| Popis: |
In many seasonally snow-covered forests, productivity is highest in the spring period when air temperature is warm enough for photosynthesis to occur and soil moisture is not limiting. Due to the relative importance of this period, even small changes in the onset date of tree activity can have large impacts on annual productivity. Therefore, understanding the environmental controls on the onset of the active season is crucial in predicting the future state of our forests. While most studies to date have focused on measuring the seasonal patterns of forest productivity using eddy-covariance or remote sensing techniques, we employed the use of thermal dissipation sap flux sensors to make year-round continuous measurements of tree activity. This approach has the advantage of allowing us to distinguish between co-located species and to assess how changes in environmental conditions are likely to affect particular tree species. After testing and verifying in a laboratory setting that thermal dissipation sap flux sensors could accurately measure transpiration during cold periods, we installed sap flux sensors in Abies concolor and Populus tremuloides at a low elevation site and A. lasiocarpa and P. tremuloides at a high elevation site to monitor seasonal transpiration patterns. Our study included two spring periods with very different precipitation and air temperature, which allowed us to compare how interannual variability in environmental conditions affected the onset of transpiration and early season transpiration patterns. We found that warmer air temperature and earlier snowmelt advanced the onset of the active season for A. concolor and A. lasiocarpa, but that a shift in timing of precipitation events contributed to a delay in P. tremuloides activity. Overall, variability in tree activity between years was greater at the low elevation site, highlighting the susceptibility of lower elevation montane forests to climate change effects. |
