Invasive species are a major driver of global change and their spread leads to the degradation of many natural ecosystems worldwide. Invasive trees and woody shrubs are known as “ecosystem engineers”, as they can profoundly change ecosystems, sometimes up to a point where a return to the original state is impossible. We have just shown that the invasive potential of Acacia longifolia is associated with its strategic use of nutrients, where trading N for P may be a major component.
Our results clearly show that soil changes pave the way to invasiveness, because they establish the relation between nutrient demand and availability, favoring to the invasive plant, and shaping the new ecosystem.
In a recent study  we explored the hypothesis that early stage invasion processes at soil level must play a fundamental role in plant establishment until invasive plants are able to outgrow the native vegetation. We examined the very early stage of invasion by a nitrogen-fixing woody legume (Acacia longifolia), growing in the canopy of native dune shrubs (Corema album) as a model system in oligotrophic primary dunes in southern Portugal. This model was chosen as soil organic matter changes can be observed more easily in oligotrophic ecosystems, which are naturally very poor in organic matter and soil nutrient concentrations.
Using foliar δ15N as a proxy for nitrogen derived from the invasive legume, we could observe a clear impact on the leaves of the native shrubs by the N-fixing invasive growing in its canopy. Also, we found far more organic matter in the soil under canopies of invaded native shrubs, compared to non-invaded native shrubs. This organic matter difference was mainly related to higher root and rhizosphere biomass and also translated into higher overall phosphorus levels in the soil. Interestingly, the effect of A. longifolia presence on C. album foliar δ15N was clearly related to this belowground biomass increase.
At the same time, the native plant was not able to respond to higher phosphorus levels under its canopy at increased foliar phosphorus concentrations, while in contrast, the invasive showed a high plasticity in foliar phosphorus concentrations, exhibiting a positive correlation with belowground phosphorus concentrations.
How Invasive Plants Outcompete Native Plants For Nutrients
These findings are in line with earlier observations in these highly oligotrophic systems, which pointed out that the invasive might exhibit a higher phosphorus use efficiency and thereby outperforms the native vegetation by using less phosphorus per biomass produced. The resulting biomass increase then has other secondary effects, such as competition for light, water, and other nutrients, which ultimately lead to an increased invasion pressure and a breakdown of the native plant community.
Apart from plant-derived biomass changes below ground, we also found higher levels of the silt-clay fraction under invaded native shrubs. This finding was somewhat unexpected, as we did not anticipate strong effects on soil structure and particle distribution itself. While the changes were small, they might nevertheless have a substantial impact, as this fraction is normally connected to high microbial activity and water and nutrient retention. However, this fraction was not only increased per se but also exhibited higher organic matter contents, depleted δ13C and increased δ15N signatures.
The higher δ15N signatures point out that the intensification of this fraction is clearly related to the invasive legume and while the observations of stronger depletion in δ13C warrants further research into this soil compartment, it could be associated with a higher fungal presence in the soil. Interestingly, this fraction was also related to the other belowground biomass changes, as well as the foliar effects observed. Thus, it might play a crucial role in the early invasion process and, together with the ratio between organic matter or phosphorus in the biotic compartment (e.g. roots and rhizosphere) versus the abiotic soil compartment (e.g. soil fractions), could be a good early indicator for invasion.
In conclusion, we found considerable differences between belowground biomass pools under invaded vs. non-invaded native shrubs and connected them to the invasives’ presence, even at a stage where plants were still small and aboveground effects minimal. These findings suggest that even when aboveground invasion pressure on the system is low, it is imperative for ecosystem conservation to remove young plants, as they might alter soil functioning already at an early stage of invasion.
- Ulm, F., Jacinto, J., Cruz, C., & Máguas, C. (2017). How to outgrow your native neighbour? Belowground changes under native shrubs at an early stage of invasion. Land Degradation & Development.
- Ulm, F., Hellmann, C., Cruz, C., & Máguas, C. (2017). N/P imbalance as a key driver for the invasion of oligotrophic dune systems by a woody legume. Oikos, 126(2).
The study, How to Outgrow Your Native Neighbour? Belowground Changes under Native Shrubs at an Early Stage of Invasion was recently published in the journal Land Degradation & Development.