Aboveground net primary production in drylands is tightly linked to annual precipitation amount. Higher precipitation, resulting from either natural variability or experimental irrigation, leads to higher production. Natural or experimental droughts yield opposite trends. We hypothesized that changes in production due to changes in precipitation result from changes in phenology, or the timing of plant activity (i.e., growing-season length), while keeping instantaneous rate of production constant. The alternative hypothesis is that phenology is fixed and instantaneous rate of productivity is variable. A combination of flexible phenology and productivity rate is also possible. 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 phenology 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 peak growth, where shrubs peaked May-Aug and grasses Aug-Sept. Our fixed vs. flexible phenology hypothesis was supported in the drought treatment and partially supported in the irrigated treatment. Drought lowered the peak photosynthetic capacity of shrubs but did not alter the growing season length (fixed phenology response). Conversely, Grasses maintained peak photosynthetic capacity but showed a significantly shorter growing period length (flexible phenology response). Irrigation also had differential effects. Grasses showed increased photosynthetic capacity compared to the control with no change in growing-season length (fixed response). Irrigated shrubs had a counter-intuitively lower photosynthetic capacity compared to the control, but higher than drought, with no change to growing-season length (fixed response). These results indicate that shrubs and grasses respond differently to precipitation changes. Grasses exhibit a combination of flexible and fixed phenology responses depending on water availability whereas shrubs maintain a fixed response. Overall, drought had a stronger effect on plant phenology than increased precipitation. Shrub response to competition (irrigated) vs. water availability (drought) may explain the differential responses to the treatments compared to the control and provide further insight to the drivers of woody plant encroachment in semi-arid grasslands.