Tree invasion in secondary grasslands diminishes herbaceous biomass and diversity: A study of mechanisms behind the process

By Antonella Ferraina, Cecilia D. Molina, Noemí Mazía, Gervasio Piñeiro, Isabel Miranda & Enrique José Chaneton

Experimental design used to evaluate the effect of tree invasion and shading on grasslands. Photo credit: Antonella Ferraina

Woody-plant invasion in grasslands is a global problem widely reported during the last decade. The Argentinean Pampas region was historically characterized by the lack of trees, which were introduced during the colonial period as a source of shade, firewood and ornamental species. Nowadays, several tree species are invasive in the region, mainly Gleditsia triacanthos, Morus alba, Melia azedarach, Ligustrum lucidum and Pinus sp. In particular, Gleditsia triacanthos (honey locust) was introduced in the mid-nineteenth century and is currently the most aggressive exotic woody-plant species in the region. Understanding the mechanisms driving plant structure is critical both for the knowledge of biological invasion and also for the conservation and management of this threatened and poorly conserved biome.

Resource competition is one of the main mechanisms in altering the structure and functioning of invaded grassland communities. However, changes in light conditions promoted by trees may have positive, negative or neutral effects depending on grass and tree characteristics and climate conditions. In addition, woody plants can alter soil conditions by leaving a soil legacy in invaded patches affecting the plant community composition in the future. Soil legacies involve changes in soil seed bank, litter quality and quantity, soil microbiota, nutrients and organic matter.

The objective of the present work was to evaluate the relative importance of the changes in light conditions generated by the invader tree Gleditsia triacanthos and the historical soil legacy of vegetation (grassland or woody-patch origins). This would allow disentangling the mechanism modulating above-and below-ground biomass, species composition and diversity of a secondary grassland.

Experimental design used to evaluate the effect of tree invasion and shading on grasslands. In the diagram, the color of the plots correlates with the background color of the origin (woody –W, or grassland –G). Source: this paper.

To do so, we established a long-term manipulative experiment by reciprocally transplanting soil plots coming from two origins, namely grasslands (G) and invaded woody patches (W), into three different habitats, open grasslands (OG), woody patches (WP), and artificially shaded grasslands (SG). The artificial-shading treatment was employed to simulate light conditions beneath a tree canopy, and to evaluate if the observed effects on the grassland community were based only on modifications in the light conditions or on other additional mechanisms.

Our results showed that grassland biomass (above- and below-ground), herbaceous litter and diversity were all lower in woody patches habitats, and shaded grasslands habitats showed intermediate values for plots of both origins. These biomass differences between open grassland and woody patches disappeared when the contributions of woody litter and tree roots were included. Additionally, this invasive deciduous tree promoted herbaceous changes mainly driven by the displacement of C4 grasses and forbs along with an increase in C3 grasses, since during spring and summer, tree shading could exclude light-demanding C4 grasses. Against expectations, we did not find strong evidence to relate the changes in community structure with a soil legacy, although only below-ground biomass changed with plot origin.

Aboveground herbaceous relative biomass in different plant functional types in OG, SG and WP habitats and with different plot origin: woody (W) and grassland (G). Source: this paper.

This is a plain language summary for the paper by Antonella Ferraina, Cecilia D. Molina and colleagues, published in the Journal of Vegetation Science ( Both Antonella Ferraina and Cecilia D. Molina jointly prepared this blog post.