Trees contribute largely to the C-fixation by transforming atmospheric CO2 to biomass and wood. Their importance in the carbon cycle is evident as their photosynthetic assimilation is one of the largest terrestrial carbon fluxes, with their standing biomass representing the largest C-pool in the terrestrial biosphere. Understanding how tree physiology and growth respond to long-term environmental change is pivotal in predicting the magnitude and direction of the terrestrial carbon sink.

Lay summary
Tree growth is strongly dependent on climate, which is reflected in tree rings. Warm years with sufficient water supply (precipitation) are good growth conditions resulting in wide tree rings. In cold or dry years growth is reduced, visible in narrow tree ring width (TRW). Consequently, TRW is used to reconstruct past climate courses.

Another physiological phenomenon, which can be used as a climate indicator is the behavior of stable isotopic ratios. During photosynthesis plants discriminate against the heavier 13C isotope in favor of the lighter 12C. At the same time the lighter and more abundant H216O transpires more readily than the heavier H218O. Therefore plants contain more of the heavier 18O compared to the feeding water. The degree of the 13C discrimination is strongly controlled by water availability and air humidity. The 18O content is also controlled by air humidity but more strongly by temperature.

Stable isotopes also provide important information on physiological processes during the interaction between the plant and its environment. This information is instrumental in understanding and modeling the impact of the environment on tree responses. Therefore, the combination of TRW and stable isotopes enhances the information extracted from tree rings.

i-TREE is an interdisciplinary research framework, which enables leading dendro-climatologists, plant physiologists, isotope specialists, and global carbon cycle modelers to capitalize on their existing synergies. The object of their research is to reduce uncertainties related to the estimation of tree/forest growth in the context of changing natural environments. Cross-cutting themes in our project are tree rings, stable isotopes, and mechanistic modeling. Goals are:

(i)             To establish a European network of tree-ring based isotope time-series for the analysis of year-to-year tree ring series reflecting long-term tree physiological changes.

(ii)            To gain specific information on the acclimation potential of trees by means of laboratory and field experiments. These results will facilitate the improvement of mechanistic models for simulating tree responses to environmental variations. Tree responses are reflected in tree growth and tree-ring isotope ratios.

(iii)          To implement these improvements into a dynamic, global vegetation model, and to validate and refine this model so that it represents plant physiological processes.

(iv)          To attribute long-term variation in tree growth to plant physiological and environmental drivers, and to identify how our refined knowledge revises predictions of the coupled carbon-cycle climate system.

With i-TREE we will contribute to the following objectives:

(i)             Advanced quantifications of long-term variation in tree growth across Central Europe,

(ii)            Novel long-term information on key physiological processes that underlie variations in tree growth, and

(iii)          Improved carbon cycle models that can be employed to revise predictions of the coupled carbon-cycle climate system.

Hence i-TREE will significantly contribute towards a seamless understanding of the responses of terrestrial ecosystems to long-term environmental change and will ultimately help to reduce uncertainties about the magnitude and direction of both the past and future terrestrial carbon sinks.