Remote sensing of grassland communities: integrated effects of soil nutrients and habitat age

Research output: ThesisDoctoral Thesis (compilation)

Abstract

Semi-natural grasslands are characterized by high fine-scale plant species richness. The richest grassland
communities, with many habitat-specialist species, are found in ancient grassland sites that have a long continuity
of grazing management and low levels of soil nutrients. Grazed grasslands were widespread in the historical
landscape. Agricultural intensification over the last two centuries has led to a reduction of grassland area: ancient,
species rich grasslands now occur only as small and isolated fragments in the landscape. Young grasslands may
also develop, under grazing management, on previously arable fields. These younger sites have high soil nutrient
levels, making them unsuitable for grassland specialist species. However, leaching and biomass-removal by
grazing management mean that, over time, there is a progressive reduction of nutrient levels that is tracked by a
succession of plant communities – as the habitat conditions become more favourable for nutrient intolerant
species.
The aim of this thesis was to investigate the ability of spectral remote sensing to capture variation in plant
community composition in dry, grazed grasslands. The study system consists of differently-aged grassland sites,
within a succession from former arable land to ancient semi-natural grasslands, on the Baltic island of Öland,
Sweden. Analyses of the relationships between field-collected data on plant community composition, and data on
spectral reflectance were based on regression methods such as PLSR, and individual species’ responses to (field-
measured) soil nutrient concentrations and reflectance were analysed using Bayesian joint species distribution
modelling. Spectral data were acquired using airborne hyperspectral sensors and the multispectral satellite
WorldView-2.
The spectral reflectance of heterogeneous grassland canopies represents whole plant communities, consisting of
multiple individuals and species. Grassland canopy reflectance can be indirectly related to the community
composition, assuming that the spectral reflectance can detect variation in the environmental conditions that drive
the species assembly. Species that are adapted to different habitats are characterized by differences in the
structural and functional properties that determine their physical appearance and spectral characteristics.
Responses of individual species to particular environmental conditions result in plant communities with similar
preferences, and similar spectral characteristics.
The strongest gradient of canopy reflectance in the studied grasslands represented variation in photosynthetic
absorption in the blue and red wavelengths, and reflectance in NIR and SWIR. This gradient was associated with
species’ preferences for ammonium availability and soil quality, and was largely characterized by the NDVI.
However, the fact that the environmental preferences of individual species are not fully described by single
gradients means that single gradients of reflectance will have a limited capability to capture patterns in grassland
plant communities. Analysis of soil data showed that species’ distributions were explained primarily by their
different preferences for soil phosphorus concentrations. Species responses to soil phosphorus were not
associated with the main spectral gradient (or the NDVI) but were, instead, associated with reflectance in the
green and red-edge regions. Low reflectance in the green regions, and high reflectance in SWIR, are likely to be
useful spectral characteristics for identifying old, species rich and phosphorus-poor grasslands.
Although the grassland canopy reflectance can explain variation in important environmental gradients, grassland
community composition is not only dependent on the habitat conditions. Plant communities of old grasslands may
include species that rarely occur in younger grasslands as a result of ecological processes, such as dispersal, that
may not contribute to the spectral characteristics of the vegetation canopy.

Details

Authors
Organisations
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Physical Geography

Keywords

  • Semi-natural grassland, Plant ecology, Remote sensing, Nitrogen, Phosphorus, Spectral reflectance, grassland specialists, Succession
Original languageEnglish
QualificationDoctor
Awarding Institution
Supervisors/Assistant supervisor
Award date2020 Mar 27
Place of PublicationLund
Publisher
  • Lund University
Print ISBNs978-91-985015-3-7
Electronic ISBNs978-91-985015-4-4
Publication statusPublished - 2020 Feb 21
Publication categoryResearch

Bibliographic note

Defence details Date: 2020-03-27 Time: 10:00 Place: Lecture hall "Pangea" (229), Geocentrum II, Sölvegatan 12, Lund External reviewer(s) Name: Diekmann, Martin Title: Professor Affiliation: University of Bremen, Bremen, Germany --- ,

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