Abstract
Pulse-labelling of trees with stable or radioactive carbon (C) isotopes offers the unique opportunity to trace the fate of
labelled CO2 into the tree and its release to the soil and the atmosphere. Thus, pulse-labelling enables the quantification of
C partitioning in forests and the assessment of the role of partitioning in tree growth, resource acquisition and C sequestration.
However, this is associated with challenges as regards the choice of a tracer, the methods of tracing labelled C in tree
and soil compartments and the quantitative analysis of C dynamics. Based on data from 47 studies, the rate of transfer differs
between broadleaved and coniferous species and decreases as temperature and soil water content decrease. Labelled
C is rapidly transferred belowground—within a few days or less—and this transfer is slowed down by drought. Half-lives of
labelled C in phloem sap (transfer pool) and in mature leaves (source organs) are short, while those of sink organs (growing
tissues, seasonal storage) are longer. 13C measurements in respiratory efflux at high temporal resolution provide the best
estimate of the mean residence times of C in respiratory substrate pools, and the best basis for compartmental modelling.
Seasonal C dynamics and allocation patterns indicate that sink strength variations are important drivers for C fluxes. We propose
a conceptual model for temperate and boreal trees, which considers the use of recently assimilated C versus stored C.
We recommend best practices for designing and analysing pulse-labelling experiments, and identify several topics which we
consider of prime importance for future research on C allocation in trees: (i) whole-tree C source–sink relations, (ii) C allocation
to secondary metabolism, (iii) responses to environmental change, (iv) effects of seasonality versus phenology in and
across biomes, and (v) carbon–nitrogen interactions. Substantial progress is expected from emerging technologies, but the
largest challenge remains to carry out in situ whole-tree labelling experiments on mature trees to improve our understanding
of the environmental and physiological controls on C allocation.