilian vegetation classification system (IBGE, 2012). All forest fragments except for F3 and F5, were inventoried between 2008 and 2012 and
Results
The ordination of the climatic variables indicated that the Araucaria Forest areas in this study are distributed along a climatic gradient that was expressively synthesized by PCA Axis 1 (Fig. 2b). This axis explained 85% of the total observed inertia and was the only significant one (Fig. 2a). The variables that were most highly correlated with this axis were the maximum temperature of the warmest month (mtwm, loading = 0.24), the mean temperature of the warmest quarter (mtwq, loading = 0.24),
Discussion
At the regional level, the dynamics of the Araucaria Forest areas varied mainly in terms of mortality and basal area loss rates, which were mostly influenced by landscape patch metrics. Small sized fragments with smaller interior areas had the highest rates of mortality and basal area loss. The high values for these rates suggest instability, which is typical of areas in a phase of disruption (Machado and Oliveira Filho, 2010, Mews et al., 2011, Silva et al., 2011) or high turnover (Laurance et
Conclusions and considerations for forest management
At a regional scale, anthropic fragmentation had a significant impact on the dynamics patterns observed in the Araucaria Forest. Larger, less edge-dominated forest areas were relatively less susceptible to high mortality and the loss of basal areas. Considering our results, we would recommend (i) the protection of small and large tracts of forest fragments; and (ii) incentives to promote the silviculture of the most relevant timber species, the light demanding Araucaria angustifolia, in the
Acknowledgments
The authors thank Fundo de Apoio à Manutenção e ao Desenvolvimento da Educação Superior of Santa Catarina (FUMDES) for granting a scholarship to AG. We also thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for granting a research productivity scholarship to ACS (Grant No. 309461/2014-6) and PH (Grant No. 456060/2014-6).
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