Date Published: April 24, 2019
Publisher: Public Library of Science
Author(s): Alessandro Filazzola, Amanda Rae Liczner, Michael Westphal, Christopher J. Lortie, Jian Liu.
The mechanisms supporting positive ecological interactions are important. Foundation species can structure desert biodiversity by facilitating seedbanks of annual plants, but the direct and indirect mechanisms of shrub effects on seedbank have not been experimentally decoupled. We conducted the first test of shrubs increasing seedbank densities through direct effects on the seedbank (i.e. shrub seed-trapping, animal-mediated dispersal) and indirect effects by facilitating the annual plant community (i.e. seed deposition, annual seed-trapping). Two distinct desert ecosystems were used to contrast transient seedbank densities in shrub and open microsites by manipulating annual plant density and the presence of the persistent seedbank. We measured transient seedbank densities at the end of the growing season by collecting soil samples and extracting seeds from each respective treatment. Transient seedbank densities were greatest in shrub canopies and with relatively higher annual plant densities. The persistent seedbank contributed to transient seedbank densities only in one desert and in the open microsite. Shrubs indirectly increased seedbank densities by facilitation the seed production of the annual plants. Therefore, shrubs are increasing seedbank independently of the annual plant community, likely through trapping effects, and dependently by facilitating seed production of the annuals. These findings provide evidence for a previously undescribed mechanism that supports annual seedbanks and thus desert biodiversity. We also identify shrubs as being significant drivers of desert plant communities and emphasize the need to consider multiple mechanisms to improve our ability to predict the response of ecosystems to change.
Seedbanks contain the biodiversity of annual plants in deserts, but their density is variable. Different subsets of plant species germinate in response to precipitation variation with the remaining species present within the seedbank [1,2]. Mechanisms that maintain and contribute to desert seedbanks are thus responsible for the long-term persistence of species within the annual community. These mechanisms can be classified into seed production, seed persistence (i.e. granivory or decomposition), and seed dispersal [3–5]. Understanding the mechanisms that contribute to seedbank densities can improve our ability to predict how desert communities will respond to change. However, previous research exploring the mechanisms that contribute to seedbanks has been generally focused on climate patterns , disturbance , or animal interactions . Dominant plants can also play a dominant role in structuring seedbanks, although these relationships are relatively understudied . Dominant plants in deserts, such as shrubs, can increase the density of seedbanks within the canopy relative to open spaces through trapping dispersing seeds [3,5,10]. Exploring the mechanisms that shrubs contribute to seedbanks is important because it could be a significant driver that maintains desert biodiversity.
Positive interactions are important for structuring the communities of desert plants . Here, shrubs consistently increased seedbank densities both dependently and independently of facilitation effects on the annual plant community. Our predictions that shrubs increase seedbank densities and increased annual seed deposition were supported. The density of annual plants was also related to increased seedbank densities in both shrub and open plots. Indirect shrub facilitation by annual plants trapping seeds was not a significant effect that contributed to transient seedbank densities, but this could be due to the artificial annuals that did not effectively mimic the seed trapping effect of annual plants. The persistent seedbank did not significantly contribute to the transient seedbank in either desert or microsite except the open microsite in Panoche Hills. Within our study, shrubs did not increase the persistent seedbank in either desert system and thus were not likely increasing the temporal heterogeneity of seedbanks. Instead, these findings support that persistent seedbank densities were similar throughout time and that the supply was balanced with the consumption of the persistent seedbank. Trapping by shrubs, but not annuals, and indirect facilitation by shrubs on annual plant reproduction are dominant mechanisms that increase densities and spatial heterogeneity of seedbanks.