Physiological responses to fertilization recorded in tree rings: isotopic lessons from a long-term fertilization trial

• Published in: Ecological applications 2009-06, Vol.19 (4), p.1044-1060
• Main Authors: Brooks, J. Renee, Coulombe, Rob
• Format: Article
• Language: English
• Subjects: basal area increment; Carbon - metabolism; carbon isotope; Carbon Isotopes - metabolism; Douglas-fir; Earlywood; Fertilization; fertilizer application; Fertilizers; forest trees; Forestry; growth; Growth rings; Isotopes; Latewood; Leaf area; leaf area dynamics; long term experiments; Nitrogen; nitrogen additions; nitrogen fertilizers; Oxygen - metabolism; oxygen isotope; Oxygen Isotopes - metabolism; photosynthesis; Plants; Pseudotsuga - physiology; Pseudotsuga menziesii; stable isotopes; Stomatal conductance; Time Factors; transpiration; tree growth; tree rings; Trees; Trees - physiology; Washington; water stress; Wood - growth & development
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/08-0310.1

Nitrogen fertilizer applications are common land use management tools, but details on physiological responses to these applications are often lacking, particularly for longterm . responses over decades of forest management. We used tree ring growth patterns and stable isotopes to understand long-term physiological responses to fertilization using a controlled fertilization experiment begun in 1964 in Washington State (USA), in which three levels of nitrogen fertilizer were applied: 157, 314, and 471 kg/ha. Basal area increment (BAI) increased more than fourfold in the highest treatment to twofold in the lowest, and a significant increase in BAI was observed for 20 years. Latewood Δ¹³C sharply decreased by 1.4‰ after fertilization and was significantly lower than controls for four years, but no differences existed between fertilization levels, and the effect disappeared after four years, indicating that intrinsic water use efficiency ($A/g_s $) increased in response to fertilization. Earlywood Δ¹³C showed similar trends but was more variable. Latewood δ¹ɸO increased significantly above controls by ~ 2‰ in all treatments, but the duration differed with treatment level, with the effect being longer for higher levels of fertilization and lasting as long as nine years after fertilization. Because source water and relative humidity were the same between experimental plots, we interpreted the δ¹⁸O increase with treatment as a decrease in leaf-level transpiration. Earlywood δ¹⁸O did not show any treatment effects. Because the Pacific Northwest has a mediterranean climate with dry summers, we speculated that fertilization caused a substantial increase in leaf area, causing the trees to transpire themselves into drought stress during the late summer. We estimate from the δ¹⁸O data that stomatal conductance ($g_s $) was reduced by — 30%. Using the Δ¹³C data to estimate assimilation rates (A), A during the late season was also reduced by 20-30%. If leaf-level A decreased, but BAI increased, we estimated that leaf area on those trees must have increased by fourfold with the highest level of treatment within this stand. This increase in leaf area resulting from fertilization caused a hydraulic imbalance within the trees that lasted as long as nine years after treatment at the highest levels of fertilization.