Long-term tree growth rate, water use efficiency, and tree ring nitrogen isotope composition of Pinus massoniana L. in response to global climate change and local nitrogen deposition in Southern China

We aimed to investigate long-term tree growth rates, water use efficiencies (WUE), and tree ring nitrogen (N) isotope compositions (delta N-15) of Masson pine (Pinus massoniana L.) in response to global climate change and local N deposition in Southern China. Tree annual growth rings of Masson pine were collected from four forest sites, viz. South China Botanical Garden (SBG), Xi Qiao Shan (XQS) Forest Park, Ding Hu Shan (DHS) Natural Reserve, and Nan Kun Shan (NKS) Natural Reserve in Southern China. The mean annual basal area increment (BAI), WUE, and delta N-15 at every 5-year intervals of Masson pine during the last 50 years were determined. Regression analyses were used to quantify the relationships of BAI and WUE with atmospheric carbon dioxide concentration ([CO2]), temperature, rainfall, and tree ring elemental concentrations at the four study sites. Tree BAI showed a quadratic relationship with rising [CO2]. The tipping points of [CO2] for BAI, the peaks of BAI when the critical [CO2] was reached, occurred earlier at the sites of SBG, XQS, and DHS which were exposed to higher temperature, N deposition, and lower mineral nutrient availability, as compared with the tipping points of [CO2] for BAI at the site of NKS which had higher rainfall, lower temperature, and better nutritional status. The average tipping point of [CO2] at the four sites for the BAI response curves was 356 ppm, after which, the BAI would be expected to decrease quadratically with rising [CO2]. The multiple regressions of BAI confirmed the relationships of long-term tree growth rate with rainfall, tree WUE, and nutrients and delta N-15 in tree rings. Nonlinear relationships between BAI and tree ring delta N-15 at DHS and negatively linear one at NKS reflected the fertilization effect of N deposition on tree growth rate initially, but this effect peaked or became negative once the forest approached or passed the N saturation. Nonlinear relations of tree WUE with rising [CO2] and summer temperature were also observed. The tipping points of [CO2] for tree BAI were not uniform, but depended on the site conditions, such as hydrological and nutritional constraints. Nonlinear relationships should be considered for predicting the dynamics of long-term tree growth rate and above-ground forest carbon (C) stock in response to future global climate change (particularly rising [CO2]) and local N deposition.