Do precipitation pulses drive anomalous carbon dioxide ecosystem exchange in temperate ecosystems?

Increased greenhouse gas concentrations have been shown to be primarily responsible for recent increases in global temperatures. Because this effect is mediated by climate feedbacks between terrestrial ecosystems and atmospheric greenhouse gases, studying greenhouse gas exchanges between the atmosphere and surface ecosystems has become an area of greater focus for scientists across several disciplines. Recent studies in semi-arid systems have identified that precipitation pulses can lead to increased emissions of carbon dioxide from the soil. However, this has not been demonstrated in temperate ecosystems. Here we seek to ask how positive or negative anomalies of net ecosystem exchange (NEE) of carbon dioxide are related to precipitation occurrence, magnitude, timing, whether these relationships are direct or lagged, and what are the likely mechanisms for any significant correlations.

Eight years of eddy-covariance observed NEE were collected from two ecosystems in northern Wisconsin. The long-term daily diurnal cycle were removed from these data, matched with precipitation observations from a nearby airport gauge, and analyzed in MATLAB. For both a northern hardwood forest and shrub wetland, NEE was found to increase (decreased uptake by biosphere / greater emissions to atmosphere) significantly, relative to average diurnal values, during and shortly after precipitation pulses of at least 2.5 millimeters (0.1 inches) in one hour. Further, larger hourly precipitation pulses resulted in larger spikes in NEE across both sites, in a linear manner. The relationships between NEE and other variables, such as sunlight exposure, pressure, and wind, warrant further research, as rainfall events involve a host of atmospheric processes beyond the introduction of water to the ecosystem.