Phosphorus and nitrogen limitations to photosynthesis in Rocky Mountain bristlecone pine (Pinus aristata) in Colorado.

We examined Pinus aristata Engelm. stands in four locations in Colorado: Almagre Mountain, Black Mountain, Goliath Peak and Quartzville. All stands are located at 3200-3700 m and face south-southeast. We measured maximum mass-based assimilation rates (A(max)) and nitrogen (N) and phosphorus (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. Leaf mass per area (LMA) was also determined for each foliar age class from each site. Foliar age, P and N concentrations, and the P:N ratio explained the most variation in A(max) when data from all sites were combined. Leaf mass per area did not vary with foliar age class. Both P and N limit A(max), although P appears to be more limiting. The critical P:N ratio is approximately 0.12. Results for Black Mountain differed from the other sites, as A(max) was not correlated with age and was negatively correlated with LMA and P. Current findings showed no evidence of N saturation at the Front Range sites (Almagre Mountain and Goliath Peak); however, because P is a limiting nutrient, increased anthropogenic N availability at sites in the Front Range may cause adverse effects on photosynthesis, and perhaps growth, in the future.