Abstract
The spatial distribution of individuals is a fundamental property of most species and constitutes essential information for the development of restoration and conservation strategies, especially for endangered plant species. In this paper we describe the spatial distribution of different size classes of the endangered tropical tree Guaiacum sanctum and the effect of canopy cover on spatial aggregation. Adult G. sanctum were located and mapped in a 50 ha plot in Palo Verde National Park, Costa Rica. Seedlings, saplings and juveniles were mapped to the nearest centimetre and permanently marked in three 50 x 50 m subplots. Within each subplot spatial aggregation was assessed using Ripley's K statistic and canopy opening readings were performed every 5 m using a densitometer. Kriging spatial interpolation and Monte Carlo simulations were used to determine if average canopy cover differed among size classes. Individuals of G. sanctum were spatially aggregated at all size classes with seedlings being the most frequent size class in all subplots. Seedlings were found predominantly in areas with significantly higher canopy cover. In contrast, juveniles were more likely found in areas with higher light availability. The high number of seedlings, saplings, and juveniles relative to adults suggests that populations of G. sanctum in PVNP are expanding. Light availability and canopy structure are important factors shaping the spatial distribution of this species. The contemporary demographic structure of G. sanctum is dependent on forest gap dynamics and changes in human disturbance during the past 25 years.
Highlights
Spatial distribution and patterning of plants is an important characteristic of communities and is a fundamental property of most species. Hutchinson (1953) determined that at least five causal factors shape the spatial pattern of plant species: 1) environmental factors such as nutrients or light availability, 2) reproductive factors including propagule dispersal and seasonality, 3) interspecific or social factors such as territoriality, predation and competition, 4) intraspecific components such as competition and density dependent factors and 5) stochastic variation in any of these causal factors
As a typical species of tropical dry forest, canopy cover and desiccation avoidance should play an important role in its recruitment and survival, we expect that canopy cover should have a measurable effect on the spatial distribution of this species
In this study we described the spatial distribution of different size classes of G
Summary
Spatial distribution and patterning of plants is an important characteristic of communities and is a fundamental property of most species. Hutchinson (1953) determined that at least five causal factors shape the spatial pattern of plant species: 1) environmental factors such as nutrients or light availability, 2) reproductive factors including propagule dispersal and seasonality, 3) interspecific or social factors such as territoriality, predation and competition, 4) intraspecific components such as competition and density dependent factors and 5) stochastic variation in any of these causal factors. Guaiacum sanctum (Zygophyllaceae) is a slow growing tropical and subtropical dry forest tree, distributed from Southern Central America to Northern Mexico and Florida, and throughout the Greater Antilles (Holdridge & Poveda 1975). This species, known as Lignumvitae, has been heavily exploited for its hard wood and medicinal value. Recent work on G. sanctum showed high levels of intra population genetic diversity and a lack of fine-scale genetic structure (Fuchs & Hamrick 2010b), congruent with high rates of seed dispersal and seed mixture from multiple maternal plants Based on these results, we hypothesize a random spatial distribution of G. sanctum seedlings. We used spatial statistics to analyse the effect of light availability on regeneration and the spatial distribution of age classes of this endangered tropical tree
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