Woody-plant encroachment is a global phenomenon with major environmental and socio-economic ramifications. Causes for this extensive land-cover change are the topic of active debates centering around the roles of historic changes in climate, atmospheric CO2 enrichment, and disturbance (e.g. fire, grazing). For the first time, we were able to separate the effect of climate and land-use change (bundling grazing and disturbances) on woody-plant encroachment at the continental scale. We reconstructed the expansion of an ensemble of species of the genus Prosopis, mesquite, across the Southern Great Plains and Hot Deserts of Continental Unites States during the 19th-20th centuries, using herbarium records going back to 1900, and historical temperature and precipitation data. In addition, we predicted mesquite distribution for the next century using scenarios derived from Global Circulation Models.
Results/Conclusions
Mesquite cover increased from 1900 to 2000 by 839,268 square km equivalent to 1.7 times the area of the State of Texas. Future mesquite cover will further increase by an additional 1 million km2 by the year 2100. Most of the past expansion occurred in the state of Texas and California. Future expansion will happen in the states of Kansas, Nevada, Oklahoma, and Utah. Our results indicated that 33% of the 1900-2000 expansion was attributed to climate change, while the rest was due to land-use change. On the contrary, climate change will account for 67 % of the predicted expansion for the year 2100. The magnitude of this phenomenon highlights consequences for ecosystem services, while the causes provide hints for control strategies.