Comparisons of radial growth and tree-ring cellulose 13C for Pinus sylvestris var. mongolica in natural and plantation forests on sandy lands

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 C-13 (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 C-13(corr) of plantation trees, corrected for atmospheric changes in C-13, significantly increased before and after 1990. However, no significant variations in tree-ring C-13(corr) for natural trees before and after 1990 indicated no obviously water stress. Intrinsic water-use efficiency (iWUE, calculated from tree-ring C-13) significantly increased before and after 1990 for both forests. Significant negative relationship between BAI and C-13(corr) 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 C-13(corr), 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.