Letter to the Editor

The potential usage of our data is not a matter for this journal, but we are pleased to respond to the other issues raised: 1. Trends in rain use efficiency (RUE), i.e. ratio of vegetation production to rainfall, might be an indicator of land condition in arid and semi-arid rangelands, but not in commercial forested or cultivated areas. In these areas negative trends in observed RUE might be due to management practices such as logging and crop rotation rather than degradation. Cultivated and forested areas should therefore not be reported in the statistics or mapped in the global map. This first comment raises two issues: First, the long-running argument about the use of RUE as an indicator of land degradation (see Prince et al., 2007). We are not using RUE as an indicator of land condition; we are using long-term trends of NDVI. RUE is used to separate NDVI trends caused by drought (what we call a false alarm) by: a. identifying areas where rainfall determines biomass productivity (those pixels where there is a positive relationship between productivity and rainfall); b. for these areas, where NDVI declined but RUE increased, we attribute declining productivity to declining rainfall; those areas are masked; c. for the remaining areas, i.e. those with a positive relationship but declining RUE and, also, all areas where rainfall does not determine productivity (where there is a negative relationship between NDVI and rainfall), NDVI trend has been calculated as RUE-adjusted NDVI; d. land degradation is indicated by a negative trend of RUEadjusted NDVI. Subsequently, we quantify this as RUEadjusted net primary productivity. Secondly, Wessels argues that RUE, and by implication NDVI trend, is not applicable to managed systems. We disagree on two counts. (1) We argue that the 25-year trend reflects more than season-to-season management actions. This is substantiated by the use of the GIMMS dataset for processes operating even over climatologically relevant periods (e.g. Alcaraz-Segura et al., 2009; Simoniello et al., 2008; Verbyla, 2008). (2) More importantly, if management actions reduce long-term biomass production, then this is land degradation by our definition. For instance, a long-term decline in soil organic matter under cultivation as a result of reduced biomass production is, surely, land degradation and has contributed about one-quarter of the excess CO2 in the atmosphere (Lal et al., 2007). 2. In the southern hemisphere, a calendar year can not be used when calculating seasonal NPP and seasonal rainfall to test their correlation. In the summer rainfall areas of the southern hemisphere the growth season starts in the latter months of year one and continues into the months of the following calendar year. Using a calendar year will severely distort the relationship between NPP and rainfall often resulting in negative correlations, not to mention trends in RUE. Growth season sum NPP and rainfall should be calculated across calendar years according to the hydrological growing season. Response. For the individual countries collaborating in the LADA programme, we did use the hydrological year; in South Africa, for instance, we used October to September (Bai & Dent, 2007). We used the calendar year for the global generalization – having established that this makes no discernable difference to the 25-year trend at the global scale (Figure 1). Even in the case of individual countries, such as South Africa, the national-scale map calculated using the October to September year is not very different from the map calculated using the calendar year (Figure 2). 3. Rainfall is not a limiting factor to net primary production NPP in humid areas (Nemani et al., 2003). In these humid areas there is unlikely to be a statistically significant correlation between rainfall and NPP and thus the RUE method is not applicable. Therefore the comment that ‘78% of degradation by area is in humid areas’, is largely misleading. Response. Rainfall does make a difference to NPP in humid areas. Droughts occur there as well. We calculated the relationship between NDVI and annual rainfall (Figure 3); the RUE adjustment was made for those areas where there is a positive relationship between NDVI and rainfall. RUE-adjusted NDVI includes areas where there is no correlation with rainfall and also areas where there is a correlation with rainfall but which also show a negative trend of Soil Use and Management, March 2009, 25, 93–97 doi: 10.1111/j.1475-2743.2009. 00197.x