Nurse plant shading is more important than soil fertility for dryland plant recruitment and diversity

”Islands” of plants surrounded by little vegetation cover in drylands are
thought to arise when larger plants facilitate recruitment via
“nurse-plant” effects. Simultaneously, plant-soil feedback processes can
increase soil nutrients beneath shrubs, leading to spatial patterns known
as “islands of fertility.” These phenomena shape dryland plant
communities; however, their relative influences on plant density and
diversity remain poorly quantified. Using reciprocal soil transplants
among pairs of unvegetated interspaces and neighboring shrubs in
combination with different cover treatments—open interspace, shaded
interspace, intact shrub canopy, and trimmed shrub canopy—this study aimed
to disentangle the influences of shading from soil properties in native
plant recruitment under the foundational shrub species Larrea tridentata
and Neltuma glandulosa in the northern Chihuahuan Desert, NM, USA.
Seedling density varied significantly across the three study years in
response to interannual climate variability and was consistently higher
under L. tridentata than N. glandulosa. Cover treatments, soil provenance,
shrub species, and spatial factors (plot location) significantly affected
seedling density, species richness, and community structure. Shading from
shrubs or shade clothes had a stronger effect on density and richness than
soil provenance or shrub species. Seedling density and richness were also
higher in soil from beneath shrubs, regardless of shrub species, location
(native or transplanted locations), or time (study year) since initial
translocation. The density of seedlings emerging from shaded interspace
soil was initially similar to that observed under intact shrubs,
indicating the importance of shading by nurse plants for recruitment.
However, the effect of shade-cloth canopy surrogates on seedling density
in interspaces switched from positive to negative by year three of the
study, likely due to seed bank depletion. The significant influence of
shading highlights the role of environmental factors in shaping plant
communities of this dryland system. In contrast, the evidence of seed
depletion combined with the influence of plot location suggests a
concurrent influence of stochastic community assembly processes.
Understanding how these processes interact to shape spatial and temporal
patterns of vegetation community assembly and potentially reinforce
alternate ecological states remains an important focus in dryland
ecosystems.