An official website of the United States government
Summary Steady‐state photosyntheticCO2responses (A/Cicurves) are used to assess environmental responses of photosynthetic traits and to predict future vegetative carbon uptake through modeling. The recent development of rapidA/Cicurves (RACiRs) permits faster assessment of these traits by continuously changing [CO2] around the leaf, and may reveal additional photosynthetic properties beyond what is practical or possible with steady‐state methods.Gas exchange necessarily incorporates photosynthesis and (photo)respiration. Each process was expected to respond on different timescales due to differences in metabolite compartmentation, biochemistry and diffusive pathways. We hypothesized that metabolic lags in photorespiration relative to photosynthesis/respiration andCO2diffusional limitations can be detected by varying the rate of change in [CO2] duringRACiR assays. We tested these hypotheses through modeling and experiments at ambient and 2% oxygen.Our data show that photorespiratory delays cause offsets in predictedCO2compensation points that are dependent on the rate of change in [CO2]. Diffusional limitations may reduce the rate of change in chloroplastic [CO2], causing a reduction in apparentRACiR slopes under highCO2ramp rates.MultirateRACiRs may prove useful in assessing diffusional limitations to gas exchange and photorespiratory rates.
more »
« less
Spring photosynthetic onset and net CO 2 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 airborneCO2from 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 (DOY111 ± 7) with a 29 ± 6 day lag until photosynthetic onset. North Slope tundra thaws 3 weeks later (DOY133 ± 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 globalCO2inversions using aCASA‐GFEDmodel 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 AlaskanCO2observations. 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 orCO2inversions, 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.
more »
« less
- PAR ID:
- 10062996
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 24
- Issue:
- 8
- ISSN:
- 1354-1013
- Page Range / eLocation ID:
- p. 3416-3435
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Rich fens are common boreal ecosystems with distinct hydrology, biogeochemistry and ecology that influence their carbon (C) balance. We present growing season soil chamber methane emission (FCH4), ecosystem respiration (ER), net ecosystem exchange (NEE) and gross primary production (GPP) fluxes from a 9‐years water table manipulation experiment in an Alaskan rich fen. The study included major flood and drought years, where wetting and drying treatments further modified the severity of droughts. Results support previous findings from peatlands that drought causes reduced magnitude of growing seasonFCH4,GPPandNEE, thus reducing or reversing their C sink function. Experimentally exacerbated droughts further reduced the capacity for the fen to act as a C sink by causing shifts in vegetation and thus reducing magnitude of maximum growing seasonGPPin subsequent flood years by ~15% compared to control plots. Conversely, water table position had only a weak influence onER, but dominant contribution toERswitched from autotrophic respiration in wet years to heterotrophic in dry years. Droughts did not cause inter‐annual lag effects onERin this rich fen, as has been observed in several nutrient‐poor peatlands. WhileERwas dependent on soil temperatures at 2 cm depth,FCH4was linked to soil temperatures at 25 cm. Inter‐annual variability of deep soil temperatures was in turn dependent on wetness rather than air temperature, and higherFCH4in flooded years was thus equally due to increased methane production at depth and decreased methane oxidation near the surface. Short‐term fluctuations in wetness caused significant lag effects onFCH4, but droughts caused no inter‐annual lag effects onFCH4. Our results show that frequency and severity of droughts and floods can have characteristic effects on the exchange of greenhouse gases, and emphasize the need to project future hydrological regimes in rich fens.more » « less
-
Abstract Widespread changes in arctic and boreal Normalized Difference Vegetation Index (NDVI) values captured by satellite platforms indicate that northern ecosystems are experiencing rapid ecological change in response to climate warming. Increasing temperatures and altered hydrology are driving shifts in ecosystem biophysical properties that, observed by satellites, manifest as long‐term changes in regionalNDVI. In an effort to examine the underlying ecological drivers of these changes, we used field‐scale remote sensing ofNDVIto track peatland vegetation in experiments that manipulated hydrology, temperature, and carbon dioxide (CO2) levels. In addition toNDVI, we measured percent cover by species and leaf area index (LAI). We monitored two peatland types broadly representative of the boreal region. One site was a rich fen located near Fairbanks, Alaska, at the Alaska Peatland Experiment (APEX), and the second site was a nutrient‐poor bog located in Northern Minnesota within the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment. We found thatNDVIdecreased with long‐term reductions in soil moisture at theAPEXsite, coincident with a decrease in photosynthetic leaf area and the relative abundance of sedges. We observed increasingNDVIwith elevated temperature at theSPRUCEsite, associated with an increase in the relative abundance of shrubs and a decrease in forb cover. Warming treatments at theSPRUCEsite also led to increases in theLAIof the shrub layer. We found no strong effects of elevatedCO2on community composition. Our findings support recent studies suggesting that changes inNDVIobserved from satellite platforms may be the result of changes in community composition and ecosystem structure in response to climate warming.more » « less
-
Summary Traditionally, leaves were thought to be supplied withCO2for photosynthesis by the atmosphere and respiration. Recent studies, however, have shown that the xylem also transports a significant amount of inorganic carbon into leaves through the bulk flow of water. However, little is known about the dynamics and proportion of xylem‐transportedCO2that is assimilated, vs simply lost to transpiration.Cut leaves ofPopulus deltoidesandBrassica napuswere placed in eitherKCl or one of three [NaH13CO3] solutions dissolved in water to simultaneously measure the assimilation and the efflux of xylem‐transportedCO2exiting the leaf across light andCO2response curves in real‐time using a tunable diode laser absorption spectroscope.The rates of assimilation and efflux of xylem‐transportedCO2increased with increasing xylem [13CO2*] and transpiration. Under saturating irradiance, rates of assimilation using xylem‐transportedCO2accounted forc.2.5% of the total assimilation in both species in the highest [13CO2*].The majority of xylem‐transportedCO2is assimilated, and efflux is small compared to respiration. Assimilation of xylem‐transportedCO2comprises a small portion of total photosynthesis, but may be more important whenCO2is limiting.more » « less
-
PremiseHerbarium specimens have been used to detect climate‐induced shifts in flowering time by using the day of year of collection (DOY) as a proxy for first or peak flowering date. Variation among herbarium sheets in their phenological status, however, undermines the assumption thatDOYaccurately represents any particular phenophase. Ignoring this variation can reduce the explanatory power of pheno‐climatic models (PCMs) designed to predict the effects of climate on flowering date. MethodsHere we present a protocol for the phenological scoring of imaged herbarium specimens using an ImageJ plugin, and we introduce a quantitative metric of a specimen's phenological status, the phenological index (PI), which we use inPCMs to control for phenological variation among specimens ofStreptanthus tortuosus(Brassicaceeae) when testing for the effects of climate onDOY. We demonstrate that includingPIas an independent variable improves model fit. ResultsIncludingPIinPCMs increased the modelR2relative toPCMs that excludedPI; regression coefficients for climatic parameters, however, remained constant. DiscussionOur protocol provides a simple, quantitative phenological metric for any observed plant. IncludingPIinPCMs increasesR2and enables predictions of theDOYof any phenophase under any specified climatic conditions.more » « less
