Small-scale invasion pattern, spread rate, and lag-phase behavior of Rhamnus frangula L.

Abstract

We tested the ability of the simple reaction–diffusion spread model to quantify the rate of spread of Rhamnus frangula L., a non-indigenous shrub, at the scale of a single forest stand in southeastern New Hampshire. Cut stems of R. frangula were collected in a Cartesian grid pattern within the invaded stand and aged. A contour map of the invasion pattern was produced using kriging. The empirical invasion rate was calculated as the linear distance per year since initial invasion. The intrinsic rate of increase and diffusion coefficient were estimated in the field and the model prediction calculated. Observed mean rate of linear expansion was measured to be 6.7 m per year with a standard deviation of 1.24 m per year, while the predicted linear asymptotic rate of spread was calculated be 6.3 m per year. Thus, the simple reaction–diffusion model yielded a very accurate prediction of actual spread rate. There may have been a lag-phase in the invasion at this site as the spread rate was found to be slower in the early stages of invasion compared to later stages. At the scale of single stands, the simple reaction–diffusion model may be accurate enough to predict the spread of new invaders without recourse to complex life-history spread rate models. Additionally, if lag-phases in spread rate occur with regular frequency, then attempts to retrospectively predict which species may be invasive will be hampered by incorrectly classifying a future invader as currently non-invasive.