Abstract
Significant alterations of cambial activity might be expected due to climate warming, leading to growing season extension and higher growth rates especially in cold-limited forests. However, assessment of climate-change-driven trends in intra-annual wood formation suffers from the lack of direct observations with a timespan exceeding a few years. We used the Vaganov-Shashkin process-based model to: (i) simulate daily resolved numbers of cambial and differentiating cells; and (ii) develop chronologies of the onset and termination of specific phases of cambial phenology during 1961–2017. We also determined the dominant climatic factor limiting cambial activity for each day. To asses intra-annual model validity, we used 8 years of direct xylogenesis monitoring from the treeline region of the Krkonoše Mts. (Czechia). The model exhibits high validity in case of spring phenological phases and a seasonal dynamics of tracheid production, but its precision declines for estimates of autumn phenological phases and growing season duration. The simulations reveal an increasing trend in the number of tracheids produced by cambium each year by 0.42 cells/year. Spring phenological phases (onset of cambial cell growth and tracheid enlargement) show significant shifts toward earlier occurrence in the year (for 0.28–0.34 days/year). In addition, there is a significant increase in simulated growth rates during entire growing season associated with the intra-annual redistribution of the dominant climatic controls over cambial activity. Results suggest that higher growth rates at treeline are driven by (i) temperature-stimulated intensification of spring cambial kinetics, and (ii) decoupling of summer growth rates from the limiting effect of low summer temperature due to higher frequency of climatically optimal days. Our results highlight that the cambial kinetics stimulation by increasing spring and summer temperatures and shifting spring phenology determine the recent growth trends of treeline ecosystems. Redistribution of individual climatic factors controlling cambial activity during the growing season questions the temporal stability of climatic signal of cold forest chronologies under ongoing climate change.
Highlights
The seasonal cycle of global temperature shows prominent yearto-year variability and low-frequency trends in phase and amplitudes (Mann and Park, 1996; Stine et al, 2009; Cornes et al, 2017)
We focused on two research questions: (i) What is the precision of the VSmodel simulations of cambial dynamics? and (ii) How did the cambial dynamics at the treeline respond to climate change during last five decades? For addressing the first question, 8 years of xylogenesis monitoring of Norway spruce was used to verify intra-annual dynamics predicted by the VS-model
The algorithm of the VS-model uses the cumulative temperature threshold to estimate the date of spring onset of cambial cell growth and, in the step, calculates daily resolved kinetics of cell growth and differentiation based on the cell position in the radial file and climatically driven external growth rates (Vaganov et al, 2006)
Summary
The seasonal cycle of global temperature shows prominent yearto-year variability and low-frequency trends in phase (timing of seasons) and amplitudes (Mann and Park, 1996; Stine et al, 2009; Cornes et al, 2017). This variance is reflected in intraannual dynamics of climatically driven biological processes. Northern and high-elevation forests experience prominent year-to-year changes in cambial dynamics (process of xylem tissue development), phenology (timing of cambial activity) and kinetics (speed of xylem cell differentiation). Short observations of cambial dynamics in cold regions limit our understanding of trends and inter-annual changes of cambial dynamics at multi-decadal timescales
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