Effects of habitat disturbance and rainfall on a dominant medicinal dry forest tree

Abstract

The interactive effects of anthropogenic disturbance and climatic variables can shape the demography of plant populations, but they remain poorly understood especially for long-lived species like tropical trees. Understanding these interactions is critical for designing forest management strategies in the face of both growing anthropogenic pressures and climate change. We explored the effects of habitat disturbance and interannual variation in rainfall on the population dynamics of Amphipterygium adstringens (Anacardiaceae), an ecologically dominant tree species endemic to the tropical deciduous forests of Mexico, that is heavily harvested for its medicinal bark. Specifically, we compared A. adstringens vital rates and long-term population growth rates of unharvested populations in a disturbed versus a protected tropical deciduous forest site; and explored the effects of inter-annual variation in annual rainfall on A. adstringens by comparing vital rates and population growth rates across three years with differing levels of rainfall. We established four plots in each of the two sites and monitored survival, growth and reproduction of 251 individuals over three years. We found that anthropogenic disturbance and rainfall differentially affected the vital rates of A. adstringens. Survival did not differ across sites or years. Growth tended to be lower and fecundity higher, in the protected site, but these effects varied across years and as a function of plant size. No seedlings were found in the disturbed site over the three years study period. Integral projection models showed that projected population growth rates (λ values) in the protected site were higher and much less variable over time than those of the disturbed site. Lambda values in the disturbed site were <1, indicating long-term population decline. In both sites, the rainiest year showed the highest population growth rate but the increase in λ was much higher in the disturbed site than in the protected site. These findings suggest that the population dynamics of A. adstringens are heavily affected by habitat disturbance forest management regime and that the impacts of interannual variability in rainfall or other environmental conditions may be exacerbated in disturbed conditions. The persistence of A. adstringens populations over time will require the exclusion of livestock as well as the implementation of silvicultural practices that will lead to seedling establishment and survival. This study provides key data for the management and conservation of A. adstringens, a highly valued medicinal species.