Basal area growth, carbon isotope discrimination, and intrinsic water use efficiency after fertilization of Douglas-fir in the Oregon Coast Range
Cornejo-Oviedo, E., S. Voelker, D. Mainwaring, D. Maquire, F. Meinzer, AND J. Renee Brooks. Basal area growth, carbon isotope discrimination, and intrinsic water use efficiency after fertilization of Douglas-fir in the Oregon Coast Range. FOREST ECOLOGY AND MANAGEMENT. Elsevier Science Ltd, New York, NY, 389:285-295, (2017).
Tree rings and stable isotopes contained within tree rings contain extremely useful information about changes in growth over time in response to environmental fluctuations. Climate change and forest management can alter long-term growth which is recorded in the tree rings. Separating the influences of climate change, elevated CO2, and nitrogen availability on long-term growth records is challenging. However, experimental research on operational N fertilization of Douglas-fir in the PNW began in the early 1950s in mid-rotation natural stands. These long-term experiments can help us to understand the influences of fertilization separately from climate and elevated CO2. Nitrogen is the most important limiting nutrient in terrestrial ecosystems in most of the world, and fertilization is a common practice in most agricultural businesses including forestry. Despite this widespread N application in forests and favorable average tree growth response in the PNW, approximately one third of fertilized Douglas-fir stands do not respond to N fertilization. This study has shown that in this Douglas fir plantations on highly productive sites can also respond positively to moderate (operational) levels of nitrogen fertilization, but the duration of growth response appears short, compared to responses on low productivity sites. The challenge ahead is to develop diagnostic criteria for a priori detection of N response potential on relatively high quality sites that have historically demonstrated high response variability in regional fertilization trials. This information can then be used to separate forest growth responses to operational practices such as fertilization from climate responses. This paper contributes to ACE CIVA 130.
Many hectares of intensively managed Douglas-fir (Pseudotsuga menziesii Mirb. Franco) stands in western North America are fertilized with nitrogen to increase growth rates. Understanding the mechanisms of response facilitates prioritization of stands for treatment. The objective of this study was to test the hypothesis that the short-term basal area growth response to a single application of 224 kg N ha-1 as urea was associated with reduced stable carbon isotope discrimination (∆13C) and increased intrinsic water use efficiency (iWUE) in a 20-yr-old plantation of Douglas-fir in the Oregon Coast Range, USA. Increment cores were measured to estimate earlywood, latewood, and total basal area increment over a time series from 1997 to 2015. Stable carbon isotope discrimination and iWUE were estimated using earlywood and latewood stable carbon isotope concentrations in tree-ring holocellulose starting seven years before fertilization in early 2009 and ending seven years after treatment. A highly significant interaction effect between fertilization treatment and year was found for total basal area growth and earlywood basal area increment. Fertilized trees showed significant total basal area growth and earlywood basal area increment in the first (2009) and second (2010) growing seasons after fertilization in 2009. A marginally significant fertilization effect was found for latewood basal area increment only in the first growing season after treatment. A significant interaction effect was also found for ∆13C and iWUE in earlywood and latewood. Fertilization significantly reduced earlywood ∆13C and increased earlywood iWUE in the first and second growing seasons after fertilization. Only a marginally significant fertilization effect was detected for latewood ∆13C and iWUE in the second growing season after treatment. Previous studies of nitrogen fertilization of Douglas-fir forests have reported increased growth and iWUE at very low productivity sites and at relatively high fertilization rates. This study has shown that Douglas fir plantations on highly productive sites can also respond positively to more moderate (operational) levels of nitrogen fertilization, but the duration of growth response appears shorter.