Effects of climate change on tree water use efficiency, nitrogen availability and growth in boreal forest of northern China

Purpose Climate change, particularly rising atmospheric carbon dioxide (CO2) concentration, can influence carbon (C) and nitrogen cycling (N) in different forest ecosystems. The major objective of this study was to quantify the long-term physiological and tree growth trends ofLarix gmeliniiplantation in a boreal environment of northern China in response to elevated atmospheric CO(2)concentration and other climatic conditions. Materials and methods Tree rings were extracted from fourLarix gmeliniisample trees, located in a boreal plantation forest of Mohe City, Heilongjiang Province, China. Tree rings were measured with a mean annual basal area increment (BAI), while tree ring stable C isotope composition (delta C-13) and N isotopic composition (delta N-15) as well as total C and N concentration were measured on mass spectrometer at 3-year intervals. Tree intrinsic water use efficiency (iWUE) was calculated using tree rings delta C-13 and atmospheric delta C-13 data. Multiple regression analysis was used to quantify the BAI and WUE relationships with atmospheric CO(2)concentration, temperature, precipitation and humidity of the study site. Results and discussion The results showed a quadratically decrease in relative humidity over the past 60 years with rising temperature, indicating the initial increasing water availability, which is peaked, but increasing water limitation thereafter. Tree iWUE continued to increase as atmospheric CO(2)concentration (C-a) increased. Tree BAI showed a quadratic relationship with atmospheric CO2(C-a), increasing initially, but peaking at the critical threshold of 352.5 ppm or in 1986, and decreasing with theC(a)thereafter. Tree ring delta N-15, an index of N availability, also responded non-linearly to the risingC(a), increasing initially with theC(a), but peaking at the criticalC(a)of 348-367 ppm and decreasing thereafter with the risingC(a), indicating the decreasing N availability in the last 20-30 years after theC(a)continued to rise. Conclusions The iWUE ofLarix gmeliniicontinued to increase under risingC(a), but this increased iWUE did not translate into tree growth consistently due to increasing water and N limitation in the boreal forest ecosystems under intensifying climate change in the last 20-30 years.