Pinus contorta invasions increase wildfire fuel loads and may create a positive feedback with fire

Abstract

Invasive plant species that have the potential to alter fire regimes have significant impacts on native ecosystems. Concern that pine invasions in the Southern Hemisphere will increase fire activity and severity and subsequently promote further pine invasion prompted us to examine the potential for feedbacks between Pinus contorta invasions and fire in Patagonia and New Zealand. We determined how fuel loads and fire effects were altered by P. contorta invasion. We also examined post-fire plant communities across invasion gradients at a subset of sites to assess how invasion alters the post-fire vegetation trajectory. We found that fuel loads and soil heating during simulated fire increase with increasing P. contorta invasion age or density at all sites. However, P. contorta density did not always increase post-fire. In the largest fire, P. contorta density only increased significantly post-fire where the pre-fire P. contorta density was above an invasion threshold. Below this threshold, P. contorta did not dominate after fire and plant communities responded to fire in a similar manner as uninvaded communities. The positive feedback observed at high densities is caused by the accumulation of fuel that in turn results in greater soil heating during fires and high P. contorta density post-fire. Therefore, a positive feedback may form between P. contorta invasions and fire, but only above an invasion density threshold. These results suggest that management of pine invasions before they reach the invasion density threshold is important for reducing fire risk and preventing a transition to an alternate ecosystem state dominated by pines and novel understory plant communities.

Pine Plantations and Invasion Alter Fuel Structure and Potential Fire Behavior in a Patagonian Forest-Steppe Ecotone

“…A closed pine canopy was only observed at the high invasion level, with an average age of establishment of 20.0 ± 2.7 years. This is only 4.5 years more than the average age of establishment at intermediate invasion levels, which showed extremely low horizontal continuity at height classes above 0.5 m. This is consistent with a recent study conducted in native grasslands in the Chilean Patagonia, where fuel build-up generated by P. contorta invasions started growing exponentially between 15 and 20 years after invasion initiation [16]. Although we did not simulate fire behavior in low and intermediate levels of invasion, their fuel characteristics implies a much lower fire hazard compared to high invasion levels [46].…”

“…However, once this proposed threshold is surpassed, fires may radically increase their intensity and severity in pine-invaded areas. The existence of this threshold for P. contorta invasions in the Patagonian steppe was previously shown by Taylor et al [16], where simulated soil heating due to fire drastically increased after the invasion reaches 10 years [16]. In addition to increased fire hazard, high invasion levels significantly raise pine removal costs [47] and magnify the chances of positive fire-vegetation feedbacks [16], thus decreasing the probabilities of successful restoration of the native community.…”

“…In this region, the most commonly planted species is Pinus ponderosa Douglas ex C. Lawson and the most widespread invasive pine species is Pinus contorta Douglas, both of which experience frequent fire in their native environments and thus have life-history traits that generally make them well adapted to fire [15]. Furthermore, both species have fast growth rates and thus rapid fuel load accumulation, particularly in the introduced range [16,17]. Lastly, frequent observations of fires originating or spreading rapidly in non-native plantations and neighboring invaded areas are generating concerns as to whether pine plantations and invasions increase native vegetation's flammability and may qualitatively alter the historical fire regime [9,18].…”

“…For instance, well managed plantations where tree thinning treatments were rigorously conducted seem less prone to high severity fires compared to those that were not thinned [19]. Similarly, pine invasions only seem to generate positive fire-invasion feedbacks after certain pine density thresholds are surpassed [16]. Thus, it is necessary to evaluate the relative fire hazard of pine plantations under different scenarios of management and different stages of invasion in comparison to the native vegetation being replaced.…”

“…Only recently, two studies have explored the changes in fuel loads and structure generated by pine invasions in southern South America [16,20]. However, neither addressed changes in flammability caused by plantations, which are also a main component of these landscapes, nor did they assess the impacts of different management practices on the relative flammability of these scenarios.…”