Mingcai Li

Foliar δ15N variations with stand ages in temperate secondary forest ecosystems, Northeast China

Abstract Natural abundance of 15N changes strongly with gradients in soil or environmental conditions across large spatial and temporal scales, but variation in δ15N with stand age at intermediate scales is poorly understood. We analyzed soil and foliar δ15N along a secondary forest chronosequence in Northeast China in mid-growing seasons 2008 and 2009, to address two questions: how does foliar δ15N vary with stand age; and are the variations driven by compositional differences in species among stands, or by consistent changes in δ15N with plant growth forms, and within species. The results showed that community-level foliar δ15N depleted as stand age increased, and these responses were remarkably consistent within three contrasting plant growth forms (herbs, shrubs, and trees), and within individual species. In spite of the three plant growth forms sharing similar responses to the stand age, tree species showed smaller variations in foliar δ15N along stand ages than herbs and shrubs. Soil δ15N also significantly depleted with increasing stand age, which may drive the variations of foliar δ15N. In addition, mycorrhizal fungi discrimination against 15N may also partly affect the patterns of foliar δ15N along stand ages. The results clearly indicate that differences in foliar δ15N among different stand ages are dominantly driven by the δ15N variations at the species levels, which reflects the variations of soil δ15N and mycorrhizal association intensity and association type, not by compositional difference in species among stands.

Occurrence and altitudinal pattern of C4 plants on Qinghai Plateau, Qinghai province, China

The natural occurrence and altitudinal pattern of species with C4 photosynthesis were investigated on Qinghai Plateau, Qinghai province by using stable carbon isotopes in plant leaves and using additional data from references. A total of 58 species belonging to 10 families and 34 genera were identified using C4 photosynthetic pathway, which is only 1.66 % of total 3 500 plant species in Qinghai province. The leading two families, i.e. Gramineae (23 species) and Chenopodiaceae (22 species) contain 77.6 % of all C4 plants in the studied area. The number of C4 species increased from 1 600 to 2 400 m a.s.l. and then decreases quickly till 4 400 m a.s.l. with one half of C4 species distributing from 2 200 to 2 800 m a.s.l. (48 %). Eight plant species were found above 4 000 m a.s.l., but the distribution of these species is limited to the south of Qinghai province (low latitude area) where annual mean temperature is above 0 °C, suggesting that low temperature may generally limit the distribution of C4 plants.

Comparisons of radial growth and tree-ring cellulose δ13C for Pinus sylvestris var. mongolica in natural and plantation forests on sandy lands

ABSTRACT To reveal the effects of water stress on the decline in introduced Mongolian pine (Pinus sylvestris var. mongolica) plantations, radial growth (basal area increment, BAI) and tree-ring cellulose δ13C (carbon isotope composition) were compared in an introduced Mongolian pine plantation and a natural Mongolian pine forest during 1965–2009. Results showed that the BAI of plantation trees increased until 1990, followed by an abrupt decline from 1990 to 1996 and maintained a stable level thereafter. In contrast, no sign of decreased growth was found for natural trees. Tree-ring δ13Ccorr of plantation trees, corrected for atmospheric changes in δ13C, significantly increased before and after 1990. However, no significant variations in tree-ring δ13Ccorr for natural trees before and after 1990 indicated no obviously water stress. Intrinsic water-use efficiency (iWUE, calculated from tree-ring δ13C) significantly increased before and after 1990 for both forests. Significant negative relationship between BAI and δ13Ccorr or iWUE was found for plantation trees after 1990 when precipitation, Palmer Drought Severity Index (PDSI) and groundwater level decreased, suggesting water stress impact on growth. For plantation trees, PDSI was positively and negatively correlated with BAI and δ13Ccorr, respectively, after 1990. For natural trees, BAI had no relationship with PDSI after 1990. These findings suggested that a decrease in PDSI (e.g. decreased precipitation and higher temperature) and groundwater level had made plantation trees face a more serious water stress compared with that of natural trees, which decreased tree growth. However, other stressors such as nutrient limitation may also contribute to growth decline.

Effects of thinning on nitrogen status of a larch plantation, illustrated by 15N natural abundance and N resorption

ABSTRACT Larch (Larix spp.) is widely distributed in the boreal and temperate areas. Nitrogen (N) is considered as the major limiting element for these areas. Thinning is a common forest management practice. Thus, it is imperative to obtain a better understanding on how thinning could affect N status of larch plantations, and thus optimize the thinning intensity for sustainable forest management. In this study, we measured N concentrations and 15N natural abundance (δ15N) of needles and surface soil (0–10 cm) in a larch plantation with T0, T25 and T50 treatments (0%, 25% and 50% thinning intensities, respectively) in Northeast China. We found that needle and soil δ15N in T25 was the lowest, and the highest in T50. No significant differences were observed for needle and soil N concentrations among the thinning intensities. T25 exhibited the highest N resorption efficiency, indicating highest N use efficiency. Overall, N cycling in T25 was more closed than the control, and with lower soil N availability, while N cycling in T50 was more opened. Our study indicates that foliar 15N natural abundance is sensitive to thinning and can be potentially used to optimize thinning intensity from the perspective of N cycling.

Foliar carbon isotope discrimination and related traits along light gradients in two different functional-type tree species

To understand how different plant functional types respond to light intensities, foliar carbon isotope discrimination (Δ13C) and related traits, i.e., specific leaf area (SLA), mass- and area-based nitrogen concentrations (Nmass and Narea), leaf dry mass content (LDMC) of two evergreen coniferous and three deciduous broad-leaved species, were measured under four light intensities. Foliar Δ13C and SLA increased significantly from full- to low-light conditions for all species. These indicate that species studied could increase their light capture capacity under low-light conditions, leading to lower water-use efficiency (higher 13C discrimination). There were significant differences in the responses of foliar Nmass or Narea to light variations in the two functional types, indicating that different functional-type tree species may have different N-use strategies to adapt to the light variations. It was found that there were large functional-type-dependent differences with regard to the relationships between foliar Δ13C and other leaf traits. Our findings suggest that all tree species could change foliar morphology to increase their light-harvesting ability under low-light conditions at the expense of decreasing their water-use efficiency. However, large differences in N-use strategy may exist between deciduous and evergreen species, which may be vital for the survival of these two functional-type tree species in a shaded understory. More important, our findings reveal that changes in Δ13C are not directly related to foliar N if N investment does not proportionally increase the photosynthetic capacity; this should be considered when exploring the relationships of nitrogen concentrations with Δ13C.