Abstract
Biological soil crusts (BSCs) are thin microbiological vegetation layers that naturally develop in unfavorable higher plant conditions (i.e., low precipitation rates and high temperatures) in global drylands. They consist of poikilohydric organisms capable of adjusting their metabolic activities depending on the water availability. However, they, and with them, their ecosystem functions, are endangered by climate change and land-use intensification. Remote sensing (RS)-based studies estimated the BSC cover in global drylands through various multispectral indices, and few of them correlated the BSCs’ activity response to rainfall. However, the allocation of BSCs is not limited to drylands only as there are areas beyond where smaller patches have developed under intense human impact and frequent disturbance. Yet, those areas were not addressed in RS-based studies, raising the question of whether the methods developed in extensive drylands can be transferred easily. Our temperate climate study area, the ‘Lieberoser Heide’ in northeastern Germany, is home to the country’s largest BSC-covered area. We applied a Random Forest (RF) classification model incorporating multispectral Sentinel-2 (S2) data, indices derived from them, and topographic information to spatiotemporally map the BSC cover for the first time in Central Europe. We further monitored the BSC response to rainfall events over a period of around five years (June 2015 to end of December 2020). Therefore, we combined datasets of gridded NDVI as a measure of photosynthetic activity with daily precipitation data and conducted a change detection analysis. With an overall accuracy of 98.9%, our classification proved satisfactory. Detected changes in BSC activity between dry and wet conditions were found to be significant. Our study emphasizes a high transferability of established methods from extensive drylands to BSC-covered areas in the temperate climate. Therefore, we consider our study to provide essential impulses so that RS-based biocrust mapping in the future will be applied beyond the global drylands.
Highlights
Our results prove that even with scarce data availability, but rather with the combination of expert knowledge for gathering training and testing data and high-resolution optical satellite imagery, we were able to successfully detect Biological soil crusts (BSCs) and map their activity in response to rainfall by transferring methods and meaningful BSC indices previously developed for arid and semi-arid drylands to a temperate dry acid grassland in Central Europe
Satellite-based Normalized Difference Vegetation Index (NDVI) measurements were revealed as supportive for quantifying the response of BSCs to rainfall events and their phenological cycle
The high cloud cover probability leads to a considerable loss in data availability for observing the quick response of BSCs to rainfall and differences between dry and wet cycles, constraining a continuous monitoring
Summary
While predominantly occurring within arid and semi-arid drylands, BSCs can be found globally throughout different climate zones [4] They influence ecosystems by limiting the water availability for soil biota, nutrient cycles, and vascular vegetation [5] and highly contribute to the global carbon and nitrogen budgets [5]. BSCs are reported as very sensitive to environmental changes, to rising temperature and decreasing humidity [8] They are highly endangered by climate change and land-use intensification, whereby the global BSC cover is expected to decrease by 25 to 40% from 2005 to 2070 [3]. This implicates the loss of their ecosystem functions. Acquiring more accurate information on their spatiotemporal distribution and dynamic is considered essential for developing sustainable management and protection strategies [1,9,10]
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