In this study, we investigated the effects of forest structure on the liana community and the effect of distance to the forest edge in a fragmented tropical forest. We found that changes in forest structure arising from forest fragmentation, particularly canopy PAI, strongly influenced the species composition of lianas. High PAI values could indicate lower light under the canopy (Ma et al. 2021), higher vegetation indices in the understory (Maeda et al. 2022; Supplementary Fig. 1), and more significant structural complexity, so the restricted occurrence of some species in these sites could indicate a lower tolerance to shading (Lomwong et al. 2023).
However, distance from the edge had a direct or indirect influence, mediated by canopy PAI, on the liana community's abundance, biomass, and number of species (α diversity). In contrast, distance from the edge only influenced composition (diversity β).
High diversity and site effect
Even considering the spatial restriction of the TLS coverage, which provided detailed data on the BDFFP fragment’s forest structure, we included a significant fraction of the local diversity. In this study, we evaluated 123 spp. and 49 morphotypes, corresponding to half of the species and 65% of the families recorded throughout the BDFFP, even though the area sampled represents only 4.6% of the entire area inventoried by the Liana project. When we compare the number of species found in our study area, which attained a larger area than Laurance et al. (2001), the number of species was almost double (47.7%) for 2.88 ha in the BDFFP. And for a region 80 km away, at the Ducke Reserve, our study comprises a third (33.1%) of the total lianas species inventoried there (Rocha et al. 2022).
We found a strong site effect during exploring and analyzing the species composition data (Supplementary Information). This effect has already been described for trees, palms, and bats in the BDFFP (Laurance et al. 2017). Before forest fragmentation, the sites located to the East, West, and Central areas of the Conservation Unit already showed differences in species composition and dynamics, and the successional trajectory after forest fragmentation made these parameters even more distinct. This process has been attributed to the different matrices of each site (Laurance et al. 2017). We propose two other possible explanations to understand the divergence in liana composition based on the distribution and dispersal of species by drainage basins and by different topographic characteristics.
The sampling sites are in different hydrographic basins. The drainage of Colosso is via the rivers Preto da Eva and Urubu (Uatumã basin) while for Dimona drainage is to the Cuieiras river (Rio Negro basin) (Amazonas 2019; Costa et al. 2023). In the Ducke Reserve (80 km away from the location of this study), there is also a difference in composition among various taxonomic groups, mainly trees, shrubs, lianas, birds, frogs, and fish. This difference was explained by the distributions of these organisms in the different drainage basins, with rare species or those with a restricted distribution occurring in only one or another drainage basin (Costa et al. 2008), roughly the same basins as the BDFFP.
Another possible explanation raised, but not tested, is the difference in altitude of the terrains, which may consequently be related to a hydrological, soil texture and nutrient gradient (Schietti et al. 2014), with Dimona at lower elevation and Colosso at a higher elevation. For the BDFFP areas, Laurance et al. (2001) found that the abundance of lianas varied with soil edaphic factors, such as fertility, texture, and slope. In the Ducke Reserve, terrain height above nearest drainage (HAND) is a compositional descriptor for two of the most representative liana families: Bignoniaceae and Fabaceae (Schietti et al. 2014; Gerolamo et al. 2022; Rocha et al. 2022).
In the studies mentioned above, Bignonia aequinoctialis (Bignoniaceae) was found at shorter distances from the water table. In our study, around 91% of the B. aequinoctialis occurred in the Dimona reserve. The species Adenocalymma validum (Bignoniaceae) is only present at Colosso and is found at larger distances from HAND. This is also true for Senegalia altiscandens (Fabaceae), which has a high abundance in plots at Colosso as compared to all 43 ha of the BDFFP (Robyn Burnham, unpublished data).
Canopy structure and distance to forest edge affect liana community
Forest edge creation is one of the crucial factors in understanding the structure and composition of the liana assemblage in fragmented tropical forests, with direct and indirect relationships mediated by forest structure altered by forest fragmentation. Certainly, edge creation is one of the transformative factors driving forest structural changes (Maeda et al. 2022), leading to a collapse in tree biomass (Laurance et al. 2017), as well as changes in microclimate patterns found in forest fragments (Camargo and Kapos 1995; Nunes et al. 2022). There is evidence that these transformations are persistent and can be detected even if the process of forest fragmentation occurred decades ago, which is the case in our study area (Almeida et al. 2019; Maeda et al. 2022).
Our study reinforces the negative relationship between the absolute abundance of lianas and distance from the edge, which has been repeatedly recorded (Laurance et al. 2001; Magrach et al. 2014; Campbell et al. 2018; Ofosu-Bamfo et al. 2022; Piovesan et al. 2022). However, we also found that liana abundance is slightly more related to canopy structure than to distance to edge.
Laurance et al. (2001), despite reinforcing the relationship with distance from the edge, already suggested that the relationship was not as strong as they expected, with tree biomass and the rate of change of tree biomass being more critical factors in that context, and these factors are related to the structure of the forest canopy. Considering the changes in forest structure caused by forest fragmentation and edge creation, distance to edge is not in and of itself what influences the structure of the liana assemblage: they are also influenced by forest structure (Blanchard et al. 2023).
Forest disturbances, such as the opening of clearings and consequent increased light penetration, can enhance the regrowth capacity of some species (Ledo and Schnitzer 2014; Rocha et al. 2020). In synergy with the process of forest fragmentation (Almeida et al. 2019), this could explain the greater abundance of lianas in these regions with less complex canopies, i.e., more open, and less dense canopies. Lianas would act as gap maintainers and niche constructors, seeing as in these regions, there is a higher abundance of lianas and a higher number of species (Schnitzer et al. 2021).
The success in increasing the abundance (Laurance et al. 2014) and density of stems (Rocha et al. 2020), while potentially spreading intensely and shading all or part of the tree canopy, reduces the growth rate of trees, keeping the canopy low. In this way, the increased abundance of lianas is influenced by, and influences, forest structure and tree species composition (Schnitzer et al. 2021).
Despite the influence of canopy PAI on the abundance of lianas, when we include taxonomic identity in the individuals (β diversity), this relationship disappears, and only the negative relationship with distance to edge remains. Thus, the direct effect of distance to edge may be mediated by other processes not analyzed in this study, such as the microclimatic variations that exist in the gradient of distance related to the edge (Camargo and Kapos 1995).
In addition to being one of the factors responsible for changes in forest structure after the death of large trees, the edge effect is also responsible for a decrease in humidity and an increase in temperature in forest fragments because of physical damage caused by strong winds and desiccation (Camargo and Kapos 1995). The edge effect can also be exacerbated by the effect of seasonality on the leaf phenology of canopy trees, reducing canopy vegetation and allowing higher light penetration into the lower strata, creating an even hotter environment (Nunes et al. 2022).
Functional traits related to the ability to tolerate microclimatic variations, such as variations in soil moisture and temperature, are essential factors in tree distribution, leading to a shift in forest dynamics (Engelbrecht et al. 2007). As we find different compositions of liana species in the gradient of distance to the edge, there may be functional groups whose characteristics generate different responses to microclimatic variations, modulating the occurrence of species. Understanding these responses seems important and alerts us to future approaches, especially related to climate change (Vogado et al. 2022), increased seasonality due to extreme events such as El Niño, and increased forest fragmentation.
Finally, comparing the results obtained for alpha and beta diversity with another study carried out in the same area, we found opposite patterns, in which 22 years ago, there were a more significant number of species at the edge, but the composition did not change in the gradient of distance to edge (Laurance et al. 2011). However, it is essential to note that the context of forest fragmentation at that time differs from what we find now. Although persistent, the effects of the forest fragmentation process have been attenuated over the years (Almeida et al. 2019), even with the periodic cutting of the matrix vegetation to maintain the isolation of the forest fragments.
In this way, it is possible that more than four decades after the beginning of the forest fragmentation process, the fragments have reached a more dynamic equilibrium, with the age of the edge being an essential factor in the dynamics of liana community (Laurance et al. 2001).
Biomass and support capacity for lianas
This work sheds light on a relevant factor in fragmented forests, the altered forest structure, to disentangle the indirect effects that influenced the structure and composition of the liana assemblage. The influence of the factors analyzed was low for all analyses performed, so other environmental factors may have a more substantial influence on the liana assemblage.
For liana biomass, studies are showing that it is related to edaphic characteristics, such as soil fertility and texture, and in the second instance, to the rate of change in above-ground tree biomass (Laurance et al. 2001), which may explain the relationship we found with distance to the forest edge. For the diversity and distribution of liana species, there is a strong relationship with the abiotic environmental characteristics, such as topographic gradient, water availability, and solar radiation, both in tropical forests (Schietti et al. 2014; Gerolamo et al. 2022; Rocha et al. 2022) and subtropical forests (Li et al. 2022).
We expected that the higher availability of thin trees would increase the physical support capacity for lianas to grow and develop, thus increasing the assemblage's abundance and biomass. However, we were unable to validate this pattern. Van der Heijden and Phillips (2008) found a positive but weak relationship between the abundance of lianas and the density of thin trees (DBH ≥ 10 cm), and the explanation for this finding was that, at a geographical scale, this predictor did not seem important, but perhaps locally it could be. In our study, we did not find this importance even locally.
Based on the literature, we concluded that the influence of the availability of physical support on the abundance and distribution of lianas tends to depend on a topographic gradient between lowlands and plateaus because of the functional traits of the stem (Rocha et al. 2022) and the life stage of the individual (Nogueira et al. 2011). In addition, the climbing guild is an essential parameter to analyze because there may be a significant relationship with tree size for the population of some species whose climbing strategy is based on tendrils and hooks (Laurance et al. 2001; Yang et al. 2018).
Lianas are a highly heterogeneous group of organisms that respond differently to forest fragmentation (Piovesan et al. 2022), have distinct functional and hydraulic traits (Meunier et al. 2021; Coppieters et al. 2022; Rocha et al. 2022) and growth strategies (Cai et al. 2007), so detecting community-level relationships is a significant challenge. Studies based on lower taxonomic levels, such as genus and species, combined with exploring functional traits, are the most promising path towards a better understanding of the other mechanisms involved in increasing the density and distribution of liana species.