Pinus halepensis invasion in mountain pampean grassland: effects of feral horses grazing on seedling establishment.

This research investigates how the ongoing presence of feral horses affects the increasing abundance of Pinus halepensis, a type of pine, in the mountain pampean grasslands of Argentina. The study found that the combination of direct grazing effects and environmental changes indirectly caused by the horses’ presence boosts the establishment and growth of the pine in these grasslands.

Investigating Pinus halepensis Invasion

The main focus of this research was the growing prevalence of Pinus halepensis in mountain pampean grasslands and its association with the continuous presence of feral horses. Pinus halepensis, also known as an Aleppo Pines, is not a native species to these grasslands and its proliferation is seen as an invasion that can disrupt the local ecosystem. The researchers hypothesized that both direct and indirect effects of feral horse grazing may influence the growth and establishment of P. halepensis.

• The study involved a field experiment, considering the response of seedling emergence, survival, and growth to the depletion of herbaceous vegetation and soil disturbance. These factors were studied in sites with contrasting grazing histories to provide a comprehensive understanding.

The Effects of Feral Horse Grazing on Seedling Establishment

The research revealed that the grazing habits of feral horses shape the environment in several ways that can enhance the proliferation of P. halepensis.

• Direct effects include heavy defoliation of perennial grasses due to horses’ grazing. This reduces the natural competitors for light, water, and nutrients, allowing P. halepensis to establish itself more easily.
• Indirect effects include a decrease in species and functional richness, diversity and evenness in the plant communities due to grazing. This reduction in competition again favors the establishment of the pines.
• Environmental changes, such as high percentages of bare ground and a low grass biomass as a result of constant grazing, likewise facilitate the seedling establishment.

Conclusion

The research demonstrated that the invasion of P. halepensis in the mountain pampean grasslands of Argentina is largely a result of the continuous presence of feral horses. The horses’ grazing habits both directly and indirectly enhance the growth and establishment of the non-native pine species. The findings support the concept of the invasional meltdown hypothesis and weakened biotic resistance as the mechanisms involved in P. halepensis invasion. This suggests the need for more informed management of these areas to prevent such invasions and maintain the integrity of the grassland ecosystem.