Spring in the boreal environment: observations on pre- and post-melt energy and CO2 fluxes in two central Siberian ecosystems

A range of observations points towards earlier onset of spring in northern high latitudes. However, despite the profound effects this may have on vegetation–atmosphere exchange of carbon (NEE), vegetation–atmosphere physical coupling, or the location of the tundra–taiga interface, the number of studies that investigate winter–spring transition fluxes in contrasting northern vegetation types is limited. Here, we examine spring ecosystem–atmosphere energy and carbon exchange in a Siberian pine forest and mire. Divergent surface albedo before and during snow-melt resulted in daytime net radiation (Rn) above the forest exceeding Rn above the mire by up to 10 MJ m–2. Until stomata could open, absorbed radiation by the green pine canopy caused substantial daytime sensible heat fluxes (H > 10MJ m–2). H above the mire was very low, even negative (< –2 MJ m–2), during that same period. Physiological activity in both ecosystems responded rapidly to warming temperatures and snow-melt, which is essential for survival in Siberia with its very short summers. On days with above-zero temperatures, before melt was complete, low rates of forest photosynthesis (1–2 umol m–2 s–1) were discernible. Forest and mire NEE became negative the same day, or shortly after, photosynthesis commenced. The mire lagged by about two weeks behind the forest and regained its full carbon uptake capacity at a slower rate. Our data provide empirical evidence for the importance the timing of spring and the relative proportion of forest vs. mire has for late winter/spring boundary-layer growth, and production and surface–atmosphere mixing of trace gases. Models that seek to investigate effects of increasingly earlier spring in high latitudes must correctly account for contrasting physical and biogeochemical ecosystem–atmosphere exchange in heterogeneous landscapes.