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Impact of Climate Change on Vegetation Phenology Using Multi-Channel Photography

Alex Stein

The term ‘phenology’ is used to describe the seasonal repetition of phenomena during a plant’s lifecycle, such as germination, growth, leaf budding, flowering and leaf fall. These phenomena are affected by the genetics of the different plant species and are directed to certain seasons by environmental conditions such as temperature, water and light. Global climate change has a crucial impact on the phenology of many plant species, and as a result, on the composition, function and services provided by ecosystems to humans.

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In recent decades, climate change has taken place at a relatively rapid rate to that which characterised the earth in the past – in the last decade in Israel we have witnessed desiccation and mortality of oaks, native and planted pines and other species, which can be linked to climate change. Since changes in plant phenology are rapid and probably are the plants’ first response to climate change, it is very important to monitor them routinely and continuously. For example, phenological monitoring should be the first to reveal signs of stress in trees, stemming from consecutive drought years. During 2014–15, remote sensing data were collected as part of a scientific study aiming to monitor the impact of climate change on plants’ lifecycles – vitality, leaf budding, leaf fall and flowering, in order to obtain an early

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warning about desiccation and mortality of the vegetation. Monitoring was performed by daily photography with a multi-channel digital camera (that provides information at a number of wavelengths), installed at the top of a tower, as well as with a regular digital camera. The two cameras provide photographs around midday, where the digital camera photographs within

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the visible spectrum only, while the multispectral camera photographs at five different wavelengths in the visible and near-infra red ranges.

From the remote sensing data we can calculate indices that represent the physiological and phenological state of the vegetation in the region, such as the normalised difference vegetation (NDVI) index , based on the

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multispectral camera data. In parallel, from the digital camera data we can calculate relatively simple indices based on the values of the digital numbers (DN) in the visible spectrum only.

During the first research year (April 2014 – April 2015), physiological measurements (water potential, stomatal conductivity and chlorophyll content) were performed every two months on 24 representative trees and shrubs, in order to link the spectral indices to the biophysical measurements.

As part of the analysis of the multispectral camera data, annual time series of NDVI were constructed for each of the monitored trees and for the entire study area. These time series presented characteristic phenological patterns for evergreen Mediterranean tree species, with higher NDVI values during spring. The physiological measurements that were performed in parallel during this year showed a physiological response of the trees to the seasonal water deficit, expressed by a decrease in the values of water potential and limited stomatal opening, mainly during summer. Similarly, examination of the correlation between NDVI values and leaf chlorophyll content showed that the relationship between these indices is not significant, probably due to the methodological difficulty in appropriately representing the entire tree canopy during the sampling season.

A follow-up study will focus on analysing additional spectral indices from the cameras operating in the field, in combination with phenological observations in the study area as a tool for understanding and explaining the processes observed using remote sensing technology.

The study took place in collaboration with Prof. Arnon Karnieli and the staff of the Remote Sensing Laboratory of the Institutes for Desert Studies at Ben-Gurion University of the Negev.

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