Long-term ecological trends of small secondary forests of the atlantic forest hotspot: A 30-year study case

https://doi.org/10.1016/j.foreco.2021.119043Get rights and content

Highlights

  • We monitored a small forest fragment in the Atlantic Forest hotspot for 30 years.

  • Long-term trends on trees biomass, density and functional composition were evaluated.

  • We found structural and compositional changes towards advanced sucessional stages.

  • Long-term trends were marked by forest biomass increase due to full protection.

  • Small fragments play important role in carbon uptake and biodiversity protection.

Abstract

Forest dynamics are driven by a myriad of factors and link directly to ecosystem services. The dynamics of the highly degraded Brazilian Atlantic Forest (AF) hotspot is mostly driven by its anthropogenic context. Currently, most of the AF region is composed of small disturbed secondary forest fragments that have received partial or full environmental protection with recent environmental norms. Such protection prevents further disturbances and consequently shifts the course of forest dynamics and ecosystem services in AF. Here, we assess long-term forest dynamics trends of a small secondary AF fragment to test the hypothesis that disturbed AF fragments are reaching advanced successional stages. We used a unique dataset of 126 permanent subplots (totaling 5.04 ha) monitored for 30 years (8 inventories between 1987 and 2017) of a disturbed secondary AF fragment (6.35 ha) that has been fully protected since 1986. In each inventory, we measured all living trees with a diameter at breast height (DBH) ≥ 5 cm and counted survivors, recruits and dead individuals between intervals. We monitored the temporal trends of (i) structural variables (biomass, tree density and species richness), (ii) functional composition (wood density and maximum potential size), (iii) and species dominance. Our results indicate that, now under full protection, the fragment is shifting towards a late successional stage: we observed increasing biomass, decreasing tree density (self-thinning) and increasing abundance of late-successional species with conservative strategies. Species richness and dominant species composition were stable throughout the monitoring period, suggesting an absence of substantial changes in community assembly. These results underscore the role played by small secondary AF fragments in ecosystem service provision (e.g., carbon uptake and shelter for biodiversity) and point to their forest dynamics trends.

Introduction

Forest dynamics results from the combination of tree demographic processes, environmental factors (e.g., climate, soil, topography), transient disturbances of different spatial scales and origins (e.g., falling trees, fire, logging) and biotic interactions (Lewis et al., 2009, McDowell et al., 2020, Seidl et al., 2017). The importance of these different drivers on forest dynamics may vary depending on local and regional ecological contexts, disturbance regime and ecosystem responses to changes in these factors (McDowell et al., 2020, Seidl et al., 2017). In Brazil, the diversity of biogeographic regions and vegetation types have historically experienced different types of disturbances and are, naturally, inserted in different ecological contexts (Ab’Saber, 2003). Therefore, the importance of environmental (climate, soil, fire) and anthropogenic (deforestation, fragmentation) factors on forest dynamics varies considerably across biogeographic regions, requiring a more specific approach to understand and analyze their patterns (Bueno et al., 2018, Eisenlohr et al., 2015, Esquivel-Muelbert et al., 2019, Maia et al., 2020, Neves et al., 2017). In heavily human-influenced regions, such as the Atlantic Forest hotspot in eastern Brazil, anthropogenic factors interact with macroscale effects (e.g., precipitation, temperature and atmospheric CO2; known to drive the dynamics of less degraded regions, such as the Amazon) and produce complex forest dynamics patterns (Esquivel-Muelbert et al., 2019, Hubau et al., 2020, Souza and Longhi, 2019, Sullivan et al., 2020).

The dynamics of the Atlantic Forest region of Brazil is strongly influenced by recent disturbance histories (Cirne-Silva et al., 2020, Dalmaso et al., 2020, Oliveira-Filho et al., 1997, Souza and Longhi, 2019). Although these forests had been occupied by indigenous peoples since pre-Columbian times, it was after European settlement in the 16th century that the Atlantic Forest began to face its most severe degradation: today, after successive economic cycles that took place in the region, only 12.4% of its original extent remains (Cruz et al., 2020, Joly et al., 2014, Ribeiro et al., 2009; SOS Mata Atlântica and INPE, 2019). The remaining Atlantic Forest cover is mainly composed of small secondary forest fragments with low connectivity inserted in agricultural landscapes, vulnerable to edge effects and specific anthropogenic disturbances, such as selective logging, livestock trampling and pesticides (Joly et al., 2014, Rezende et al., 2018, Ribeiro et al., 2009). As a result, land-use and disturbance histories mediate climate and soil effects on the dynamics and successional pathways of these forests (Cirne-Silva et al., 2020, Dalmaso et al., 2020, Santos et al., 2018, Souza and Longhi, 2019).

Due to the highly degraded status of its forests, in the last decades, public policies have aimed to protect the remaining Atlantic Forest vegetation; for example, the “Atlantic Forest Law” (Brasil, 2006) deals with the use and protection of native vegetation in the region, while the Forest Code establishes limits for the conservation of Atlantic Forest remnants in rural properties (Brazil, 2012). Due to these legal restrictions, remnants already regarded as legal reserves or permanent preservation areas were granted partial conservation, suffering occasional anthropogenic disturbances from the surrounding agricultural and fragmentation contexts (Arroyo-Rodriguez et al., 2017, Joly et al., 2014, Ribeiro et al., 2009). These forest fragments have since undergone ecological changes resulting from their partial or full protection, as shown by medium- and long-term studies in the region (10–20 years of monitoring). For example, structural variables and, in some cases, species composition trend towards stability, while biomass and, in some cases, tree mortality, tend to increase (Cirne-Silva et al., 2020, Rocha et al., 2020, Santos et al., 2018, Villanova et al., 2019). However, longer-term observations are lacking from the literature, which could allow the assessment of the consistency of these patterns already observed in partially conserved fragments. This type of investigation could hint on future successional pathways of more recently protected fragments.

In addition, most studies are limited to structural attributes, ignoring community functional trends, which are key for comparatively assessing ecosystem function among communities from different regions (Esquivel-Muelbert et al., 2019, Feeley et al., 2020, Poorter et al., 2019). Besides, post-disturbance successional trajectories are expected to vary across the different vegetation types in the Atlantic Forest, which range from rainforests to semideciduous and deciduous seasonal forests (Joly et al., 2014, Neves et al., 2017). The successional pathways experienced throughout this vegetation gradient reflect the main local ecological drivers and result in specific temporal shifts in functional composition of early- and late-successional species (e.g., acquisitive or conservative strategies) that establish in these forests (Connell and Slatyer, 1977, Dalmaso et al., 2020, Poorter et al., 2019). For example, early-successional species in rainforests of the region tend to display acquisitive traits and late-successional species, conservative traits; at the same time, dry forests species display opposing strategies (i.e., early-successional conservative species and late-successional acquisitive species) (Chave et al., 2009, Poorter et al., 2019). However, further studies are needed to confirm these patterns, in addition to clarifying the successional trajectories of other vegetation types such as semideciduous forests, which are subject to climatic seasonality and present deciduousness (Neves et al., 2017), but it has high carbon stocks and productivity in relation to dry forests (Maia et al. 2020).

The dynamics trends of secondary forests in the Atlantic Forest region also allows assessment of essential ecosystem service provision in a region that encompasses 17 Brazilian states, 72% of the Brazilian population and 70% of the gross domestic product (Joly et al., 2014; SOS Mata Atlântica and INPE, 2019). These fragments provide essential ecosystem services such as water quality maintenance, air and climate quality control, carbon storage and biodiversity maintenance (Arroyo-Rodriguez et al., 2017, Joly et al., 2014, Maia et al., 2020, Matos et al., 2020). The exceptionally high biodiversity and endemism found in the Atlantic Forest, combined with its highly degraded state, have placed it among the global hotspots for biodiversity conservation and the priority areas for restoration (Joly et al., 2014, Matos et al., 2020, Myers et al., 2000, Mittermeier et al., 2004, Strassburg et al., 2020).

Here, we provide a detailed study case of a semideciduous forest fragment in the Atlantic Forest (AF) region of Brazil. We draw from 30 years of observations (8 inventories between 1987 and 2017) of a fully protected fragment, covering almost 80% of its total extent. The current situation of our study site reflects that of most AF remnants, which have suffered past disturbances and, today, benefit from partial or full legal protection (Brasil, 2006, Brazil, 2012). In this study case, we investigate the fragment’s long-term trends of tree community structure (density, biomass and species richness), functional composition (wood density and potential tree size) and ecological dominance. Our two main goals were: (i) to infer temporal trends of AF fragments in advanced successional stages and (ii) to evaluate whether these trends are driven by local soil conditions and edge effects. We hypothesize that: (i) the community is trending towards an advanced successional stage and (ii) these trends are influenced by local soil conditions and edge effects. Specifically, we expect to find an overall increase in biomass and decrease in tree density (McDowell et al., 2020, Oldeman, 1983), an increase in the abundance of late-successional species with conservative strategies (e.g., higher wood density and larger potential tree size) (Connell and Slatyer, 1977; Poorter et al., 2019) and, consequently, significant changes in dominant species composition. We discuss these trends within two contexts: global and local climate change effects, for their influences on vegetation dynamics (McDowell et al., 2020). We also discuss the role of these small secondary fragments for the provision of essential ecosystem services within the region’s landscape context, such as carbon storage and shelter for biodiversity.

Section snippets

Study area

The study area is a forest fragment of 6.35 ha located in the Federal University of Lavras campus (UFLA), in the municipality of Lavras, Minas Gerais, Brazil (Fig. 1). The relief is flat and the relative slope angles range from 5% to 15%, with an average altitude of 925 m a.s.l (Junqueira Jr. et al., 2017). Vegetation in the fragment is classified as Atlantic montane seasonal semideciduous forest (Oliveira-Filho et al., 1997). Massive deforestation began in the second half of the 19th century

General community temporal trends

We observed different trends for the vegetation variables throughout the time series (the average values of each year of measurement are in Table S3). We observed a significant temporal trend of increasing biomass and decreasing tree density in the community, implying a significant increase in the average biomass per tree (Fig. 3; Table 1); i.e., a smaller number of larger trees began to characterize the community. The temporal increase in biomass occurred mainly in the larger diameter classes,

Long-term community trends towards an advanced successional stage

Our results confirmed our hypothesis that the study site is trending towards an advanced successional stage. These unique long-term results may also reflect the conservation status and ecosystem service provision of other secondary forests in the Atlantic Forest (AF) region of Brazil. We found a trend of increasing biomass and decreasing tree density in the site, leading to a scenario of increasing average tree size over the 30 years of monitoring. We also observed long-term changes in

CRediT authorship contribution statement

Cléber R. Souza: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft. Vinicius A. Maia: Data curation, Investigation, Methodology, Writing - review & editing. Natália Aguiar-Campos: Data curation, Investigation, Writing - review & editing. Alisson B.M. Santos: Data curation, Investigation, Writing - review & editing. André Ferreira Rodrigues: Data curation, Writing - review & editing. Camila L. Farrapo: Data curation, Validation, Writing -

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

To the Federal University of Lavras (UFLA), the State of Minas Gerais Research Foundation (FAPEMIG), the National Council for Scientific and Technological Development (CNPq) and the Coordination for the Improvement of Higher Education Personnel (CAPES) for all the support.

Funding sources.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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