Untitled DocumentTransect study: vegetation water use from sap flux measurements, with focus on snow-dominated hydrologic regimes and responses to climate/vegetation change
Macro Theme Area:
Basin Scale Water Balance [Project ID: B19]
PI:
Joe McConnell
CO-PI(s):
N/A
Basin focus:
Rio Grande
Specific area in
basin /
field sites:
Pine and mixed conifer sites in the Valles Caldera
Summary/Goals: The primary purpose of our research is to use replicated sapflow measurements in trees at the two Valles Caldera sites in New Mexico to characterize and quantify tree transpiration throughout the year and to link interpretation of sap flow to related SAHRA meteorology and hydrology measurements at the two sites. In a comparative study, we are making similar measurements of sap flow in the northern Sierra Nevada of CA.
Activities and outcomes during past year:
As we showed at the 2005, 2006, and 2007 annual meetings, we have installed sap flow measurement devices in 8 trees (2 probes per tree) at the pine and mixed conifer sites in the Valles Caldera. Though discontinuous, measurements started in July, 2005 and are nearly continuous from March, 2006 through December, 2008 so we have captured nearly three complete annual growth periods. We will continue to make these measurements throughout the next year (at least), with a primary aim to make year-round measurements at the two Valles Caldera sites as well as at the UC-Berkeley Sagehen Field Station in the Sierra Nevada. A back-of-the-envelop calculation based on the current set of measurements suggests that ~30% of annual precipitation is remitted to the atmosphere via tree transpiration and that tree transpiration is closely linked to soil moisture availability (collaboratively with other SAHRA investigators). One important finding in 2006 was that the tree transpiration at the pine site in the Valles Caldera began in late April, shut down by mid-May when soil moisture dropped below ~0.13, and then started again when the summer monsoon began in early July. All 8 trees at the Pine site were essentially inactive in the hydrologic cycle from mid-May to early July. Comparisons to the much more typical 2007 and 2008 water years show that the pine and conifer trees normally transpire vigourously during the May-July period when moisture is available in the soil and the air dry prior to onset of the summer monsoon in July. Representatives of our group at DRI have made 5 separate visits to the Valles Caldera during 2007 and 2 in 2008. This extra effort has paid off since we have obtained nearly complete, accurate data on sap flow at both sites for nearly all of the 2007 and 2008 growing seasons. We also participated in the workshop in Jemez Springs in August 2007 and in the annual meeting in Tucson in October 2007.
Finally, we are in the process of writing a manuscript with Dr. J. Kirchner and his PhD student on the SAHRA-funded sapflow measurements that we have been making in the UC Sagehen Creek field station in the Sierra Nevada. We expect to submit this manuscript in early 2009. Co-located measurements of sapflow, meterology, stream flow and shallow ground water levels throughout a two-year period show rapid exchanges between the stream, shallow ground water, and trees that vary with season.
Plans for the upcoming year:
Continue to maintain and expand instruments to get full annual records. Continue to work with SAHRA collaborators to interpret the results. The sapflow (and soil moisture) data that we have developed were used in 2 papers in the Ecohydrology special issue and we are working with Marcy Litvak and her students at the U. of New Mexico in a joint interpretation of soil moisture, sapflow, and eddy flux measurments. Our aim continues to be to quantify the amount of water being transpired through the trees throughout the year and to characterize the controlling parameters (e.g., soil moisture, soil temperature beneath the snow pack, humidity, radiation). In October 2006, we added basic met instruments to our system at the mixed conifer site (air temperature, relative humidity, wind speed and direction, solar radiation, limited soil moisture and soil temperature), with the met instruments approximately in the middle of the tree canopy. We also installed a satellite download system. Some of the measurements downloaded by the satellite are available on a public website (http://www.wrcc.dri.edu/cgi-bin/rawMAIN.pl?nmvcnm). We are working with SAHRA collaborators to make the most of the remote download capability - both for remote monitoring of instrument health (e.g., available power) and for field safety. In October 2007, we added also 3 soil moisture and temperature sensors in each of two depth profiles at the lower Pine site. These data are available to all Sahra collaborators.
To expand sapflow measurements in the Sierra Nevada and better understand the impact of tree transpiration on the hydrologic balance in snow dominated regions, we plan to add sensors to trees at the UC-Santa Barbara snow research site on Mammoth Mountain, CA in August 2009.
