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. Opposite trends result from natural or experimental droughts. We hypothesized that changes in production due to changes in precipitation result from changes in 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 predicted that shrubs are more likely to exhibit fixed phenology response to precipitation and grasses a flexible phenology response. Shrubs typically outperform native grasses in terms of photosynthetic rates and growing season length across a range of soil moisture conditions. Our objective was to understand the temporal relationship between 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 color information (i.e., greenness, an estimate of photosynthetic capacity) through time.
Through the use of statistics and art, we aimed to unravel phenological responses to precipitation through an alternative form of data visualization. Shrubs and grasses showed different greenness peaks throughout their phenology cycle, where the peak for shrubs occurred during the months May-Jul (control) and for grasses during Aug-Oct (control). Our hypotheses were partially rejected. Drought treatments did not change the photosynthetic capacity in shrubs compared to the control but counterintuitively lengthened the growing period (from May-Jul to Apr-Aug). For grasses, photosynthetic capacity decreased compared to the control while the growing period remained unchanged. Shrubs in irrigation treatments showed decreased photosynthetic capacity during one month (June) compared to both control and drought treatments whereas irrigated grasses showed increased photosynthetic capacity for half of August. Irrigation had no effect on growing season lengths for either shrubs or grasses. These results indicate that shrubs and grasses respond differently to precipitation changes, and drought has a stronger effect on plant phenology than increased precipitation. This study is similar to larger scale research using satellite NDVI, but its fine-scale photos are more relevant for addressing management questions at the landscape level.