The effects of water and shade treatments on photosynthesis and root-rhizosphere respiration in young ponderosa pine

Graduation date: 2006Water treatments were applied to young ponderosa pine trees in the Eastern Cascades, Oregon during the 2003 growing season, and shade treatments were applied during the 2003 and 2004 growing seasons to understand how photosynthesis and soil respiration, particularly the root/rhizosphere fraction (R[subscript rrh]), would respond to increased moisture availability through watering and possibly decreased belowground supply of current photosynthate due to shading. This forest typically experiences high soil water deficits in July and August that limit photosynthesis and respiration. Watered trees were did not achieve a greater photosynthetic capacity than control (un-watered) trees, despite sustained sap flux. Soil respiration was [approximately] 140% greater in watered/un-shaded trees in 2003, but the R[subscript rrh] fraction did not show an increase until 2004. This was explained by a 30% increase in fine root biomass by 2004 and almost doubled root specific respiration rates (respiration per gram of fine root) by then end of the 2003 growing season. After an entire year of shading, carboxylation (V[subscript cmax]) of shade-developed needles was higher than un-shaded needles in early spring (May), but decreased compared to un-shaded trees by late June; Amax was consistently lower for shaded compared to un-shaded trees. Soil respiration was coupled to canopy processes, decreasing gradually through the growing season, but no effect was found on R[subscript rrh] fraction of that respiration. The ratio of foliage mass developed during treatments to fine root mass indicated that carbon allocation may have shifted belowground in shaded trees. The interaction of shade and water reduced moisture stress by decreasing moisture stress experienced in the canopy and increasing soil water availability, which allowed for minimal maintenance of photosynthesis and drastically increased soil respiration 250% compared to control trees. The long-term implications are that photosynthesis and soil respiration are strongly driven by current and previous years' photosynthetic potential and moisture availability. Predicting the response of R[subscript rrh] and its contribution to ecosystem carbon exchange to future climate change may be underestimated without accounting for environmental conditions of the previous year and carry over effects on carbon allocation and root biomass accumulation