Lining Song

Modeling shallow groundwater levels in Horqin Sandy Land, North China, using satellite-based remote sensing images

Abstract The objective of this study is to establish a method using remote sensing and inverse modeling techniques to rapidly determine the groundwater levels in Horqin Sandy Land, North China. First, the tasseled cap wetness (TCW) data derived from Landsat images with the corresponding soil water content (SWC) via field investigations were processed, and their statistical relationships were established. The determination coefficient of the linear regression was 0.72, indicating a good agreement between the TCW and SWC data. Second, the principles of how groundwater affected the near-surface soil moisture are discussed. The critical condition that the groundwater could seep upward through capillaries to the near-surface was applied to the relationship between the SWC and the groundwater levels. Finally, the relationship between the TCW and the groundwater levels was established and an empirical inverse model was developed. The results were verified using 82 groundwater level samples obtained by observation wells and vertical electrical sounding methods. The determination coefficient between the groundwater levels derived from the empirical model and the field measurements was 0.80, demonstrating that the inverse model closely reflected the actual groundwater levels. The established method could be used to rapidly determine the shallow groundwater levels of the study area with reliable results and may be applicable to aeolian desert areas with low vegetation cover.

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.

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.