Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

Abstract

[1] This study investigates the impacts of canopy structure specification on modeling net radiation (R n ), latent heat flux (LE) and net photosynthesis (A n ) by coupling two contrasting radiation transfer models with a two-leaf photosynthesis model for a maturing loblolly pine stand near Durham, North Carolina, USA. The first radiation transfer model is based on a uniform canopy representation (UCR) that assumes leaves are randomly distributed within the canopy, and the second radiation transfer model is based on a gappy canopy representation (GCR) in which leaves are clumped into individual crowns, thereby forming gaps between the crowns. To isolate the effects of canopy structure on model results, we used identical model parameters taken from the literature for both models. Canopy structure has great impact on energy distribution between the canopy and the forest floor. Comparing the model results, UCR produced lower R n , higher LE and higher A n than GCR. UCR intercepted more shortwave radiation inside the canopy, thus producing less radiation absorption on the forest floor and in turn lower R n . There is a higher degree of nonlinearity between A n estimated by UCR and by GCR than for LE. Most of the difference for LE and A n between UCR and GCR occurred around noon, when gaps between crowns can be seen from the direction of the incident sunbeam. Comparing with eddy-covariance measurements in the same loblolly pine stand from May to September 2001, based on several measures GCR provided more accurate estimates for R n , LE and A n than UCR. The improvements when using GCR were much clearer when comparing the daytime trend of LE and A n for the growing season. Sensitivity analysis showed that UCR produces higher LE and A n estimates than GCR for canopy cover ranging from 0.2 to 0.8. There is a high degree of nonlinearity in the relationship between UCR estimates for A n and those of GCR, particularly when canopy cover is low, and suggests that simple scaling of UCR parameters cannot compensate for differences between the two models. LE from UCR and GCR is also nonlinearly related when canopy cover is low, but the nonlinearity quickly disappears as canopy cover increases, such that LE from UCR and GCR are linearly related and the relationship becomes stronger as canopy cover increases. These results suggest the uniform canopy assumption can lead to underestimation of R n , and overestimation of LE and A n . Given the potential in mapping regional scale forest canopy structure with high spatial resolution optical and Lidar remote sensing plotforms, it is possible to use GCR for up-scaling ecosystem processes from flux tower measurements to heterogeneous landscapes, provided the heterogeneity is not too extreme to modify the flow dynamics.

Estimating average tree crown size using spatial information from Ikonos and QuickBird images: Across-sensor and across-site comparisons

“…Tree crown size is not only a critical canopy structural parameter for understanding forest ecosystem function (Song et al, 2009), but it is also significantly related to DBH, the major structural parameter that is often used to infer numerous other structural parameters (Gholz et al, 1979;Grier & Logan, 1977;Jenkins et al, 2004). Table 1 shows that there is a significant allometric relationships between CD and DBH on a species-specific basis.…”

“…Consideration of between-crown gaps is a major step toward more realistic characterizations of forest canopies from the traditional turbid medium representation where leaves are assumed to be randomly distributed in the entire canopy space without crown boundaries. Song et al (2009) found that a uniform canopy representation can lead to overestimation of radiation interception, transpiration, and carbon assimilation in the canopy. Therefore, tree crown size is essential for more accurate understanding of ecosystem function.…”

“…These exchanges determine the nature of goods and services that terrestrial ecosystems can offer and depend on canopy structure (Chen & Coughenour, 1994;Ni et al, 1997;Song et al, 2009). Tree crown size is one of the key canopy structural parameters because crowns are the space within which leaves are attached to the trees.…”

A photosynthesis‐based two‐leaf canopy stomatal conductance model for meteorology and air quality modeling with WRF/CMAQ PX LSM

“…Stomata control the amount of water transpired by vegetation so that stomatal conductance and its scaling from leaf to canopy are key processes in estimating ET. Following approaches developed for global climate models (GCMs) [ Dai et al ., ; Cox et al ., ; Bonan et al ., ; Clark et al ., ; Kowalczyk et al ., ; Oleson et al ., ] and ecosystem productivity models [ Campbell and Norman , ; Medlyn et al ., ; Song et al ., ; Evers et al ., ; Baker et al ., ], we implemented a coupled photosynthesis‐based stomatal conductance model with sunlit and shaded leaves in the PX LSM for coupling ET estimation with CO 2 assimilation.…”

Modeling photosynthesis of discontinuous plant canopies by linking the Geometric Optical Radiative Transfer model with biochemical processes

“…Recent efforts have been made to develop and evaluate a simplified GORT model for the use in coupled global dynamic terrestrial ecosystem models Yang et al, 2010). Despite these successful applications, the current version of the GORT model does not have analytical solutions for radiation absorption by sunlit and shaded leaves, though previous studies have tried to solve the process of multiple scattering between canopy and background in an iterative manner (Song et al, 2009). However, sunlit and shaded leaves must be treated separately in photosynthesis modeling because flux densities of photosynthetically active radiation (PAR) incident on leaf surfaces are different (He et al, 2013).…”