Fencing promotes fast recovery of demographic processes after grazing-driven collapse in Bursera graveolens forests

Abstract

Global change pressures are jeopardizing the functioning and structure of most tropical forests and clouding the future for their biodiversity and provided services. Although the impact of direct destruction through deforestation and fragmentation is currently in the research portfolio, overgrazing, which is more diffuse and generalized but chronic, especially in the seasonal dry forests, has been by far less addressed. Overgrazing can lead to a dramatic change in natural regeneration, often triggering the collapse of dominant species.

The almost monospecific dry forests of Bursera graveolens, one of the most representative forest of the Tumbesian region, are on the verge of extinction due to both direct destruction and chronic overgrazing and regeneration collapse in forest remnants. Here, we evaluated the impact of a very simple measure, the installation of fences, on the regeneration of ecological processes affecting the dominant species. We mapped and measured all Bursera individuals with a height greater than or equal to 30 cm in six 1-ha plots, 3 within and 3 outside fenced areas. Using spatial point pattern analysis, we inferred the importance of demographic and ecological processes affecting adults and juveniles in fenced and unfenced areas.

The spatial structure of adults was similar in fenced and unfenced areas, varying from random to aggregated patterns, showing that fences did not interfere with ecological processes affecting adult trees. On the contrary, we found 2765 juveniles in the three fenced plots but none in free ranging areas. Juveniles showed heterogeneous clustered patterns, and their distribution and growth were negatively influenced both by the presence and the height of adult trees. On average, there was an exclusion zone of 10 m around adult trees were recruitment of juveniles was limited. Competition among juveniles appeared to be negligible.

All in all, these results suggest that, in addition to recruitment, two of the main mechanisms that rule tree population dynamics in tropical forests, i.e., dispersal limitation and a Janzen-Connell-like mechanism favoring recruiting far from adult trees have been immediately restored in the fenced area. This shows that fencing is a viable tool for a fast regeneration and conservation of Bursera forests.