Elevation alters carbon and nutrient concentrations and stoichiometry in Quercus aquifolioides in southwestern China.

Elevation is a complex environmental factor altering temperature, light, moisture and soil nutrient availability, and thus may affect plant growth and physiology. Such effects of elevation may also depend on seasons. Along an elevational gradient of the Balang Mountain, southwestern China, we sampled soil and 2-year old leaves, 2-year old shoots, stem sapwood and fine roots (diameter<5mm) of Quercus aquifolioides at 2843, 2978, 3159, 3327, 3441 and 3589m a.s.l. in both summer and winter. In summer, the concentrations of tissue non-structural carbohydrates (NSC) did not decrease with increasing elevation, suggesting that the carbon supply is sufficient for plant growth at high altitude in the growing season. The concentration of NSC in fine roots decreased with elevation in winter, and the mean concentration of NSC across tissues in a whole plant showed no significant difference between the two sampling seasons, suggesting that the direction of NSC reallocation among plant tissues changed with season. During the growing season, NSC transferred from leaves to other tissues, and in winter NSC stored in roots transferred from roots to aboveground tissues. Available soil N increased with elevation, but total N concentrations in plant tissues did not show any clear elevational pattern. Both available soil P and total P concentrations in all plant tissues decreased with increasing elevation. Thus, tissue N:P ratio increased with elevation, suggesting that P may become a limiting element for plant growth at high elevation. The present study suggests that the upper limit of Q. aquifolioides on Balang Mountain may be co-determined by winter root NSC storage and P availability. Our results contribute to better understanding of the mechanisms for plants' upper limit formation.

Homeostatic levels of nonstructural carbohydrates after 13 yr of drought and irrigation in Pinus sylvestris.

Nonstructural carbohydrates (NSCs) are important for the growth and survival of trees. Drought may lead to a decrease in tree growth and to NSC depletion, whereas increased soil moisture in otherwise dry ecosystems may increase growth and NSC concentrations. A long-term (13 yr) irrigation experiment was conducted in a Pinus sylvestris-dominated forest located at the dry margin of the species in southern Switzerland. We measured the relative leaf area, growth, NSCs, needle δ13 C, [N] and [P] in trees on control and irrigated plots. Irrigation resulted in higher growth rates and carbon isotope discrimination, but did not alter NSC levels. Growth and NSC decreased with decreasing leaf area in both treatments, but NSC did not correlate with leaf-level gas exchange indices, such as foliar δ13 C, [N] or [P]. A legacy effect was shown, as trees with initially low leaf area had limited ability to respond to prolonged irrigation. The NSC constancy across treatments provides evidence that carbohydrate storage may stay constant when climate changes are sufficiently slow to allow acclimation. Moreover, we speculate that total leaf area, rather than leaf gas exchange per unit leaf area, drives the variation in whole-tree carbohydrate dynamics in this system.

Short-term parasite-infection alters already the biomass, activity and functional diversity of soil microbial communities.

Native parasitic plants may be used to infect and control invasive plants. We established microcosms with invasive Mikania micrantha and native Coix lacryma-jobi growing in mixture on native soils, with M. micrantha being infected by parasitic Cuscuta campestris at four intensity levels for seven weeks to estimate the top-down effects of plant parasitism on the biomass and functional diversity of soil microbial communities. Parasitism significantly decreased root biomass and altered soil microbial communities. Soil microbial biomass decreased, but soil respiration increased at the two higher infection levels, indicating a strong stimulation of soil microbial metabolic activity (+180%). Moreover, a Biolog assay showed that the infection resulted in a significant change in the functional diversity indices of soil microbial communities. Pearson correlation analysis indicated that microbial biomass declined significantly with decreasing root biomass, particularly of the invasive M. micrantha. Also, the functional diversity indices of soil microbial communities were positively correlated with soil microbial biomass. Therefore, the negative effects on the biomass, activity and functional diversity of soil microbial community by the seven week long plant parasitism was very likely caused by decreased root biomass and root exudation of the invasive M. micrantha.