Plant responses to rainfall frequency and intensity variations from field to global scales

Regardless of annual rainfall amount changes, daily rainfall events are becoming more intense but less frequent across Earth’s land surfaces. Larger rainfall events and longer dry spells ­have complex and sometimes opposing effects on plant photosynthesis and growth, challenging abilities to understand broader consequences on the carbon cycle. Cross-scale analyses are ultimately needed to quantify responses of vegetation function to fewer, larger rainfall from different data sources, disentangle the complex driving mechanisms of the plant responses, and scale findings from field to global scales.

Here, we ask, to what degree is global vegetation function sensitive to shifts in daily rainfall frequency and intensity, especially when compared with variations in annual rainfall totals? Is global vegetation function (and terrestrial carbon uptake via photosynthesis) higher or lower in years with less frequent, more intense rainfall?

First, we collate field, model, and satellite studies that investigate the effects of fewer, larger rainfall events, while controlling for annual rainfall amounts. Plant function responses vary between -28% to 29% (5th to 95th percentile) in years with fewer, larger rainfall events compared to nominal years, with the sign of response contingent on climate; productivity increases are more common in dry ecosystems (46% positive; 20% negative), whereas responses are typically negative in wet ecosystems (28% positive; 51% negative) in years with fewer, larger rainfall events. Field scale analyses and analytical models applied to site data reveal that non-linear plant responses to soil moisture are a major mechanism responsible for these differences in sign. Second, using vegetation indices from four different satellites and a statistical approach, we draw similar conclusions about the changing sign of response across dry to wet ecosystems. Furthermore, the satellite analysis reveals that global vegetation is sensitive to daily rainfall variability across 42% of Earth’s vegetated land surfaces. Surprisingly, vegetation is almost (95%) as sensitive to daily rainfall variability as vegetation is to annual rainfall totals.

These findings across scales suggest that daily rainfall variability impacts on terrestrial ecosystems are likely having a substantial impact on the global carbon cycle and food security. Observational results, included mechanisms revealed in these analyses, are pivotal for benchmarking models and an analysis on this topic is ongoing.