Aboveground net primary production (ANPP) in drylands is tightly linked to mean annual precipitation amount, where higher precipitation leads to higher production, while droughts yield opposite trends. We hypothesized that changes in ANPP due to changes in precipitation result from shifts in growing season length, while keeping the instantaneous rate of production constant. The alternative hypothesis is that phenology is fixed and the instantaneous rate of productivity is variable. A combination of these two hypotheses is also possible: systems could exhibit flexible season length and instantaneous productivity rates. We tested this phenological sensitivity hypothesis between two plant functional types: shrubs and grasses. We predict that shrubs are more likely to exhibit fixed growing-season length response to precipitation and grasses a flexible response. Our objective was to understand the temporal relationship between plant growing-season length and precipitation changes over a multi-year period. We installed time-lapse cameras just outside of plots in the Jornada LTER (NM, USA) across a range of experimental rainfall manipulations (-80%, 0, +80%) and analyzed the daily images for quantitative greenness (an estimate of photosynthetic capacity) through time.
Results/Conclusions
Shrubs and grasses showed differences in the onset and duration of growth, where shrubs exhibited growth from May-Aug and grasses Aug-Sept. Our results were contrary to the fixed vs. flexible phenology hypothesis: an increase in water availability was significantly and positively related to increase in grass greenness, however, treatments did not alter the growing season length (fixed phenology response). Conversely, there were no treatment effects on shrub greenness or growing-season length. We also found a significant positive relationship between grass greenness and ANPP, indicating an ecophysiological rather than phenological driver of changes in grass productivity. Overall, drought had a stronger, negative effect on grass greenness than increased precipitation. Grass sensitivity to low water availability, combined with phenological differences from shrubs, further suggests risk of grassland to shrubland transitions in drylands with increasing probabilities in the frequency and magnitude of droughts.