The Mexican and southwest USA Threatened Conifer Region stretches from central southern California south to the Guatemalan border. In the north a broadly Mediterranean climate prevails with summer droughts a major limiting factor. In Mexico, conifers occur in a range of climates that include cold subalpine areas of the highest volcanoes and the lush, lowland tropical forests. Most species are found in the pine-oak woodlands that once covered extensive areas of the many mountain ranges in Mexico. This region is important for conifer diversity and is the main centre for the genus Pinus with over 50 species which represents 50% of all known species. Unfortunately this region has the highest number of threatened taxa with more than 60 mostly being threatened mainly by widespread over-exploitation, fire and the conversion of forests for agriculture. Many species are narrow endemics of relictual origin with small populations close to the tops of mountains. These taxa are particularly vulnerable to the impacts of changes to precipitation associated with climate change.