Plant production decreases more than litter decomposition with rising aridity in drylands

Climate change increases aridity in many drylands worldwide, which has
significant consequences for ecosystem functioning and may reduce carbon
sequestration. However, responses of major carbon cycle processes in
drylands, including primary production and decomposition, to increasing
aridity remain poorly understood. In this study, we assessed the
quantitative effects of precipitation and the underlying impacts of
functional traits on aboveground net primary production (ANPP) and plant
litter decomposition in herbaceous Mediterranean plant communities. A
dataset encompassing a wide range of precipitation (~50-1000 mm) was
generated by selecting four field sites along a natural precipitation
gradient, applying rainfall manipulations, and conducting the experiment
over 3-9 years. Across the entire dataset, ANPP and decomposition
decreased non-linearly with declining precipitation, showing steeper
reductions at low compared to high precipitation levels. Notably, above
~400 mm, the two processes followed a similar pattern, but below this
threshold, ANPP dropped more rapidly, while decomposition decreased less
and remained relatively high. Plant functional traits associated with low
growth rates exacerbated the reduction of ANPP at the drier sites, whereas
higher litter quality at these sites compared with the wetter sites
enabled relatively high rates of litter decomposition. The latter findings
were confirmed by a litter transplantation study, where litter from the
arid site decomposed faster at all sites compared to litter from the
wetter sites. In addition, dryland decay mechanisms, such as
photodegradation and microbial degradation enabled by non-rainfall water
sources might have mitigated some of the dryness-related decrease in
decomposition. Increasing climate change-induced aridity in drylands may
drive long-term shifts in herbaceous vegetation composition toward
smaller, less productive species that produce more labile litter. This
trend is expected to accelerate the decline in production more than the
decline in decomposition, likely reducing carbon sequestration.