Picture this: it’s the 20th century and people are planting pine trees all over Chile. Pines of the genus Pinus, that is, which is very different from the native Araucaria araucana (the monkey puzzle tree). The latter might very well be called the ‘Chilean pine’ by some, but lacked the versatility for those wanting efficient wood production and erosion protection. So: in come the Pinus-pines, to help the economy!
These might have looked like good ideas – and perhaps still do too many – but now the tables have drastically turned: these pines have spread like wildfire and are causing havoc on the ecosystem. Indeed, those pine trees have the tricky habitat not to remain where they are, and especially in the native Chilean landscape of Araucaria forests and Patagonian steppe, pine trees come across very little resistance. As a result, the once so characteristic landscape is now a tangled mess of pine branches and trunks.
But what’s happening beneath these dense canopies? How much are the microclimate and soil suffering from these stubborn pine trees? And, even more importantly, what is the impact of these changes on the native vegetation? In a recent study led by our Chilean partners from the University of Concepcíon, we set out to investigate this effect for invasions of lodgepole pine (Pinus contorta). We measured everything from temperature to soil pH to nutrient availability across a gradient of pine biomass, from scattered individuals to dense entanglements.
Our results – published in ‘Diversity – were alarming – the impact was as huge as we suspected based on visual assessment alone. The more pine trees, the worse it got. The local micro-environment was drastically altered, resulting in a loss of light and nutrients, decreased pH, and increased litter depth, amongst others. But our biggest worry? The native plant diversity was virtually wiped out in the most densely invaded plots.
Our study revealed that it is probably the interplay of all these environmental factors – all so dramatically changed from background conditions – that explains the progressive drop in native plant species in the understory. Indeed, few native species likely have the necessary flexibility to deal with changes in all these defining environmental characteristics at once. The mechanisms behind the loss of biodiversity of native species associated with plant invasion would thus not only depend on the competition exerted by P. contorta, but also on the modifications that this species exerts on the abiotic environment. Moreover, these microenvironmental changes can have significant effects on other functional groups (e.g., pollinators, decomposers) with important consequences for the whole trophic network of the invaded ecosystems.
So, what’s the plan of action? Can we get rid of these pine trees, and if so, what happens to the microenvironmental conditions? How long will it take for the ecosystem to bounce back? Once that question is answered, it is of course still a matter of finding affordable solutions to get those stubborn pine trees out of there…
García, R. A., Fuentes-Lillo, E., Cavieres, L., Cóbar-Carranza, A. J., Davis, K. T., Naour, M., Lembrechts, J.J. & Pauchard, A. (2023). Pinus contorta Alters Microenvironmental Conditions and Reduces Plant Diversity in Patagonian Ecosystems. Diversity, 15(3), 320.