Shrub and grass ANPP
The ANPP of each shrub species varied differently among treatments and
years. In the wet year (2017), ANPP of A. prolifera in the
nitrogen addition plots doubled ANPP in the control plots
(p =0.04) (Fig. S6). Moreover, the ANPP of A. prolifera was
higher in 2017 than in 2016 in the +N treatment
(p <0.001) and was higher in 2017 than in the others
studied years in the +NW treatment (p <0.05). The water
addition treatment only affected A. volckmannii such that its
ANPP increased in the +W plots (p =0.005) and, in addition, this
species increased ANPP during the wet year, being more than double in
2017 than in the rest of the years (p <0.01) (Fig. S6).
The ANPP of S. filaginoides decreased in +NW compared to C
(p =0.001) and was higher in 2017 than in 2019 in all treatments,
with intermediate values in 2016 and 2018 (p <0.05)
(Fig. S6).
Total shrub ANPP in the +N, +W and +NW treatments were not significantly
different compared to the control (Table 3, Fig. 4A). Shrub ANPP was
about two folds higher in 2017 as a consequence of the anomalous
precipitation in late March of that year than in the other study years,
regardless of treatments (p≤0.01) (Table 3, Fig. 4A). Grass ANPP was not
affected by the treatments until two years after the start of the
experiment (Fig. 4B). From 2015 to 2019, grass ANPP was higher in +N and
+NW than in C (p <0.02) (Table 3, Fig. 4B). There was no
difference between grass ANPP in +W and C (p >0.5).
When comparing within-treatment grass ANPP across years, we found no
interannual differences in the control (Fig 4B). However, grass ANPP in
nitrogen addition plots was higher in 2017 and 2018 than in 2014
(p <0.02) and ANPP in +NW treatment was higher in 2017
than in 2014 (p =0.03) (Fig. 4B). Community-level ANPP (shrubs +
grasses) increased significantly in the treatments with nitrogen
addition (p <0.001) compared to the control (Table 3,
Fig. 4C). The total ANPP was higher in the wet year (2017) than in 2016
(p =0.001) and 2019 (p <0.001) (Fig. 4C).