Abstract
The distribution of tree species in tropical forests is generally related to the occurrence of disturbances and shifts in the local environmental conditions such as light, temperature, and biotic factors. Thus, the distribution of pioneer tree species is expected to vary according to the gap characteristics and with human disturbances. We asked whether there was variation in the distribution of a pioneer species under different environmental conditions generated by natural disturbances, and between two forests with contrasting levels of human disturbance. To answer this question, we studied the distribution patterns and population persistence of the pioneer tree species Croton floribundus in the size and age gap range of a primary Brazilian forest. Additionally, we compared the plant density of two size-classes between a primary and an early successional human-disturbed forest. Croton floribundus was found to be widespread and equally distributed along the gap-size gradient in the primary forest. Overall density did not vary with gap size or age (F-ratio = 0.062, P = 0.941), and while juveniles were found to have a higher density in the early successional forest (P = 0.021), tree density was found to be similar between forests (P = 0.058). Our results indicate that the population structure of a pioneer tree species with long life span and a broad gap-size niche preference varied between natural and human-disturbed forests, but not with the level of natural disturbance. We believe this can be explained by the extreme environmental changes that occur after human disturbance. The ecological processes that affect the distribution of pioneer species in natural and human-modified forests may be similar, but our results suggest they act differently under the contrasting environmental conditions generated by natural and human disturbances.
Highlights
Pioneer species are thought to present different population structures under different stages of forest succession or levels of human disturbances (Matthes 1992; Swanson et al 2011; Tabarelli et al 2012)
In early successional forests, which have regenerated after human disturbances, the distribution patterns, survival, and reproduction of pioneer species are believed to change due to the shifts in the local environmental conditions, such as light, temperature, and biotic factors (Matthes 1992; MartınezGarza and Howe 2003; Swanson et al 2011; Tabarelli et al 2012; Lohbeck et al 2014)
A possible explanation for this could be that while this gap has disturbance modes, gap area, and gap age (Table 1) similar to the other gaps analyzed, it was not surrounded by any C. floribundus trees within a radius of 34 m (Fig. 2B)
Summary
Pioneer species are thought to present different population structures under different stages of forest succession or levels of human disturbances (Matthes 1992; Swanson et al 2011; Tabarelli et al 2012). In early successional forests, which have regenerated after human disturbances, the distribution patterns, survival, and reproduction of pioneer species are believed to change due to the shifts in the local environmental conditions, such as light, temperature, and biotic factors (Matthes 1992; MartınezGarza and Howe 2003; Swanson et al 2011; Tabarelli et al 2012; Lohbeck et al 2014). These expectations are based on the exploiter-mediated coexistence model in patches, which a 2015 The Authors.
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