NIR hyperspectral imaging combined with chemometrics for the monitoring of water patterns during dehydration of nonvascular epiphytic communities

Description

The present work is aimed at developing water maps for understanding how a micro- ecosystem composed by nonvascular epiphytic communities (mainly, lichens and bryophytes), manages water and, consequently, how the thalli can prolong water availability depending on the morphology of the species and the composition of the community itself. For the first time, in fact, the nonvascular epiphytic community has been studied during the dehydration process with the objective of understanding the relationship between bryophytes and lichens and their competition or facilitation regarding water management. To achieve this aim, the water status of bark, lichens (crustose and foliose) and bryophytes was monitored by means of near-infrared (NIR) hyperspectral imaging (HSI), working in the spectral range from 1000 to 2500 nm (SWIR3 – Specim, Finland). This spectroscopic technique appears to be the most suitable from both a spectral and a spatial perspective. From the spectral perspective, in fact, NIR spectroscopy presents strong absorption bands ascribable to the O-H bond and, so, water shows an unmistakable signature in this spectral region. Moreover, the application of an imaging technique allows to identify spatial patterns, a key aspect when a non-homogeneous sample is analysed. Furthermore, the non-destructive and non-contact nature of the technique offers the advantage of enabling the monitoring of the same sample for the entire duration of the process without interferences. Regarding the sampling protocol, it was possible to analyse pure communities (composed mainly by a single organism), binary and ternary mixtures, for a total of 32 samples. All of the micro-ecosystem were naturally supported by chestnut bark. Samples of non-colonised (or as less colonised as possible) bark were also analysed. The samples were first submitted to a full hydration protocol and, then, let dry at room temperature (19°C) and humidity (41%) for about 12 h. During the dehydration period, NIR-HSI acquisition was carried out every 45 minutes for 15 sampling points, obtaining 480 images. Before image acquisition at each sampling point, the samples were weighed to quantify their dehydration curve, relative to their weight, prior to the hydration phase. The system, in fact, over the period of time relative to the dehydration cycle, can be considered constant regarding its chemical composition and so the variations along NIR spectra can be ascribable mainly to water content variation. On the images, an exploratory data analysis was performed, by means of principal component analysis (PCA), with the aim of comparing the drying trajectory in the orthogonal space defined by the lowest-order components with the dehydration curve. The study of these correspondences on pure communities opens the possibility to understand the water management of each organism and then to use such information in the study of more complex microclimates (binary and ternary mixtures). The projection of the microclimate images in the space defined by the pure samples allows to map changes in the water status of each complex sample, focusing on lichens and bryophytes and on their interaction. The role of the bark support was also considered. After this exploratory step, a supervised strategy is proposed, with the aim of extracting not only the water pattern inside the microclimate but also a numerical parameter expressing the water content in the different regions of interest inside a single map. The study of these communities and their relative behaviour in water stress condition plays a crucial role in understanding how these organisms are facing the climate change and the subsequent water scarcity or overabundance.

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