Abstract
Rapid warming in northern ecosystems over the past four decades has resulted in earlier spring, increased precipitation, and altered timing of plant–animal interactions, such as herbivory. Advanced spring phenology can lead to longer growing seasons and increased carbon (C) uptake. Greater precipitation coincides with greater cloud cover possibly suppressing photosynthesis. Timing of herbivory relative to spring phenology influences plant biomass. None of these changes are mutually exclusive and their interactions could lead to unexpected consequences for Arctic ecosystem function. We examined the influence of advanced spring phenology, cloud cover, and timing of grazing on C exchange in the Yukon–Kuskokwim Delta of western Alaska for three years. We combined advancement of the growing season using passive-warming open-top chambers (OTC) with controlled timing of goose grazing (early, typical, and late season) and removal of grazing. We also monitored natural variation in incident sunlight to examine the C exchange consequences of these interacting forcings. We monitored net ecosystem exchange of C (NEE) hourly using an autochamber system. Data were used to construct daily light curves for each experimental plot and sunlight data coupled with a clear-sky model was used to quantify daily and seasonal NEE over a range of incident sunlight conditions. Cloudy days resulted in the largest suppression of NEE, reducing C uptake by approximately 2 g C m−2 d−1 regardless of the timing of the season or timing of grazing. Delaying grazing enhanced C uptake by approximately 3 g C m−2 d−1. Advancing spring phenology reduced C uptake by approximately 1.5 g C m−2 d−1, but only when plots were directly warmed by the OTCs; spring advancement did not have a long-term influence on NEE. Consequently, the two strongest drivers of NEE, cloud cover and grazing, can have opposing effects and thus future growing season NEE will depend on the magnitude of change in timing of grazing and incident sunlight.
Highlights
Arctic warming over the past several decades has been nearly twice as rapid as at lower latitudes (Elmendorf et al 2012) leading to new environmental conditions in northern ecosystems (Hinzman et al 2013)
Mean air temperature when the open-top chambers (OTC) were in place was 0.7 °C, 1.0 °C, and 2.4 °C warmer in the advanced compared to the ambient season treatment in 2014, 2015, and 2016, Figure 3
The advanced growing season reduced daily C uptake between 1.3 and 1.6 g C m−2 d−1, but only while the OTCs were in place
Summary
Arctic warming over the past several decades has been nearly twice as rapid as at lower latitudes (Elmendorf et al 2012) leading to new environmental conditions in northern ecosystems (Hinzman et al 2013) These new states include changes to precipitation regimes and cloud cover (Schweiger 2004, Zhang et al 2013, Klein et al 2015), a greening of the landscape with earlier spring (Hoye et al 2007, Tape et al 2012, Ju and Masek 2016), and altered phenology of plant–animal interactions (Brook et al 2015, Doiron et al 2015). Delayed herbivory with respect to plant phenological stage can enhance C uptake in a coastal Alaskan wetland by allowing plants to accumulate greater aboveground biomass (Choi et al 2019) before herbivores consume leaves (Kelsey et al 2018, Leffler et al 2019), and timing of herbivory influences forage quality and fecal N which are important drivers of N cycling (Beard et al 2019)
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