At the community level, the effect of the shrub on herbaceous biomass
(RII mean ± SE: 0.02 ± 0.03)
and richness
(RIImean ± SE: −0.06 ± 0.02) was almost neutral among all treatments, exceptRII on richness at W40N0 treatment which was slightly negative
(see result of sample t-test in Fig. 2).
Watering and N fertilization did
not significantly affect RII on biomass and richness (Table S1).
The RDA results showed that height and LDMC contributed to explain how
RII varied at the species level in response to watering and N
fertilization (R2 = 0.22; Fig. 3a). Four groups
of species were identified with the cluster analysis conducted on RDA
species scores (Fig. 3b and Table S2). Axis 1 opposed mostly group C,
including the tallest species (all rhizomatous perennial grasses, e.g.,L. secalinus and P. flaccidum ), to group A including the
shortest ones (mostly annuals, e.g.,Euphorbia
humifusa and C. puberulum ), while group B (including mostly
annuals, e.g., Incarvillea sinensis and S. viridis ) and
group D (including all bunchgrasses, e.g., Cleistogenes
squarrosa, Agropyron mongolicum, and Stipa glareosa )
contained species of intermediate heights. Axis 2 opposed mostly group
D, including species with the highest LDMC values, to group B with the
lowest ones, while groups A and C contained species with intermediate
LDMC values.
Watering and N fertilization had contrasting effects on the biomass of
the four cluster groups in the quadrats away from the shrubs (Fig. 4).
The biomass of Group A increased with watering (F = 21.85, P< 0.001), in particular from W20 to W40 treatments (see Tukey
test in Fig. 4). The biomass of groups B and C increased with N
fertilization
(F
= 6.77, P = 0.03 and F = 15.73, P < 0.001,
respectively), in particular from N10 to N60 treatments for both groups.
The biomass of Group D only
decreased
with important N fertilization in the W40 treatments
(significant
water × N interaction; F = 11.28, P = 0.02). Contrasting
responses to watering and N fertilization among cluster groups were also
observed for RII . RII of group A decreased with N
fertilization
(from
overall neutral RII values in N0 (0.15
± 0.13) and N10 (−0.10 ± 0.13)
treatments to negative RII values in N60
(−0.41
± 0.12) treatments; F = 10.92, P = 0.004), whereas that of group
C increased with watering (from neutral RII values in W0 (0.02 ±
0.12) and W20 (0.11 ± 0.14) treatment to positive RII values in
W40 (0.41 ± 0.09) treatment; F = 6.98, P = 0.03; see the results
of Tukey test in Fig. 4). RII of Group D strongly increased with
N fertilization in the W40 treatment, but slightly decreased with N
fertilization in the two other water treatments (significant water × N
interaction; F = 21.24, P < 0.001; see the results of
Tukey test in Fig. 4). Watering and N fertilization had no significant
effects on RII of group B, which values were overall slightly
negative (−0.17 ± 0.06). The responses of RII s for each cluster
group were consistent with the positions of these groups and the
direction of treatment arrows in the RDA (Fig. 3 and Fig. 4). This was
in particular the case for group C, located on the left of axis 1, in
the same direction than all W40 treatment arrows, and for group A,
located on the right of axis 1, in the opposite direction than all N60
treatment arrows. Finally, group D was located on the upper part of axis
2 in the same direction than the W40N60 treatment arrow and the opposite
direction of the W40N0 one.