Leaf chlorophyll content as a proxy for leaf photosynthetic capacity

Abstract

Improving the accuracy of estimates of forest carbon exchange is a central priority for understanding ecosystem response to increased atmospheric CO levels and improving carbon cycle modelling. However, the spatially continuous parameterization of photosynthetic capacity (Vcmax) at global scales and appropriate temporal intervals within terrestrial biosphere models (TBMs) remains unresolved. This research investigates the use of biochemical parameters for modelling leaf photosynthetic capacity within a deciduous forest. Particular attention is given to the impacts of seasonality on both leaf biophysical variables and physiological processes, and their interdependent relationships. Four deciduous tree species were sampled across three growing seasons (2013-2015), approximately every 10 days for leaf chlorophyll content (Chl ) and canopy structure. Leaf nitrogen (N ) was also measured during 2014. Leaf photosynthesis was measured during 2014-2015 using a Li-6400 gas-exchange system, with A-Ci curves to model Vcmax. Results showed that seasonality and variations between species resulted in weak relationships between Vcmax normalized to 25°C (Vcmax25) and N (R = 0.62, P < 0.001), whereas Chl demonstrated a much stronger correlation with Vcmax25 (R = 0.78, P < 0.001). The relationship between Chl and N was also weak (R = 0.47, P < 0.001), possibly due to the dynamic partitioning of nitrogen, between and within photosynthetic and nonphotosynthetic fractions. The spatial and temporal variability of Vcmax25 was mapped using Landsat TM/ETM satellite data across the forest site, using physical models to derive Chl . TBMs largely treat photosynthetic parameters as either fixed constants or varying according to leaf nitrogen content. This research challenges assumptions that simple N -Vcmax25 relationships can reliably be used to constrain photosynthetic capacity in TBMs, even within the same plant functional type. It is suggested that Chl provides a more accurate, direct proxy for Vcmax25 and is also more easily retrievable from satellite data. These results have important implications for carbon modelling within deciduous ecosystems.

Estimation of Leaf Photosynthetic Capacity From Leaf Chlorophyll Content and Leaf Age in a Subtropical Evergreen Coniferous Plantation

“…Previously, V cmax normalized to 25 °C (V cmax25 ), a proxy for Rubisco content, was assumed to be constant over time according to PFT in most ecosystem models (Houborg et al, ; Wang et al, ; Y. G. Zhang, Guanter, et al, ). However, research has demonstrated both seasonal variations (Alton, ; Croft et al, ; Grassi et al, ; Medvigy et al, ) and large differences between species and even within the same PFT (Croft et al, ; Dillen et al, ). Perhaps the largest efforts to model V cmax25 have been through its relationship with leaf nitrogen content (N) (Kattge et al, ; Walker et al, ), due to the large amount of total leaf N (15–35%) that is allocated to Rubisco protein in C 3 plants (Evans, ; Feng & Dietze, ; Field & Mooney, ; Sage et al, ; Walcroft et al, ).…”

“…Recent research has demonstrated the potential for leaf chlorophyll content as a proxy for V cmax25 (Croft et al, ; Houborg et al, ; Houborg et al, ; Luo et al, ). Houborg et al () established a semi‐mechanistic relationship between V cmax25 and chlorophyll (leaf nitrogen as an intermediate variable) in cropland ecosystems.…”

“…Houborg et al () established a semi‐mechanistic relationship between V cmax25 and chlorophyll (leaf nitrogen as an intermediate variable) in cropland ecosystems. Croft et al () directly built a relationship between photosynthetic capacity (V cmax25 and J max25 ) and leaf chlorophyll content in a temperate deciduous forest. Alton () derived V cmax25 from the relationship between the MERIS Terrestrial Chlorophyll Index (MTCI), a vegetation index related to leaf chlorophyll content, and J max25 .…”

“…However, leaf chlorophyll content was higher in 1‐year‐old leaves than in the current year's leaves (Katahata et al, ). Previous studies have focused on testing the leaf chlorophyll‐V cmax25 relationship in croplands and deciduous forests (Croft et al, ; Houborg et al, ), with little investigation into this relationship in other PFTs and for different leaf ages.…”

Canopy-Level Photochemical Reflectance Index from Hyperspectral Remote Sensing and Leaf-Level Non-Photochemical Quenching as Early Indicators of Water Stress in Maize

“…Leaf chlorophyll and carotenoid contents are not good indicators of water stress, because leaf chlorophyll pigments do not have direct response to water stress. Instead, leaf chlorophyll content is highly related to illumination [28], leaf nitrogen and RuBisCo (ribulose 1, 5-bisphosphate carboxylase/oxygenase) enzyme [43]. When water stress increased, carotenoid content increased while the chlorophyll content remained constant or decreased, leading to the decrease in the Chl/Car ratio.…”