This study has provided relevant insights into the successional trajectory effects of fragmentation on the tree community of a small forest remnant surrounded by human-modified landscapes. Under 17 years of community monitoring, the dynamics' analysis revealed a continuous reduction in the overall tree abundance and shifts in trait composition favouring adult species with resource-acquisitive, pioneer life-strategies (reduction in tree maximum height and in species with simple leaves and a slight increase in the percentage of small seeds). However, for the adult tree community (DBH > 10 cm), we found a significant increase in wood density. Indeed, the last monitoring interval (2011–2017) showed a strong increase in total basal area and a significant increase in aboveground biomass (reaching higher values than found for the first survey). Based on these findings, our hypothesis was partly accepted, as natural succession is being negatively affected, except for aboveground biomass.
Other studies in Neotropical small forests have shown tree communities dominated by a small set generalist species in forest fragments with one to 10 hectares (Laurance 2001; Laurance et al. 2006b). After analyzing the long-term dynamics of a forest fragment with 4 ha, we found a decrease in Hmax and simple leaves for the adult tree community and a decrease in simple leaves for the overall tree community. During succession, pioneer species are expected to be replaced by shade-tolerant ones, with the community gradually accumulating species diversity and ecological functions (Guariguata and Ostertag 2001). This trajectory is changed when strong environmental filters lead to the selection of species that share the adaptation strategies required to colonize and survive in a changed post-disturbance habitat (Tabarelli et al. 2008).
In disturbed habitats, the establishment of late-successional species is compromised until the stressful conditions had reduced to an acceptable state (Lebrija-Trejos et al. 2010). Edge creation, e.g., promotes microclimatic and structural changes within the forest with severe consequences to tree communities (Murcia 1995), such as desiccation which act as a climate filter favouring mostly generalist pioneer species (Pierce et al. 2017). Leaf type reflects adaptations to reduced water availability and therefore a gradual increase in species with simple leaves is expected in the course of succession towards more mature and undisturbed forests (Wright et al. 2017; Gei et al. 2018). Consequently, we found a gradual increase of trees with compound leaves, a strategy to prevent the excessive water loss as plants with pinnate or bipinnate leaves can release individual leaflets (and not complete leaves) during severe water stress, increasing heat dissipation and regulating temperature (Wright et al. 2017; Gei et al. 2018).
When considering the adult tree community, we also found a decrease in species Hmax. The reduction of adult trees’ Hmax is also expected in communities under disturbance (Carreño-Rocabado et al. 2012). The tree canopy damage allows trees to get more access to light, which no longer consists of a limiting-resource (Ruiz-jaen and Potvin 2011). These results suggest the presence of strong post-fragmentation edge effects, leading to an increase in pioneer species while shade-tolerant species are negatively impacted (Zambrano et al. 2020). As a consequence, adult tree species may not invest in increasing their height as much as other life strategies (King et al. 2006). Besides, it is also expected a great loss of large trees due to physiological stress and wind turbulence (Oliveira et al., 2008, also see Laurance et al., 1988, 2000).
The increase in wood density throughout time in the adult tree community can also be related to disturbed habitats as it guarantees trunk resistance to physical damages by herbivores, pathogens and woody debris (Van Gelder et al. 2006; Mccarthy-Neumann and Kobe 2008) and also an indication of hydraulic safety (Sterck et al. 2006). Previous studies have shown the lack of wood density variation with fragmentation and fragment forest size, even for secondary forests (Magnago et al. 2014; Liu et al. 2019). However, since wood densities of species can be affected by environmental changes due to fragmentation (Laurance et al. 2006a, b; Nock et al. 2009), the average wood densities we used from the literature (Chave et al. 2009; Zanne et al. 2009) would miss any such variation. Our functional diversity analyses could be refined in future analyses using wood densities derived from in situ measurements, and also, when herbarium collections permit, fruit and seed sizes as continuous rather than categorical variables.
The gain in basal area in the community indicates their species’ high productivity (Finegan et al. 2015). Indeed, the higher basal area is translated in gains in biomass, which showed a significant increase through time. The values for AGB found in this study are comparable to large undisturbed rainforests (Pyles et al. 2018). Previous studies have reported that fragmentation has no significant effect on biomass, with the productivity of the shade-intolerant trees having a high role in the accumulation of biomass (Magnago et al. 2014; Liu et al. 2019).
Proximity to edge and small fragment size favours a turnover with trees typical from late-successional stages being replaced by pioneer and especially the short-lived species (Nascimento and Laurance 2004; Tabarelli et al. 2004; Laurance et al. 2006a; Michalski et al. 2007; Santos et al. 2012). Consequently, forest regeneration potential might be lower in these fragments, with detrimental consequences for tree diversity and regeneration dynamics in the long term. However, the process of forest succession can take many years of centuries, therefore longer monitoring is needed to check whether the conditions found in the studied fragment matches the retrogressive succession hypothesis or is rather a transient, regeneration dynamic in human-modified landscapes that might lead to advanced successional stages (Guariguata and Ostertag 2001; Letcher and Chazdon 2009; Ewers et al. 2016; Muscarella et al. 2016).