Where does the forest come back from? Soil and litter seed banks and the juvenile bank as sources of vegetation resilience in a semiarid Neotropical forest

By María Lucrecia Lipoma, Valentina Fortunato, Lucas Enrico & Sandra Díaz

Above: Litter sample collection for the identification of seeds in the litter seed bank. Bellow: Juvenile plant bank of the dominant woody species of the ecosystem of the Chaco forest, Aspidosperma quebracho-blanco. Photo credit: Lucrecia Lipoma.

Resilience, the ability of ecosystems to recover after disturbances, has received a lot of attention in recent years and we are making progress towards the understanding what makes an ecosystem resilient, and how resilience can be threatened. These are some of the questions that motivate our studies in the Chaco forest ecosystem of Argentina, the most extensive seasonally dry forest in South America.

The existence of biodiversity reservoirs from which species recruit after disturbances is a key factor for the recovery of terrestrial ecosystems. If these reservoirs have “memory” of the reference vegetation, they will act as sources of resilience and contribute to its regeneration. However, are different reservoirs equally important for different ecosystems? The soil seed bank is by far the most studied reservoir. It is a widespread practice in vegetation studies to collect and identify species in the soil seed bank, as an effort to understand past and present vegetation dynamics. It is, however, also very common to find many species in the vegetation that are not present in the soil seed bank. This is especially true for woody ecosystems, where other reservoirs, like the litter seed bank (seeds that are trapped in the litter and do not penetrate the soil layer) and the juvenile bank (individuals that remain immature until they grow to adult size) are also important in the regeneration process. So, the first question we aimed to answer was: where do woody species regenerate from in the Chaco forest?

The Chaco forest ecosystem has experienced important disturbances related to land-use in the last few decades, which have affected the vegetation and could have consequences for regeneration. Since an ecosystem’s resilience can be compromised when its sources of resilience are affected by disturbances, our second question was: is land use affecting the reservoirs in the Chaco forest?

With these questions in mind, we described the vegetation of different communities that belong to the Chaco forest and had different histories of land use. We collected soil and litter samples and identified seeds from each reservoir, and we also recorded the species present in the bank of juvenile plants.

The Chaco Forest ecosystem in the area of the Natural Reserve “Chancani” in the northwest of Córdoba, Argentina. Photo credit: Lucas Enrico.

We found that all woody species from the Chaco forest were found in the litter and the juvenile banks, indicating that regeneration in this ecosystem strictly depends on the existence of these reservoirs, but seems not to be related with the existence of soil seed banks. We also identified that, although the litter and the juvenile banks are very similar to the vegetation of the Chaco forest in terms of species composition, such similarity decreases as land use intensifies, compromising the ability of these reservoirs to act as sources of resilience in a future.

According to our results, ecosystems that have suffered low intensity of land use will have more chances to recover, mainly because they preserve the “memory” of the past vegetation in the litter and juvenile banks. However, if the regeneration is more dependent on these reservoirs, it could be highly vulnerable to disturbances acting at the ground level, such as fire, and seed predation or herbivory on seedlings, which often do not affect seeds buried in the soil. These ideas will bring new questions for future studies to improve our understanding of the dynamic of this emblematic ecosystem.

This is a plain language summary for the paper of Lipoma et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12842). The post was prepared by M. Lucrecia Lipoma & Lucas Enrico.