COS 212-4 - Population dynamics of Caribbean pine (Pinus caribaea var. hondurensis) in a frequently burned, humid savanna

Background/Question/Methods

Fire is proposed as an important determinant of woody plant population dynamics in savannas via its limitation of seedling to sapling transitions. In the humid, lowland savannas of Belize, the influence of fire on the overstory tree, Caribbean pine (Pinus caribaea var. hondurensis), has not been studied in detail. To fill this gap, we used data from natural stands of Caribbean pine to model population dynamics under different fire regimes. In early 2017, we installed permanent monitoring plots in open and shrubby savanna environments in Paynes Creek National Park. Within each plot, we tagged and measured the heights of all seedling/juvenile pines (< 2.5cm diameter at breast height, DBH) and the DBH of all larger pines. We recensused plots in 2018, 2019, and 2020 to assess growth and survival in years with and without fire and obtained observations from the literature on reproduction. We used integral projection models to examine the contributions of different life stages to population growth under fire regimes of varying frequency. We modeled four populations: pines in open savanna and in shrub, each grouped by individuals that were measured after intervals with very recent fire and after intervals with fire over a year previously.

Results/Conclusions

All populations appeared to be declining slowly, resulting from an interaction between survival and growth rates, especially at small sizes. Although growth rates did not differ, survival of small pines was higher with fire return intervals of more than a year, but larger individuals were more likely to suffer mortality than at shorter return intervals. Model sensitivities indicated that lambda was more sensitive to changes in individual growth rates than in survival rates; increasing the slope of the growth parameter by 5% increased all population growth rates to 1. Pines between 10 and 20 cm DBH made an influential contribution to population growth. These results indicate that enough individuals need to recruit into and surpass this size class for pine populations to grow. Our models suggest that annual fires are unlikely to favor Caribbean pine population growth and that research is needed on factors affecting growth rates in natural stands of lowland Caribbean pine. Identifying the conditions that favor rapid establishment and recruitment into size classes above the threshold we identified is an important research need.