PARENT SESSION
Oral Session #76: Plant Ecology: Nutrient Uptake, Movement, Use. Presiding: J. Schade.
Friday, August 10, 2001. 8:00 AM to 11:45 AM. Hall of Ideas G.

Nitrogen and phosphorus photosynthetic colimitation in Rocky Mountain bristlecone pine.

BOYCE, RICHARD¹, LARSON, JENNIFER¹, SANFORD, ROBERT¹, ¹

ABSTRACT- Nitrogen is considered to be the primary nutrient limiting photosynthesis in plants. However, phosphorus has been shown to be colimiting in some conifers. We examined Rocky Mountain bristlecone pine (Pinus aristata) growing in two locations: Mt. Goliath, near Denver, CO, receiving relatively high anthropogenic N deposition, and Quartzville, near Breckinridge, CO, receiving relatively low amounts. We measured maximum mass-based photosynthetic rates (A_max) and N and P foliar concentrations on six foliar age classes, from which instantaneous photosynthetic N and P use efficiencies (PNUE and PPUE, respectively) and P:N ratios were estimated. A_max was weakly correlated with N but strongly correlated with P. PNUE was correlated with P:N and less stongly with P, whereas there was a nonsignificant negative correlation with N. PPUE was not correlated with any factor. N, P, A_max, PNUE and PPUE all decreased with increasing foliar age. There were no statistically significant site differences, but P concentrations by age class were consistently lower at Mt. Goliath. These results indicate that both P and N limit photosynthesis in this species. Because bristlecone pine often occupies substrates with low P availability, increased anthropogenic N deposition that decreases foliar P:N may adversely affect photosynthesis.

KEY WORDS: Pinus aristata, photosynthesis, Colorado, phosphorus