Intra-canopy leaf trait variation facilitates high leaf area index and compensatory growth in a clonal woody encroaching shrub

Leaf trait variation enables plants to utilize large gradients of light availability that exist across canopies of high leaf area index (LAI), allowing for greater net carbon gain while reducing light availability for understory competitors. While these canopy dynamics are well understood in forest ecosystems, studies of canopy structure of woody shrubs in grasslands are lacking. To evaluate the investment strategy used by these shrubs, we investigated the vertical distribution of leaf traits and physiology across canopies of Cornus drummondii, the predominant woody encroaching shrub in the Kansas tallgrass prairie. We also examined the impact of disturbance by browsing and grazing on these factors. Our results reveal that leaf mass per area (LMA) and leaf nitrogen per area (Na) varied approximately threefold across canopies of C. drummondii, resulting in major differences in the physiological functioning of leaves. High LMA leaves had high photosynthetic capacity, while low LMA leaves had a novel strategy for maintaining light compensation points below ambient light levels. The vertical allocation of leaf traits in C. drummondii canopies was also modified in response to browsing, which increased light availability at deeper canopy depths. As a result, LMA and Na increased at lower canopy depths, leading to a greater photosynthetic capacity deeper in browsed canopies compared to control canopies. This response, along with increased light availability, facilitated greater photosynthesis and resource-use efficiency deeper in browsed canopies compared to control canopies. Our results illustrate how C. drummondii facilitates high LAI canopies and a compensatory growth response to browsing-both of which are key factors contributing to the success of C. drummondii and other species responsible for grassland woody encroachment.

Earth Observations from DSCOVR/EPIC Instrument

The NOAA Deep Space Climate Observatory (DSCOVR) spacecraft was launched on February 11, 2015, and in June 2015 achieved its orbit at the first Lagrange point or L1, 1.5 million km from Earth towards the Sun. There are two NASA Earth observing instruments onboard: the Earth Polychromatic Imaging Camera (EPIC) and the National Institute of Standards and Technology Advanced Radiometer (NISTAR). The purpose of this paper is to describe various capabilities of the DSCOVR/EPIC instrument. EPIC views the entire sunlit Earth from sunrise to sunset at the backscattering direction (scattering angles between 168.5° and 175.5°) with 10 narrowband filters: 317, 325, 340, 388, 443, 552, 680, 688, 764 and 779 nm. We discuss a number of pre-processingsteps necessary for EPIC calibration including the geolocation algorithm and the radiometric calibration for each wavelength channel in terms of EPIC counts/second for conversion to reflectance units. The principal EPIC products are total ozone O3amount, scene reflectivity, erythemal irradiance, UV aerosol properties, sulfur dioxide SO2 for volcanic eruptions, surface spectral reflectance, vegetation properties, and cloud products including cloud height. Finally, we describe the observation of horizontally oriented ice crystals in clouds and the unexpected use of the O2 B-band absorption for vegetation properties.

Influence of Snow on the Magnitude and Seasonal Variation of the Clumping Index Retrieved from MODIS BRDF Products

The foliage Clumping Index (CI) is an important vegetation structure parameter that allows for the accurate separation of sunlit and shaded leaves in a canopy. The CI and its seasonality are critical for global Leaf Area Index (LAI) estimating and ecological modelling. However, the cover of snow tends to reduce the reflectance anisotropy of the vegetation canopy and thus probably influences CI estimates. In this paper, we investigate the influence of snow on the magnitude and seasonal variation of the CI retrieved from Moderate-resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) products based on field-measured CI and statistics from the global MODIS CI product. We find that the backup algorithm can effectively correct abnormally large CI values and obtain more reasonable CI retrievals than the main algorithm without any constraints in snow-covered areas. Validation indicates that the time-series CI product shows the potential in investigating the trajectories of the clumping effect in snow seasons. For evergreen forests, the clumping effect is relatively stable throughout the year; however, for deciduous vegetation types, CI values tend to display significant seasonal variations. This study suggests that the latest version of the global MODIS CI product, in which the backup algorithm is used to process the snow-covered pixels, has improved accuracy for CI retrievals in snow-covered areas and thus is probably more suitable as the input parameter for ecological and meteorological models.

Midday Depression vs. Midday Peak in Diurnal Light Interception: Contrasting Patterns at Crown and Leaf Scales in a Tropical Evergreen Tree

Crown architecture usually is heterogeneous as a result of foraging in spatially and temporally heterogeneous light environments. Ecologists are only beginning to identify the importance of temporal heterogeneity for light acquisition in plants, especially at the diurnal scale. Crown architectural heterogeneity often leads to a diurnal variation in light interception. However, maximizing light interception during midday may not be an optimal strategy in environments with excess light. Instead, long-lived plants are expected to show crown architectures and leaf positions that meet the contrasting needs of light interception and avoidance of excess light on a diurnal basis. We expected a midday depression in the diurnal course of light interception both at the whole-crown and leaf scales, as a strategy to avoid the interception of excessive irradiance. We tested this hypothesis in a population of guava trees (Psidium guajava L.) growing in an open tropical grassland. We quantified three crown architectural traits: intra-individual heterogeneity in foliage clumping, crown openness, and leaf position angles. We estimated the diurnal course of light interception at the crown scale using hemispheric photographs, and at the leaf scale using the cosine of solar incidence. Crowns showed a midday depression in light interception, while leaves showed a midday peak. These contrasting patterns were related to architectural traits. At the crown scale, the midday depression of light interception was linked to a greater crown openness and foliage clumping in crown tops than in the lateral parts of the crown. At the leaf scale, an average inclination angle of 45° led to the midday peak in light interception, but with a huge among-leaf variation in position angles. The mismatch in diurnal course of light interception at crown and leaf scales can indicate that different processes are being optimized at each scale. These findings suggest that the diurnal course of light interception may be an important dimension of the resource acquisition strategies of long-lived woody plants. Using a temporal approach as the one applied here may improve our understanding of the diversity of crown architectures found across and within environments.

Estimation of leaf area index and its sunlit portion from DSCOVR EPIC data: Theoretical basis

This paper presents the theoretical basis of the algorithm designed for the generation of leaf area index and diurnal course of its sunlit portion from NASA's Earth Polychromatic Imaging Camera (EPIC) onboard NOAA's Deep Space Climate Observatory (DSCOVR). The Look-up-Table (LUT) approach implemented in the MODIS operational LAI/FPAR algorithm is adopted. The LUT, which is the heart of the approach, has been significantly modified. First, its parameterization incorporates the canopy hot spot phenomenon and recent advances in the theory of canopy spectral invariants. This allows more accurate decoupling of the structural and radiometric components of the measured Bidirectional Reflectance Factor (BRF), improves scaling properties of the LUT and consequently simplifies adjustments of the algorithm for data spatial resolution and spectral band compositions. Second, the stochastic radiative transfer equations are used to generate the LUT for all biome types. The equations naturally account for radiative effects of the three-dimensional canopy structure on the BRF and allow for an accurate discrimination between sunlit and shaded leaf areas. Third, the LUT entries are measurable, i.e., they can be independently derived from both below canopy measurements of the transmitted and above canopy measurements of reflected radiation fields. This feature makes possible direct validation of the LUT, facilitates identification of its deficiencies and development of refinements. Analyses of field data on canopy structure and leaf optics collected at 18 sites in the Hyytiälä forest in southern boreal zone in Finland and hyperspectral images acquired by the EO-1 Hyperion sensor support the theoretical basis.

Quantitative characterization of clumping in Scots pine crowns

BACKGROUND AND AIMS

Proper characterization of the clumped structure of forests is needed for calculation of the absorbed radiation and photosynthetic production by a canopy. This study examined the dependency of crown-level clumping on tree size and growth conditions in Scots pine (Pinus sylvestris), and determined the ability of statistical canopy radiation models to quantify the degree of self-shading within crowns as a result of the clumping effect.

METHODS

Twelve 3-D Scots pine trees were generated using an application of the LIGNUM model, and the crown-level clumping as quantified by the crown silhouette to total needle area ratio (STAR(crown)) was calculated. The results were compared with those produced by the stochastic approach of modelling tree crowns as geometric shapes filled with a random medium.

KEY RESULTS

Crown clumping was independent of tree height, needle area and growth conditions. The results supported the capability of the stochastic approach in characterizing clumping in crowns given that the outer shell of the tree crown is well represented.

CONCLUSIONS

Variation in the whole-stand clumping index is induced by differences in the spatial pattern of trees as a function of, for example, stand age rather than by changes in the degree of self-shading within individual crowns as they grow bigger.

Nitrogen-addition effects on leaf traits and photosynthetic carbon gain of boreal forest understory shrubs

Boreal coniferous forests are characterized by fairly open canopies where understory vegetation is an important component of ecosystem C and N cycling. We used an ecophysiological approach to study the effects of N additions on uptake and partitioning of C and N in two dominant understory shrubs: deciduous Vaccinium myrtillus in a Picea abies stand and evergreen Vaccinium vitis-idaea in a Pinus sylvestris stand in northern Sweden. N was added to these stands for 16 and 8 years, respectively, at rates of 0, 12.5, and 50 kg N ha(-1) year(-1). N addition at the highest rate increased foliar N and chlorophyll concentrations in both understory species. Canopy cover of P. abies also increased, decreasing light availability and leaf mass per area of V. myrtillus. Among leaves of either shrub, foliar N content did not explain variation in light-saturated CO2 exchange rates. Instead photosynthetic capacity varied with stomatal conductance possibly reflecting plant hydraulic properties and within-site variation in water availability. Moreover, likely due to increased shading under P. abies and due to water limitations in the sandy soil under P. sylvestris, individuals of the two shrubs did not increase their biomass or shift their allocation between above- and belowground parts in response to N additions. Altogether, our results indicate that the understory shrubs in these systems show little response to N additions in terms of photosynthetic physiology or growth and that changes in their performance are mostly associated with responses of the tree canopy.

Variation in light-intercepting area and photosynthetic rate of sun and shade shoots of two Picea species in relation to the angle of incoming light

We investigated the effects of sun- and shade-shoot architecture on the photosynthetic rates of two Picea species by applying light from various angles in the laboratory. Compared with sun shoots, shade shoots were characterized by lower mass allocation per light-intercepting area, less leaf mass per shoot mass, less mutual shading among leaves and more efficient allocation of chlorophyll to photosynthesis. The shoot silhouette to total leaf area ratio (STAR(ϕ)) decreased with increasing shoot inclination angle (ϕ, the shoot axis angle relative to the projection plane) and was consistently higher for the shade shoots. Morphological and physiological characteristics of the shade shoots resulted in maximum rates of net photosynthesis at ϕ = 0° (P(max,0)) similar to that of the sun shoots when expressed on a leaf mass, total leaf area and chlorophyll basis. When the angle of incoming light was varied, P(max,ϕ) per total leaf area (P(max,ϕ )/A(T)) of the shade shoots increased linearly with increasing STAR(ϕ), while P(max,ϕ) per shoot silhouette area did not change. In contrast, the response of the sun shoots was non-linear, and an optimum angle of incoming light was determined. Our results suggest that shade-shoot morphology is adaptive for utilizing diffuse light incoming from various angles, while sun-shoot morphology is adaptive for avoiding the negative effects of strong direct radiation and for enhancing light diffusion into the canopy. We propose that the angle of incoming light should be taken into account when estimating photosynthetic rates of sun shoots of conifer trees in the field.

Sunflecks in trees and forests: from photosynthetic physiology to global change biology

Sunflecks are brief, intermittent periods of high photon flux density (PFD) that can significantly improve carbon gain in shaded forest understories and lower canopies of trees. In this review, we discuss the physiological basis of leaf-level responses to sunflecks and the mechanisms plants use to tolerate sudden changes in PFD and leaf temperature induced by sunflecks. We also examine the potential effects of climate change stresses (including elevated temperatures, rising CO(2) concentrations and drought) on the ability of tree species to use sunflecks, and advocate more research to improve our predictions of seedling and tree carbon gain in future climates. Lastly, while we have the ability to model realistic responses of photosynthesis to fluctuating PFD, dynamic responses of photosynthesis to sunflecks are not accounted for in current models of canopy carbon uptake, which can lead to substantial overestimates of forest carbon fixation. Since sunflecks are a critical component of seasonal carbon gain for shaded leaves, sunfleck regimes and physiological responses to sunflecks should be incorporated into models to more accurately capture forest carbon dynamics.

Light interception efficiency explained by two simple variables: a test using a diversity of small- to medium-sized woody plants

• Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. • We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). • The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. • These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.

Analysis of the sensitivity of absorbed light and incident light profile to various canopy architecture and stand conditions

We analyzed the effect of simplifying assumptions in canopy representation of radiation transfer models, comparing modeled diffuse non-interceptance and photosynthetic photon flux density with measurements at different layers of complex pine-broadleaved canopy with large seasonal variation of leaf area index. The most detailed model included clumping of trees (i.e., stand density) and a vertical specification of leaf angle distribution and shoot clumping. A less detailed model replaced the vertically specified variables with their means. The most parsimonious model accounted for neither shoot clumping nor stand density. The vertical specification of shoot clumping and leaf angle distribution only slightly improved vertical and seasonal openness and light estimates over using mean values. Further simplification had little effect on total absorbed light but was more risky for estimates of the vertical distributions of openness and light absorbed by the canopy, which will affect photosynthesis estimates due to the non-linearity of photosynthetic light response. Including woody surfaces in winter, when leaf area was low, was essential for reproducing the measurements correctly. A sensitivity analysis showed that ignoring (i) shoot clumping could result in a substantial overestimation of total absorbed light with errors increasing with decreasing leaf area and (ii) stand density in sparse stands could lead to substantial overestimation of total absorbed light, and the effect is largely independent of leaf area. Also, (iii) the effect of changing leaf angle distribution increased with decreasing leaf area, and was larger and more persistent along the leaf area range with increasing shoot clumping. Overall, accounting for the effect of tree clumping on absorbed light is most important in stands composed of species where leaves are not very clumped (e.g., broadleaved). However, even in forests with highly clumped shoots (i.e., coniferous), an accurate estimation of absorbed light distribution in stands requires incorporation of stand density in the model.

Effects of hydraulic architecture and spatial variation in light on mean stomatal conductance of tree branches and crowns

In a Pinus taeda L. (loblolly pine) plantation, we investigated whether the response to vapour pressure deficit (D) of canopy average stomatal conductance (G(S)) calculated from sap flux measured in upper and lower branches and main stems follows a hydraulically modelled response based on homeostasis of minimum leaf water potential (Psi(L)). We tested our approach over a twofold range of leaf area index (L; 2-4 m(2) m(-2)) created by irrigation, fertilization, and a combination of irrigation and fertilization relative to untreated control. We found that G(S) scaled well from leaf-level porometery [porometry-based stomatal conductance (g(s))] to branch-estimated and main stem-estimated G(S). The scaling from branch- to main stem-estimated G(S) required using a 45 min moving average window to extract the diurnal signal from the large high-frequency variation, and utilized a light attenuation model to weigh the contribution of upper and lower branch-estimated G(S). Our analysis further indicated that, regardless of L, lower branch-estimated G(S) represented most of the main stem-estimated G(S) in this stand. We quantified the variability in both upper and lower branch-estimated G(S) by calculating the SD of the residuals from a moving average smoothed diurnal. A light model, which incorporated penumbral effects on vertical distribution of direct light, was employed to estimate the variability in light intensity at each canopy level in order to explain the increasing SD of both upper and lower branch-estimated G(S) with light. The results from the light model showed that the upper limit of the variability in individual branch-estimated G(S) could be attributed to incoming light, but not the variation below that upper limit. A porous medium model of water flow in trees produced a pattern of variation below the upper limit that was consistent with the observed variability in branch-estimated G(S). Our results indicated that stems acted to buffer leaf- and branch-level variation and might transmit a less-variable water potential signal to the roots.

Growth towards light as an adaptation to high light conditions in Chara branches

Growth of plants or plant organs towards more light is commonly interpreted as an adaptation to low light conditions. Here, we show for the first time, in a study of charophyte branches, a growth-based orientation towards light functioning as a mechanism to protect the plant from excessive light. Two Chara species were exposed to five different intensities of photosynthetically active radiation and species traits and pigmentation were measured. Branches of plants exposed to higher light intensities were convergent and pointed steeply upwards, whereas those exposed to lower light intensities grew nearly straight and were less inclined. Only branches that increased in length during the experiments reacted to differences in light intensity. This indicates that branch orientation is determined by a light-dependent growth reaction. Orientation of charophyte branches towards light is accompanied by a decrease in chlorophyll a (Chla) content and a lower Chla : carotenoid ratio, which clearly indicates that the plant is taking protective measures against potentially damaging excess light conditions. We suggest that the growth-based orientation of Chara branches towards light may protect sexual organs, which grow on adaxial branch sides, from light damage. In addition, the upward orientation of branches might lead to increased light transmission within dense charophyte beds, thus enabling an enhanced gross production.