Chlorophyll meters for monitoring foliar nitrogen in three tree species from arid Central Asia

Leaf chlorophyll meters can be used to support easy and efficient crop nitrogen (N) management. They can be used on tree species native to Central Asia that have been suggested for afforesting salt-affected croplands, but its use demands a priori calibration as the relationship between chlorophyll meter measurements and leaf N contents is crop/species specific. Data collected with a SPAD-502 leaf chlorophyll meter during 2006–2008 was used to generate a calibration dataset for the leaf N status of three tree species. The second-degree polynomial relationships between the SPAD-502 readings and total leaf chlorophyll and N content had higher predictive power than linear relationships for all species, although with differences among them. The validation was completed with the same species grown at a different location (difference in space) and with older trees (difference in age). The comparison between predicted and spectrophotometrically determined leaf N content showed a root mean square error (RMSE) of 25 mg g⁻¹ DM, which corresponds to a relative RMSE of 10% for Elaeagnus angustifolia. For Gleditsia triacanthos and Robinia pseudoacacia they were 11 mg g⁻¹ DM and 16%, 14 mg g⁻¹ DM and 10%, respectively. The findings demonstrate that the SPAD-502 can with confidence be used for non-destructive assessment of tree foliar N status of these three species, although only within the range of the SPAD-502 values used for the calibration. Options for the use of the SPAD-502 for forest management in agroforestry-based land use systems in Central Asia are addressed.