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

Abstract

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 of Larix gmelinii plantation in a boreal environment of northern China in response to elevated atmospheric CO2 concentration and other climatic conditions. Tree rings were extracted from four Larix gmelinii sample 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 (δ13C) and N isotopic composition (δ15N) 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 δ13C and atmospheric δ13C data. Multiple regression analysis was used to quantify the BAI and WUE relationships with atmospheric CO2 concentration, temperature, precipitation and humidity of the study site. 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 CO2 concentration (Ca) increased. Tree BAI showed a quadratic relationship with atmospheric CO2 (Ca), increasing initially, but peaking at the critical threshold of 352.5 ppm or in 1986, and decreasing with the Ca thereafter. Tree ring δ15N, an index of N availability, also responded non-linearly to the rising Ca, increasing initially with the Ca, but peaking at the critical Ca of 348–367 ppm and decreasing thereafter with the rising Ca, indicating the decreasing N availability in the last 20–30 years after the Ca continued to rise. The iWUE of Larix gmelinii continued to increase under rising Ca, 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.