American coniferous forests are spring dormant
A precipitation-dependent cycle of activity allows mountain coniferous forests in the Southwest United States to survive extreme dry seasons.
This was discovered by the micrometeorologist Constance Brown-Mitic at the University of Indiana in Bloomington and her colleagues. With the help of 30 meter high meteorological measuring towers, they studied the water and carbon dioxide exchange of forests in the Santa Catilina Mountains near Tucson in Arizona. Normally, the activity cycle of most coniferous forests is temperature dependent, so they are dormant in winter and photosynthesize and grow in summer. However, the analysis of the data collected by the researchers over two and a half years revealed that the photosynthetic activity of the fir and pine trees in the observed region is rather determined by the availability of water.
During the two-month dry season in May and June, the trees slow down photosynthesis drastically, they take up very little carbon dioxide and at the same time give off hardly any moisture. However, in early July, at the beginning of the two-month rainy season in the American Southwest, the trees immediately come to life again. They then remain in the active phase well into winter, because the solar radiation at these latitudes allows photosynthesis for a sufficiently long time, the ground only freezes superficially and sufficient moisture is available through rain or melting snow. When this is finally exhausted, the trees return to their "spring dormancy".
The forested mountain ranges in Arizona and New Mexico, some 1,000 to 3,000 meters high, are isolated from one another like islands by dry desert valleys. They are one of the main sources of water for the surrounding regions where water scarcity is a constant problem. Extreme drought prevailed in this area, especially when the measurements were being carried out in the years 2002 to 2004. It remains to be seen whether coniferous forests at lower altitudes can adapt to longer dry periods, for example due to climate change, in the same way as mountain forests. (vs)