Foliar carbohydrate differs between Picea crassifolia and Sabina przewalskii with the altitudinal variation of Qilian Mountains, China

Nonstructural carbohydrates (NSC) and nitrogen metabolism strongly influence growth and development in plants. The biosynthesis of cellulose and lignin (structural carbohydrates, SC) depends largely on the supply of NSC. We desire to examine which hypothesis, carbon limitation or growth limitation, best fits the alpine plant response between NSC, SC, carbon (C), nitrogen (N) and altitude. We compared the foliar concentrations of carbohydrates, C and N between the leaves of Picea crassifolia (lower-elevation tree-line species) and Sabina przewalskii (high-elevation tree-line species) in their response to changing elevation. Our site was located in the mid-northern area of Qilian Mountains, China. We found that foliar soluble sugar (SG) concentrations were significantly higher in P. crassifolia than in S. przewalskii at the 2,700-3,400 m level. Foliar NSC levels in P. crassifolia increased at the 2,700-3,100 m level, indicating that growth was limited gradually resulting in a surplus of NSC (to conform to GLH), subsequently decreasing at the 3,100-3,400 m level, the assimilation declined leading to C deficit (to conform to CLH). SC (SC metabolism disorders at 3,100-3,400 m), C, N and starch were significantly lower in P. crassifolia than in S. przewalskii. Conversely, foliar SG concentration shows a fall-rise trend with increasing elevation for S. przewalskii. SC concentration in S. przewalskii leaves decreased with an increase of elevation and has a significantly positive correlation to N concentration marking the assimilation of plants. Therefore, the high-elevation tree-line species (S. przewalskii) utilize or store more foliar SG leading to a decrease of SG concentration for survival and growth/regrowth in an adverse environment, higher total C, N, SC, starch contents and lower NSC level. Also, their change trends along the elevational gradient in leaves of S. przewalskii indicate better assimilation strategies for SG use under environmental stress compared to P. crassifolia. This indicates that foliar C metabolism along the elevation follows the principle of the growth-limitation hypothesis (GLH) or carbon limitation hypothesis (CLH), which depends on the acclimation of different alpine life-forms to the environment.