Nitrogen allocation in ponderosa pine seedlings exposed to interacting ozone and drought stresses

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Abstract
Summary: Nitrogen (N) resorbed from foliage before leaf abscission is a significant source of N for new leaves. The widely distributed photochemical air pollutant ozone (O3) accelerates foliar senescence and leaf abscission and therefore could interfere with the process of N resorption. The objective of this study was to determine the effects of O. and drought stress on the N economy of ponderosa pine (Pinus ponderosaLaws.), a species highly susceptible to O Potted ponderosa pine trees (2‐yr‐old) were exposed for 3 yr to three concentrations of O3and two soil water regimes in open‐top chambers in the Sierra Nevada mountains of California. One‐third of the trees was harvested at the end of each exposure season and each age class of needles, branches, stems, coarse roots, fine roots, and leaf litter was analysed for N. Exposure to a seasonal O3mean of 88 ppb increased N concentrations in pine foliage, particularly in current‐year needles. Current‐year foliage from well‐watered (WW) trees exposed to elevated O averaged 34% higher N than that from trees in charcoal‐filtered (CF) chambers. Drought‐stressed (DS) trees from all O3treatments averaged 10% higher N in current‐year needles than WW seedlings. Resorbed N from older age classes of needles from WW trees increased from 32% of current‐year needle N in CF seedlings to 51% of current‐year N in the high O3treatment. Nitrogen resorbed from older needles of DS trees increased from 15% in CF to 38% of current‐year N in the high O3treatment. Thus O3‐injured ponderosa pine seedlings increased resorption of N from older needles and increased partitioning of N to current‐year foliage. The increased N in current‐year needles facilitated increased rates of photosynthesis in these needles, thus partly compensating for the O3‐induced loss of older leaves.