Spectral analysis software improves confidence in plant and soil water stable isotope analyses performed by isotope ratio infrared spectroscopy (IRIS)

Analyses of the stable isotope ratios of carbon (&#948;13C), nitrogen (&#948;15N) and oxygen (&#948;18O) nowadays are widespread tools to identify effects of climate or weather on the growth of trees or to reconstruct climate conditions from &#948;-values in tree rings. Less often, they have been used to identify effects of edaphic conditions on the growth and water relations of trees. Here, I present three examples of differentiation between the effects of weather and edaphic factors on the growth of mature forest trees in three different biomes.(1)&#160;In an old-growth Nothofagus forest dominated by N.&#160;alpina growing in a Sub-Mediterranean climate type at the western slopes of the Andes Mountains in south-central Chile, the growth of the trees was mainly affected by high temperature and low precipitation in spring and summer irrespective of the substrate&#8217;s age (Miocene to 3500 years B.P.), but was enhanced by higher concentrations of plant-available phosphorus (P) in the soil. The negative correlation between growth and temperature and between &#948;13C and &#948;18O in the tree rings was interpreted as an impairment of net carbon assimilation by anomalously warm weather conditions during the growing season. (2) In a mature Norway spruce (Picea abies) plantation&#160; in the lower mountain ranges of western Germany, stem diameter increment was mostly unaffected by the soil moisture conditions, but, in accordance with modelled soil water potentials and transpiration rates, the &#948;18O ratios were lower in rings of trees growing at the wettest microsite (Gleysol) than in tree rings from the microsite with moderately moist soil (Cambisol). Thus, &#948;18O ratios of tree rings may help to identify microsites differing in soil water availability along small-scale gradients of soil moisture under homogeneous climatic conditions and to explain the occurrence of particular tree species along those gradients in forest stands. (3)&#160;Along an elevation gradient (900&#8212;2000&#160;m a.s.l.) of a mature secondary Pinus kesiya forest in the tropical south-central highlands of Vietnam, the basal area increment of the trees was not immediately correlated with temperature or precipitation, but with the C:N ratios and the &#948;15N signature of the upper mineral soil (negatively) and with the concentrations of plant-available P and of "base" cations (calcium, magnesium, potassium) in the soil (in a positive manner). This led to the conclusion that lower temperatures at higher elevations exert an indirect effect on tree growth by inducing higher C:N ratios and by reducing the rate of N and P mineralization. Mineralization may have been further hampered by lower concentrations of "base" cations (upon enhanced leaching by precipitation) and by a negative feedback from low availability of mineralized N and P at higher elevations.Against the background of climate change, in particular, combining growth and stable isotope analyses with additional important plant functional traits such as wood density and hydraulic conductance could even increase the power of predictions based on wood increment and isotope ratios.

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