The photosynthetic capacity, nutrient content, and nutrient use efficiency of different needle age-classes of black spruce (Piceamariana) found in interior Alaska

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Abstract
The photosynthetic capacity of different needle-age classes of black spruce was studied using 14C-labeling on whole branches at a site near Fairbanks, Alaska. Black spruce had highest photosynthetic rates in the 1-yr-old tissue with the 4 most current age-classes showing similar high rates before declining with age. Older needles were found to maintain 40% of maximum photosynthetic rates after 13 seasons. Highest level of N was found in current needles with the first 8 age-classes maintaining high levels before declining to 70% of maximum values in the oldest needles. P levels were much higher in current tissue with a gradual decline after the 1-yr-old tissue to a value of 55% of maximum values in the oldest tissue. Nutrient use efficiency was found to be highest in the 1-yr-old tissue and declined with needle age. The loss of nutrient use efficiency may be due to the use of older tissue as aboveground nutrient stores, a decline in the efficiency of the photosynthetic apparatus of the needles, or an increase in the leaf specific weight. Black spruce has adapted to the low nutrient availability with low photosynthetic rates and low nutrient contents. Greater longevity of the needles would maximize the photosynthetic return per unit N invested in the needles.