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 Amax when data from all sites were combined. Leaf mass per area did not vary with foliar age class. Both P and N limit Amax, 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.