The Mediterranean climate of California’s Sierra Nevada Mountains supports a dense conifer forest that contains some of the largest trees in the world. Well-established ecological relationships, such as the Miami Model, predict relatively low NPPs for these forests (~250 g/m2/yr to 1300 g/m2/yr) due to winter cold limitation and summer drought. However, the observed rates of NPPs are quite high (up to 2000 g/m2/yr), raising the question of what environmental conditions and plant adaptations promote such a high NPP. We hypothesize that the trees in these forests are neither as cold-limited nor water-limited as surface weather station data suggest. Eddy covariance observations at the top of a 55 m tall micrometeorological tower located at 2050 m elevation indicate daytime CO2 uptake continues year round, and is not limited by winter cold or summer drought. Comparisons of temperature measurements on the tower with operational balloon soundings indicate that tree canopies are often in the free troposphere, which buffers the temperatures they experience and moderates winter cold limitation and summer evapotranspiration.
Kelly, A.E., Goulden, M.L. (2009): Climate controls on anomalously high productivity in the mixed conifer forests of the Sierra Nevada. Fall meeting, American Geophysical Union, December 2009. 90(52). Abstract B33A-0359..