Stealth invasions on the rise: rapid long-distance establishment of exotic pines in mountain grasslands of Argentina

Abstract

Pine tree invasions threaten many natural ecosystems of the Southern Hemisphere, modifying their structure and functioning through shifts in fire regimes, water balance, and biodiversity. The magnitude of such impacts depends on how much of the landscape has been invaded, thus a better understanding of the dispersal ability of pines and predictions of their future invasions are needed. Here we depict the spatio-temporal patterns of Pinus elliottii and Pinus taeda invading a new environment away from planted plots (i.e., invasion front), and discuss the underlying mechanisms that lead to a very concerning, yet poorly documented, pine invasion in central Argentina. Combining high-resolution imagery, allometric field data, and dendrochronology, we reconstructed the pine invasion into mountain grasslands from its onset in 1990. We found that even though the maximum density of invading pines (80 trees ha⁻¹) was very low compared to adjacent plantation (1000 trees ha⁻¹), density decreases exponentially with distance from the plantation edge. Remarkably, invading pines were found throughout the sampling plots showing high dispersal capacity, with no differences in age with increasing distance. The observed low density and spatially widespread exotic pine establishment, create a stealth type of invasion that is difficult to perceive in its early stages and challenging to manage once large areas are compromised. As invasion continues, long-distance dispersal will possibly become a major agent of landscape transformation and may lead to large pine-dominated neo-ecosystems, such as the savanna-like formation described here that replaced native grasslands in only three decades.