Coping with changing plant-plant interactions in restoration ecology: Effect of species, site, and individual variation

Prepared by Pedro J. Garrote, Antonio R. Castilla & Jose M. Fedriani

The Mediterranean dwarf palm (Chamaerops humilis) spreads along the typical landscape of study sites in Doñana National Park (south-western Spain). (Photo credit: Pedro J. Garrote)

Most ecological interactions can usually be positive, negative, or neutral, depending on the balance between benefits and costs for their participants. Understanding the ecological and applied implications of these shifts along a continuum is critical for increasing our knowledge of ecosystem functioning and how we can use it to revert the global degradation of habitats. A good example of ecological interactions applied to ecosystem restoration is the use of plant-plant interactions to ensure the revegetation success, especially in stressful environments such the arid, semi-arid or alpine habitats. The participants of these interactions are nurse plants that can facilitate the survival of beneficiary plants. However, it is well-known that these interactions usually change from facilitation to competition and vice versa depending on the ecological context. For instance, plant species A can have positive effects on species B but negative on species C, or these effects can vary between different localities or plant individuals. Therefore, if we can profoundly understand these shifts in plant-plant interactions along the facilitation-competition continuum, we could improve restoration programs by increasing the success of revegetation actions.

To illustrate the potential shifts that we can detect in plant-plant interactions, we chose as study species the Mediterranean dwarf palm (Chamaerops humilis), which is an endemic species from the Western Mediterranean basin and is considered as a keystone species in multiple ecological processes (e.g. food resource, shelter for vertebrates and invertebrates, plant-plant facilitation). Thus, we conducted field experiments in two human-degraded sites within Doñana National Park in south-western Spain. In these experiments, we evaluated seed and seedling survival of seven common woody plant species beneath and outside the dwarf palm. To do this, we selected 15 dwarf palms and 15 adjacent open spaces in both study sites, where first, we offered 7200 seeds of the seven target species and counted the remaining seeds after four days. Second, we sowed 3,600 seeds of the seven target species and monitored seedling emergence, survival, and recruitment once a month for one year.

Jose M. Fedriani (left) and Pedro J. Garrote (right) during the monitoring of field experiments in spring 2019. (Photo credit: Antonio R. Castilla)

Our results show how a nurse species, the dwarf palm, can have positive, negative, or neutral effects on woody plants depending on the species, the site, and the individual. Thus, the dwarf palm had negative effects on seed survival. By contrast, the dwarf palm strongly facilitated woody species since none of them showed greater survival and recruitment in open spaces than beneath it. The most interesting result is that the net effect of dwarf palm-woody species interaction shifted along plant life stages and depended enormously on the strong variation among the chosen individual palms. Facing these results, we emphasize the importance of considering the inter-individual variation along the facilitation-competition continuum and its implications for the restoration of human-degraded ecosystems. We also propose multiple management recommendations from an individual level to plant communities that could critically increase the success of revegetation actions in restoration programs.

This is a plain language summary for the paper of Garrote et al. published in Applied Vegetation Science (