Age-related nutrient content and carbon isotope composition in the leaves and branches of Quercus aquifolioides along an altitudinal gradient

Quercus aquifolioides Rehder & E.H. Wilson, an evergreen alpine and subalpine species, occupies a wide range of habitats in the Wolong Nature Reserve, southwestern China. We measured age-related carbon (C) and nutrient (N, P, K, Mg and Ca) contents, C/N, carbon isotope composition (delta C-13) and specific leaf area (SLA) in the leaves and branches of Q. aquifolioides trees along an altitudinal gradient ranging from 2,000 to 3,600 m. The results showed that both age and altitude significantly affected the morphological and physiological properties of Q. aquifolioides. Young tissues possessed higher contents of N, P, K and Mg, lower Ca contents, both on a dry mass basis (subscript M'') and on a unit area basis (subscript A''), and lower C/N and delta C-13 values than did the old ones. The levels of N-M and delta C-13 increased with increasing altitude above 2,800 m, but decreased with increasing altitude below 2,800 m. In contrast, C/N and SLA showed opposite patterns, and other nutrient contents, including P-M, K-M, Ca-M and Mg-M, exhibited irregular changes with elevation. On the other hand, delta C-13 was positively correlated with NM in both leaves and branches, and negatively correlated with SLA in leaves along the altitudinal gradient. Our results also showed that both the Mg-M level of leaves and the Ca-M level of branches, besides the functional correlations between the N-M level and the structure of leaves, are responsible for or accompanied by variation in delta C-13. In addition, delta C-13 was negatively correlated with C/N in both leaves and branches along an altitudinal gradient. It follows that high-altitude plants achieve higher water use efficiency (WUE) at the expense of decreasing nitrogen use efficiency (NUE, derived from C/N), whereas plants at 2,800 m can maintain relatively higher NUE but lower WUE. These characteristics probably reflect the physiological potential of Q. aquifolioides for vigorous growth and metabolism at the optimum altitude (around 2,800 m). With increasing distance from the optimum altitude, NUE decreases. The observed intraspecific variation in the trade-off between WUE and NUE may partially explain the altitudinal distribution of Q. aquifolioides in relation to moisture and nutrient availability.