Seasonality of temperate forest photosynthesis and daytime respiration

Abstract

Terrestrial ecosystems currently offset one-quarter of anthropogenic carbon dioxide (CO2) emissions because of a slight imbalance between global terrestrial photosynthesis and respiration. Understanding what controls these two biological fluxes is therefore crucial to predicting climate change. Yet there is no way of directly measuring the photosynthesis or daytime respiration of a whole ecosystem of interacting organisms; instead, these fluxes are generally inferred from measurements of net ecosystem-atmosphere CO2 exchange (NEE), in a way that is based on assumed ecosystem-scale responses to the environment. The consequent view of temperate deciduous forests (an important CO2 sink) is that, first, ecosystem respiration is greater during the day than at night; and second, ecosystem photosynthetic light-use efficiency peaks after leaf expansion in spring and then declines, presumably because of leaf ageing or water stress. This view has underlain the development of terrestrial biosphere models used in climate prediction and of remote sensing indices of global biosphere productivity. Here, we use new isotopic instrumentation to determine ecosystem photosynthesis and daytime respiration in a temperate deciduous forest over a three-year period. We find that ecosystem respiration is lower during the day than at night-the first robust evidence of the inhibition of leaf respiration by light at the ecosystem scale. Because they do not capture this effect, standard approaches overestimate ecosystem photosynthesis and daytime respiration in the first half of the growing season at our site, and inaccurately portray ecosystem photosynthetic light-use efficiency. These findings revise our understanding of forest-atmosphere carbon exchange, and provide a basis for investigating how leaf-level physiological dynamics manifest at the canopy scale in other ecosystems.

Photosynthetic productivity and its efficiencies in ISIMIP2a biome models: benchmarking for impact assessment studies

“…Even with updated micrometeorological techniques such as the eddy-covariance method (Baldocchi et al 2001), GPP cannot be directly measured and must be estimated from net CO 2 flux using appropriate separation algorithms (Reichstein et al 2005). Beer et al (2010) scaled up field-based GPP data using a statistical model approach to produce a global map of GPP, although the assumptions used in the estimation can introduce certain biases (Wehr et al 2016). GPP has also been estimated by satellite remote sensing based on the relationship between canopy-absorbed solar radiation and GPP, using the light-use efficiency approach (e.g.…”

Species-specific temporal variation in photosynthesis as a moderator of peatland carbon sequestration

“…Annual CO 2 flux balances from EC measurements are prone to significant systematic bias, sometimes in excess of 30 % but usually between 10 and 30 % of the cumulative flux (e.g., Baldocchi, 2003;Rannik et al, 2006). The underestimation of the EC fluxes implicit in the unclosed energy balance (70 % for Siikaneva-1, unpublished data) might be partly compensated for by the Kok effect, which might be more significant than previously thought, as indicated by Wehr et al (2016). Our P G estimates include errors related to the LAI development measurements, visual species cover estimation, the conversion from cover to LAI and the laboratory measurements of photosynthetic parameters.…”

The CarbonTracker Data Assimilation System for CO<sub>2</sub> and <i>δ</i><sup>13</sup>C (CTDAS-C13 v1.0): retrieving information on land–atmosphere exchange processes

“…These long-term estimates are combined with the quadratic gastransfer velocity from 3 hourly ECMWF ERA-Interim wind fields (Wanninkhof, 1992) to create fluxes on a 1 • × 1 • grid at a 3 hourly temporal resolution. An additional trend was applied to the fluxes to ensure that increases in anthropogenic uptake are proportional to increases in atmospheric CO 2 levels (see http://www.esrl.noaa.gov/gmd/ccgg/carbontracker).…”

“…In so-called double-deconvolution methods this particular trait is used to untangle the global land carbon budget from the ocean carbon budget (Keeling et al, 1989;Tans et al, 1993;Ciais et al, 1995). More recently, the 13 C isotope was used to study the diurnal cycle of GPP and TER (Wehr et al, 2016) and was used as a tracer of water use efficiency to study long-term responses to CO 2 increases in tree rings (van der Sleen et al, 2015), and attempts are underway to do the same based on atmospheric records. On regional scales, variations in the ratio of 13 CO 2 / 12 CO 2 (typically reported as δ 13 C in ‰ relative to the VPDB reference ratio) reflect changes in discrimination processes associated with photosynthetic uptake of carbon by plants (e.g., Farquhar et al, 1989;Fung et al, 1997;Scholze et al, 2003;Rayner et al, 2008).…”