We are operating four eddy covariance flux towers along an elevation gradient (a climosequence) in the Sierra CZO to understand the relationships between climate, plant biogeography, plant production, evaptranspiration, and water balance. Flux towers are located in Oak/Pine woodland (1320' elevation, annual mean T 14.4o C, mean precip 510 mm), Ponderosa Pine forest (3900' elevation, T 10.9o C, precip 870 mm), Midmontane forest (6600' elevation, T 8.9o C, precip 1010 mm), Subalpine forest (8900' elevation, T 4.1o C, precip 1080 mm).
Key findings include: (1) Drought stress decreases and winter cold stress increases with elevation. The midmontane forest occupies a climatological sweet spot where neither drought nor cold curtail photosynthesis. (2) The year‐round growing season at the midmontane sites causes large GPP and Et. The subalpine forest is dormant in winter, resulting in much reduced GPP and Et and increased water yield. (3) This shift in winter activity reflects a basic difference in plant physiology. Midmontane trees have high rates of winter photosynthesis at T > 6o C; subalpine trees are dormant throughout winter, even when T > 6o C. (4) Climate warming should increase winter photosynthesis and Et in midmontane, since there will be more days above 6o C. Climate warming may have little effect on winter photosynthesis and Et in Subalpine. (5) A key question is whether and how quickly midmontane species will expand into the current subalpine belt. Upslope migration would markedly increase winter Et at these elevations and decrease water yield
Goulden, M., Winston, G., Kelly, A., Meadows, M., and Bales, R. (2011): Relationships between elevation, photosynthesis and evapotranspiration in the Sierra CZO. National CZO Program 2011 All Hands Meeting.