Interannual variability of NDVI and its relationship to climate for North American shrublands and grasslands

Our objective was to analyse the interannual variability of different chara cteristics of the seasonal dynamics of NDVI and their relationships with cl imatic variables for grassland and shrubland sites of North America. We sel ected twenty-five sites located in relatively undisturbed areas. We analyse d the variability of seven traits derived from the annual dynamics of the N DVI at each site: the annual integral, the difference between maximum and m inimum NDVI, the dates of the inflection points of a double logistic model fitted to the NDVI curve, the difference between these dates, the date of m aximum NDVI, and the coefficient of determination of the double logistic mo del. The temporal variability of traits that integrated aspects of primary produ ctivity over the year was lower than those related to seasonality. This sug gests that from year to year, grassland and shrubland ecosystems would diff er more in the timing of production and senescence than in the total amount of carbon fixed. The integral of NDVI showed less temporal variability tha n annual precipitation. The coefficient of variation of both precipitation and the NDVI integral were positively related. The slope of the relationshi p was significantly lower than 1, indicating that the variability of ecosys tem function is a lower proportion of the variability of annual precipitati on in areas with a high relative variability of this climatic variable than in areas of low variability The variability of most of the NDVI traits ana lysed showed a negative and, in general, non-linear relationship with annua l precipitation. The same kind of relationship has been reported elsewhere for annual precipitation and its coefficient of variation. Mean annual prec ipitation has been reported as the main control of above-ground net primary production in grassland and shrubland ecosystems. Our results suggest that this climatic Variable is also associated with the interannual variability of carbon gains, such as the primary production and its seasonality.