Abstract
In water limited environments precipitation is often considered the key factor influencing vegetation growth and rates of development. However; other climate variables including temperature; humidity; the frequency and intensity of precipitation events are also known to affect productivity; either directly by changing photosynthesis and transpiration rates or indirectly by influencing water availability and plant physiology. The aim here is to quantify the spatiotemporal patterns of vegetation responses to precipitation and to additional; relevant; meteorological variables. First; an empirical; statistical analysis of the relationship between precipitation and the additional meteorological variables and a proxy of vegetation productivity (the Normalized Difference Vegetation Index; NDVI) is reported and; second; a process-oriented modeling approach to explore the hydrologic and biophysical mechanisms to which the significant empirical relationships might be attributed. The analysis was conducted in Sub-Saharan Africa; between 5 and 18°N; for a 25-year period 1982–2006; and used a new quasi-daily Advanced Very High Resolution Radiometer (AVHRR) dataset. The results suggest that vegetation; particularly in the wetter areas; does not always respond directly and proportionately to precipitation variation; either because of the non-linearity of soil moisture recharge in response to increases in precipitation; or because variations in temperature and humidity attenuate the vegetation responses to changes in water availability. We also find that productivity; independent of changes in total precipitation; is responsive to intra-annual precipitation variation. A significant consequence is that the degree of correlation of all the meteorological variables with productivity varies geographically; so no one formulation is adequate for the entire region. Put together; these results demonstrate that vegetation responses to meteorological variation are more complex than an equilibrium relationship between precipitation and productivity. In addition to their intrinsic interest; the findings have important implications for detection of anthropogenic dryland degradation (desertification); for which the effects of natural fluctuations in meteorological variables must be controlled in order to reveal non-meteorological; including anthropogenic; degradation.
Highlights
IntroductionThe effect of climate variation on vegetation productivity has been studied in many drylands [1,2,3]and elsewhere [4,5,6]
The effect of climate variation on vegetation productivity has been studied in many drylands [1,2,3]and elsewhere [4,5,6]
For the transition dates of greenup, maturity and senescence, the comparison between the Advanced Very High Resolution Radiometer (AVHRR) and MODIS [79] (Figure 2) measurements revealed a good agreement with root mean square
Summary
The effect of climate variation on vegetation productivity has been studied in many drylands [1,2,3]and elsewhere [4,5,6]. During the last few decades, two sequences of extremely dry years, in 1972–1973 and again in 1983–1984, struck the Sahel-part of a longer drought that lasted from the end of the 1960s to the mid-1990s [9,10] This unusual dry spell was not limited to the Sahelian eco-climatic zone but extended to regions more to the south as well [10]. Since 1994, annual rainfall totals recovered and varied around the mean of the standard 30-year climatological period of 1931–1960 [9,11] These fluctuations in rainfall at intra-annual, interannual and decadal time scales make the Sahelian region of Africa the most dramatic example of climate variation that has been directly measured [12]. The Sahel provides a valuable natural experiment on the effects of climatic variations on vegetation production
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