State Water Resources Research Institute Program

Project ID: 2009AZ297B
Title: The Ecohydrology and Management of Pinus Ponderosa Forests in the Southwest
Project Type: Research
Start Date: 3/01/2009
End Date: 2/28/2010
Congressional District: AZ 001
Focus Categories: Hydrology, Management and Planning, Groundwater
Keywords: Snow, rain, isotope, hydrogen, oxygen, release effect
Principal Investigators: Koch, George; Mullin, Lucy Penn
Federal Funds: $ 10,000
Non-Federal Matching Funds: $ 19,999
Abstract: Management initiatives and a changing climate present new challenges to understanding the role of southwestern forests in regional hydrology. The headwaters of many watersheds in the Southwest consist of ponderosa pine (Pinus ponderosa)-dominated forest uplands that supply 70-90% of annual streamflow in this region and therefore are an important link in watershed-atmosphere interactions. Fire exclusion, heavy grazing, and high seedling recruitment over the last century have produced a forest structure characterized by high tree density and decreased herbaceous vegetation that is vulnerable to stand-replacing fires and subsequent soil erosion. Thinning of ponderosa pine forests to reduce fire risk and restore ecological health is a major management initiative in the Southwest. As current treatments around communities expand to larger scales it becomes important to understand their effects on the components of forest water balance. Concurrently, there is a need to understand the implications of potential changes in precipitation regimes on water use and yield by southwestern forests, which currently receive inputs of winter snow and monsoonal summer rains. The proposed study will determine how restoration thinning of ponderosa pine forests alters the components of evapotranspiration and sources of plant-transpired water.

This study will address two major questions. First, how are total evapotranspiration and its components altered by forest thinning treatments? For this portion of our research, an eddy covariance system will measure stand-level evapotranspiration at a control site and a thinned site. Within this whole-ecosystem measurement, transpiration of individual trees of different size classes will be determined separately using sapflow systems. A combination of root trenching and soil lysimeters will be used to estimate understory transpiration and surface evaporation. We hypothesize that stand-level evapotranspiration will be greater in the thinned site due to increased water availability stimulating overstory tree transpiration, increased throughfall precipitation increasing herbaceous understory transpiration, and higher soil irradiation increasing soil evaporation.

For our second question, we ask: How does stand density influence reliance on winter versus summer precipitation? To investigate this question, hydrogen and oxygen stable isotope analysis of water will be used to first determine the local isotopic signatures of winter and summer precipitation inputs. Because winter and summer precipitation events in the Southwest are dominated by different major air circulation systems, these two inputs have distinct isotopic signatures. Next, by measuring the isotopic signature of xylem sap in trees in control and thinned plots throughout the year, we can determine the relative reliance on water inputs received as winter snows versus summer rains. We hypothesize that deeper soil waters will be largely derived from winter precipitation, that winter soil water recharge will be greatest in thinned stands, and that these water sources will be primarily used by large trees. Overall, we predict that thinned stands will be more reliant on winter precipitation due to less crown interception of snow inputs in combination with greater soil surface evaporation and herbaceous transpiration of ephemeral summer inputs.

Results of this study will improve understanding of water use and water balance by a major forest type in the Southwest. Moreover, it will provide important insights to how an increasingly widespread management practice - restoration thinning - alters ecosystem-scale water use and therefore potential water yield of southwestern ponderosa pine forests. Finally, by examining the contribution of winter and summer precipitation inputs to tree transpiration, the proposed study may provide insights to how changing precipitation patterns might influence the productivity and water balance of this major vegetation type of the Southwest.

Progress/Completion Report, 2009, PDF

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