Modeling the spread of an invasive vine: Consequences for carbon cycling in a post-agricultural landscape
In many regions around the world, agricultural de-intensification and abandonment has been followed by forest recovery. In others regions, however, this process has been followed by the proliferation of a handful of alien and native species that are altering the structure and function of ecosystems. Among these species, vines stand out due to their capacity to rapidly blanket forest edges and smother plant canopies of pasture, crop and second-growth over vast areas thus potentially impacting carbon cycling. Here we ask how does the spread and subsequent establishment and growth of an invasive vine, namely Pueraria phaseoloides, alter the size of aboveground carbon pools in a tropical landscape undergoing agricultural de-intensification and abandonment. To address this question we developed a simple cellular automata model in which 1) initial site of occupation depended on the probability of occurrence of P. phaseoloides derived from biophysical variables and presence of vine patches mapped from satellite imagery, 2) biomass accumulation changed according to a logistic growth function, and 3) dispersal occurred via stem elongation to neighboring susceptible sites if S – an index of susceptibility to vine invasion based on land use – was greater than a given susceptibility threshold, Sth.
After 50 years, 14% (Sth = 30) and 12% (Sth = 40) of the sites were invaded by P. phaseoloides. This change in the number of occupied sites was accompanied by changes in the number (from 824 to 906) and size (min-max: 1-672 and 1-203 pixels) of vine patches. Thus, increased susceptibility to invasion resulted in more but smaller vine patches. In these simulated landscapes, P. phaseoloides accumulated a total of 3.5 x104 (Sth = 30) and 3.0 x104 (Sth = 40) Mg of aboveground biomass. In the absence of P. phaseoloides development of secondary forests in the invaded area could have contributed to the accumulation of ~ 19 x104 Mg of aboveground biomass. Thus, invasion by P. phaseoloides can significantly reduce aboveground carbon storage in tropical landscapes undergoing agricultural de-intensification and abandonment.