Abstract
Human-driven peatland drainage has occurred in Europe for centuries, causing habitat degradation and leading to the emission of greenhouse gases. As such, in the last decades, there has been an increase in policies aiming at restoring these habitats through rewetting. Alder (Alnus glutinosa L.) is a widespread species in temperate forest peatlands with a seemingly high waterlogging tolerance. Yet, little is known about its specific response in growth and wood traits relevant for tree functioning when dealing with changing water table levels. In this study, we investigated the effects of rewetting and extreme flooding on alder growth and wood traits in a peatland forest in northern Germany. We took increment cores from several trees at a drained and a rewetted stand and analyzed changes in ring width, wood density, and xylem anatomical traits related to the hydraulic functioning, growth, and mechanical support for the period 1994–2018. This period included both the rewetting action and an extreme flooding event. We additionally used climate-growth and climate-density correlations to identify the stand-specific responses to climatic conditions. Our results showed that alder growth declined after an extreme flooding in the rewetted stand, whereas the opposite occurred in the drained stand. These changes were accompanied by changes in wood traits related to growth (i.e., number of vessels), but not in wood density and hydraulic-related traits. We found poor climate-growth and climate-density correlations, indicating that water table fluctuations have a stronger effect than climate on alder growth. Our results show detrimental effects on the growth of sudden water table changes leading to permanent waterlogging, but little implications for its wood density and hydraulic architecture. Rewetting actions should thus account for the loss of carbon allocation into wood and ensure suitable conditions for alder growth in temperate peatland forests.
Highlights
Forest peatlands are widely distributed around the globe and represent 20–25% of global peatland cover (Zoltai and Martikainen, 1996)
We focused our analyses on the period between 1994 and 2018, which encompasses the period running from 10 years before the last rewetting action in the area until present, including the extreme flooding event in 2011 (Figure 2)
Negative effects of waterlogging on alder growth have been previously reported in other studies (RodríguezGonzález et al, 2010; Tulik et al, 2020) and the rewetting action that took place in 2003 at our study sites leading to a sudden and permanent flooding of an adjacent stand caused the death of all alder trees there (Bönsel, 2006)
Summary
Forest peatlands are widely distributed around the globe and represent 20–25% of global peatland cover (Zoltai and Martikainen, 1996). In the last decades, rewetting policies have been introduced with the aim to restore drained peatlands and their ecosystem functions (Cris et al, 2014; Tanneberger et al, 2021) but the extent of their long-term success is still largely unknown In peatland forests, this success heavily depends on the acclimation capacity of the tree species to increased water tables. It is a pioneer species that can fix atmospheric nitrogen, thanks to its symbiosis with the bacterium, Frankia alni (McVean, 1956; Claessens et al, 2010); it is a peat forming species (Barthelmes, 2009), and it is of economic value for timber production (Claessens et al, 2010) Even though it is a waterlogging-tolerant species (Glenz et al, 2006; Niinemets and Valladares, 2006), several studies have reported the negative effects of prolonged flooding and water level fluctuations on alder radial growth (Laganis et al, 2008; Douda et al, 2009; Rodríguez-González et al, 2010; Tulik et al, 2020) and high water table levels are known to decrease the wood quality of alder (Barthelmes, 2009). Understanding how alder growth, functioning, and subsequent survival may be affected by rewetting actions is paramount to design effective protocols that restore the ecosystem functions of alder wet forests and preserve their wood production potential as much as possible
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