Tropical forests play a central role in global climate regulation, carbon cycling, and the maintenance of terrestrial biodiversity, while exhibiting high structural and functional complexity across broad environmental gradients (Pan et al., 2011; Malhi et al., 2010; Malhi et al., 2014). Their distribution and functioning are shaped by the interaction between climate, topography, and edaphic conditions, resulting in pronounced spatial heterogeneity in floristic composition, productivity, and ecological processes (Girardin et al., 2014). In this context, climatic and altitudinal gradients within the tropics provide an ideal framework for evaluating forest ecosystem responses to variations in temperature and water availability, factors of particular relevance under scenarios of global climate change (Malhi et al., 2010).

Within this broad array of ecosystems, tropical seasonally dry forests (TSDF) represent one of the most distinctive, vulnerable, and yet least studied forest biomes in the Neotropics (Pennington et al., 2009; Dirzo et al., 2011; Dryflor et al., 2016). These forests are characterized by pronounced climatic seasonality, with extended dry periods that impose strong physiological constraints and favor the dominance of species and functional strategies adapted to water stress, such as deciduousness, high wood density, and high water-use efficiency (Sánchez-Azofeifa et al., 2014; Poorter et al., 2016). Despite these limiting environmental conditions, TSDF harbor high levels of floristic diversity, endemism, and ecological uniqueness, often concentrated within relatively small geographic areas, which has led to their recognition as global priorities for conservation and scientific research (Pennington et al., 2009; Dryflor et al., 2016).

From a structural and functional perspective, TSDF are commonly dominated by families such as Fabaceae (Leguminosae) and Bignoniaceae, whose species exhibit key adaptations related to biological nitrogen fixation, drought tolerance, and the efficient exploitation of soils with high nutrient variability (Guzman et al., 2021). However, despite their ecological importance, these forests have been identified as among the most threatened tropical ecosystems worldwide, with less than 10% of their original extent remaining in many countries. This dramatic reduction is primarily driven by conversion to agricultural and livestock activities, habitat fragmentation, and their limited representation within formal protected area networks (Miles et al., 2006; Hoekstra et al., 2005; Dryflor et al., 2016).

In Peru, inter-Andean tropical seasonally dry forests comprise three major floristic groups—coastal, inter-Andean, and eastern forests, reflecting pronounced environmental and biogeographic heterogeneity (Linares-Palomino et al., 2011; Linares-Palomino et al., 2022). These ecosystems span a wide altitudinal range that, in some regions of northern Peru, exceeds 2,000 m a.s.l., particularly in inter-Andean valleys associated with the Marañón River basin (Linares-Palomino et al., 2011; Marrufo Huamán, 2023). In this region, TSDF exhibit exceptionally high levels of endemism, estimated at approximately 33% of woody plant species, along with pronounced floristic turnover (Marcelo-Peña et al., 2016; Marcelo-Peña et al., 2025). These patterns have been attributed to the complex Andean topography and historical processes of climatic isolation that have promoted the evolutionary differentiation of plant communities (Särkinen et al., 2012; Marcelo-Peña et al., 2025).

Nevertheless, despite their well-recognized biological importance, scientific research on TSDF in Peru remains limited, particularly with regard to systematic ecological studies and analyses across altitudinal gradients. This constraint hampers a comprehensive understanding of their dynamics and limits the development of effective conservation strategies (Delgado-Paredes et al., 2020; Marcelo-Peña et al., 2025). This knowledge gap is reflected in the scarcity of long-term monitoring plots in dry forests, in contrast to the relatively large number of studies and permanent plots established in montane forests. In the department of Amazonas, for example, several floristic assessments based on monitoring plots have been conducted in montane forests (Añazco Urbina et al., 2021; Culqui et al., 2024; Sánchez Zamora et al., 2024), whereas only a single study has specifically focused on the establishment of vegetation monitoring plots in dry forests (Tarrillo Cieza, 2024).

In this context, the establishment of evaluation plots in TSDF of northern Peru represents a strategically important initiative to improve ecological knowledge of these highly vulnerable ecosystems. Given the high biodiversity of the Amazonas department and the urgent need to generate baseline information to support conservation efforts, the installation of 16 study plots was undertaken at the altitudinal boundaries between TSDF and montane forest ecosystems in the district of Sonche, Chachapoyas Province. This effort aims to contribute to the characterization of vegetation structure and composition in the dry forests of northeastern Peru.

The results of this study provide relevant empirical evidence on the ecological role of seasonally dry tropical forests (SDTFs) at altitudinal limits and their transition toward montane forests, a component that has historically been underrepresented in Andean ecological research (Cueva et al., 2019). Although tropical dry forests have been widely recognized for their high functional specialization, pronounced climatic seasonality, and high proportion of endemic species, most studies have focused on latitudinal gradients or lowland environments (Siyum, 2020). In contrast, altitudinal gradients linking these ecosystems with montane forest formations have received limited attention, constraining our understanding of the ecological processes that structure such transitions (Murga-Orrillo et al., 2021).

In the Sonche district, SDTFs exhibited a relatively homogeneous floristic structure, characterized by low tree species richness and strong dominance by a few species, notably Vachellia aroma, Inga adenophylla, Clusia rigida, and Myrsine oligophylla (Aguirre Mendoza et al., 2021a, 2021b). This pattern reflects communities dominated by species highly tolerant to water stress (Dirzo et al., 2011; Galván-Cisneros et al., 2023; Rodríguez and Oyuela-Andino, 2023), with high relative density, small to intermediate diameters, and a predominance of young individuals (Montenegro-Hoyos et al., 2022; Ramírez et al., 2022). Consequently, the observed structural dominance suggests a simplified forest organization typical of Andean and Neotropical SDTFs, where a small number of species control most of the space and resources, limiting vertical stratification and promoting rapid regeneration dynamics (Pennington et al., 2000).

The vertical structure of SDTFs further supports this interpretation, as individuals between 0 and 10 m in height predominate. In dry environments, limited water availability constrains vertical stem growth and favors species adapted to seasonal stress conditions (Pennington et al., 2000). These adaptations include functional strategies such as deciduous phenology, small or semi-persistent leaves, and deep root systems that optimize water use but do not necessarily promote the development of tall, complex canopies (Miles et al., 2006). However, this pattern may also reflect continuous recruitment coupled with lower survival of mature trees under prolonged drought conditions. Consistently, the inverted “J-shaped” diameter distribution recorded in SDTFs is characteristic of forests with high regeneration rates and elevated mortality of large individuals, a pattern widely documented in dry forests across Latin America (Reis et al., 2017).

In contrast, montane forests exhibited greater floristic heterogeneity and more evenly distributed Importance Value Index (IVI) values among multiple species, including Aiouea montana, Clethra revoluta, Ilex quitensis, Ilex scopulorum, Hieronyma oblonga, and Clusia multiflora. This pattern reflects less restrictive environmental conditions, associated with higher precipitation and moderate temperatures (Cueva et al., 2019). Tree species richness peaked between 2,300 and 2,500 m a.s.l., following a unimodal pattern characteristic of montane ecosystems, where diversity reaches its maximum at mid-elevations (Li et al., 2023). This pattern has been attributed to a combination of favorable climatic conditions, increased moisture availability, and high structural heterogeneity, which facilitate the coexistence of species with both tropical and montane affinities (Sánchez Zamora et al., 2024). Conversely, at elevations above 2,500 m a.s.l., diversity declines progressively due to thermal constraints and more severe environmental filters that limit the establishment of less-adapted species (Bañares-de-Dios et al., 2024).

The transition between SDTFs and montane forests along the altitudinal gradient highlights the central role of water availability and elevation-modulated climatic conditions in shaping forest composition and architecture, in agreement with patterns observed in other Andean gradients (Blundo et al., 2012). Beta diversity analysis using the Sørensen index confirmed a clear floristic differentiation between both ecosystems; however, this transition is not abrupt. Instead, an ecological continuum is evident, where typical dry forest species coexist with montane forest elements, particularly in ecotonal plots such as S08. This pattern of floristic overlap has been widely documented in tropical altitudinal gradients and reflects the interaction of climatic, edaphic, and topographic factors rather than the presence of rigid environmental thresholds (Veintimilla et al., 2019; Guzmán et al., 2025). In this context, altitude modulates the hydric filters characteristic of dry forests through its interaction with thermal gradients and changes in soil structure, allowing the persistence of mixed communities (Krömer et al., 2013).

From a biogeographic perspective, the tree species composition recorded at altitudinal limits supports the hypothesis that montane dry forests function as contact zones and potential ecological corridors between contrasting bioclimatic regions (Blundo et al., 2012). Nevertheless, most available studies lack spatial designs capable of capturing this complexity, as they are based on isolated plots or inventories without altitudinal continuity (Malizia et al., 2020). In this regard, the scarcity of permanent plots in dry forests of northern Peru limits the assessment of temporal dynamics, species turnover rates, and responses to climatic variability—processes that require continuous sampling along environmental gradients (Baez et al., 2015; Malizia et al., 2020).

Multiple correspondence analysis (MCA) reinforced these patterns by revealing clear associations between dominant plant families and specific environmental conditions. In lower SDTF zones, Fabaceae and Clusiaceae were associated with warm and dry environments, characterized by fast-growing species with efficient water use (Aguirre Mendoza et al., 2021a, 2021b; Galván-Cisneros et al., 2023). At intermediate elevations, Asteraceae and Primulaceae were linked to smaller-stature trees, suggesting nutrient limitations that constrain structural development (Montenegro-Hoyos et al., 2022). In montane forests, Lauraceae, Melastomataceae, Clethraceae, and Aquifoliaceae dominated the floristic composition, reflecting adaptations to cold and humid environments and to organic matter–rich soils (Tejedor Garavito et al., 2014; Guzmán et al., 2022).

From a conservation perspective, our results support a shift toward strategies that recognize the altitudinal gradient as an integrated ecological unit, in which the conservation of SDTFs and montane forests is complementary for maintaining connectivity, ecosystem resilience, and the provision of ecosystem services under climate change (Killeen and Solorzano, 2008). The conservation of ecological corridors between 1900 and 2,700 m a.s.l. is a priority to facilitate species movements and altitudinal range shifts associated with global warming (Meza Mori et al., 2020; Meza-Mori et al., 2025). SDTFs are not merely the lower end of the altitudinal gradient, but rather a distinct and vulnerable component of the Andean ecological system, structured by restrictive environmental conditions that favor communities dominated by a few drought-adapted species (Pennington et al., 2009). Their degradation would have disproportionate effects on landscape connectivity, hydrological regulation, and the ecological stability of the entire system (Miles et al., 2006; Malhi et al., 2014; Dryflor et al., 2016).

Likewise, the values of the Importance Value Index (IVI) provide a robust empirical basis for linking observed structural patterns with concrete actions in ecological restoration and climate change adaptation. In the 1900–2,100 m a.s.l. range (seasonally dry tropical forest), low diversity and high dominance of Vachellia aroma and Inga adenophylla reflect functionally simplified communities that are nonetheless critical for system stability. Their high abundance, drought tolerance, wide distribution, and capacity for biological nitrogen fixation (Bax et al., 2021) make them priority candidates for the revegetation of degraded soils and for facilitating the establishment of less stress-tolerant species (Fremout et al., 2021; de Araújo et al., 2024). However, these species are highly vulnerable to the projected increase in the duration and intensity of dry periods, which may increase mortality and reduce natural regeneration (Campo et al., 2023). In the transitional zone between 2,100 and 2,300 m a.s.l., the pronounced ecotonal character is expressed in a relatively balanced community structure, indicating high sensitivity to relatively small changes in temperature and precipitation at the climatic boundary between dry and montane forests (Elsen et al., 2018). In the mid-montane forest (2300–2,500 m a.s.l.), where the unimodal peak of diversity is concentrated, climate vulnerability increases due to the dominance of species adapted to environmentally stable conditions, with low thermal variability and predictable hydrological regimes (Gillani et al., 2025). Furthermore, in the high montane forest (2500–2,700 m above sea level), species adapted to cold and humid climates face a high risk of local extinction due to the lack of available altitudinal space to migrate in the face of the upward shift of climatic niches (Mata-Guel et al., 2023).

Finally, this study presents limitations that should be considered when interpreting the results. Increased sampling effort would improve the representativeness of tree diversity, particularly in transition zones between SDTFs and montane forests (Chao et al., 2014). Moreover, the absence of soil analyses and biomass estimates constrains our understanding of the functional processes regulating forest structure and productivity (Pan et al., 2011; Poorter et al., 2015). Future research should integrate edaphic variables, carbon stocks, and population dynamics to more accurately assess the ecological resilience of these ecosystems under climate change.

This study demonstrates that tree diversity along the altitudinal gradient in the district Sonche does not follow a non-linear pattern, but rather a unimodal one, with a maximum between 2,300 and 2,500 m a.s.l., near the transition zone between SDTF and montane forest. In this context, this diversity peak, together with the systematic shifts in species dominance, confirms the existence of a well-defined altitudinal floristic succession and a high level of structural complexity within the ecosystem. Furthermore, our results allow a precise identification of both the most sensitive segments of the gradient and the structurally most important species, which are simultaneously the most vulnerable to climate change. At lower elevations (1900–2,100 m), communities dominant by Vachellia aroma and Inga adenophylla which are key to the stability of the SDTF, are highly sensitive to the projected increase in the duration and intensity of dry periods, potentially increasing mortality and limiting regeneration. Similarly, in the mid-montane forest (2300–2,500 m), where the diversity maximum is concentrated, dominant species adapted to historically stable environmental conditions show high vulnerability to rising temperatures and changes in precipitation seasonality. Finally, in the upper montane forest (2500–2,700 m), strictly montane species that sustain canopy structure under cold and humid climates face a high risk of local extinction due to the lack of available altitudinal space for upward migration. Taken together, we propose that conservation should focus on the entire altitudinal gradient as an integrated functional unit, as the loss of dry forest at lower elevations or montane forest at higher elevations would compromise ecological integrity, landscape connectivity, and the provision of ecosystem services throughout the system.