Abstract
We evaluated the response of vegetation’s photosynthetic activity to drought conditions from 1998 to 2014 over Romania and the Republic of Moldova. The connection between vegetation stress and drought events was assessed by means of a correlation analysis between the monthly Standardized Precipitation Evaporation Index (SPEI), at several time scales, and the Normalized Difference Vegetation Index (NDVI), as well as an assessment of the simultaneous occurrence of extremes in both indices. The analysis of the relationship between drought and vegetation was made for the growing season (from April to October of the entire period), and special attention was devoted to the severe drought event of 2000/2001, considered as the driest since 1961 for the study area. More than three quarters (77%) of the agricultural land exhibits a positive correlation between the two indices. The sensitivity of crop areas to drought is strong, as the impacts were detected from May to October, with a peak in July. On the other hand, forests were found to be less sensitive to drought, as the impacts were limited mostly to July and August. Moreover, vegetation of all land cover classes showed a dependence between the sign of the correlation and the elevation gradient. Roughly 60% (20%) of the study domain shows a concordance of anomalously low vegetation activity with dry conditions of at least 50% (80%) in August. By contrast, a lower value of concordance was observed over the Carpathian Mountains. During the severe drought event of 2000/2001, a decrease in vegetation activity was detected for most of the study area, showing a decrease lasting at least 4 months, between April and October, for more than two thirds (71%) of the study domain.
Highlights
The destruction caused by natural disasters has significant economic, environmental and social impacts [1]
Negative correlations between the Normalized Difference Vegetation Index (NDVI) and Standardized Precipitation Evaporation Index (SPEI) were more frequent in May and June, mostly for the Carpathians, though positive correlations during these months were noted as well (Figure 2)
In June, most of the correlations between the NDVI and SPEI were positive in the extra-Carpathian regions, and the areas in which this was the case increased in size as the time scale increased
Summary
The destruction caused by natural disasters has significant economic, environmental and social impacts [1]. Several approaches to assessing the impact of drought on vegetation activity have been proposed, including: (i) analyses of the hydraulic properties of vegetation, on either a species or a biome level [26,27]; (ii) assessment of the impacts on tree growth based on tree-ring data [28,29]; (iii) analyses of crop yields [30,31]; (iv) phenological studies [32,33]; and v) monitoring vegetation using satellite data [34,35,36] The latter approach is advantageous, since it allows repeated measurements with high spatial resolution to be made over large areas and at different times [37]. The impacts of drought on vegetation activity and on crop yields have been evaluated using so-called vegetation indices, obtained from remote sensing information, such as the Normalized Differences Vegetation Index (NDVI), at local, regional or global scales [25,35,38,39,40,41,42,43]
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