bioRxiv preprint doi: https://doi.org/10.1101/2020.09.25.313213; this version posted October 14, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 1 Douglas fir and Norway spruce admixtures to European beech along a site gradient in Northern 2 Germany – Are soil nutrient conditions affected? 3 Estela Covre Foltran1, Christian Ammer2,3, Norbert Lamersdorf 1 4 1 Soil Science of Temperate Ecosystems - University of Göttingen 5 2 Silviculture and Forest Ecology of the Temperate Zones - University of Göttingen 6 3 Center for biodiversity and sustainable land use - University of Göttingen 7 Author contact: [email protected] 8 Abstract 9 10 Background The establishment of economically valuable conifers into a matrix of native 11 broadleaved tree species may serve as model systems that combine economic interests and 12 nature conservation. However, it is not clear which effects the enrichment by conifers has on 13 soil properties. 14 Methods Our study analyzed pure mature European beech (Fagus sylvatica), Douglas fir 15 (Pseudotsuga menziesii) and Norway spruce (Picea abies) stands as well as mixtures of beech 16 with either Douglas fir or spruce along a soil and climate gradient in Northern Germany. We 17 determined chemical soil properties of the O-horizon and upper mineral soil horizons. Soil pH, 18 concentrations and storage of exchangeable cations, base saturation (BS) as well total P 19 contents were analyzed. 20 Results We observed lowest pH and BS in spruce stands while beech showed higher BS. The 21 impact of Douglas fir on soil chemistry varied depending on the site. Under Douglas fir-beech 22 mixtures, mineral soil pH and BS were higher than under the respective Douglas-fir stands at 23 nutrient-poor sandy soils. While spruce and its admixture deplete soil exchangeable Ca and Mg 24 more than Douglas fir mixed with. Total soil exchangeable K under mixed stands were among 25 the highest, independent of the site condition. 26 Conclusions Overall, our study suggest that the enrichment of beech stands by Douglas fir does 27 not cause unexpected and detrimental changes of soil acidity and does not strongly affect soil 28 exchangeable base cation reserves when compared to European beech. Instead, admixtures of 29 Douglas-fir seem to lead to smaller changes in pH, CEC and BS than those of Norway spruce. 30 Therefore, forest management may consider mixtures of European beech and Douglas fir as a 31 reasonable management option without apprehending negative effects on soil chemistry. 32 Keywords: mixed forests; species-identity; broadleaves; conifers; Fagus sylvatica; 33 Pseudotsuga menziesii; Picea abies. 1 bioRxiv preprint doi: https://doi.org/10.1101/2020.09.25.313213; this version posted October 14, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 34 1. Introduction 35 The development of forest soils is a complex process driven by abiotic and biotic factors 36 (Binkley and Fisher, 2020). The existing geologic parent material, climate conditions and the 37 topography of a given site are essential to the formation of a soils. However, those key 38 characteristics develop very slowly. Much faster processes are caused by the presence (or 39 absence) of particular tree species which are known to alter the development of soils, at times 40 scales of decades (van Breemer et al, 1998 ). In fact, biotic factors are important for soil 41 formation (Busse et al., 2019), impacting soil biological, soil physical and soil chemical 42 processes and characteristics (Cremer and Prietzel, 2017). Thus, forest stand type potentially 43 leads to distinct impacts on forest soil chemical and soil physical processes, as well as on soil 44 biodiversity (Heitz and Rehfuess 1999; Ammer et al. 2006, Prietzel and Bachmann 2012)r. 45 In almost all temperate forest regions, but increasingly also in the boreal zone and some tropical 46 areas, the present tree species composition is a result of modern forest management practices 47 (Vesterdal et al., 2013). Compositionally and structurally diverse forests represent an important 48 element of approaches to deliver a wide range of ecosystem goods and services (Forrester et 49 al., 2017, Felipe-Lucia et al. 2018). Moreover, the establishment and management of mixed 50 stands is also discussed as an effective measure to adapt forests stands to climate change 51 (Ammer 2017) and other global challenges such as air pollution and invasive species (Bauhus 52 et al., 2009). Actually, reports of frequent droughts, windthrow and bark beetle infestations 53 around Europe, induced by climate change, make the wide-spread use of native conifers, e.g. 54 Norway spruce (Picea abies) in Central Europe, increasingly problematic (Dobor et al., 2020; 55 Kölling and Zimmermann, 2007; Seidl et al. 2007). However, single-species plantations are 56 still dominating forests planted for wood and fibre production (Coll et al., 2018; Liu et al., 57 2018) supplying up to 33% of the total industrial roundwood in the world. Nevertheless, for 58 various reasons this forest type is under increasing pressure (Williams 2011, Bremer and Farley 59 2010). For example, Felton et al., (2010) reviewed negative ecological and environmental 60 impacts of monoculture plantations of Norway spruce (Picea abies). They showed that these 61 plantations are less resistance to biotic and abiotic disturbances than mixed stands. 62 However, in many parts of the world conifers are important in economic terms. In Europe, for 63 example, between 1992 and 2002 conifers accounted for 72% of the total roundwood 64 production (Koulelis 2009), which underlines their importance for sawmill industry. One 65 option for reconciling production-oriented goals with conservational interests may be found in 66 mixtures of highly productive native or non-native conifer and less productive native 2 bioRxiv preprint doi: https://doi.org/10.1101/2020.09.25.313213; this version posted October 14, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 67 broadleaved species (Hildebrandt et al. 2010, Oxbrough et al. 2016). For example, in Central 68 Europe enrichments of European beech (Fagus sylvatica) stands with native Norway spruce 69 (Picea abies) or non-native Douglas fir (Pseudotsuga menziesii menziesii), may result in 70 mixtures that provide income and cope better with the envisaged hazards (Neuner et al. 2015). 71 Coastal Douglas fir is considered a suitable alternative forest tree species to Norway spruce in 72 Central Europe, since the latter is heavily affected by climate changes resulting in drought 73 stress and subsequent bark beetle attacks (Hlásny & Turčáni 2013). Thus, on many sites this 74 species will not be able to be cultivated successfully any more (Kölling et al. 2009). In contrast, 75 Douglas fir is characterized by fast growth, good wood properties and a high tolerance to heat 76 and drought, which makes it a highly profitable tree species at appropriate sites across Europe 77 (Kownatzki, 2011) apart form already dry sites (Eckart et al. 2019). 78 Ecological characteristics, such as growth rate of mixed species stands are often intermediate 79 in comparison with pure stands of the corresponding species (Augusto et al., 2015; Rothe and 80 Binkley, 2001). Belowground facilitation processes through complementary effects of the 81 different tree species in mixed stands have been reported by several authors and may explain 82 higher productivity of mixed stands when compared with pure stands in some cases (Ammer 83 2019). Cremer and Prietzel, (2017) investigated the effects of tree species composition on 84 mineral soil base saturation and pH and found that overall tree species mixtures appear to 85 improve soil base cation stocks. However, mixture effects on forest soil chemistry vary 86 depending on tree species identity, climatic factors and soil type (Augusto et al., 2015; 87 Vesterdal and Raulund-Rasmussen, 1998). Tree species identity may also have an important 88 impact on the ecosystem level, e.g. soil C stock , C:N ratio, and pH, particularly in the O- 89 horizon and top mineral soil layers (Augusto et al., 2015; Vesterdal et al., 2013, 2008). 90 From a management point of view the selection of tree species with desired characteristics, e.g. 91 complementary traits for resource use, is one of the most important silvicultural decisions 92 (Schall and Ammer 2013). Creating mixtures depending on the soil nutrient status requires 93 careful tree species selection rather than increasing tree species diversity per se (Dawud et al., 94 2017). Conifers are known to increase C stocks, while many broadleaves species are able to 95 increase base saturation at top-mineral soil (Cremer and Prietzel, 2017). However, not much is 96 known how native European beech and non-native Douglas fir interact and shape soil 97 chemistry. Douglas fir often shows high fine root density in deeper soil layers (Calvaruso et 98 al., 2011). Therefore, it might decrease nutrient leaching and cations losses which would differ 99 from pattern that have been observed under the native conifer Norway spruce (Oulehle et al. 100 2007). If so, this would indicate a conifer species identity effect. To our knowledge, however, 3 bioRxiv preprint doi: https://doi.org/10.1101/2020.09.25.313213; this version posted October 14, 2020.