INTRODUCTION

The decline of insects in the last 50 years is well documented (Hallmann et al., 2017; Kotze & O'Hara, 2003; Wendorff & Schmitt, 2019), driven at least in part by climate change and loss of habitat (Wagner et al., 2021). Montane insect species in particular are in peril (Dirnbock et al., 2011; Sánchez-Bayo & Wyckhuys, 2019) as temperature variation at higher elevations due to climate change can result in a loss of habitat. For example, studies have shown that contractions of lower elevation ranges may not correspond to an upward shift of higher elevational ranges (Dahlhoff et al., 2019; Moret et al., 2016). Other montane insects may ‘run out of mountain’, that is, there may be no habitable terrain above where they currently exist that is available for colonization (Dieker et al., 2011; Wilson et al., 2005). Montane saproxylic Coleoptera, or beetles that depend in some part of their life cycle on dead or dying wood (sensu Speight, 1989), are of particular interest because they are ecologically important and taxonomically and functionally diverse (Nieto & Alexander, 2010). Saproxylic Coleoptera not only play important roles in nutrient recycling but also include many feeding guilds, including predators, parasites, fungivores, detritivores, myxomycophages (slime mould feeders), wood-consumers and omnivores (Gimmel & Ferro, 2018). Saproxylic Coleoptera are often used as biodiversity indicators for wider forest ecosystem functioning (Burns et al., 2014; Karpiński et al., 2021). Therefore, understanding the spatial dynamics of montane saproxylic Coleoptera communities at the tree-line is fundamental to forecasting the change in biodiversity patterns in a shifting landscape.

Following the trend of many montane flora and fauna (Rahbek, 2005), Coleopteran biodiversity generally decreases with increasing elevation (Corcos et al., 2018; Franc et al., 2007) or displays a hump-shaped distribution along an elevational gradient (Tykarski, 2006). Coleoptera biodiversity can increase with increasing elevation, but this is rare (Dolson et al., 2021). These trends vary among taxonomic and functional groups and across geographical areas and depend on spatial scale and elevational gradient range (Chamberlain et al., 2016; Colwell et al., 2004; McCain, 2009). Numerous variables are known to drive community structure along an elevational gradient, especially temperature, which can delay timing of flight and elongate life cycles of bark beetles and other herbivorous insects (Bale et al., 2002; Reymond et al., 2013). Rising temperatures are expected to shorten generation times of some pest bark beetles, such as Ips typographus (Linnaeus, 1758), at higher elevations (Jakoby et al., 2019). However, little is known about saproxylic Coleoptera community structure at the tree-line, an important ecotone.

In most mountains, the delineation between a forest margin and shrub-only terrain is a matter of scale, as canopies can open gradually or with a sharp transition depending on slope and other environmental factors (Holtmeier & Broll, 2007). The tree-line is generally defined as the point in which the dominant stem of a tree no longer grows above 3 m (Körner, 2012), and in the last 100 years, the tree-line delineation in some mountains has migrated upwards (Harsch et al., 2009). The relationship between the tree-line and climate change is difficult to untangle from other biotic and abiotic variables; soil temperature, local, current and historic land use and abiotic site conditions can play a role in limiting tree growth at a specific elevation (Hofgaard, 1997; Holtmeier & Broll, 2005; Körner, 2012). Research shows that the rise in the tree-line in the Pyrenees mountains is likely influenced more strongly by land abandonment rather than climate change (Ameztegui et al., 2016; Batllori & Gutiérrez, 2008). There is little debate, however, that the eastern Pyrenean tree-line is migrating upwards and that the population of the dominant tree at the Pyrenean tree-line, the Pinus mugo complex (hereafter Pinus mugo), has become denser and less patchy over the last 50 years (Batllori et al., 2010; Batllori & Gutiérrez, 2008), although these two spatial phenomena are driven by different factors (Feuillet et al., 2020).

In this work, we examined taxonomic and functional saproxylic and non-saproxylic Coleoptera community responses to stand and landscape characteristics at the tree-line and 200–300 meters below the tree-line in a forest in the eastern Pyrenees. Other studies have examined saproxylic Coleoptera community responses to stand level characteristics in Mediterranean mountains (Parisi et al., 2020), Scandinavian forests (Brunet & Isacsson, 2009a, 2009b; Gibb et al., 2006), boreal Canadian forests (Saint-Germain et al., 2006) and the Swiss Alps (Schiegg, 2000, 2003); but to our knowledge, this is the first study to examine saproxylic Coleoptera community responses to landscape and stand characteristics specifically at the tree-line. In this study, we expected the following outcomes: (1) abundance and richness of saprotroph Coleoptera (beetles that feed upon dead or decaying organic matter) and saproxylic Coleoptera (beetles that depend on dead or decaying wood at some point in their life cycle) to be closely linked to the volume of dead wood and large trees and (2) forest characteristics that are related to higher levels of sunlight, volume of dead wood and density of larger trees to predict higher taxonomic abundance, taxonomic richness, functional feeding guild abundance and functional feeding guild richness. This study was conducted as part of a larger research programme monitoring climate change in high elevation Andorra (Bookwalter et al., 2023).

中文翻译：

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使用分类学和功能行会方法了解树线的鞘翅目群落

介绍

过去 50 年昆虫数量的减少有据可查（Hallmann 等人，  2017 年；Kotze 和 O'Hara，  2003 年；Wendorff 和 Schmitt，  2019 年），至少部分是由气候变化和栖息地丧失造成的（Wagner 等人）等，  2021）。山地昆虫物种尤其面临危险（Dirnbock et al.,  2011；Sánchez-Bayo & Wyckhuys,  2019），因为气候变化导致高海拔地区的温度变化可能导致栖息地丧失。例如，研究表明，较低海拔范围的收缩可能并不对应于较高海拔范围的向上移动（Dahlhoff 等人，  2019 年；Moret 等人，  2016 年））。其他山地昆虫可能会“跑出山外”，也就是说，它们目前存在的地方上方可能没有可用于定居的宜居地形（Dieker et al., 2011；  Wilson et al.,  2005）。山地腐木鞘翅目，或在其生命周期的某些部分依赖于死亡或垂死木材的甲虫（sensu Speight，  1989）特别令人感兴趣，因为它们在生态上非常重要，并且在分类学和功能上多样化（Nieto＆Alexander，  2010）。土腐鞘翅目不仅在养分循环中发挥重要作用，而且还包括许多摄食群体，包括捕食者、寄生虫、食真菌动物、碎屑动物、粘菌噬菌体（粘菌饲养者）、木材消费者和杂食动物（Gimmel & Ferro、 2018）。腐木鞘翅目经常被用作更广泛的森林生态系统功能的生物多样性指标（Burns 等，  2014；Karpiński 等，  2021）。因此，了解林线处山地腐木鞘翅目群落的空间动态对于预测不断变化的景观中生物多样性模式的变化至关重要。

遵循许多山地动植物群的趋势（Rahbek，  2005），鞘翅目生物多样性通常随着海拔的增加而减少（Corcos等，  2018；Fran等，  2007）或沿海拔梯度呈现驼峰状分布（Tykarski） ，  2006）。鞘翅目生物多样性会随着海拔的升高而增加，但这种情况很少见（Dolson et al.,  2021）。这些趋势因分类和功能组以及不同地理区域而异，并取决于空间尺度和海拔梯度范围（Chamberlain 等，  2016；Colwell 等，  2004；McCain，  2009 ））。已知有许多变量会沿着海拔梯度驱动群落结构，尤其是温度，这可以延迟树皮甲虫和其他食草昆虫的飞行时间并延长生命周期（Bale 等人，2002 年；Reymond 等人，  2013年 ）。气温上升预计会缩短一些害虫树皮甲虫的世代时间，例如高海拔地区的Ipstypographus（Linnaeus，1758）（Jakoby 等，  2019）。然而，人们对重要生态交错带林线处的腐木鞘翅目群落结构知之甚少。

在大多数山区，森林边缘和灌木地形之间的划分是一个规模问题，因为树冠可以逐渐打开或急剧过渡，具体取决于坡度和其他环境因素（Holtmeier & Broll，2007  ）。林线一般定义为树木的主干不再生长超过3 m的点（Körner，  2012），并且在过去的100年中，一些山脉的林线轮廓已经向上迁移（Harsch）等人，  2009）。林木线与气候变化之间的关系很难与其他生物和非生物变量分开。土壤温度、当地、当前和历史的土地利用以及非生物场地条件可能会限制特定海拔的树木生长（Hofgaard， 1997 年；霍尔特迈尔和布罗尔，  2005；科尔纳，  2012）。研究表明，比利牛斯山脉林木线的上升可能更强烈地受到土地废弃的影响，而不是气候变化的影响（Ameztegui 等人，  2016 年；Batllori 和 Gutiérrez，  2008 年）。然而，几乎没有争议的是，东比利牛斯山脉的林木线正在向上迁移，并且比利牛斯山脉林木线的主要树木——黑松松复合体（以下简称黑松）的种群数量在整个比利牛斯山林线线上变得更加密集和不那么分散。过去 50 年（Batllori 等人，  2010 年；Batllori 和 Gutiérrez，  2008 年）），尽管这两种空间现象是由不同因素驱动的（Feuillet et al.,  2020）。

在这项工作中，我们研究了东比利牛斯山脉森林中树线和树线以下 200-300 米处的分类学和功能性腐木和非腐木鞘翅目群落对林分和景观特征的反应。其他研究还考察了地中海山脉（Parisi 等，  2020）、斯堪的纳维亚森林（Brunet & Isacsson，  2009a，2009b；Gibb 等，  2006）、加拿大北方森林（Saint-Germain）中腐木鞘翅目群落对林分水平特征的反应。等，  2006 ）和瑞士阿尔卑斯山（Schiegg，  2000，2003））；但据我们所知，这是第一项研究腐木鞘翅目群落对景观和林分特征（特别是林线）的反应的研究。在这项研究中，我们预计会出现以下结果：（1）腐生鞘翅目（以死亡或腐烂的有机物为食的甲虫）和腐木鞘翅目（依赖于死亡或腐烂的有机物的甲虫）的丰度和丰富度(2) 与较高水平的阳光、死木体积和较大树木的密度相关的森林特征预测更高的分类丰度、分类丰富度、功能性摄食行会丰度和功能性摄食行会丰富度。这项研究是监测安道尔高海拔地区气候变化的大型研究计划的一部分（Bookwalter 等，  2023）。