Introduction

Global warming and more frequent and extreme droughts are critical environmental drivers for forest dynamics because of their impacts on atmospheric carbon dioxide (CO₂) acquisition through photosynthesis and tree carbon dynamics (Ciais et al., 2005; Rödenbeck et al., 2020). However, although nearly all studies caution that higher temperatures and droughts are increasingly co-occurring (Overpeck, 2013; Sun et al., 2019), little is known about their combined impacts on tree photosynthetic properties.

Exposure to elevated air temperature but sufficient soil moisture supply should increase leaf-level photosynthetic properties in the long term because of enhanced photochemical reactions, as long as temperatures do not exceed the photosynthesis optimal temperature (Dreyer et al., 2001; Way et al., 2015; Dusenge et al., 2019). Hence, warming can translate into higher light-saturated assimilation (A_sat) (e.g. Prieto et al., 2009) and more efficient leaf-level photochemistry. For instance, warming has been associated with higher maximum rate of carboxylation by Rubisco (Vc_max) and maximum photosynthetic electron transport rate (J_max), especially when plants are limited by cold temperatures (e.g. Way & Oren, 2010). However, once the optimal temperature threshold of photosynthesis is exceeded, exposure to heat stress could cause the separation of the light-harvesting complexes from the photosystem II (PSII) reaction center or the denaturation of proteins (Gounaris et al., 1984; Yamane et al., 1997). The consequences are reduced A_sat, Vc_max, and J_max with high temperatures (Medlyn et al., 2002; Dai et al., 2021; Dusenge et al., 2021), a reduction of the maximum photochemical efficiency (F_v/F_m) and decreasing chlorophyll (Chl) concentrations. Variation in these responses largely depends on the species' temperature range. Húdoková et al. (2022) observed that heat waves of +5°C above daily average during the summer reduce A_sat, PSII, and increase the degradation of Chl in temperate European beech trees (Fagus sylvatica L.), while sessile oak, a Mediterranean tree species growing in warmer and drier environments, showed no effect. Thus, species adapted to higher temperature may better resist future conditions, while more sensitive ones may be seriously threatened ecologically.

Low soil moisture can impact the leaf-level photosynthesis through stomatal regulation and nonstomatal processes (Flexas & Medrano, 2002). Reduced moisture decreases stomatal conductance, and thus leaf photosynthetic CO₂ assimilation due to a reduction of CO₂ diffusion and thus lower intracellular CO₂ concentration in the leaf (C_i) (Gallé & Feller, 2007; Zhou et al., 2014), leading to a possible photo-damage to PSII (Powles, 1984; Epron & Dreyer, 1993). Independently of stomatal closure, biochemical processes will be affected during prolonged soil moisture reduction, thereby limiting leaf-level photosynthesis properties via a downregulation of Rubisco activity and content (Parry, 2002). Hence, impacts of low soil moisture could include a reduction of A_sat, Vc_max, J_max, and F_v/F_m (Zhou et al., 2014; Santos et al., 2018). For instance, Arend et al. (2013, 2016) showed reduced A_sat and impaired PSII photochemistry in drought-exposed beech and downy oak trees (Quercus pubescens Willd.), including stronger impacts in beech because of its higher vulnerability to low moisture. Indeed, beech is considered rather isohydric compared to oak as this species will close its stomates at less negative leaf water potentials (Klein, 2014) and is generally assumed to be negatively affected by drought (Gessler et al., 2006).

How species respond to the combined effects of rising temperature and low soil moisture depends on the species tolerance, the range of environmental conditions, and how extreme temperature and precipitation changes are. Some studies have reported strong exacerbated effects of combined warming and high soil moisture stress (Contran et al., 2013; Arend et al., 2016). However, combined effects are not always occurring as plants can acclimate leaf-level photosynthetic properties in response to the environment (e.g. Arend et al., 2011). For example, Grossiord et al. (2017) observed that piñon pine and juniper trees growing in semi-arid environments where drought is a recurrent stress have stronger leaf-level responses to soil moisture than to warming with no exacerbation of their combined effects. Moreover, prolonged exposure to warming and low soil water availability can lead to whole-plant structural acclimation (i.e. shift in biomass allocation), allowing plants to maintain leaf-level gas exchange under more stressful conditions (Thomas, 2000; Bréda et al., 2006; Schönbeck et al., 2022). Thus, drought and warming can have distinct or similar effects on the photosynthetic properties depending on species-specific responses to temperature and soil moisture, making projections about their combined effect a big challenge for plant ecologists and land managers (Mittler, 2006; Rennenberg et al., 2006).

In addition to the direct impacts of chronic warming and reduced soil water on individual trees, plant performance in natural ecosystems is driven by the dynamics of the whole forest, including the interactions between trees (Grossiord, 2019). Depending on the species identity, climate, and site conditions, these interactions can have either positive, negative, or no feffects on plant functioning (Hooper et al., 2005; Grossiord et al., 2014a,b,c; Jucker et al., 2016). More specifically, interactions between tree species with complementary structural (e.g. different rooting depths and canopy heights) and functional traits (e.g. phenology, hydraulic traits) have often been associated with mitigating effects during extreme events like droughts (e.g. Grossiord et al., 2014a,b,c; Anderegg et al., 2018; Grossiord, 2019). The potential drivers of such positive effects are associated with niche complementarity and the local environmental conditions created by tree neighbors (Paquette & Messier, 2011; Liang et al., 2015). Local neighbors can increase canopy stratification, and thus decrease the risk of light-damage (Mensah et al., 2018; Kothari et al., 2021), potentially leading to higher A_sat during warmer and drier climate. Similarly, having a more diverse litter input (i.e. litter provided by numerous species) can increase soil nutrient content (e.g. Zak et al., 2003). Higher soil fertility can enhance tree-level photosynthetic properties (Gessler et al., 2017; Gillespie et al., 2020). However, to our knowledge, no study has experimentally tested how tree species interactions mitigate leaf-level photosynthetic responses to warming and reduced soil moisture acting alone or combined over multiple years.

We, thus, aimed to understand how the interactions of two co-existing, broadly distributed but contrasting tree species, i.e. European beech and downy oak, influence their leaf-level photosynthetic responses to air warming and reduced soil moisture. We exposed young trees in intra- or inter-specific interactions to air warming and reduced soil moisture alone or combined. The tree species were selected because of their distinct functional strategies to deal with warmer and drier climate. European beech is a temperate species growing in moist environments, with mean annual temperatures varying between 2 and 14°C (Durrant et al., 2016), which tolerates shady conditions. Downy oak is growing in direct light in warmer sub-Mediterranean regions (mean annual air temperatures varying between 5 to 17°C; Pasta et al., 2016). Previous work showed that the interaction between beech and oak can lead to belowground water source partitioning (Zapater et al., 2011; Grossiord et al., 2014a,b,c) and higher productivity (Jourdan et al., 2020) at the tree-level, suggesting a mitigating effect of their interaction. However, when environmental conditions become too stressful, an adverse effect of the interaction between these species on leaf-level gas exchange has also been observed (e.g. Didion-Gency et al., 2021).

Our objectives were to (1) determine how warming and reduced soil moisture acting alone or combined influenced the leaf-level photosynthetic properties (i.e. A_sat, J_max, Vc_max, F_v/F_m and Chl) in beech and oak, and (2) assess how the interactions among these two species impact their leaf-level photosynthetic properties to warming and soil moisture reduction. If grown in intra-specific composition, we expect (1) oak to have higher leaf-level photosynthetic properties with warming compared to ambient conditions while beech will have lower leaf-level photosynthetic properties because of its stronger vulnerability to high temperatures. Reduction in soil moisture will decrease the photosynthetic properties of both species but with larger reductions expected in beech. Combined warming and low soil moisture will exacerbate photosynthetic responses observed under reduced soil moisture acting alone because of enhanced water stress. If grown in inter-specific interactions, we expect (2) warming to enhance the photosynthetic properties of oak and mitigate the reduction in beech compared to those under intra-specific interactions. Tree responses to soil moisture reduction in inter-specific interactions (i.e. acting alone or combined with warming) will be less impacted than in intra-specific interactions.

中文翻译：

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温暖和干燥条件对树木光合特性的影响以及物种相互作用的作用

介绍

全球变暖和更频繁的极端干旱是森林动态的关键环境驱动因素，因为它们会影响通过光合作用和树木碳动态获取大气中的二氧化碳 (CO ₂ )（Ciais等人，2005 年；Rödenbeck等人，2020 年）。然而，尽管几乎所有研究都警告说高温和干旱越来越多地同时发生（Overpeck，  2013 年；Sun等人，2019 年），但人们对它们对树木光合特性的综合影响知之甚少。

由于光化学反应增强，长期暴露于升高的空气温度但充足的土壤水分供应应该会增加叶级光合特性，只要温度不超过光合作用的最佳温度（Dreyer等人，2001 年；Way等人，2001 年） 。 , 2015 年；Dusenge等人，2019 年）。因此，变暖可以转化为更高的光饱和同化作用 ( A _sat ) (eg Prieto et al ., 2009 ) 和更有效的叶级光化学。例如，变暖与 Rubisco 较高的最大羧化速率有关（Vc _max ) 和最大光合电子传输速率 ( J _max )，尤其是当植物受低温限制时 (eg Way & Oren,  2010 )。然而，一旦超过光合作用的最佳温度阈值，暴露于热应激可能导致光捕获复合物与光系统 II (PSII) 反应中心分离或蛋白质变性 (Gounaris等人，1984 年；Yamane等人等人，1997 年）。结果是高温下的 A _sat、 V c _max和J _{max降低（Medlyn}等人., 2002 ; 戴等，2021；Dusenge等人，2021 年），最大光化学效率 ( F _v / F _m ) 的降低和叶绿素 (Chl) 浓度的降低。这些反应的变化很大程度上取决于物种的温度范围。Húdoková等人。( 2022 ) 观察到夏季高于每日平均水平 +5°C 的热浪降低了 A _sat和 PSII，并增加了温带欧洲山毛榉树 ( Fagus sylvatica ) 中 Chl 的降解L.），而无柄橡树（一种生长在温暖和干燥环境中的地中海树种）则没有显示出任何影响。因此，适应更高温度的物种可能更好地抵抗未来的条件，而更敏感的物种可能会受到严重的生态威胁。

低土壤水分会通过气孔调节和非气孔过程影响叶片光合作用 (Flexas & Medrano,  2002 )。水分减少会降低气孔导度，因此由于 CO ₂扩散减少导致叶片光合作用 CO _{2同化，从而降低叶片中细胞内 CO 2}浓度 ( C _i )（Gallé & Feller，  2007 年；Zhou等人，2014 年），导致可能对 PSII 造成光损伤 (Powles,  1984 ; Epron & Dreyer,  1993). 独立于气孔关闭，生化过程将在长期土壤水分减少期间受到影响，从而通过下调 Rubisco 活性和含量来限制叶级光合作用特性 (Parry,  2002 )。因此，低土壤水分的影响可能包括A _sat、V c _max、J _max和F _v / F _m的减少（Zhou等人，2014 年；Santos等人，2018 年）。例如，Arend等人。( 2013 年, 2016 年)在暴露于干旱的山毛榉和柔软的橡树 ( Quercus pubescens Willd.) 中显示A _sat降低和 PSII 光化学受损，包括对山毛榉的更强影响，因为它对低水分的脆弱性更高。实际上，与橡木相比，山毛榉被认为是等水的，因为该物种会在叶水势较低时关闭气孔（Klein，  2014 年），并且通常被认为会受到干旱的负面影响（Gessler等人，2006 年）。

物种如何应对温度升高和土壤湿度低的综合影响取决于物种的耐受性、环境条件的范围以及极端温度和降水变化的程度。一些研究报告了变暖和高土壤水分胁迫共同加剧的强烈影响（Contran等人，2013 年；Arend等人，2016 年）。然而，综合效应并不总是发生，因为植物可以响应环境适应叶级光合特性（例如 Arend等人，2011）。例如，Grossiord等人。( 2017) 观察到，在干旱是反复出现的半干旱环境中生长的松树和杜松树对土壤水分的叶片水平反应比对变暖的反应更强，而不会加剧它们的综合影响。此外，长时间暴露于变暖和低土壤水分可利用性可导致整株植物结构适应（即生物量分配的转变），使植物能够在压力更大的条件下维持叶片水平的气体交换（Thomas，  2000；Bréda等人，2006 年；Schönbeck等人，2022 年). 因此，根据物种对温度和土壤水分的特定反应，干旱和变暖可能对光合特性产生不同或相似的影响，这使得预测它们的综合影响对植物生态学家和土地管理者来说是一个巨大的挑战（Mittler，  2006 年；Rennenberg等人） ., 2006 ).

除了长期变暖和土壤水分减少对个别树木的直接影响外，自然生态系统中的植物性能还受到整个森林动态的驱动，包括树木之间的相互作用（Grossiord，  2019 年）。根据物种特性、气候和场地条件，这些相互作用可能对植物功能产生积极、消极或无影响（Hooper等人，2005 年；Grossiord等人，2014a、b、c；Jucker等人。 , 2016). 更具体地说，具有互补结构（例如不同的生根深度和冠层高度）和功能特性（例如物候学、水力特性）的树种之间的相互作用通常与干旱等极端事件期间的缓解作用有关（例如 Grossiord等人，2014a，b、c；Anderegg等人，2018 年；Grossiord，  2019 年）。这种积极影响的潜在驱动因素与生态位互补性和树木邻居创造的当地环境条件有关（Paquette & Messier，  2011 年；Liang等人，2015 年）). 当地邻居可以增加冠层分层，从而降低光害的风险（Mensah等人，2018 年；Kothari等人，2021 年），这可能导致在温暖和干燥的气候下更高的 A _{sat 。}同样，拥有更多样化的凋落物输入（即由众多物种提供的凋落物）可以增加土壤养分含量（例如 Zak等人，2003 年）。更高的土壤肥力可以增强树木的光合作用特性（Gessler等人，2017 年；Gillespie等人，2020 年）). 然而，据我们所知，还没有研究通过实验测试树种相互作用如何减轻叶级光合作用对变暖和土壤水分减少的单独或多年联合作用的反应。

因此，我们旨在了解两种共存、分布广泛但截然不同的树种（即欧洲山毛榉和绒毛橡树）的相互作用如何影响它们对空气变暖和土壤水分减少的叶级光合反应。我们将处于种内或种间相互作用的幼树单独或组合暴露于空气变暖和土壤水分减少的环境中。选择这些树种是因为它们具有独特的功能策略，可以应对更温暖和更干燥的气候。欧洲山毛榉是一种生长在潮湿环境中的温带树种，年平均气温在 2 至 14°C 之间变化（Durrant等人，2016 年）), 可以容忍阴暗的条件。在温暖的亚地中海地区（年平均气温在 5 至 17°C 之间变化；Pasta等人，2016 年），绒毛橡树在直射光下生长。之前的研究表明，山毛榉和橡树之间的相互作用可以导致地下水源分区（Zapater等人，2011 年；Grossiord等人，2014a、b、c）和更高的生产力（Jourdan等人，2020 年）) 在树级别，表明它们的交互具有缓解作用。然而，当环境条件变得过于紧张时，也观察到这些物种之间的相互作用对叶级气体交换的不利影响（例如 Didion-Gency等人，2021）。

我们的目标是 (1) 确定变暖和土壤水分减少如何单独或组合影响叶级光合特性（即A _sat、J _max、V c _max、 F _v / F _m和 Chl) 在山毛榉和橡树中，以及 (2) 评估这两个物种之间的相互作用如何影响它们的叶级光合作用特性以变暖和减少土壤水分。如果在特定成分内生长，我们预计 (1) 与环境条件相比，橡树在变暖时具有更高的叶级光合特性，而山毛榉将具有更低的叶级光合特性，因为它对高温的脆弱性更强。土壤水分的减少将降低这两个物种的光合作用特性，但预计山毛榉的减少幅度更大。由于水分胁迫增加，变暖和低土壤水分的结合将加剧在土壤水分减少的情况下观察到的光合作用反应。如果在种间相互作用中生长，我们预计 (2) 变暖会增强橡树的光合作用特性，并与特定种内相互作用相比减轻山毛榉的减少。与种内相互作用相比，树木在种间相互作用（即单独作用或与变暖相结合）中对土壤水分减少的反应受到的影响较小。