Shrubs indirectly increase desert seedbanks through facilitation of the plant community

Abstract

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.

Soil seed bank characteristics in relation to different shrub species in semiarid regions

“…Previous studies showed that the size of a shrub could impact the arrival of other plant seeds (Pugnaire & Lázaro, 2000) because larger shrub canopy surfaces can provide greater facilitative effects on plants. Shrubs with higher canopy surface and taller can intercept more solar radiation (Maestre & Cortina, 2005), have higher soil nutrients (Zhang, Yang, Wang, & Zhao, 2015), or lower evapotranspiration (Kidron & Gutschick, 2013) creating a favorable microclimate for seed production, particularly by annuals (Filazzola et al, 2019). As a result, compared to the other shrubs, taller and larger canopy in A. scoparia might increase SSB density through higher seed production by other plants.…”

Restoring closed trails in the Sonoran Desert: interactions of seed timing, seed source, and ripping

“…For the soil bank treatment, on 16 November 2016, we collected 0.5 L of topsoil, including the litter on top, to a depth of 6 cm from each of 10 individual shrubs or trees in a parallel band 5 and 20 m from the segments. Soils under shrubs and trees, and the litter, foster high seed bank densities in arid environments (Guo et al 1998; Filazzola et al 2019). Seeds in arid environments are densest in the top 0–1 cm but can be unevenly distributed throughout a 10‐cm profile with smallest and largest seeds on the surface and medium to large seeds settling lower in the profile (Guo et al 1998).…”

Dominant plant facilitation can generate indirect competition in a South‐American desert plant community

“…Increased competition within understories likely acts as a mechanism promoting two‐phase vegetation mosaics (i.e. understorey and open microsites), because these microsites can sometimes regulate or filter the emergence of dominant plants via the classical direct effects of nurse plants, but also via indirect effects mediated by neighbourhoods, in combination with other spatiotemporally variable mechanisms such as seed trapping or rainfall (Giladi et al, 2013; Cipriotti et al, 2014; Cipriotti & Aguiar, 2015; Filazzola et al, 2019). Importantly, understorey microsites in stressful environments can open doors to invasions through facilitation and biotic acceptance (Flory & Bauer, 2014; Badano et al, 2015; Lucero et al, 2019), which have the potential to compete and negatively impact the facilitator species (Rodríguez‐Buriticá & Miriti, 2009; Filazzola et al, 2020).…”

“…Increased competition within understories likely acts as a mechanism promoting two-phase vegetation mosaics (i.e. understorey and open microsites), because these microsites can sometimes regulate or filter the emergence of dominant plants via the classical direct effects of nurse plants, but also via indirect effects mediated by neighbourhoods, in combination with other spatiotemporally variable mechanisms such as seed trapping or rainfall (Giladi et al, 2013;Cipriotti et al, 2014;Cipriotti & Aguiar, 2015;Filazzola et al, 2019).…”