Abstract
Wood growth constitutes the main process for long-term atmospheric carbon sequestration in vegetation. However, our understanding of the process of wood growth and its response to environmental drivers is limited. Current dynamic global vegetation models (DGVMs) are mainly photosynthesis-driven and thus do not explicitly include a direct environmental effect on tree growth. However, physiological evidence suggests that, to realistically model vegetation carbon allocation under increased climatic stressors, it is crucial to treat growth responses independently from photosynthesis. A plausible growth response function suitable for global simulations in DGVMs has been lacking. Here, we present the first soil water-growth response function and parameter range for deciduous and evergreen conifers. The response curve was calibrated against European larch and Norway spruce in a dry temperate forest in the Swiss Alps. We present a new data-driven approach based on a combination of tree ring width (TRW) records, growing season length and simulated subdaily soil hydrology to parameterize ring width increment simulations. We found that a simple linear response function, with an intercept at zero moisture stress, used in growth simulations reproduced 62.3% and 59.4% of observed TRW variability for larch and spruce respectively and, importantly, the response function slope was much steeper than literature values for soil moisture effects on photosynthesis and stomatal conductance. Specifically, we found stem growth stops at soil moisture potentials of -0.47MPa for larch and -0.66MPa for spruce, whereas photosynthesis in trees continues down to -1.2MPa or lower, depending on species and measurement method. These results are strong evidence that the response functions of source and sink processes are indeed very different in trees, and need to be considered separately to correctly assess vegetation responses to environmental change. The results provide a parameterization for the explicit representation of growth responses to soil water in vegetation models.
Highlights
Trees sequester CO2 long-term through biomass formation
We present a new data-driven approach based on a combination of tree ring width records, growing season length and simulated sub-daily soil hydrology to parameterise ring width increment simulations
We found that a simple linear response function, with an intercept at zero moisture stress, used in growth simulations reproduced 62.3% and 59.4% of observed tree ring width variability for larch and spruce respectively and, importantly, the response function slope was much steeper than literature values for soil moisture effects on photosynthesis and stomatal conductance
Summary
Trees sequester CO2 long-term through biomass formation. More than three trillion trees (Crowther et al, 2015) sequester about 18% of annual anthropogenic emissions (Pan et al, 2011; Le Quere et al, 2018) in their stems. Wood growth is a crucial process in maintaining the climate mitigation effect of terrestrial ecosystems. Tree ring width is the indicator of annual stem radial increment. Variable tree ring widths at environmentally limiting sites provide important long-term evidence of how tree growth responds to the environment (Douglass, 1914) and simul taneously how much carbon was captured in the stem. Despite tree rings being used to reconstruct climate through correlation, our mechanistic understanding of growth processes and their environmental drivers remains limited (Cuny et al, 2015)
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