13 Apr 2016

Spatial and Temporal Variations in the Response of the Vegetation Indices to Surface Temperature

Revalin Herdianto, March 2013
The University of Western Australia

Abstract
Temperature and water are two environmental factors that greatly affect vegetation dynamics. Vegetation response to variations in these factors is very complex and is modulated by topographic gradients. Moreover, the response is affected by vegetation adaptation to the local environment. Temperate deciduous broadleaf forests are an ideal land use class to study the response patterns due to their conspicuous seasonality driven by climates. This thesis examined response signatures of deciduous broadleaf forests dominated by oak (Quercus spp.) in South Korea (referred to as Korea), Bosnia, West Virginia (USA) and patches of Nothofagus gunnii in Tasmania (Australia) to variations in temperature and precipitation as viewed by remotely sensed vegetation indices. The overall aim of the thesis is to identify response signatures of vegetation indices to surface temperature with variations in altitudinal gradients and precipitation.

In the first phase of the work, time series of enhanced vegetation index (EVI) and normalised difference vegetation index (NDVI) with land surface temperature (LST) were analysed with precipitation patterns in Sancheong catchment, Korea, with varying elevation, using 1000 m pixels. This stage aimed to obtain the response patterns of vegetation indices to surface temperature with temporal variations in precipitation along a topographic gradient. In the second stage, the analysis was extended to Bosnia, West Virginia, and Tasmania using EVI only, with 250 m pixels. This stage aimed to obtain the response of vegetation index to surface temperature with spatial variations in precipitation patterns and vegetation composition. Analysis using precipitation was undertaken on cumulative precipitation prior to peak EVI to examine the effect of summer water availability and inter seasonal storage.

This study found a non linear and positive correlation, with a threshold in inter-annual EVI-LST relationship in Korea, Bosnia, and West Virginia. EVI response to LST is different below and above the threshold. While the threshold was set at an EVI of 0.25 in Korea using 1000 m pixels, the value increased to 0.30 when using 250 m pixels in the same site. In Bosnia and West Virginia, the threshold remained at EVI 0.25, but the distinction between below and above the threshold was less obvious. In Tasmania, the threshold was not present. This was due to lower winter temperature ranges in Korea, Bosnia, and West Virginia with longer periods of low temperature. This result shows that vegetation growing in similar temperature regimes exhibit a convergence in their response to temperature variations, independent of species composition.

When the analysis was undertaken at a smaller temporal scale, i.e., annually, the EVI-LST relationship demonstrated hysteresis loops, which exhibit different sensitivity to precipitation patterns in different regions. The loop width, denoted by the horizontal (LST) gap between the ascending and descending path, is associated with annual precipitation in Korea. Beside, variations in loop shape can be explained by cumulative precipitation before peak EVI, which indicates a storage effect from precipitation in previous season. In Bosnia and West Virginia, both annual precipitation and cumulative precipitation only partly explain the variations in the loop shape. Analysis on evapotranspiration relative to precipitation using a Budyko curve suggested that Bosnia and West Virginia are more potentially in water deficit, while Korea is less likely in this status. The lack of response signatures in Bosnia and West Virginia may be related to deeper root systems that enable the use of water reserve. Tasmania is in an energy-limited environment, and growth in this region is not limited by water availability. These results confirm previous studies on vegetation adaptive response to local climatic conditions. The response patterns can be traced by its annual and inter-annual EVI-LST relationship in deciduous broadleaf forest environment such as in Korea.

This study has successfully obtained the response signatures of vegetation indices to surface temperature with variations in spatial and temporal precipitation patterns in Korea, as aimed in the study. Yet, this leads to the importance of further research to formulate the effect of precipitation from previous season and water storage to the shape of hysteresis loop.

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