Scale-dependent interactions in coastal biogeomorphic landscapes: pioneer both inhibits and facilitates primary foredune builder across spatial scales

Ecosystem engineers often drive landscape formation and vegetation succession by modifying environmental conditions. Along the Northwestern European coast, dune formation is classically believed to be pioneered by sand couch (Elytrigia juncea), followed by the primary foredune builder marram grass (Calamagrostis arenaria, formerly Ammophila arenaria) once sufficiently large dunes have formed. However, these ideas lack experimental validation, leaving the specific relationship between sand couch, the landscape they form, and marram grass establishment unknown. Here, we experimentally investigated this relationship by planting 975 young marram grass plants in two sand couch dominated, embryonic dune systems. Using structural equation models, we examined the effect of sand couch and the landscape attributes (i.e., elevation, distance to sea and elevation change) on marram grass establishment. Results reveal indirect local inhibition by sand couch via landscape modifications on survival of establishing marram grass. Specifically, sand couch elevated the environment, and elevated areas in turn eroded more in winter, which was the key factor reducing marram grass survival. At the landscape scale, however, we observed the highest survival in relatively stable and sheltered microsites formed because of sand couch dune building. While the indirect inhibition by sand couch through elevation was found at both locations, the direct effects of sand couch on survival (i.e., neutral in exposed and positive in sheltered conditions) and shoot formation (i.e., negative in exposed and neutral in sheltered conditions) were location and thus context dependent. Similar to scale-dependent interactions found in other biogeomorphic landscapes (e.g., salt marshes, seagrass, mussel beds), we suggest that interspecific facilitation acts on larger spatial scales rather than the commonly suggested small-scale facilitation through created elevation. More specifically, we suggest that dune formation by pioneer species leads to the creation of stable and sheltered microsites beneficial for establishment of later successional species at the landscape-scale.

生态系统工程师（Ecosystem engineers）通常通过改变环境条件，驱动景观形成与植被演替。在欧洲西北海岸，沙丘形成的经典认知认为，先驱物种为沙生蔗草（Elytrigia juncea）；待沙丘初具规模后，由作为前丘主要建造者的欧洲沙芦草（Calamagrostis arenaria，原学名Ammophila arenaria）接替占据主导。然而，这些观点缺乏实验验证，导致沙生蔗草、其塑造的景观与欧洲沙芦草定植之间的具体关联尚不明确。本研究通过在两处以沙生蔗草为主的胚胎沙丘系统中，种植975株欧洲沙芦草幼苗，实验探究了这一关联。我们采用结构方程模型（structural equation models），分析了沙生蔗草及景观属性（即海拔、距海距离与海拔变化）对欧洲沙芦草定植的影响。结果显示，沙生蔗草通过改变景观特征，对定植中的欧洲沙芦草产生了间接的局部抑制作用：具体而言，沙生蔗草抬升了局部环境的海拔，而高海拔区域在冬季更易遭受侵蚀，这正是降低欧洲沙芦草存活率的关键因素。但在景观尺度上，我们观察到在沙生蔗草塑造沙丘所形成的相对稳定且受遮蔽的微生境中，欧洲沙芦草的存活率最高。尽管两处研究位点均发现沙生蔗草通过海拔变化产生的间接抑制作用，但沙生蔗草对存活率的直接影响（暴露环境下为中性，遮蔽环境下为正向）以及对分株形成的影响（暴露环境下为负向，遮蔽环境下为中性）则因位点而异，即存在情境依赖性。与其他生物地貌景观（如盐沼、海草床、贻贝床）中发现的尺度依赖型相互作用类似，我们认为种间促进作用更多发生在较大的空间尺度上，而非以往普遍认为的通过抬升海拔实现的小型促进作用。更具体地说，我们提出：先驱物种塑造的沙丘会在景观尺度上创造出稳定且受遮蔽的微生境，这有利于后续演替物种的定植。