Location differences in senescence and leaf macronutrient concentrations for selected tree species in the savannah woodlands of Botswana

Abstract

Trees are a core component of savannah vegetation content, with vital ecosystem functions. Therefore, determining location differences in their senescence time and leaf macronutrient (MN) concentrations can yield indicators of differences in habitat quality. In this work, senescence time and MN levels in three common tree species (Colophospermum mopane, Grewia flava, Vachellia erioloba) in the semi-arid savannah woodlands of Botswana were studied, at three temperature and rainfall-contrasted study sites. Chlorophyll (Chl) levels in sample tree leaves were measured weekly in the March–May (autumn) period using a chlorophyll meter that utilised the blue + red absorption and green reflectance Chl properties. In the same period, sampling tree Green Leaf Index (GLI) values were computed from periodic (approximately every five days) cloud-free 10 m resolution Sentinel-2 MSI images. Some image dates coincided with the Chl measurement dates, which facilitated predictive modelling using GLI values. Leaf MN concentrations were determined once-off, during the peak phenology period. Graphical plots of measured Chl and GLI values indicated the onset of senescence, through the commencement of sustained reductions. Trees in the hotter, more arid site generally had earlier senescence and lower nitrogen and phosphorous concentrations than same species trees in cooler, higher rainfall sites.