Partitioning of water flux in a Sierra Nevada ponderosa pine plantation

Abstract

The weather patterns of the west side of the Sierra Nevada Mountains (cold, wet winters and hot, dry summers) strongly influence how water is partitioned between transpiration and evaporation and result in a specific strategy of water use by ponderosa pine trees (Pinus ponderosa) in this region. To investigate how year-round water fluxes were partitioned in a young ponderosa pine ecosystem in the Sierra Nevada Mountains, water fluxes were continually measured from June 2000 to May 2001 using a combination of sap flow and eddy covariance techniques (above- and below-canopy). Water fluxes were modeled at our study site using a biophysical model, FORFLUX. During summer and fall water fluxes were equally partitioned between transpiration and soil evaporation while transpiration dominated the water fluxes in winter and spring. The trees had high rates of canopy conductance and transpiration in the early morning and mid-late afternoon and a mid-day depression during the dry season. We used a diurnal centroid analysis to show that the timing of high canopy conductance and transpiration relative to high vapor pressure deficit (D) shifted with soil moisture: during periods of low soil moisture canopy conductance and transpiration peaked early in the day when D was low. Conversely, during periods of high soil moisture canopy conductance and transpiration peaked at the same time or later in the day than D. Our observations suggest a general strategy by the pine trees in which they maximize stomatal conductance, and therefore carbon fixation, throughout the day on warm sunny days with high soil moisture (i.e. warm periods in winter and late spring) and maximize stomatal conductance and carbon fixation in the morning through the dry periods. FORFLUX model estimates of evaporation and transpiration were close to measured/calculated values during the dry period, including the drought, but underestimated transpiration and overestimated evaporation during the wet period.

Introduction

The west side of the Sierra Nevada is characterized by a Mediterranean climate, with cold, wet winters and hot, dry summers. These weather patterns impose restrictions on transpiration rates: in winter water is abundant but near-freezing temperatures are limiting while in summer sunlight and temperatures are optimal but water is limiting. Given these constraints, how water is partitioned, i.e. transpired or evaporated from surfaces, is critical in west side Sierra Nevada forests. Ponderosa pine (Pinus ponderosa) is the most abundant conifer west of the Rocky Mountains and is a dominant tree species in the mixed-conifer forests of the Sierra Nevada Mountains (SNEP, 1996). Given the extent of the Sierra Nevada forests and abundance of ponderosa pine, the specific strategy of water use by this species is of particular interest.

Modeling gas exchange in ecosystems that experience seasonal drought stress has proven to be a difficult challenge (Law et al., 2000). Since it is prohibitively expensive to measure water fluxes at a multitude of sites, models are an important tool in understanding and predicting how water is partitioned in different ecosystems. This is especially true given the need to understand how vegetation will respond to projected scenarios of climate change. Nikolov (Nikolov, 1997a, Nikolov, 1997b; Nikolov et al., 1995) developed FORFLUX, a biophysical model, to study the exchange of water vapor, CO₂, O₃, and energy between terrestrial ecosystems and the atmosphere. The model mechanistically couples major ecosystem processes controlling the flows of carbon and water. The FORFLUX model has been used with relative success in a subalpine forest (Zeller and Nikolov, 2000), lodgepole pine (Pinus contorta), limber pine (Pinus flexilis), and quaking aspen (Populus tremuloides) (Nikolov et al., 1995), along with cotton and grapevine fields (Nikolov, 1997a). The model is unique in that it simulates the feedback between canopy transpiration rate and stomatal sensitivity to drought and freezing, and uses this information to constrain conductance. These features make FORFLUX a promising model for ecosystems that experience seasonal drought stress. However, the ability of this model to predict trace gas exchange in a drought-stressed forest ecosystem has remained untested.

To investigate how year-round water fluxes were partitioned in a young ponderosa pine ecosystem in the Sierra Nevada Mountains, we continually measured water fluxes from June 2000 to May 2001 using two methods: (1) measuring sap flow in trees; and (2) using sub-canopy and above-canopy eddy covariance techniques to determine soil and total canopy water flux, respectively. We calculated canopy conductance from sap flow data using an inverse Penman–Monteith equation. Environmental variables including air temperature, soil moisture, vapor pressure deficit, and photosynthetically active radiation were measured continuously. We modeled water fluxes at our study site using FORFLUX. Finally, we conducted a diurnal centroid analysis of transpiration, canopy conductance, and vapor pressure deficit to investigate how water status affects the diurnal patterns of transpiration and canopy conductance.