Relationship between aboveground productivity and annual water
inputs (ambient precipitation and experimental water addition)
Arid and semiarid ecosystems are considered highly sensitive to
precipitation changes (Haverd et al., 2016; Huxman et al., 2004; Maestre
et al., 2012). Several studies show a linear relationship between
interannual ANPP and precipitation (Sala et al., 2012; Wang et al.,
2014). Recently, however, Knapp et al. (2017) proposed that with the
inclusion of precipitation extremes, the common linear relationship may
change to a nonlinear one, reflected in “concave up” or “concave
down” ANPP-precipitation relationships. Consistent with the linear
model, we found symmetric relationships with ANPP of each functional
group and total ANPP under all treatments, even when our data set
included an anomalous year. In previous studies in a similar Patagonian
steppe (Fernández et al., 1991; Jobbágy & Sala, 2000), at least grass
ANPP was not associated with precipitation.
The sensitivity of grass ANPP to annual water inputs was higher in the
+N and +NW treatments than in the C and the +W treatment. Therefore, our
results suggest that increasing N availability can enhance ANPP response
to increased precipitation, as shown by other studies (Ma et al., 2020;
Meng et al., 2022; Zhang et al., 2021). When water availability is not a
limiting factor for plant growth, as was observed for grasses in our
study, plants often exhibit increased biomass production in response to
improved soil N availability. Differences in the slopes of the
ANPP-water input relationships suggest that grasses in the control plots
were generally less sensitive to annual precipitation than shrubs. It
provides evidence that soil water redistribution during wet years
benefits deep-rooted species more than grasses, as grasses are less
water-limited because shallow soil layers are recharged during the fall
and winter seasons.
At the community level, although N enrichment increased productivity, it
did not improve the sensitivity of ANPP to precipitation, probably due
to the combined effects of precipitation and nitrogen addition on shrubs
and grasses, highlighting the importance of community structure. The
lower intercept value of the function fitted to the +W treatment
compared to the control suggests higher nutrient limitation by
increasing plant N demand or N losses (Mudge et al., 2017; Ren et al.,
2017). However, in previous works in the Patagonian steppe, Carbonell
Silletta et al. (2022) found no change in soil nutrient availability
with water addition and Cavallaro et al. (2023) found increased rates of
transpiration and photosynthesis in +W plots. This suggests that by
increasing water input without nitrogen addition, more photosynthates
are allocated to other functions rather than aboveground growth.
Conclusions
Our results show that nitrogen addition rather than water addition
improves community ANPP, mainly due to the effects on the grasses and to
the shift in the dominance of P. ligularis to P. humilis .
Our study revealed that the relationship between ANPP and annual
precipitation can be described by linear models across dry and wet
years. Grasses and shrubs responded differently to increased
precipitation, with shrubs being more sensitive than grasses to wet
years. However, under increased nutrient availability, the response of
grass ANPP to annual water input was amplified (i.e., steeper slope of
the ANPP-water input relationship), whereas shrubs showed no change in
ANPP with nitrogen addition. Therefore, the higher sensitivity of shrub
ANPP to increased precipitation compared to that of grasses suggests
that under climate change scenarios with increased precipitation, shrubs
could dominate the community, whereas if this change occurs with high N
deposition, an encroachment of grasses by a single species may take
place in the long term.
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Table 1. Family, life form, leaf phenology, plant height and maximum
rooting depth for species of Patagonian steppe included in this study