Introduction

Most temperate forest trees associate with ectomycorrhizal (ECM) fungi (Martin et al., 2016) that provide them nutrients and water in exchange for photosynthetically fixed carbon (C; Smith & Read, 2008). In Central European forests, numerous ECM species co-exist (Buée et al., 2009; Nguyen et al., 2020; Khokon et al., 2023), showing a variable degree of host specialization (Lang et al., 2011; van der Linde et al., 2018; Voller et al., 2023). Ectomycorrhizal fungi enwrap the root tips with extraradical mycelium from which hyphae extend into the soil. In a common mycorrhizal network (CMN), roots of two or multiple plants, from the same or different species, are connected by mycelia of shared ECM species (Finlay & Read, 1986; Simard & Durall, 2004; Beiler et al., 2010; Horton, 2015). Several studies claim that CMNs are responsible for plant-to-plant direct exchange of C (e.g. Simard et al., 1997a,c; Klein et al., 2016; Cahanovitc et al., 2022). While the fungal translocation of C is well-recognized in mycoheterotrophic plants (Bidartondo, 2005; Courty et al., 2011), the relevance of C and nutrient transfer through CMNs among autotrophic plants is subject of an intense ongoing debate (Hoeksema, 2015; Henriksson et al., 2023; Karst et al., 2023; Robinson et al., 2024). Using isotope labeling, the CO₂ assimilated by a ‘donor’ plant was detected in the roots of one or more neighboring ‘recipient’ plants (e.g. Simard et al., 1997a,c; Cahanovitc et al., 2022). Ectomycorrhizal roots contain conspicuous amounts of fungal tissue (Smith & Read, 2008); therefore, C detected in recipient roots could be retained in the fungal tissue of the ECM root tip and may not be actually translocated to the plant tissue (Robinson & Fitter, 1999). Due to the technical difficulties in separating fungal and plant tissues of the ECM roots, studies investigating this aspect are still lacking. In this study, we conducted an innovative experiment separating the fungal-colonized tissue from the plant transport tissue by dissecting the ECM root tip to find out whether C is transferred to the plant or whether it remains in the fungus.

Investigations about interplant C exchange have thus far focused on sympatric tree species (e.g. Simard et al., 1997a; Klein et al., 2016; Cahanovitc et al., 2022). It is not known whether C can be exchanged between native and non-native tree species interacting in a mixed forest. However, it is important to gain knowledge on this aspect because non-native species are often introduced in forest plantations as an active adaptation measure to global climate change (Ammer et al., 2018).

The conifer Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is used in forest plantations in temperate regions around the world, but its natural distribution is western North America (Spellmann et al., 2015; Thomas et al., 2022). It was introduced to Europe in the 19^th century due to its high timber production and nowadays has become the most common non-native tree species in Central European forests (Brus et al., 2019). To preserve local biodiversity, Douglas-fir is generally admixed with indigenous tree species, such as the broadleaf European beech (Fagus sylvatica L.). Forest floor diversity in mixed Douglas-fir-beech forests is similar to that of beech forests (Glatthorn et al., 2023), but the impact on forest functions is not well-understood. Rog et al. (2020) reported that phylogenetically related tree species with more similar mycorrhizal communities shared more C than less related tree species. Thus, we expected that conspecific trees exchange more C than heterospecific pairs, such as beech and Douglas-fir.

Both Douglas-fir and European beech associate with ECM fungi. In European forests, where Douglas-fir was introduced, local ECM species colonized Douglas-fir roots (Le Tacon et al., 1984; Parladé et al., 1995; Dučić et al., 2009). Therefore, it is possible that the same ECM species colonize both the non-native Douglas-fir and the native European beech, potentially establishing a CMN. However, in a study comparing pure and mixed beech and Douglas-fir forest stands, the relative abundance of ECM species was lower in the soil of pure Douglas-fir and mixed forests than in pure beech forests (Likulunga et al., 2021), suggesting that in Douglas-fir stands soil exploration by ECM hyphae is reduced.

Here, we investigated the translocation of C between tree species with widely separated ranges, that is European beech, native to Central Europe, and Douglas-fir, native to North America. We used saplings growing in conspecific and heterospecific pairs and traced the newly assimilated C from beech saplings (donors) to either beech or Douglas-fir saplings (recipients) with ¹³C stable isotope labeling. Specifically, we addressed the following hypotheses:

1. C from CO₂ assimilated by beech is transferred belowground to the roots of neighboring trees, but it is retained in the fungal structures within the ectomycorrhiza.
2. The amount of belowground C transfer is related to the overlap of ECM fungi in mycorrhizal networks. Therefore, beech trees, which share more ECM species, translocate more C with each other than with Douglas-fir.

中文翻译：

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花旗松的外生菌根真菌保留了来自邻近欧洲山毛榉的新同化碳

 介绍

大多数温带森林树木与外生菌根 (ECM) 真菌相关（Martin等， 2016 ），这些真菌为它们提供营养和水，以换取光合固定碳（C；Smith & Read， 2008 ）。在中欧森林中，多种 ECM 物种共存（Buée等人， 2009 年；Nguyen等人， 2020 年；Khokon等人， 2023 年），表现出不同程度的寄主专业化（Lang等人， 2011 年；van der Linde等人， 2018 ；Voller等人， 2023 ）。外生菌根真菌用根外菌丝包裹根尖，菌丝从根尖延伸到土壤中。在共同菌根网络（CMN）中，来自相同或不同物种的两种或多种植物的根通过共享 ECM 物种的菌丝体连接（Finlay & Read， 1986 ；Simard & Durall， 2004 ；Beiler等， 2010 ）霍顿， 2015 ）。多项研究声称 CMN 负责植物与植物之间的碳直接交换（例如 Simard等人， 1997a ， c ；Klein等人， 2016 ；Cahanovic等人， 2022 ）。虽然 C 的真菌易位在真菌异养植物中得到了广泛认可（Bidartondo， 2005 ；Courty等人， 2005）。， 2011 ），自养植物中通过 CMN 进行碳和养分转移的相关性一直是激烈争论的主题（Hoeksema， 2015 ；Henriksson等， 2023 ；Karst等， 2023 ；Robinson等， 2024 ）。使用同位素标记，在一个或多个邻近“受体”植物的根部检测到“供体”植物同化的CO ₂ （例如Simard等人， 1997a ， c ；Cahanovitc等人， 2022 ）。外生菌根含有大量的真菌组织（Smith & Read， 2008 ）；因此，在受体根中检测到的 C 可能保留在 ECM 根尖的真菌组织中，并且实际上可能不会转移到植物组织中（Robinson & Fitter， 1999 ）。由于分离 ECM 根的真菌和植物组织存在技术困难，目前仍缺乏这方面的研究。在这项研究中，我们进行了一项创新实验，通过解剖 ECM 根尖，将真菌定植组织与植物转运组织分离，以查明 C 是否转移到植物中或是否保留在真菌中。

迄今为止，有关株间碳交换的研究主要集中在同域树种上（例如 Simard等， 1997a ；Klein等， 2016 ；Cahanovic等， 2022 ）。目前尚不清楚在混交林中相互作用的本地树种和非本地树种之间是否可以交换碳。然而，获得这方面的知识很重要，因为人工林中经常引入非本地物种，作为对全球气候变化的积极适应措施（Ammer等人， 2018 ）。

针叶树花旗松 ( Pseudotsuga menziesii (Mirb.) Franco) 用于世界各地温带地区的人工林，但其自然分布在北美西部（Spellmann等， 2015 ；Thomas等， 2022 ）。由于木材产量高，它于¹⁹世纪被引入欧洲，如今已成为中欧森林中最常见的非本地树种（Brus等人， 2019 ）。为了保护当地的生物多样性，花旗松通常与本土树种混合，例如阔叶欧洲山毛榉（ Fagus sylvatica L.）。花旗松-山毛榉混交林的森林地表多样性与山毛榉森林相似（Glatthorn等， 2023 ），但对森林功能的影响尚不清楚。罗格等人。 ( 2020 ) 报道，具有更相似菌根群落的系统发育相关树种比相关性较低的树种共享更多的 C。因此，我们预计同种树木比异种树木交换更多的碳，例如山毛榉和花旗松。

花旗松和欧洲山毛榉都与 ECM 真菌有关。在引入花旗松的欧洲森林中，当地的 ECM 物种在花旗松根上定居（Le Tacon等人， 1984 年；Parladé等人， 1995 年；Dučić等人， 2009 年）。因此，同一种 ECM 物种有可能同时寄生在非本地花旗松和本地欧洲山毛榉上，从而有可能建立 CMN。然而，在一项比较纯山毛榉和混交林以及花旗松林的研究中，纯花旗松和混交林土壤中 ECM 物种的相对丰度低于纯山毛榉林（Likulunga等， 2021 ），表明在花旗松林中，ECM 菌丝对土壤的探索减少了。

在这里，我们研究了分布范围广泛的树种之间的碳易位，即原产于中欧的欧洲山毛榉和原产于北美的花旗松。我们使用同种和异种对生长的树苗，并用¹³ C 稳定同位素标记追踪从山毛榉树苗（供体）到山毛榉树苗或花旗松树苗（受体）的新同化 C。具体来说，我们提出了以下假设：

1. 
   山毛榉吸收的 CO ₂中的碳被转移到地下到邻近树木的根部，但它保留在外生菌根内的真菌结构中。
2. 
   地下碳转移量与菌根网络中ECM真菌的重叠程度有关。因此，与花旗松相比，共享更多 ECM 物种的山毛榉树之间的碳易位量更多。