A global satellite study of net primary productivity (NPP) from 1982 to 1999 detected a net carbon gain, primarily in the northern hemisphere. A more recent study from 2000 to 2009 indicated a net global carbon loss primarily in the southern hemisphere. This loss was attributed to the five droughts in the last 10 years. Another study indicated that some 20% of NPP is appropriated by human activities including livestock production. We ask explicitly, 1) Is there a net loss or gain of carbon from US rangelands in the last 10 years? 2) Is the US livestock herd a driver of this change? And 3) How much of NPP is appropriated by livestock from US Rangelands? In order to answer these questions we used the aridity index, an internationally agreed upon climatic definition of drylands combined with the shrubland/grassland land cover definition used by the Heinz report on the State of the Nation’s Ecosystems to define rangelands. We tested the hypothesis that the NPP of rangelands in national parks > NPP in rangelands outside the parks that were commercially grazed by livestock stratified by Bailey’s ecoregions at the section level from 2000 to 2006. We were mindful that the opposite could be true based on grazing optimization theory.
Results/Conclusions
We found that US Drylands had a net carbon gain from 2000 to 2009 despite five years of drought with a net gain of 8 million tons of biomass per year. However, we found that NPP in the parks were > by 58% than areas outside the park, meaning that commercial livestock and probably wildfires were appropriating biomass. This figure is surprisingly slighter higher than the 50% rule of thumb used by many federal agencies. We also found that the stocking rate of the national livestock herd was significantly related to US rangeland NPP where NPP decreased with increasing stocking rate from 2000 to 2008. What is responsible for the net carbon gain? At the national scale, some hypothesize that this is due to woody encroachment --an assertion that we will test in the future.