1 INTRODUCTION

River regulation threatens a wide range of river-dependent biota, including riparian plants (Poff et al., 1997). This is because riverine biota are adapted to natural flow regimes (Bunn & Arthington, 2002) and regulation alters flow, resource, and disturbance regimes (Nilsson & Svedmark, 2002) resulting in changes to physical, chemical, and biological processes (Poff & Zimmerman, 2010). While significant investment into research and management of river flows in recent years has greatly improved ecosystem outcomes for many taxa (e.g., Tonkin et al., 2020), there are many key knowledge gaps that limit recovery efforts (Horne et al., 2017).

Riparian vegetation (species associated with a waterway but excluding aquatic species) plays a critical role in connecting aquatic and terrestrial habitats, supports high biodiversity, and provides important ecosystem services (Nilsson & Svedmark, 2002). Riparian vegetation is greatly influenced by river flow due to direct effects on plant growth, survival, seed dispersal, germination, recruitment, and morphology, as well as indirect effects on environmental factors, such as geomorphology and nutrient dynamics, which contribute to plant habitat (Casanova & Brock, 2000; Catford & Jansson, 2014; Vesipa et al., 2017). The effects of flow or inundation in terms of duration, depth, timing, variability, and frequency on plants varies among species and contexts (Gattringer et al., 2018; Poff et al., 1997; Webb et al., 2012) and understanding of the effects of seasonal flow timing on herbaceous riparian plants is limited (Greet et al., 2011; Van Eck et al., 2006).

Plant life-history strategies and physiological traits (e.g., adaptations to inundation, dispersal mechanisms, phenology, and growth form) play an important role in determining where plants occur in relation to flooding gradients (Catford & Jansson, 2014; Merritt et al., 2010) and species vary in the strategies they use to survive and recover from flooding (Blom & Voesenek, 1996). Plant adaptations to inundation include those that enable photosynthesis underwater and tolerance to or avoidance of anoxia (Catford & Jansson, 2014). Tolerance to inundation tends to be lower in species adapted to drier habitats (terrestrial species) and higher in species adapted to wetter habitats (amphibious or aquatic species). This degree of tolerance determines plant species’ distribution in riparian areas, because plants tend to be inundated by different depths and durations in relation to their position on the river bank or floodplain (Johansson & Nilsson, 2002; Van Eck et al., 2004).

In temperate climates with cool-season (winter/spring) dominated rainfall, regulation often reduces natural flow peaks via reservoir capture, and increases warm-season (summer/autumn) flows for consumptive uses. This results in a flattening of seasonal flow patterns, with reduced cool-season flows and unnaturally high warm-season flows (Braatne et al., 2007; Greet et al., 2013a; Humphries & Lake, 1996). Warm-season consumptive flows for irrigation often involve large transfers of water over weeks to months (e.g., January to March, Cottingham et al., 2010). Environmental flows tend to involve smaller volumes of water released over days to weeks, aimed at reinstating natural flow regime components (e.g., cool-season flows or summer freshes), to counteract some of the negative impacts of regulation on stream biota, including those on riparian vegetation (Merritt et al., 2010; Miller et al., 2013; Nagler et al., 2020). As such, managers of both consumptive and environmental flows need to understand the effects of elevated warm-season flows.

There is evidence that this seasonal flow flattening and extended warm-season flows negatively impact vegetation in riparian zones by decreasing plant vigour (growth and survival) and recruitment (Greet et al., 2013b; Vivian et al., 2020). Cool-season flows that correspond with natural flow regimes appear to have fewer negative impacts on plant health (Kitanovic, 2019; Van Eck et al., 2006). Despite recognition that the impacts of warm-season flows are more detrimental than cool-season flows for plants in riparian zones, the underlying mechanisms remain unclear. It is therefore important to identify drivers of this seasonal effect to inform flow management so that environmental and consumptive flows are delivered in a way that minimises damage to plants in riparian zones.

Altered seasonal flow patterns can be expected to adversely affect plant species with phenologies adapted to natural flow regimes, e.g. species with seed release timing adapted to natural seasonal flow peaks (Kehr et al., 2014; Perry et al., 2020). The effects of water attributes that vary with season, such as temperature and oxygen availability (higher oxygen content in colder water) are less certain (Van Eck et al., 2005). While few published studies have experimentally isolated the mechanisms by which seasonal flow timing affects herbaceous plants, water temperature has been identified as potentially important (Van Eck et al., 2005). Plant responses to inundation water temperature can also be expected to vary between species, with less flood-tolerant species potentially more sensitive to warm water inundation (Van Eck et al., 2006). Flood-tolerant species may be able to draw on carbohydrate reserves to support increased respiration rates at higher temperatures of inundation (Van Eck at al., 2005). However, even river-associated biota, including riparian plants, have evolved in the context of natural water temperature patterns and the bioenergetic function of plants can be negatively impacted by extreme warm temperatures (Olden & Naiman, 2010). Further experimental research on the role of water temperature is needed to help understand the mechanisms behind negative effects of warm-season inundation on herbaceous plants (Vivian et al., 2020).

We aimed to improve our mechanistic understanding of herbaceous plant responses to inundation in different seasons by focussing on water temperature effects to inform flow management in temperate, regulated river systems. We tested the effects of inundation duration and inundation temperature on the growth and survival of eight herbaceous plant species common to riparian zones in south-eastern Australia. Inundation durations typical of environmental and consumptive flows and both cool and warm water were used to reflect potential cool-season and warm-season flow scenarios. We expected species to differ in their tolerance to inundation, as indicated by their survival and growth responses. We also expected warmer water to exacerbate the negative effects of inundation on survival and growth and that this would vary with species' inundation tolerance (i.e., be more pronounced in less tolerant species).

中文翻译：

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水温升高加剧了洪水对河岸草本植物的负面影响

1 简介

河流管制威胁到范围广泛的依赖河流的生物群，包括河岸植物（Poff 等，  1997 年）。这是因为河流生物群适应了自然流态 (Bunn & Arthington,  2002 )，而调控改变了水流、资源和干扰机制 (Nilsson & Svedmark,  2002 )，导致物理、化学和生物过程发生变化 (Poff & Zimmerman ,  2010 年）。尽管近年来对河流流量研究和管理的大量投资极大地改善了许多分类群的生态系统结果（例如，Tonkin 等人，  2020 年），但仍有许多关键的知识差距限制了恢复工作（Horne 等人，  2017 年） .

河岸植被（与水道相关的物种，但不包括水生物种）在连接水生和陆地生境、支持高度生物多样性和提供重要的生态系统服务方面发挥着关键作用（Nilsson & Svedmark,  2002）。由于对植物生长、存活、种子传播、发芽、补充和形态的直接影响，以及对环境因素的间接影响，例如地貌和养分动态，河岸植被受到河流流量的极大影响，这些环境因素有助于植物栖息地。 Casanova 和 Brock，  2000 年；Catford 和 Jansson，  2014 年；Vesipa 等人，  2017 年）。水流或淹没在持续时间、深度、时间、变异性和频率方面对植物的影响因物种和环境而异（Gattringer 等人，  2018 年；Poff 等人，  1997 年；Webb 等人，  2012 年）和理解季节性流量时间对草本河岸植物的影响是有限的（Greet 等人，  2011 年；Van Eck 等人，  2006 年）。

植物生命史策略和生理特征（例如，对淹没的适应、扩散机制、物候学和生长形式）在确定植物与洪水梯度相关的位置方面发挥着重要作用（Catford & Jansson,  2014 ; Merritt et al.,  2010 年）和物种在洪水中生存和恢复的策略各不相同（Blom 和 Voesenek，  1996 年）。植物对淹没的适应包括那些能够在水下进行光合作用和耐受或避免缺氧的植物（Catford & Jansson，  2014）。适应较干燥生境的物种（陆生物种）对洪水的耐受性往往较低，而适应较湿生境的物种（两栖或水生物种）对洪水的耐受性往往较高。这种耐受程度决定了植物物种在河岸地区的分布，因为相对于它们在河岸或洪泛区的位置，植物往往被不同的深度和持续时间淹没（Johansson & Nilsson,  2002 ; Van Eck et al.,  2004） .

在凉爽季节（冬季/春季）以降雨为主的温带气候中，调节通常通过水库捕获来减少自然流量峰值，并增加用于消费用途的暖季（夏季/秋季）流量。这导致季节性流量模式变平，冷季流量减少，暖季流量异常高（Braatne 等人，  2007；Greet 等人，  2013a；Humphries & Lake，  1996）。用于灌溉的暖季消耗流量通常涉及数周至数月的大量水转移（例如，1 月至 3 月，Cottingham 等人，  2010）。环境流量往往涉及几天到几周内释放的少量水，旨在恢复自然流态成分（例如，冷季流量或夏季新鲜水），以抵消监管对河流生物群的一些负面影响，包括那些河岸植被（Merritt 等人，  2010 年；Miller 等人，  2013 年；Nagler 等人，  2020 年）。因此，消耗和环境流量的管理者需要了解暖季流量增加的影响。

有证据表明，这种季节性流量变平和暖季流量延长通过降低植物活力（生长和存活）和补充（Greet 等人，  2013b；Vivian 等人，  2020）对河岸地区的植被产生负面影响。与自然流态相对应的冷季流似乎对植物健康的负面影响较小（Kitanovic，  2019；Van Eck 等，  2006）。尽管认识到暖季水流对河岸地区植物的影响比冷季水流更有害，但其潜在机制仍不清楚。因此，重要的是要确定这种季节性影响的驱动因素，以便为流量管理提供信息，以便以最大限度地减少对河岸地区植物的损害的方式提供环境和消费流量。

预计改变的季节性流动模式会对物候适应自然流动状态的植物物种产生不利影响，例如种子释放时间适应自然季节性流动峰值的物种（Kehr 等人，  2014 年；Perry 等人，  2020 年）。随季节变化的水属性的影响，例如温度和氧气可用性（较冷水中的氧气含量较高）不太确定（Van Eck 等人，  2005 年）。虽然很少有已发表的研究通过实验分离出季节性流动时间影响草本植物的机制，但水温已被确定为具有潜在的重要意义（Van Eck 等，  2005）。预计植物对淹没水温的反应也会因物种而异，耐洪性较低的物种可能对温水淹没更敏感（Van Eck 等人，  2006 年）。耐洪物种可能能够利用碳水化合物储备来支持在更高的淹没温度下增加呼吸速率（Van Eck 等人，  2005 年）。然而，即使是与河流相关的生物群，包括河岸植物，也在自然水温模式的背景下进化，植物的生物能量功能可能会受到极端温暖温度的负面影响（Olden & Naiman，  2010）。需要对水温的作用进行进一步的实验研究，以帮助了解暖季淹没对草本植物产生负面影响的机制（Vivian et al.,  2020）。

我们旨在通过关注水温效应为温带、受管制河流系统的流量管理提供信息，从而提高我们对草本植物在不同季节对洪水的反应的机械理解。我们测试了淹没持续时间和淹没温度对澳大利亚东南部河岸带常见的八种草本植物生长和存活的影响。典型的环境和消耗流量以及冷水和温水的淹没持续时间被用来反映潜在的冷季和暖季流量情景。正如它们的生存和生长反应所表明的那样，我们预计物种对淹没的耐受性会有所不同。我们还预计温暖的水会加剧淹没对生存和生长的负面影响，这会因物种而异