Abstract
Leaf chlorophyll content declines in response to environmental stressors and during natural senescence periods. Assessments of forest health and vulnerability to stressors require repeated measurements of forest health to quantify declines in chlorophyll content and identify shifts in the timing of key phenological events. Although remote sensing is ideal for making reliable and repeated forest health measurements to assess forest response to environmental change over large areas, reliable warning systems for monitoring plant health are lacking. To facilitate the estimation of chlorophyll content from remotely sensed data, we define the relationships between leaf chlorophyll content and spectral indices in European beech trees (Fagus sylvatica). We show that hand-held chlorophyll meters can be calibrated to leaf chlorophyll content providing a non-destructive method for rapid assessment of leaf chlorophyll content in the field. Spectral indices based on the red edge spectral region have the strongest relationship with leaf chlorophyll content (mND705 R2 = 0.95, RMSE = 4.70; mSR705 R2 = 0.95, RMSE = 4.71) but are unable to discriminate between photosynthetically active leaves and inactive leaves. Alternative spectral indices can discriminate effectively between photosynthetically active and inactive leaves but are less well suited to defining declines in chlorophyll content. These findings reveal key research needs for improving the use of remote sensing data for forest health assessments.
Highlights
Plant pigment concentrations are important for understanding plant-environment interactions
Spectral indices based on the red edge spectral region have the strongest relationship with leaf chlorophyll content but are unable to discriminate between photosynthetically active leaves and inactive leaves
The close relationship between SPAD values and total chlorophyll content of mature European beech leaves and low root mean square error (R2: 0.96; RMSE: 4.31) demonstrates that the SPAD-502 meter is capable of detecting declines in chlorophyll content and emphasises the value of hand-held chlorophyll meters for estimating chlorophyll content in the field [1, 24, 25]
Summary
Plant pigment concentrations are important for understanding plant-environment interactions. Plants absorb light energy for photosynthesis primarily using the chlorophyll pigments (chlorophyll a and chlorophyll b) and these essential pigments are found in the highest concentrations in healthy leaf material. The ability to characterise changes in chlorophyll concentration in vivo during stress events or senescence periods can provide important insights into plant-environment interactions, help to assess plant health, and determine growth potential. The repeated measurement of chlorophyll content over large areas provides an extremely useful mechanism to improve our understanding of plant-environment interactions and provide evidence to inform forest management, by detailing subtle changes in chlorophyll content that will enable changes in plant health and shifts in natural phenology cycles to be characterised [5, 6]
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