Updating the dual C and O isotope-Gas-exchange model: A concept to understand plant responses to the environment and its implications for tree rings.

The combined study of carbon (C) and oxygen (O) isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional responses to environmental change. The approach relies on established relationships between leaf gas exchange and isotopic fractionation to derive a series of model scenarios that can be used to infer changes in photosynthetic assimilation and stomatal conductance driven by changes in environmental parameters (CO , water availability, air humidity, temperature, nutrients). We review the mechanistic basis for a conceptual model, in light of recently published research, and discuss where isotopic observations do not match our current understanding of plant physiological response to the environment.

High-frequency stable isotope signals in uneven-aged forests as proxy for physiological responses to climate in Central Europe.

Picea abies (L.) Karst. and Fagus sylvatica (L.

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

The O signature of atmospheric water vapour (δ O ) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with O-depleted water vapour (-106.

Oxygen isotopes in tree rings are less sensitive to changes in tree size and relative canopy position than carbon isotopes.

Stable isotope ratios in tree rings have become an important proxy for palaeoclimatology, particularly in temperate regions. Yet temperate forests are often characterized by heterogeneous stand structures, and the effects of stand dynamics on carbon (δ C) and oxygen isotope ratios (δ O) in tree rings are not well explored. In this study, we investigated long-term trends and offsets in δ O and δ C of Picea abies and Fagus sylvatica in relation to tree age, size, and distance to the upper canopy at seven temperate sites across Europe.

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.

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.

Whole-tree seasonal nitrogen uptake and partitioning in adult Fagus sylvatica L. and Picea abies L. [Karst.] trees exposed to elevated ground-level ozone.

The effect of long-term exposure of twice-ambient O(3) (2 × O(3)) on whole-tree nitrogen (N) uptake and partitioning of adult beech and spruce was studied in a mixed forest stand, SE-Germany. N uptake as (15)N tracer and N pools were calculated using N concentrations and biomass of tree compartments. Whole-tree N uptake tended to be lower under 2 × O(3) in both species compared to trees under ambient O(3) (1 × O(3)).

Ground-level ozone differentially affects nitrogen acquisition and allocation in mature European beech (Fagus sylvatica) and Norway spruce (Picea abies) trees.

Impacts of elevated ground-level ozone (O(3)) on nitrogen (N) uptake and allocation were studied on mature European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.

Effects of twice-ambient carbon dioxide and nitrogen amendment on biomass, nutrient contents and carbon costs of Norway spruce seedlings as influenced by mycorrhization with Piloderma croceum and Tomentellopsis submollis.

Elevated tropospheric CO(2) concentrations may increase plant carbon fixation. In ectomycorrhizal trees, a considerable portion of the synthesized carbohydrates can be used to support the mutualistic fungal root partner which in turn can benefit the tree by increased nutrient supply. In this study, Norway spruce seedlings were inoculated with either Piloderma croceum (medium distance "fringe" exploration type) or Tomentellopsis submollis (medium distance "smooth" exploration type).