Agricultural and Forest Meteorology 249 (2018) 377–389 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Model-based analysis of the impact of diffuse radiation on CO2 exchange in a T temperate deciduous forest Min S. Leea, David Y. Hollingerb, Trevor F. Keenanc, Andrew P. Ouimetted, Scott V. Ollingerd, ⁎ Andrew D. Richardsona,e,f, a Harvard University, Department of Organismic and Evolutionary Biology, 26 Oxford Street, Cambridge, MA 02138, USA b USDA Forest Service, Northern Research Station, 271 Mast Rd, Durham, NH 03824, USA c Climate and Ecosystem Sciences Division, Lawrence Berkeley National Lab., 1 Cyclotron Rd., Berkeley, CA 94720, USA d University of New Hampshire, Earth Systems Research Center, 8 College Rd, Durham, NH 03824, USA e Northern Arizona University, School of Informatics, Computing and Cyber Systems, PO Box 5693, Flagstaff, AZ 86011, USA f Northern Arizona University, Center for Ecosystem Science and Society, PO Box 5620, Flagstaff, AZ 86011, USA ARTICLE INFO ABSTRACT Keywords: Clouds and aerosols increase the fraction of global solar irradiance that is diffuse light. This phenomenon is Diffuse radiation known to increase the photosynthetic light use efficiency (LUE) of closed-canopy vegetation by redistributing Light use efficiency photosynthetic photon flux density (400–700 nm) from saturated, sunlit leaves at the top of the canopy, to Eddy covariance shaded leaves deeper in the canopy. We combined a process-based carbon cycle model with 10 years of eddy Net ecosystem exchange covariance carbon flux measurements and other ancillary data sets to assess 1) how this LUE enhancement Deciduous forest influences interannual variation in carbon uptake, and 2) how errors in modeling diffuse fraction affect pre- Canopy photosynthesis dictions of carbon uptake. Modeled annual gross primary productivity (GPP) increased by ≈0.94% when ob- served levels of diffuse fraction were increased by 0.01 (holding total irradiance constant). The sensitivity of GPP to increases in diffuse fraction was highest when the diffuse fraction was low to begin with, and lowest when the diffuse fraction was already high. Diffuse fraction also explained significantly more of the interannual variability of modeled net ecosystem exchange (NEE), than did total irradiance. Two tested radiation partitioning models yielded over- and underestimates of diffuse fraction at our site, which propagated to over- and underestimates of annual NEE, respectively. Our findings highlight the importance of incorporating LUE enhancement under diffuse light into models of global primary production, and improving models of diffuse fraction. 1. Introduction sunlit leaves are often light saturated while shaded leaves receive little light and thus lie on the linear part of the light response curve (Roderick A key uncertainty of forest ecosystem carbon uptake in a changing et al., 2001). Under diffuse light conditions, sunlit leaves receive less climate is its differential responses to diffuse and direct beam solar direct beam PPFD but shaded leaves receive more diffuse PPFD, which radiation (Bonan, 2008; Heimann and Reichstein, 2008; Settele et al., comes from all directions of the sky and penetrates the canopy to a 2014). Cloud cover and aerosols (Cheng et al., 2016; Niyogi et al., fuller extent. Because leaves in deep shade benefit more from an in- 2004) account for most of the variability in the ratio of diffuse to global crease in PPFD than leaves in full sun suffer from an equivalent de- irradiance (hereafter referred to as diffuse fraction), and projections of crease, a more even vertical distribution of PPFD should enhance the how these will change in the future are highly uncertain (Boucher et al., photosynthetic light use efficiency (LUE) of the canopy as a whole 2013; Wild, 2009). There is also uncertainty associated with the path- (Farquhar and Roderick, 2003). Direct measurements of forest CO2 ways through which diffuse fraction influences the carbon budget. uptake have shown that canopy LUE is indeed enhanced under cloudy Diffuse fraction affects the photosynthetic photon flux density (PPFD) conditions, though estimates of enhancement vary (Alton, 2008; Alton distribution within the forest canopy, which has potentially important et al., 2007; Baldocchi, 1997; Gu et al., 2002; Hollinger et al., 1994; implications for canopy photosynthesis. Under clear sky conditions, Jenkins et al., 2007; Urban et al., 2007; Zhang et al., 2010). A number Abbreviations: fd,diffuse fraction; PPFD, photosynthetic photon flux density; NEE, net ecosystem exchange; GPP, gross primary productivity; RE, ecosystem respiration; LUE, light use efficiency ⁎ Corresponding author at: Northern Arizona University, School of Informatics, Computing and Cyber Systems, Building 90 (1295 S. Knoles Dr.), Flagstaff, AZ 86011, USA. E-mail address: [email protected] (A.D. Richardson). https://doi.org/10.1016/j.agrformet.2017.11.016 Received 29 May 2017; Received in revised form 9 November 2017; Accepted 12 November 2017 Available online 20 November 2017 0168-1923/ © 2017 Elsevier B.V. All rights reserved. M.S. Lee et al. Agricultural and Forest Meteorology 249 (2018) 377–389 of studies have also found that aerosol loading events, such as the provides insights into the relative importance of diffuse fraction for eruption of Mount Pinatubo (Farquhar and Roderick, 2003; Gu et al., accurately modeling carbon budgets at longer time scales. 2003; Mercado et al., 2009), have enhanced the terrestrial carbon sink. Finally, we tested the accuracy of two standard partitioning models However, aside from these studies of dramatic increases in diffuse used to predict diffuse fraction, and analyzed the impact of their errors fraction, there has been relatively little research on the extent to which on predictions of NEE and other carbon cycle components. While stu- interannual variation in diffuse fraction − typically stemming from dies have measured the goodness of fit between observations and pre- fluctuations in cloudiness − mediates the interannual variation in dictions of NEE that were informed by modeled diffuse fraction (Gu carbon budget. Min and Wang (2008) find that the transmittance index, et al., 2002; Rocha et al., 2004; Schurgers et al., 2015), and Gu et al. compared to temperature and precipitation, is more highly correlated (2002) compare independently parameterized models utilizing ob- (R2 > 0.65) with mean midsummer net ecosystem production at a served and modeled diffuse fraction, this is the first study to our northern hardwood forest, which suggests LUE changes with cloudiness knowledge that has compared performances of model runs differing may accumulate over the growing season in annually distinct ways and only in the accuracy of their diffuse and direct PPFD drivers. Our have meaningful influence on forest productivity in a given year. findings help gauge the importance of improving diffuse fraction Diffuse fraction also covaries with other environmental factors that models to better replicate forest carbon dynamics. impact the carbon budget. The presence of clouds and aerosols often To achieve these three objectives, we first optimized and validated a reduces the solar radiation reaching Earth’s surface, and is associated process-based model using 10 years of half-hourly eddy covariance with lower air and leaf temperature and vapor pressure deficit (Gu fluxes, observations of direct and diffuse PPFD, and other ancillary et al., 2002; Oliphant et al., 2011; Wohlfahrt et al., 2008; Zhang et al., measurements at the Bartlett Experimental Forest, a deciduous site in 2011). Less incident PPFD reduces photosynthesis, while the thermal the northeastern United States. After optimizing model parameters effects can enhance photosynthesis when ambient temperature is above using the first half of our observational record, and validating the the optimum (Baldocchi and Harley, 1995; Steiner and Chameides, model against the second half, we then prescribed various scenarios of 2005) and reduce ecosystem respiration (Alton, 2008; Gu et al., 1999; diffuse fraction to measure how model outputs and performance re- Urban et al., 2007). Lower vapor pressure deficit (VPD) associated with spond. reduced irradiance can increase stomatal conductance, enhancing leaf photosynthesis (Gu et al., 1999). Recent studies suggest the decrease in 2. Materials and methods total irradiance has a greater effect on the net ecosystem exchange (NEE) than associated changes in temperature and humidity (Alton 2.1. Site et al., 2007; Knohl and Baldocchi, 2008; Oliphant et al., 2011), though quantifying these separately remains difficult (Kanniah et al., 2012). The Bartlett Experimental Forest (BEF) (https://www.nrs.fs.fed.us/ In this paper, we investigated the impacts of diffuse radiation on ef/locations/nh/bartlett/) is a primarily deciduous forest located in the forest gross primary productivity (GPP) by combining long-term data northeastern United States (44.05° N, 71.29° W). Mean annual tem- sets (half-hourly measurements over 10 years) of eddy covariance perature is approximately 6.6°C (summer: 20°C, winter: −8°C), and fluxes and direct and diffuse PPFD with a process-oriented model. Many mean annual precipitation is approximately 1300 mm, distributed studies have estimated diffuse fraction using radiation partitioning evenly throughout the year. The soils are Spodosols, developed on models (Alton, 2008; Alton et al., 2007; Choudhury, 2001; Gu et al., glacial till derived from granite and gneiss. Soils