Abstract
Multi-temporal remote sensing information and spectral indices have been
extensively used in studies to monitor ecosystem functioning and
surface-energy budgets. However, most of these indices did not show good
results in areas covered by sparse vegetation, like most of the
Drylands. In these ecosystems, open spaces between plants are often
covered by biological soil crusts (biocrusts), i.e. communities of
cyanobacteria, algae, microfungi, lichens, mosses and other
microorganisms growing in the uppermost millimeters of the soil. Due to
their mostly dark color, biocrusts influence the spectral response of
dryland surfaces, making it necessary to assess the sensibility of
widely used spectral indices to variations in biocrusts cover. In this
study we used spectra of biocrusts, bare soil and vegetation to analyze
the effect of biocrust cover on the spectral response of heterogeneous
areas. In a second approach we investigated the impact of biocrust water
status on spectral characteristics. Based on spectral mixture analysis,
we calculated the response of a wide range of vegetation/biocrust/bare
soil landscape compositions, obtaining a total of 702 spectra. These
were used to calculate the Normalized Difference Vegetation Index
(NDVI), the Enhanced Vegetation Index (EVI), the Water Index (WI) and
surface albedo, and the effect of biocrust cover and water status on
these indices was analyzed. Biocrusts exerted a considerable effect on
vegetation indices and surface albedo, whereas WI was mostly affected by
vegetation type and cover. As biocrust cover increased, the value of
NDVI and EVI also increased, whereas albedo decreased, and these effects
were more important under low vegetation cover. Moreover, as biocrusts
almost immediately turned dark after water pulses, the effect of
biocrust cover on spectral indices increased already 30 min after
wetting. Although these results varied depending on vegetation type,
they demonstrate, that biocrusts largely affect the spectral response of
dryland surfaces, and they illustrate how this effect is reinforced by
water. Thus, biocrusts need to be considered in studies analyzing
dryland phenology, productivity and water status. Moreover, in order to
increase the accuracy of hydrological and climate forecast predictions,
biocrust effects on surface albedo, both in a dry and wet stage, need to
be included. (C) 2015 Elsevier Inc. All rights reserved.
