Spring photosynthetic onset and net CO₂ uptake in Alaska triggered by landscape thawing

Abstract

The springtime transition to regional-scale onset of photosynthesis and net ecosystem carbon uptake in boreal and tundra ecosystems are linked to the soil freeze-thaw state. We present evidence from diagnostic and inversion models constrained by satellite fluorescence and airborne CO from 2012 to 2014 indicating the timing and magnitude of spring carbon uptake in Alaska correlates with landscape thaw and ecoregion. Landscape thaw in boreal forests typically occurs in late April (DOY 111 ± 7) with a 29 ± 6 day lag until photosynthetic onset. North Slope tundra thaws 3 weeks later (DOY 133 ± 5) but experiences only a 20 ± 5 day lag until photosynthetic onset. These time lag differences reflect efficient cold season adaptation in tundra shrub and the longer dehardening period for boreal evergreens. Despite the short transition from thaw to photosynthetic onset in tundra, synchrony of tundra respiration with snow melt and landscape thaw delays the transition from net carbon loss (at photosynthetic onset) to net uptake by 13 ± 7 days, thus reducing the tundra net carbon uptake period. Two global CO inversions using a CASA-GFED model prior estimate earlier northern high latitude net carbon uptake compared to our regional inversion, which we attribute to (i) early photosynthetic-onset model prior bias, (ii) inverse method (scaling factor + optimization window), and (iii) sparsity of available Alaskan CO observations. Another global inversion with zero prior estimates the same timing for net carbon uptake as the regional model but smaller seasonal amplitude. The analysis of Alaskan eddy covariance observations confirms regional scale findings for tundra, but indicates that photosynthesis and net carbon uptake occur up to 1 month earlier in evergreens than captured by models or CO inversions, with better correlation to above-freezing air temperature than date of primary thaw. Further collection and analysis of boreal evergreen species over multiple years and at additional subarctic flux towers are critically needed.

Increased high‐latitude photosynthetic carbon gain offset by respiration carbon loss during an anomalous warm winter to spring transition

“…Solar-induced chlorophyll fluorescence (SIF) is an electromagnetic signal emitted by plants during foliar light absorption by chlorophyll (Joiner et al, 2013;van der Tol, Berry, Campbell, & Rascher, 2014). SIF is directly proportional to photosynthetic activity (Porcar-Castell et al, 2014) and was shown to be an important indicator of photosynthetic activation and growing season duration throughout northern latitude ecosystems (Jeong et al, 2018;Parazoo et al, 2018). Spaceborne SIF is less sensitive to clouds, high albedo surfaces, and non-photosynthetic vegetation than reflectance-based vegetation indices.…”

“…The net CO 2 exchange depends on the balance between CO 2 assimilation through vegetation productivity and CO 2 release through ecosystem respiration (ER), which may respond differently to seasonal climate and environmental variations. High‐latitude ecosystems are generally temperature or radiation limited, and therefore, warming has a primary control on the seasonal change in photosynthesis or respiration (Parazoo et al, ; Figure ). Climate warming promotes earlier landscape thawing, a reduction in spring snow cover, earlier onset of vegetation productivity, and longer growing seasons (Box et al, ).…”

“…Remote sensing provides another tool to evaluate land surface conditions affecting net carbon exchange, either directly through photosynthetic activity derived from satellite vegetation indices or solar‐induced chlorophyll fluorescence observations, or indirectly through satellite observed soil moisture and thermal conditions influencing ER (Schimel et al, ). However, satellite remote sensing retrievals are subject to observational errors, especially over the high northern latitudes where low solar illumination, frequent cloud cover, seasonal snow and ice cover, and heterogeneous land surface conditions can degrade sensor signal‐to‐noise and result in significant data loss (Parazoo et al, ). To overcome limitations from any single dataset and method, we used complementary in situ measurements, model simulations, and remote sensing observations to untangle the effects of competing ecosystem processes and improve understanding of high‐latitude climate–carbon feedbacks.…”

Surface Energy Budgets of Arctic Tundra During Growing Season

“…Mean annual temperature and precipitation are −8.28 °C and 304 mm, respectively. Mean July maximum daily temperature is ∼12 °C, and mean summertime precipitation is 210 mm (Parazoo et al, ; Riedel et al, ). Half‐hourly data from years 2004–2007 were made available through the FLUXNET network (Zona & Oechel, ).…”

Exposure to cold temperature affects the spring phenology of Alaskan deciduous vegetation types

“…GPP MAX ; figure 1). We also define the GPP onset date as the mean DOY when GPP is between 10% and 20% of GPP MAX for that year (Parazoo et al 2018). This definition can account for observation noise and range of transition dates from slow to rapid spring recovery in arctic tundra and boreal ecosystems.…”