Nitrogen and carbon source-sink relationships in trees at the Himalayan treelines compared with lower elevations.

No single hypothesis or theory has been widely accepted for explaining the functional mechanism of global alpine/arctic treeline formation. The present study tested whether the alpine treeline is determined by (1) the needle nitrogen content associated with photosynthesis (carbon gain); (2) a sufficient source-sink ratio of carbon; or (3) a sufficient C-N ratio. Nitrogen does not limit the growth and development of trees studied at the Himalayan treelines. Levels of non-structural carbohydrates (NSC) in trees were species-specific and site-dependent; therefore, the treeline cases studied did not show consistent evidence of source/carbon limitation or sink/growth limitation in treeline trees. However, results of the combined three treelines showed that the treeline trees may suffer from a winter carbon shortage. The source capacity and the sink capacity of a tree influence its tissue NSC concentrations and the carbon balance; therefore, we suggest that the persistence and development of treeline trees in a harsh alpine environment may require a minimum level of the total NSC concentration, a sufficiently high sugar:starch ratio, and a balanced carbon source-sink relationship.

[Understory effects on overstory trees: A review.] / æž—ä¸‹å±‚æ¤è¢«å¯¹ä¸Šå±‚ä¹”æœ¨çš„å½±å“ç ”ç©¶ç»¼è¿°.

Plant-plant interactions play a key role in regulating the composition and structure of communities and ecosystems. Studies of plant-plant interactions in forest ecosystems have traditionally concentrated on either tree-tree interactions or overstory species' impacts on understory plants. The possible effects of understory species on overstory trees have received less attention. We summarized the effects of understory species on soil physiological properties, soil fauna activities, leaf litter decomposition, and ecophysiology and growth of the overstory species. Then the effects of distur-bance on understory-overstory interactions were discussed. Finally, an ecophysiology-based concept model of understory effects on overstory trees was proposed. Understory removal experiments showed that the study area, overstory species age, soil fertility and understory species could significantly affect the understory-overstory interactions.

[Altitudinal changes in leaf mass per unit area and tissue non-structural carbohydrates content of Abies fabri on Gongga Mountain of southwest China].

Taking the northeast slope of Moxi vally and southwest slope of Kangding vally in Gongga Mountain of Southwest China as sampling sites, this paper investigated the leaf mass per unit area (LMA) and tissue non-structural carbohydrates content (NSC) of Abies fabri distributed at the positions from lower elevations to alpine treeline during the periods of dormancy and vigorous growth. The results indicated that A. fabri had higher LMA and NSC in warm-moist Moxi valley than in hot-dry Kangding valley. The LMA and NSC of treeline trees were all higher than those of the trees at lower elevations, which was more obvious during the period of vigorous growth. The overall altitudinal changing trends of LMA and NSC in Gongga Mountain of Southwest China did not support the hypothesis of 'carbon limitation could induce the formation of alpine treeline'.

Large clones on cliff faces: expanding by rhizomes through crevices.

BACKGROUND AND AIMS: Large clones of rhizomatous plants are found in many habitats, but little is known about whether such clones also occur on cliff faces where environmental conditions are extremely harsh and heterogeneous. METHODS: Using molecular (intersimple sequence repeat, ISSR) markers, the genotypic composition of a cliff-face population of Oxyria sinensis in Sichuan, China, was investigated. KEY RESULTS: The 98 O. sinensis ramets sampled belonged to 12 different genotypes (clones). The three most frequent clones were represented with 45, 22 and 12 ramets, respectively; the remaining nine were represented with only one to five ramets. The three largest clones spanned at least 2.7 m in the vertical direction and 4.6-6.9 m in the horizontal direction on the cliff face. CONCLUSIONS: On the cliff face, large clones of O. sinensis are formed by rhizomes growing along the crevices. Expansion by rhizomes may help O. sinensis to exploit the patchy resources and support establishment and growth of new ramets. Moreover, rooted ramets connected by rhizomes may effectively reduce the susceptibility of O. sinensis to rock fall and erosion and thus greatly improve the chances for long-term survival. The multi-clone structure indicates that sexual reproduction is also important for the long-term persistence of O. sinensis populations on cliffs.