Dated tree rings are used to understand tree-growth responses to climatic variability and reconstruct past climate conditions. In parallel, mechanistic models make use of experimentally derived plant-atmosphere interactions to simulate past and future environmental impact on forest productivity. Yet, substantial model inconsistencies and mismatches with empirical data highlight the need for a deeper mechanistic understanding of climate-growth relationships. One hypothesis to explain this mismatch is that cambial activity is not solely limited by photosynthetic activity, especially at temperature-limited sites where there is little evidence for carbon limitation. Novel mechanistic models are pointing to the importance of including tree hydraulics, since turgor pressure in cambial cells might dictate initiation and rate of wood formation. However, due to the lack of datasets describing intra-annual physiological processes combined with wood formation, the importance of this process remains largely unclear. We attempt to bridge this gap by comparing modelled turgor-driven xylem growth in Larix decidua Mill. and Picea abies (L.) Karst in the Swiss Alps along a 900 m elevational transact with observational data on wood formation. We combine multi-annual and sub-hourly sap flow and point dendrometer measurements into an individual-based mechanistic tree-growth model to simulate and validate inter- and intra-annual wood formation.
Results/Conclusions
The mechantisic tree-growth model was used to assess turgidity occurring in the cambial zone and tracheid enlargement during radial growth covering the growing seasons from 2008 to 2015 for 20 trees. The model has been successfully calibrated for five sites, characterized by contrasting growing season temperatures and soil moisture conditions. Results indicated that the simulated wood formation matched with weekly xylogenesis observations, with the best agreement observed for the treeline L. decidua. The hydraulic parameters derived with the model revealed that low elevation P. abies was affected by drought, showing an increased resistance in the xylem for water transport to the crown. Additionally, annual growth patterns simulated across site and species corresponded to the ring-width measurements from the monitored trees. As the results suggest that inter-annual ring-width variability can also be explained by turgor-driven growth processes, we advocate these mechanisms to be considered, next to photosynthesis, when modelling climate-growth responses. With these efforts we hope to advance the process-based understanding of how climate shapes annual tree-ring structures to improve the accuracy of reconstructing the climate of the past and predicting growth under changing climatic conditions.