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Rapid and reliable estimation of leaf area index (LAI), a crucial parameter in process-based models of vegetation cover response, is important in ecological studies. The Beer-Lambert law is widely used to calculate forest LAI, but data collection methods are time-consuming and calculations are often inaccurate. Our objective was to improve the accuracy of Beer-Lambert law-based LAI estimation by employing indirect data collection and location-specific light extinction coefficients ( K ). Canopy transmittance and LAI of two 100 m 2 temperate forest stands in southwestern Germany, one managed and one protected, was estimated using line quantum sensing (LQS) at 45,000 points per stand. The Beer-Lambert law was then inverted to estimate LAI using the measured transmittance with a K of 0.53-0.54. Hemispherical reference photographs were used as independent validation data to determine ideal K values. Experimental data demonstrated that LAI values estimated using LQS with adjusted K values were more accurate than those calculated using the basic application of the Beer-Lambert law. LQS results correlated with those determined using hemispherical photography for both the managed (R² = 0.80) and protected (R² = 0.81) stands. Overall, these findings show that adjusting K values for individual forest systems improves the accuracy of LAI estimation.•The modified method is more accurate than that using fixed K ranges.•The modified method accounts for individual ecosystems, with different K values for different environments.•The method can accurately reflect the dynamic changes of forest canopy structure, allowing integration of additional environmental measurements., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have influence the work reported in this paper., (© 2022 The Author(s).)

Abstract: The photon recollision probability in vegetation canopies, defined as the probability that a photon, after having interacted with a canopy element, will interact again, is a useful tool in remote sensing and ecological applications, enabling to link canopy optical properties at different wavelength and to estimate radiation absorption. In this work, a method is presented to estimate the photon recollision probability for horizontally homogeneous leaf canopies with arbitrary leaf angle distribution as well as for discrete crown canopies. The estimation is based on analytical approximation of the first-order recollision probability. Using the analytical solution of the two-stream equations of radiative transfer and Monte Carlo modeling, the first-order photon recollision probability is shown to slightly underestimate the mean recollision probability. Also, an approximation formula for the mean recollision probability in a horizontally homogeneous canopy is presented as a function of leaf area index. The method to calculate photon recollision probability in discrete crown canopies requires only the knowledge of total and between-crown canopy transmittance and is thus independent of the geometric-optical model used. [Copyright &y& Elsevier]

Abstract: Although it is known that many plant species that are sensitive to solar UV radiation exist in low radiation environments, little is known about the sensitivity of young tree seedlings and herbaceous forest vegetation that inhabits the low radiation environment of the forest understory. Furthermore, little is known about the UV radiation environment of this vegetation during the deciduous forest leaf-out period when solar radiation levels are the highest. The development of the forest canopy and the penetration of solar UV radiation through the canopy to the understory of a deciduous upland forest was studied between 15 April and 1 June of 2004. During the early part of the leaf-out period, the mean canopy transmittance in the PAR exceeded that in the UV-A or UV-B. By 20 days into the leaf-out period, the PAR transmittance was nearly equal to that of the UV. As the leaves in the canopy reached full expansion, the PAR, UV-A and UV-B penetration was essentially equal and small. Canopy transmittance during leaf-out was not dependent on cloud cover. Greater penetration of solar radiation in longer wavelengths within the UV was probably a result of scattering of direct beam radiation off tree bark in the canopy. The effective canopy transmittance from 15 March through the canopy leaf-out was 47%. For an expected period of UV exposure for the herbaceous layer of 15 March through 15 May, the tree seedlings and herbaceous plants received 41kJm−2 of biologically effective UV-B radiation. It is projected that the herbaceous layer vegetation will receive approximately 27% more UVBBE radiation on clear days with a decreased stratospheric ozone of 20DU and approximately 3% more UVBBE radiation with increased temperatures causing leaf-out to begin 10 days earlier than 2004. More research is needed to understand the potential interactions of these and other climate change components on the deciduous forest. [Copyright &y& Elsevier]

Urban trees are one of the most effective strategies to mitigate excessive heat stress in cities. To understand the underlying mechanisms of their cooling effect and to assess their use in urban planning, the accurate simulation of how trees interact with the ambient built environment is critical and imperative. However, the representation of urban trees in existing urban canopy models (in particular the single-layer ones) remains oversimplified. Here we develop a new Monte Carlo ray tracing method to explicitly resolve the canopy transmittance and evaluate its impact on radiative view factors between trees and regular building facets. The new method is highly accurate in reproducing analytical solutions. Sensitivity tests of radiative view factors suggest the importance of canopy transmittance in changing the radiation exchange. We then incorporate the ray tracing algorithm into the new version of the Arizona State University (ASU) Single-Layer Urban Canopy Model (ASLUM v3.1). In addition to radiation transmittance, ASLUM v3.1 explicitly resolves the radiative shading, evapotranspiration, and root water uptake of urban trees in street canyons, with significantly improved performance in predictions (especially latent heat flux) when compared to previous versions. We further apply ASLUM v3.1 to evaluate the impacts of trees with varying characteristics on urban radiation exchange and turbulent heat fluxes. Results show that urban trees reduce the net radiation of ground and wall as well as the daytime temperature via shading and transpiration, but may slightly warm the nighttime street canyons through radiative trapping effect. Image 1 • We develop a new stochastic method to include canopy transmittance in view factors. • Sensitivity of view factors to urban geometries and transmittance is evaluated. • The new urban canopy model with tree modeling outperforms its previous versions. • Radiation exchange and cooling effect of trees depend on foliage characteristics. [ABSTRACT FROM AUTHOR]