Windowed recurrence plot approach in semiarid grasslands

Being one of the essential ecosystems, grasslands represent an important ecological area for water and biodiversity conservation. In this line, remote sensing instruments are a helpful tool for assessing vegetation status. The Modified Soil-Adjusted Vegetation Index (MSAVI) time-series are used to monitor drought events and to consider the soil influence in vegetation monitoring. In this sense, Recurrence plots (RPs) techniques have been demonstrated to be one of the most capable tools to unravel the complex dynamics of the time-series analysis. This work highlights the recurrence techniques' benefits in visualising and quantifying vegetation dynamics.

We chose a study area in the centre of Spain, where the Mediterranean climate dominates. We selected the MODQ1.V006 product from the MODIS imagery collection, with a spatial resolution of 250x250m. Then, an average MSAVI time series from pixels that met predefined criteria were analysed. RPs and Cross recurrence plots (CRPs) were computed to reveal the dynamics of the time series. Furthermore, diagonal-wise profiles (DWP)  and windowed-cross recurrence plots (WCRPs) were included in the analysis at different time scales. In the end, RPs, CRPs and WCRPs are quantified through the recurrence quantification analysis (RQA).

RPs displayed different patterns depending on the studied time series. Precipitation showed a stochastic dynamic, emphasising the unstable behaviour of Mediterranean rainfalls. On the opposite, temperature revealed a diagonal-like pattern in the RP. This fact pointed out the temperature's seasonal behaviour over time. Concerning MSAVI, RP presented a mixture of both patterns.

CRPs between precipitation and MSAVI showed a delayed consequence of MSAVI to precipitation events. Contrary to precipitation, CRPs between temperature and MSAVI did not show a delayed response in the studied period. WCRPs indicated characteristic phases in the time series, revealing interactions between vegetation and climate and being different between wet and dry seasons.

RPs techniques have been demonstrated to be a valuable instrument for uncovering the complex dynamics between vegetation and climate. Therefore, they should be considered a viable alternative in the vegetation time series analysis.

 

Acknowledgements: The authors acknowledge the support of Clasificación de Pastizales Mediante Métodos Supervisados - SANTO from Universidad Politécnica de Madrid (project number: RP220220C024).

References

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Almeida-Ñauñay, A.F., Benito, R.M., Quemada, M., Losada, J.C., Tarquis, A.M., 2021. The Vegetation–Climate System Complexity through Recurrence Analysis. Entropy 23, 559. https://doi.org/10.3390/e23050559

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