The 18 O-signal transfer from water vapour to leaf water and assimilates varies among plant species and growth forms.

The 18 O signature of atmospheric water vapour (δ18 OV ) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the 18 O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with 18 O-depleted water vapour (-106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the 18 O-signal transfer by calculating the mean residence time of O in leaf water (MRTLW ) and sugars (MRTSugars ) and related it to leaf traits and physiological drivers. MRTLW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRTSugars was shorter in C3 and C4 plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRTSugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ18 OV variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ18 OV variations, which is important for the interpretation of δ18 O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.

The influence of increasing temperature and CO2 concentration on recent growth of old-growth larch: contrasting responses at leaf and stem processes derived from tree-ring width and stable isotopes.

Time series of tree-ring growth show significant increases since the early 1970s at the alpine tree line, with simultaneously increasing temperatures and atmospheric CO2 concentration. For a comprehensive understanding of this growth change, the physiological response patterns at both the leaf and stem level need to be separately analyzed and identified, and can be retrieved from tree-ring growth and isotope (δ13C, δ18O) series. In this study, we assessed the relative contribution of environmental factors to interannual tree-ring variability by multivariate linear mixed-effects models and the dual isotope approach on a dataset of tree-ring records of ~400-year-old larch (Larix decidua Mill.) from a non-water-limited high-elevation site in the Swiss Alps. The models suggest that summer temperatures and the recent lack of larch budmoth outbreaks were most important for explaining growth variations and trends, while a significant direct effect of the continuously increasing CO2 concentration could not be confirmed. In contrast, δ13C and δ18O, which are strongly influenced by fractionation changes in the leaf, clearly reflected the impact of air humidity (precipitation and vapor pressure deficit) and CO2 concentration: the increase in (δ13C-derived) intrinsic water-use efficiency over the second half of the 20th century suggests an increase in carbon assimilation as a result of enhanced CO2 concentration. The tree-ring δ18O largely reflected recent precipitation as source water, thus indicating a low variability in stomatal conductance, which was confirmed by the dual isotope approach. These leaf-level effects were not reflected in stem growth as they may have been masked by the temperature-caused growth limitation controlling the allocation of increased amounts of photosynthates into wood growth. Our approach demonstrates that the identification of different roles of environmental factors on leaf and stem processes helps to improve the assessment of site-specific changes of carbon fluxes and growth performance under future environmental conditions.

Comparison of δ(18)O and δ(13)C values between tree-ring whole wood and cellulose in five species growing under two different site conditions.

RATIONALE: We investigated the applicability of tree-ring whole-wood material for δ(18)O and δ(13)C analysis in comparison with the more time- and resource-intensive use of cellulose, by considering possible variability between (i) five different tree species (Fagus sylvatica, Quercus robur, Picea abies, Abies alba, Pseudotsuga menziesii), (ii) two sites that differ in soil moisture, and (iii) climate conditions within a 10-year period. METHODS: Stem cores of 30 individual trees (n = 3 trees per each species and site) were sampled from two sites in south Germany (Bavaria), and tree rings within sapwood of the years 2001-2010 were separated. The δ(18)O and δ(13)C values from homogenized tree-ring whole wood and from extracted cellulose were measured by mass spectrometry. Species-specific offsets in isotope values were analyzed and the responses in isotopic signature to climate variability including a single drought event were compared between whole-wood and cellulose. RESULTS: A constant offset in δ(18)O values of ca 5‰ between wood and cellulose was observed for most species independent of site conditions, with a significant difference between beech and Douglas-fir, while inter-annual variability was only observed in oak. The offset in δ(13)C values ranged between 1.45 and 1.84‰ across species, sites and years. Both materials generally showed similar strength in responses to temperature, precipitation and soil water availability, particularly for conifers. Resistance to severe drought stress--partly more strongly reflected in the δ(13)C values of cellulose--was lower for conifers than for the deciduous species. CONCLUSIONS: Wood material from the sapwood of the studied tree species is as useful as cellulose for studying environmental effects on tree-ring δ(18)O and δ(13)C values at a short-term scale as considered in most ecophysiological studies. The more variable response of oak may require further investigations.