Pine invasions in treeless environments: dispersal overruns microsite heterogeneity

Abstract

Understanding biological invasions patterns and mechanisms is highly needed for forecasting and managing these processes and their negative impacts. At small scales, ecological processes driving plant invasions are expected to produce a spatially explicit pattern driven by propagule pressure and local ground heterogeneity. Our aim was to determine the interplay between the intensity of seed rain, using distance to a mature plantation as a proxy, and microsite heterogeneity in the spreading of Pinus contorta in the treeless Patagonian steppe. Three one‐hectare plots were located under different degrees of P. contorta invasion (Coyhaique Alto, 45° 30′S and 71° 42′W). We fitted three types of inhomogeneous Poisson models to each pine plot in an attempt for describing the observed pattern as accurately as possible: the “dispersal” models, “local ground heterogeneity” models, and “combined” models, using both types of covariates. To include the temporal axis in the invasion process, we analyzed both the pattern of young and old recruits and also of all recruits together. As hypothesized, the spatial patterns of recruited pines showed coarse scale heterogeneity. Early pine invasion spatial patterns in our Patagonian steppe site is not different from expectations of inhomogeneous Poisson processes taking into consideration a linear and negative dependency of pine recruit intensity on the distance to afforestations. Models including ground‐cover predictors were able to describe the point pattern process only in a couple of cases but never better than dispersal models. This finding concurs with the idea that early invasions depend more on seed pressure than on the biotic and abiotic relationships seed and seedlings establish at the microsite scale. Our results show that without a timely and active management, P. contorta will invade the Patagonian steppe independently of the local ground‐cover conditions.

Tree Regeneration Spatial Patterns in Ponderosa Pine Forests Following Stand-Replacing Fire: Influence of Topography and Neighbors

“…Further, our observed patterns stand in contrast to other studies of conifer regeneration, wherein density was one to two orders of magnitude greater during stand initiation [41,42]. Those studies found relatively simpler patterns and no effect of topography nor biotic interactions on those patterns.…”

Landscape characteristics of non‐native pine plantations and invasions in Southern Chile

“…Furthermore, seedling establishment outside plantations in Argentina was influenced more by seed density than by biotic or abiotic factors (Pauchard et al . ). Therefore, it is likely that where plantations can establish, non‐native pines can also invade.…”

“…Although non‐native pines grow across a range of abiotic conditions (Pauchard et al . ; Tomiolo et al . ), some native ecosystems are particularly vulnerable to invasion.…”

How are monospecific stands of invasive trees formed? Spatio-temporal evidence from Douglas fir invasions

“…This pattern of stand formation has important implications for tree invasion research as well as for invasion management. One implication is that space-for-time substitutions—commonly used in plant invasion research (Zenni et al 2014; Pauchard et al 2016)—cannot be applied at the stand scale, because all trees in the monodominant stand may be, on average, of similar age, so trees near the plantation’s edge are not necessarily older. Another significant outcome was the timing of invasion onset, which occurred ca.…”

“…For non-native species formation of monodominant stands is not clear, and for invasive conifers it has been proposed to occur as a gradual process from the seed source (e.g. encroaching a similar distance every year from the original source) (Zenni et al 2014; Pauchard et al 2016). Documenting the process of stand formation of non-native trees provides basic information, which allows formulating hypotheses regarding the causal mechanisms.…”