Enhanced Ecosystem Productivity in Cloudy or Aerosol-laden Conditions Xin Xi April 1, 2008

Radiation process Players: atmospheric gases; aerosols; clouds Processes: absorption and scattering (both back and forward) in both shortwave and longwave; emission (only longwave) Role of atmospheric aerosols: Direct effect: aerosol scattering and absorption Indirect effect: aerosol particles as CCN; 1st indirect effect (enhanced cloud albedo); 2nd indirect effect (enhanced cloud lifetime and amount); semi- direct effect (reduced cloud fraction due to aerosol absorption) Implications for plant growth: 1. Available PAR (total amount) 2. Direct and diffuse components of PAR (400nm~700nm) 3. Induced environmental change (temperature, wind, humidity, CO2, etc) 4. Precipitation and soil moisture 5. Aerosol deposition on vegetation (e.g., poisoning effect) 6. Other effects e.g., photolysis (O 3, NO, etc)

Previous findings 1. Crop scientists found higher radiation use efficiency (RUE, ratio of accumulated biomass to total intercepted solar radiation) or light use efficiency (LUE, same as RUE but based on PAR) for diffuse radiation than for direct radiation. 2. Flux measurements shows higher RUE in cloudy days than clear days for coniferous and deciduous forests 3. Highest NEE rate occurs in cloudy days though incident radiation is largely reduced. …… Explanations: 1. Clouds can increase the diffuse radiation at the surface if the sky is not too cloudy. This will enhance carbon assimilation, if the photosynthesis gains of increased diffuse radiation exceed the photosynthetic loss of reduced beam direct radiation. 2. The presence of clouds can be also the causes or consequences of changes in many atmospheric factors, like air/soil temperature, moisture, latent heat and rainfall, etc 3. Reduced leaf temperature, reduced vapor pressure deficit, stomatal dynamics associated with light fluctuations, reduced soil respiration… 4. This paper focuses on the different effects of diffuse and direct radiation.

Differential photosynthetic responses to direct and diffuse radiation: At the scale of a single leaf, direct and diffuse radiation donot make any difference in terms of photosynthesis. However, over the vegetation canopy, diffuse radiation can more uniformly distribute among all leaves and thus increase the light use efficiency, while the direct radiation only reaches a small fraction of leaves and easily leads to light saturation due to the Rubisco limit, and even decrease in photosynthesis rate due to the enhanced respiration. Strategy adopted in this study: It’s impossible to compare photosynthesis rate of diffuse radiation with that of direct radiation under natural conditions, since the leaves receive direct and diffuse light at the same time. So, parameters that define the photosynthetic responses to diffuse and direct radiation are retrieved from the flux measurements through nonlinear regression, on which this study is based.

Data Flux tower measurements (eddy covariance) at 5 sites: a Scots pine forest: 23m above ground and 10m above canopy an aspen forest: 39.5m above ground and 17.5m above canopy a mixed deciduous forest: 36.9m above surface and 10m above canopy a native tallgrass prairie (C4 grass): 4.5m height a winter wheat crop: 4.5m height Datasets: half-hourly NEE, air temperature, vapor pressure deficit, soil temperature and global PAR Direct and diffuse PAR: only available at the mixed deciduous forest; others use a radiation partition model for calculations

Analysis method NEE (Fc) = Re – GPP (P) Negative NEE value means net carbon gain. Model 1: Model 2: α: initial canopy quantum yield, when global PAR incident on canopy (I t ) is zero. β: describes the closeness of to linear response (CLR) of the canopy photosynthetic curve, or the capacity to resist photosynthetic saturation at high level of PAR. Higher value of α and β indicate better light use efficiency. Model 3: Ta: air temperature; V: VPD The author divides the data into 11-day moving windows, and separate each window into two parts, one part for regression inversion, the other for validation.

Results r2r2 RMSE Index of agreement Model 2 performs better than model 1 consistently and model 3 in most cases.

Model 2 is robust for all sites, and for both regression data and validation data!

1.Initial canopy quantum yield for diffuse PAR α f is larger than that for direct PAR α r. 2.Seasonality: α f and α r change with different developmental stage.

Logarithmic scale 1.CLR coefficient for diffuse radiation β f is several order larger than that for direct radiation β r, showing that direct PAR easily causes photosynthetic saturation. 2.Seasonality of β f and β r as compared to α f and α r 3.Seasonal pattern of β f and β r indicates a shift of plant photosynthesis from a non- linear response at early growing season to a linear response in mid-growing season.

Fig2 Fig7 Air temperature and VPD affect α f and α r

Summary 1.Diffuse radiation has a less tendency to cause canopy photosynthetic saturation and results in a higher radiation use efficiency. 2.The advantage of diffuse radiation over direct radiation increase with radiation level (section 4.6). 3.Temperature and vapor pressure deficit can cause different responses in diffuse and direct canopy photosynthesis. 4.The photosynthetic responses (α f,α r, β f and β r ) to temperature and VPD differ for distinct plant species (C3 vs. C4) and canopy architecture (also Niyogi et al, 2004), and also differ from the leaf scale to the canopy scale.

by NASA Langley Research Center June 15, million tons of ash and gas (SO2)!

Data 1. Eddy covariance flux tower measurement in Harvard hardwood forest in 1992~1997; 2. Cloudless conditions, only volcanic aerosol; 3. Two methods: (1). Model 2 in the previous paper; retrieved parameters are used to calculate gross photosynthesis rate assuming normal radiation condition, which is then compared with the actual value (perturbed radiation condition) ; diffuse radiation effect only! (2). Pure statistical analysis; all factors should be considered!

Method 1 αfαrβfβrαfαrβfβr

1.Perturbed radiation regime corresponds to a higher gross photosynthesis rate. 2.But the difference decreases in 1994, when the atmosphere returns to the normal condition.

Method 2 The cloudless NEE measurement is normalized with a long-term value based on the data from 1992 to Statistical tests are conducted to compare the normalized cloudless NEE in 1992, 1993 and 1994 with a reference sample (1995~2001).

Relative difference in the enhanced percentage of NEE in 1993 for method 1 (6%) and method 2 (15%) can be explained partly by changes in temperature and thus ecosystem respiration. soil temperature in 1993 is lower than the averaged value from 1995 to 2001.

Summary 1.Both two methods shows that the increase of diffuse radiation caused by the volcanic eruption enhanced Harvard Forest photosynthesis under cloudless conditions for the two years after the eruption. 2.To extend this result to annual time scale and global scale, the aerosol effect on cloud formation should be considered, since sulfate aerosol is effective CCN and cloud can effectively produce diffuse radiation. 3.Under moderately cloudy sky, improved moisture condition and reduced solar heating (leaf temperature and soil temperature) may further enhance carbon uptake.