DOI: 10.5061/dryad.2547d7wsc
title: The importance of facilitation on community assembly disappears under severe drought stress

description: Despite the recognition of positive interactions as an important driver of
 species coexistence and community structure, the underlying mechanism of
 how facilitation affects assembly processes along stress gradients is
 poorly explored. Understanding the responses of functional diversity to
 benefactor species at the extreme end of the stress gradient could provide
 valuable insight about facilitation-involved assembly mechanisms and
 contribute to the predictions of species coexistence under climate change.
 In the drought-stressed community in the Badain-Jaran Desert, the
 responses of the local community to the nurse shrub species Calligonum
 mongolicum Turcz. were evaluated using hierarchical Bayesian models. For
 the 3-year experiment, summer rainfall in each year formed a natural
 gradient of drought stress. To evaluate the shrub’s effects on the
 assembly process along that gradient, individual samples were collected in
 pairwise under-shrub and open habitats, and four traits related to stress
 tolerance and resource acquisition were measured simultaneously. Under
 moderate drought stress, we observed shifting community-weighted means,
 broadening ranges and reducing overlaps of functional traits under shrubs.
 These effects were partly driven by a distinct microenvironment created by
 shrub plants, in particular the improvement and heterogeneity of soil
 moisture conditions. However, this influence on trait distributions was
 strongly dependent on the environmental context, and generally disappeared
 as drought stress shifted toward its driest end, almost in line with the
 decreased positive interaction assessed by plant density and species
 diversity. This study focused on water-limited community that lies at the
 driest end of drought gradient and confirmed that facilitation can drive
 the assembly process through both environmental filtering and niche
 differentiation. More importantly, these assembly mechanisms are proven to
 become less efficient under extreme drought stress, which may suggest the
 occasionally disappearing role of benefactor plants on community assembly
 and an increasing risk of biodiversity loss in the context of climate
 change.
description:   This investigation was conducted in a
 typical desert grassland at the southern edge of the Badain Jaran Desert
 from 2017 to 2019. During the investigating period, summer rainfall
 declined from 71.4 mm in 2019 to 48.6 mm in 2017 and 25.6 mm in 2018.
 Through this investigation, we studied the effect of foundation plant (<em>Calligonum
 mongolicum</em>, shrub) on local-scale assembly process along an interannual
 drought gradient in water-limited communities. Species relative abundance
 was investigated in both under-shrub and open habitats, which can be used
 to measure the effect size of shrubs on local communities under different
 stresses. Trait distributions (i.e., community-weighted mean, trait range,
 trait spacing) among habitats were measured to evaluate the effect of
 shrubs on community assembly under different drought stresses.
 Additionally, the effect of shrubs on soil moisture/water content (SWC)
 and soil organic matter (SOM) under different drought stresses were also
 investigated, which can be used to determine the underlying mechanism that
 driving the assembly process. To measure species relative abundance, a
 single 2 m × 2 m quadrat was established beneath the canopy of
 stand-alone <em>Calligonum mongolicum</em>, while another
 quadrat of the same size was placed in an open area. In these quadrats,
 species was identified, and the species-specific abundance was
 recorded. To measure functional traits at different years and
 habitats, we randomly selected 2-5 adult individuals for every observed
 understory species according to their abundance in each selected quadrat
 at both habitats. Five species were omitted from the analyses because they
 were too rare to accurately estimate trait values. To obtain robust
 estimation, more than 30 individuals for each species were sampled in
 different years and habitats. When the suitable samples of a species did
 not reach this amount, we randomly collected additional individuals from
 the same habitats in the fenced plots. Lateral spread (cm) was measured as the maximum crown
 width of individuals in the understory community. Height (cm) was measured
 as the distance between the extended upper boundary of the crown and the
 ground level. SLA (cm∙g<sup style="color:#333333;font-family:'Times New Roman' , serif;">-1</sup>) was determined as the ratio of fresh leaf area to leaf
 dry mass. LDMC (mg∙g<sup style="color:#333333;font-family:'Times New Roman' , serif;">-1</sup>) was determined as the ratio of leaf dry mass to fully
 rehydrated fresh mass. To measure soil moisture/water content (SWC), we collected
 four soil cores (5 cm diameter, 10–20 cm in depth) along the diagonal of
 each quadrat, and determined them gravimetrically after oven-drying for 48
 h at 110°C. To measure soil organic matter (SOM), we
 randomly chose 10 pairwise quadrats each year, collected four soil cores
 within each quadrat, and applied the dichromate oxidation
 method.