Distribution of juveniles of tree species along a canopy closure gradient in a tropical cloud forest of the Venezuelan Andes

Abstract

Cloud forests represent a minor portion of the world forests, though outstanding in terms of biodiversity, endemisms, and environmental services provided. Understanding the factors that drive the regeneration and species composition of these forests, and in particular how light availability affects the patterns of juvenile tree distribution in the understory, is critical for conservation and restoration programs. In this study, we determined the range-size and overlap of the abundance distribution of juveniles for 20 tree species in an Andean tropical cloud forest in Venezuela along a gradient of percentage canopy openness (%CO) used as a surrogate of light availability. The observed distribution of %CO was then compared with a bounded null model of community structure in order to test light partitioning as a driver of tree species' coexistence. We measured %CO using hemispherical photography and the abundance and size of juvenile trees in 280 plots of 1-m radius spread over a 32 ha forest area. The distribution of sites was skewed towards the lower end of the %CO gradient (0.5 to 12.8%), while species abundance sharply diminished at both ends of the gradient. Nevertheless, 15 out of 20 species had a non-random distribution in relation to %CO, with many species concentrated near the lower side of the gradient. The observed pattern of species' overlap was within the 95% confidence limits for the average overlap expected under the bounded null model. These patterns indicate that low canopy openness is the rule in this forest, in spite of the scattered tree-fall gaps, and suggest that light partitioning does not determine the tree community structure at the juvenile stage. High redundancy in light requirements among juveniles of tree species may have a positive effect on species coexistence in cloud forests, thus maintaining a high species diversity. However, other factors such as recruitment limitation and differential growth/carbon-gain among species at the juvenile stage along the light gradient could contribute to the high diversity of these ecosystems.

Diversity, structure and dynamics of tropical montane forests: Insights from permanent-plot monitoring in the Venezuelan Andes

“…In addition, our study sites are also located in an elevation belt (>2000 m a. s. l.) closer to the cloud-immersion zone where species diversity is generally lower in many Andean forests (Girardin et al 2014;Griffiths et al 2021). Indeed, most if not all of the abundant species found in our plots are species with unique adaptations to persist, survive and grow slowly under shade and relatively cool temperatures (Quevedo-Rojas et al 2016). Another factor to consider is that the Andean forests in Venezuela have a floristic composition influenced by the Orinoquia region, which tends to be drier and less diverse, whereas, for example, Andean forests of Perú, Bolivia and Ecuador are connected to the wetter and more diverse Amazonian ecoregion (Kessler et al 2011;Trujillo-C and Henao-Cárdenas 2018).…”

“…According to Ataroff and Sarmiento (2004), forests in La Mucuy correspond to the higher belt of the mountain cloud forests, consisting of dense evergreen vegetation with a complex stratified structure, average canopy tree height ranging between 18-22 m and emergent trees higher than 25 m. The forests of San Eusebio correspond to the lower belt of mountain cloud forests, with dense evergreen vegetation and at least three clearly differentiated strata. The upper/canopy stratum reaches 30 m, with emergent trees higher than 40 m. There is an intermediate stratum with tree heights between 20-25 m and a lower stratum with trees ranging between 10-15 m (Quevedo et al 2016).…”

Mixed Forest Plantations with Native Species for Ecological Restoration in Cloud Forests of the Venezuelan Andes

“…The photos were processed with Gap Light Analyzer (GLA) v. 2.0 [17]. For detailed procedures see [9,18]. For site preparation no tillage was done, so the soil was disturbed only when opening the planting holes.…”

“…In 2007, our team began research projects aimed to study systematically cloud forest species requirements. Research included the distribution of seedlings of tree species in the forest understory along a light gradient [9,10] and ecophysiological and morphological responses to contrasting light conditions [11,12]. In 2012, we began a project aimed to develop and execute a plan for the landscape ecological restoration in the cloud forest of Paramo El Tambor with participation of stakeholders to restore degraded areas and help to protect the wildlife and their habitats.…”