PARENT SESSION
Oral Session # 50: Forest Ecology III: Modeling; Nutrient Cycling.
Presiding: L Comas
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 100.

Drought sensitivity of growth in a pine forest under CO₂ enrichment and differing nutrient supplies.

McCarthy, Heather^*,1, Millar, Bonnie¹, Oren, Ram¹, ¹ Duke University, Durham, NC, USA

ABSTRACT- Basal area increment and shoot and needle elongation were monitored in a free air CO₂ enrichment experiment in a 20-year-old loblolly pine forest, during a growing season with severe drought (∼50% lower than average April through September precipitation). Combining these measurements with previous measurements during non-drought years allowed for a comparison of drought sensitivity across woody (stems and shoots) and non-woody (needles) components, exposed to differing levels of atmospheric CO₂ and soil fertility. Although the stem growth period during the dry growing season lasted longer than in growing seasons with average precipitation, annual basal area increment was significantly reduced by drought. Under drought conditions, trees at ambient CO₂ with native soil fertility had 56% lower stem growth and 51% lower shoot growth (both P<0.001) than under non-drought conditions. At the same time, needle growth was reduced by 22% (P=0.002) in the upper canopy. Under elevated CO₂ with native soil fertility there was a lesser decrease of 47% in stem growth. However, in contrast to ambient CO₂, drought caused no reduction in shoot or needle length. This indicates that elevated CO₂ may provide a buffer for drought-induced growth reductions, but that this buffer is greater within the canopy. With fertilization, drought-induced reductions in stem growth under ambient and elevated CO₂ were 55% (P<0.001) and 46%, respectively. Thus, stem growth is similarly buffered against drought by elevated CO₂ whether nutrients are added or not.

Key words: drought, Pinus taeda, growth, elevated CO₂