Sensitivity of Diffuse Light Fertilization in Southern Pine Carbon Fluxes Observed with Remotely-Sensed Vegetation Indices

Abstract

It is well established that plant canopies exhibit increased light use efficiency (LUE) under higher diffuse fraction, given constant total radiation. However, Southern pine forests have been largely overlooked in these studies, despite the fact that nearly one-third of forested land in the United States can be classified as Southern pine. Since this ecoregion contributes significantly to the national scale terrestrial carbon sink and experiences high aerosol loading and cloudiness, increasing understanding of diffuse light fertilization (DLF) could improve our ability to predict the climate mitigation potential of Southeastern forests. Here we have determined the sensitivity of gross primary productivity (GPP) to diffuse light by building remotely sensed LUE models that include increasingly explicit inputs of diffuse light, from simple approximations, to actual measurements from a diffuse radiometer, across 4 Ameriflux registered Southern pine eddy covariance sites. Our results show that including diffuse light measurements increased R2 and decreased RMSE values regardless of species or management stage, illustrating a strong influence of diffuse light fertilization in Southern pine ecosystems.

Full text

Sensitivity of Diffuse Light Fertilization in
Southern Pine Carbon Fluxes Observed with
Remotely-Sensed Vegetation Indices
Jeremy Forsythe & Tom O’Halloran - AMS May 2nd 2023

Introduction - Diffuse Light Fertilization
↑Gu, Lianhong, Dennis D. Baldocchi, Steve C. Wofsy, J. William Munger, Joseph J. Michalsky, Shawn P.
Urbanski, and Thomas A. Boden. 2003. “Response of a Deciduous Forest to the Mount Pinatubo
Eruption: Enhanced Photosynthesis.” Science 299 (5615): 2035–38.
←Niyogi, Dev, Hsin I. Chang, V. K. Saxena, Teddy Holt, Kiran Alapaty, Fitzgerald Booker, Fei Chen, et
al. 2004. “Direct Observations of the Eects of Aerosol Loading on Net Ecosystem CO2 Exchanges
over Dierent Landscapes.” Geophysical Research Letters 31 (20): 1–5.
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Introduction - Diffuse Light Fertilization Mechanism
→Knohl & Baldocchi. 2008. Eects of diuse radiation on canopy gas
exchange processes in a forest ecosystem. Journal of Geophysical Research:
Biogeosciences 113:1–17.
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Introduction - Diffuse Light Studies & Southern Pines
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Introduction - Southern Pine Productivity
↑Lu, Xiaoliang et al 2015. Land Carbon Sequestration within the Conterminous United States:
Regional- and State-Level Analyses. Journal of Geophysical Research: Biogeosciences 120: 379–98.
→McNulty, Steven et al. Forests and Climate Change in the Southeast USA. 2013. CLIMATE OF THE
SOUTHEAST UNITED STATES Chap 8.
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Methods - LUE Models & Data Sources
GPP =PAR ∗fAPAR ∗ϵ
“Big Leaf” Model “Two Leaf” Model
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Methods - Study Sites
Virginia
South
Carolina
Mature Young
Loblolly
(Pinus
taeda)
Longleaf
(Pinus
palustris)
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Hypotheses
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Results - Temperature & VPD Scalars
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Results : US-HB3 SC Young Longleaf
Site Model AIC R2 RMSE
US-HB3:
SC Young
Longleaf
Simple Model 411.27 0.78 1.22
Mid Cloud 693.49 0.81 1.40
Two Leaf -100.47 0.84 0.94
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Results : US-SB3 VA Young Loblolly
Site Model AIC R2 RMSE
US-SB3:
VA Young
Loblolly
Simple Model 1501.63 0.76 2.48
Mid Cloud 260.22 0.78 1.16
Two Leaf 162.54 0.85 1.09
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Results : US-HB2 SC Mature Longleaf
Site Model AIC R2 RMSE
US-HB2:
SC Mature
Longleaf
Simple Model 1031.66 0.73 1.63
Mid Cloud 97.53 0.78 1.04
Two Leaf 2450.00 0.71 3.22
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Results : US-SB1 VA Mature Loblolly
Site Model AIC R2 RMSE
US-SB1:
VA Mature
Loblolly
Simple Model 3146.36 0.83 5.50
Mid Cloud 723.07 0.86 1.47
Two Leaf 1088.30 0.84 1.79
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Conclusions
Site Model AIC R2 RMSE
US-HB2:
SC Mature
Longleaf
Simple Model 1031.66 0.73 1.63
Mid Cloud 97.53 0.78 1.04
Two Leaf 2450.00 0.71 3.22
US-HB3:
SC Young
Longleaf
Simple Model 411.27 0.78 1.22
Mid Cloud 693.49 0.81 1.40
Two Leaf -100.47 0.84 0.94
US-SB1:
VA Mature
Loblolly
Simple Model 3146.36 0.83 5.50
Mid Cloud 723.07 0.86 1.47
Two Leaf 1088.30 0.84 1.79
US-SB3:
VA Young
Loblolly
Simple Model 1501.63 0.76 2.48
Mid Cloud 260.22 0.78 1.16
Two Leaf 162.54 0.85 1.09
Conclusions:
1. Light use eciency models work well in
Southern pine ecosystems.
2. Diuse light fertilization eect is
apparent in Southern pine ecosystems.
3. Incorporating a quantification of the
diuse light fertilization eect into LUE
models will reduce error and increase
accuracy.
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Thank You! @JDForsythe12
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