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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.
Abstract
Root phenology influences the timing of plant resource acquisition and carbon fluxes into the soil. This is particularly important in fen peatlands, in which peat is primarily formed by roots and rhizomes of vascular plants. However, most fens in Central Europe are drained for agriculture, leading to large carbon losses, and further threatened by increasing frequency and intensity of droughts. Rewetting fens aims to restore the original carbon sink, but how root phenology is affected by drainage and rewetting is largely unknown.
We monitored root phenology with minirhizotrons in drained and rewetted fens (alder forest, percolation fen and coastal fen) as well as its soil temperature and water table depth during the 2018 drought. For each fen type, we studied a drained site and a site that was rewetted ~25 years ago, while all the sites studied had been drained for almost a century.
Overall, the growing season was longer with rewetting, allowing roots to grow over a longer period in the year and have a higher root production than under drainage. With increasing depth, the growing season shifted to later in time but remained a similar length, and the relative importance of soil temperature for root length changes increased with soil depth.
Synthesis and applications. Rewetting extended the growing season of roots, highlighting the importance of phenology in explaining root productivity in peatlands. A longer growing season allows a longer period of carbon sequestration in form of root biomass and promotes the peatlands' carbon sink function, especially through longer growth in deep soil layers. Thus, management practices that focus on rewetting peatland ecosystems are necessary to maintain their function as carbon sinks, particularly under drought conditions, and are a top priority to reduce carbon emissions and address climate change.