Journal of Ecology ( IF 5.3 ) Pub Date : 2024-05-04 , DOI: 10.1111/1365-2745.14320 Kirsten Krüger 1 , Cornelius Senf 2 , Tommaso Jucker 3 , Dirk Pflugmacher 4 , Rupert Seidl 1, 5
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1 INTRODUCTION
Gaps are the fingerprint of disturbances in forest canopies (Jucker, 2022). Disturbances are ‘any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment’ (Pickett & White, 1985). They range from the death of individual trees to large but rare mortality events, such as wind-throws, insect outbreaks or wildfires, and play a crucial role in the dynamics of forest ecosystems. Disturbances, for instance, alter forest structure and species composition for decades to centuries, depending on disturbance magnitude, frequency and severity, and modulate key ecosystem fluxes such as carbon uptake and nutrient cycling (Maltamo et al., 2014; Muscolo et al., 2014). As forest disturbance regimes are changing in response to climate change (Seidl et al., 2017), a better quantitative understanding of disturbances and the gaps they create in forest canopies is needed.
Canopy gaps span a wide range of scales across at least four orders of magnitude (from approximately 102 to 106 m2). Also, gap characteristics vary with biome and forest type, as different biomes are affected by different disturbance agents (Jentsch & von Heßberg, 2022) and different forest types are more or less prone to various types of disturbance (Griffiths et al., 2014; Seidl et al., 2020; Sommerfeld et al., 2018). In the forests of Central Europe, for example, large-scale stand-replacing disturbances are usually found within conifer-dominated forests (Potterf et al., 2023; Sommerfeld et al., 2018), while small canopy gaps typically occur in broadleaved forests (Hobi et al., 2015), with mixed forests experiencing the full spectrum of gap sizes (Frankovič et al., 2021; Nagel et al., 2021). The gaps observed in a forest canopy at any given point in time are thus the cumulative effect of all disturbance agents acting within a landscape. They are furthermore modulated by forest type-related traits, for example tree species diversity (Jactel et al., 2017) and land-use legacies (Munteanu et al., 2015; Thom et al., 2018). The variability in canopy gaps (resulting from the diverse forces that create them and the manifold responses to them) is a crucial factor creating beta diversity in forest ecosystems (Mori et al., 2018; Senf et al., 2020). However, many studies of forest mortality focus on either one end of the spectrum of gaps (e.g. either on small-scale mortality or landscape-scale disturbances) and rarely investigate canopy gap dynamics across the continuum of interactive processes that create them (Fisher et al., 2008; Hobi et al., 2015; Hurtt et al., 2016).
Forest canopy gaps are not static but change dynamically over time. A number of previous studies have analysed gap structure at a single point in time, creating important insights into structural patterns created by various gap-forming processes (Asner et al., 2013; Goodbody et al., 2020; Goulamoussène et al., 2017; Reis et al., 2022; Vehmas et al., 2011; Vepakomma et al., 2011). These studies showed that gap characteristics vary considerably among boreal and tropical forests (Goodbody et al., 2020; Goulamoussène et al., 2017). The variation in canopy openings is typically lower within forest biomes (Asner et al., 2013) than across them. Differences in gap characteristics within forest biomes are often driven by variation in soil fertility and human-induced disturbances (Reis et al., 2022). However, what remains understudied is how canopy gaps change over time. For instance, do newly formed gaps go through a phase of expansion before they eventually close? And if so, what is the relative importance of new gap formation versus gap expansion in shaping canopy dynamics? In this regard it is important to note that gap dynamics reflect the interactions inherent in forest disturbance regimes (Buma, 2015; Canelles et al., 2021). For example, wind-throw creates edges that are more susceptible to subsequent wind disturbance (Ruck et al., 2012; Zeng et al., 2009). Moreover, forest edges receive more direct radiation, leading to warmer microclimate which, can further promote disturbances (e.g. by bark beetles, Kautz et al., 2013). Understanding and quantifying the dynamic nature of gaps is important because the transient edges that they create are important habitat features (Reiner et al., 2023) and modulate manifold ecosystem processes (Pöpperl & Seidl, 2021).
Besides gap formation, gap closure is an equally important process for understanding the long-term impact of disturbances on forest ecosystems (Turner et al., 1993). The main mode of gap closure is by tree regeneration within gaps. Gaps in the forest canopy increase the resource availability on the forest floor (i.e. light, water, nutrients), which facilitates the establishment of a new cohort of trees and enhances the growth of regeneration already present on the forest floor before canopy opening (Muscolo et al., 2014; Thom et al., 2023; Zhu et al., 2014). However, lateral crown expansion of trees growing at the edge can also contribute to gap closure, either partially (in larger gaps) or completely (in small gaps) (Leitold et al., 2022; Vepakomma et al., 2011). To date, the overwhelming majority of studies on gap dynamics have focused on the process of gap formation (e.g. Asner et al., 2013; Dalagnol et al., 2019; Goodbody et al., 2020; Goulamoussène et al., 2017; Hobi et al., 2015; Koukoulas & Blackburn, 2004; Reis et al., 2022; Vehmas et al., 2011; Vepakomma et al., 2010), while studies on gap closure and different closure mechanisms remain more scarce (e.g. Blackburn et al., 2014; Coates, 2000; Fujita et al., 2003; Gorgens et al., 2023; Hunter et al., 2015; Leitold et al., 2022; Rodes-Blanco et al., 2023; Vepakomma et al., 2011). Consequently, the contribution of different mechanisms of gap closure (i.e. lateral crown expansion versus regenerating trees) and the modulating role of gap size remain poorly understood. On the one hand, we might expect small gaps to close faster than large gaps due to higher seed input (Masaki et al., 2019) and favourable microclimate (Thom et al., 2023). On the other hand, larger gaps favour the establishment of early-seral species, which are at a disadvantage in small gaps because of limited light availability, but have faster height growth (Muscolo et al., 2014). Early-seral species are thus generally able to close gaps faster than shade-tolerant, late-seral species, but require more light and thus larger gaps. Further, the influence of tree species diversity on gap closure rates remains uncertain. For mature trees, there is evidence for a positive relationship between species diversity and productivity in general (Madrigal-González et al., 2016; Williams et al., 2017) and for a positive effect of niche complementarity on tree growth in particular has been reported (Jucker et al., 2015). How these effects translate to the regeneration stage of forest development, however, remains widely unclear (Grossman et al., 2017; Lang et al., 2012).
The net outcome of gap formation and closure is an important indicator of forest change, yet remains poorly quantified. At the landscape scale, the relationship between these two rates determines whether a forest remains ecologically resilient (Senf & Seidl, 2022)—defined here as having higher rates of gap closure than gap formation. Conversely, higher rates of gap formation than closure are an indicator of ongoing structural changes in forest canopies, and can precede a possible reorganization trajectory towards alternative states (Seidl & Turner, 2022). As both disturbance and recovery are influenced by ongoing global change, studying their net outcomes can serve as an important means to monitor ongoing changes in forest canopies.
Recent developments in the field of remote sensing have transformed the spatial scale and temporal resolution at which we can study gap dynamics in forest ecosystems (Senf, 2022). Moderate-resolution optical sensors like Landsat provide insights into large-scale forest disturbance patterns (Griffiths et al., 2014; Masaki et al., 2019; Senf & Seidl, 2021), and high resolution data from aerial images and high resolution satellites allow the characterization of gap patterns on a finer scale (Fujita et al., 2003; Henbo et al., 2006). However, fine-grained analyses distinguishing individual processes of gap dynamics, such as lateral crown expansion, remain beyond the reach of current satellite sensors, as they cannot capture vertical forest dynamics well. Airborne lidar (light detection and ranging) has expanded the capacity to identify gaps beyond satellite data (Dalagnol et al., 2019; Goodbody et al., 2020), and study tree recovery at fine spatial grain. The main advantage of lidar is the use of an active sensor which provides information on the 3D structure of forest vegetation (Leitold et al., 2022; Senf & Seidl, 2022; Stritih et al., 2023; Vepakomma et al., 2011). Moreover, repeated lidar acquisitions enable researchers to track gap development over time, allowing for a fine-grained analysis of gap formation and closure (Jucker, 2022; Maltamo et al., 2014). The increasing availability of repeated lidar acquisitions and a rapidly developing processing infrastructure (Roussel et al., 2020; Silva et al., 2019) make multi-temporal lidar a powerful tool for understanding forest canopy dynamics (Blackburn et al., 2014; Dalagnol et al., 2019; Gorgens et al., 2023; Leitold et al., 2022; Rodes-Blanco et al., 2023; Vepakomma et al., 2012).
Our aim was to quantify canopy gap formation and closure in an unmanaged temperate mountain forest landscape—allowing us to focus on natural forest dynamics in the absence of human interventions—in the Northern Front Range of the European Alps. Our specific objectives were (i) to understand the rates and modes of canopy gap formation by contrasting the formation of new gaps with the expansion of existing ones, (ii) to study the rates and modes of gap closure, considering closure from tree regeneration as well as from lateral crown expansion, and (iii) assess drivers of gap closure, including elevation, forest type and aspect. We hypothesized that (H1) gap expansion is more important than the formation of new gaps due to prominent disturbance interactions in the disturbance regime of our study system, such as bark beetle spread after wind disturbance (Seidl & Rammer, 2017; Senf, Pflugmacher, et al., 2017). We furthermore expected that (H2) small gaps close faster than large ones, and that gaps in mixed forests of coniferous and broadleaved species close faster than in forests of either conifers or broadleaved species due to niche complementarity (Jucker et al., 2015). We also hypothesized that (H3) vertical ingrowth of the regenerating cohort is the predominant process of gap closure (Winter et al., 2015), while lateral crown expansion plays a substantial role only in broadleaved forests, as these have higher crown plasticity (Schröter et al., 2012). Lastly, we contrasted gap formation and closure rates at the landscape scale, hypothesizing that (H4) gaps are created faster than they close, based on the observation of strong increases in tree mortality in Europe in recent years (Patacca et al., 2022; Senf & Seidl, 2021), and the broader expectation of the emergence of more open forests under climate change (McDowell et al., 2020).
中文翻译：
间隙扩张是温带山地森林景观中树冠开口的主要驱动因素
1 简介
间隙是森林冠层干扰的指纹（Jucker，2022）。干扰是“任何时间上相对离散的事件，会破坏生态系统、群落或人口结构，并改变资源、基质可用性或物理环境”（Pickett & White，1985）。它们的范围从单棵树木的死亡到大规模但罕见的死亡事件，如风吹、昆虫爆发或野火，在森林生态系统的动态中发挥着至关重要的作用。例如，根据干扰强度、频率和严重程度，干扰会改变森林结构和物种组成数十年至数百年，并调节碳吸收和养分循环等关键生态系统通量（Maltamo 等，2014；Muscolo 等， 2014）。由于森林干扰机制正在因气候变化而发生变化（Seidl 等，2017），因此需要更好地定量了解干扰及其在森林冠层中造成的间隙。
冠层间隙的尺度范围很广，至少有四个数量级（从大约 10 2 到 10 6 m 2 ）。此外，间隙特征因生物群落和森林类型而异，因为不同的生物群落受到不同干扰因素的影响（Jentsch＆von Heßberg，2022）并且不同的森林类型或多或少容易受到各种类型的干扰（Griffiths et al.，2014； Seidl 等人，2020；Sommerfeld 等人，2018）。例如，在中欧森林中，大规模的林分更替干扰通常发生在针叶树为主的森林中（Potterf 等，2023；Sommerfeld 等，2018），而小树冠间隙通常发生在阔叶林中（Hobi 等人，2015），混交林经历了各种间隙大小（Frankovič 等人，2021；Nagel 等人，2021）。因此，在任何给定时间点在森林冠层中观察到的间隙是景观中所有干扰因素的累积效应。此外，它们还受到森林类型相关特征的调节，例如树种多样性（Jactel 等，2017）和土地利用遗产（Munteanu 等，2015；Thom 等，2018）。冠层间隙的变异性（由造成冠层间隙的多种力量以及对它们的多种反应造成）是在森林生态系统中产生β多样性的关键因素（Mori等人，2018年；Senf等人，2020年）。然而，许多关于森林死亡率的研究都集中在差距范围的一端（例如，小规模死亡率或景观规模干扰），而很少研究在创造它们的相互作用过程的连续过程中的冠层间隙动态（Fisher等人） .，2008；Hobi 等人，2015；Hurtt 等人，2016）。
森林冠层间隙不是静态的，而是随时间动态变化的。之前的许多研究分析了单个时间点的间隙结构，对各种间隙形成过程产生的结构模式产生了重要的见解（Asner等人，2013年；Goodbody等人，2020年；Goulamoussène等人，2017年） ；Reis 等人，2022；Vepakomma 等人，2011）。这些研究表明，北方森林和热带森林之间的间隙特征差异很大（Goodbody 等人，2020 年；Goulamoussène 等人，2017 年）。森林生物群落内树冠开口的变化通常低于森林生物群落之间的变化（Asner et al., 2013）。森林生物群落内间隙特征的差异通常是由土壤肥力的变化和人为干扰引起的（Reis等人，2022）。然而，尚未得到充分研究的是树冠间隙如何随时间变化。例如，新形成的缺口在最终缩小之前是否会经历一个扩张阶段？如果是这样，新的间隙形成与间隙扩大在塑造树冠动态方面的相对重要性是什么？在这方面，值得注意的是，差距动态反映了森林干扰机制固有的相互作用（Buma，2015；Canelles 等，2021）。例如，风抛产生的边缘更容易受到随后的风扰动（Ruck 等人，2012 年；Zeng 等人，2009 年）。此外，森林边缘接受更多的直接辐射，导致小气候变暖，从而进一步促进干扰（例如树皮甲虫，Kautz et al., 2013）。理解和量化间隙的动态性质非常重要，因为它们产生的瞬态边缘是重要的栖息地特征（Reiner等人，2023）并调节多种生态系统过程（Pöpperl＆Seidl，2021）。
除了间隙形成之外，间隙闭合对于理解干扰对森林生态系统的长期影响同样重要（Turner 等，1993）。间隙闭合的主要方式是通过间隙内的树木再生。森林冠层的间隙增加了森林地面上的资源可用性（即光、水、养分），这有助于建立新的树木群，并增强树冠开放之前森林地面上已经存在的再生的生长（Muscolo 等）等人，2014；Thom 等人，2023；Zhu 等人，2014）。然而，生长在边缘的树木的侧向树冠扩张也可能有助于间隙闭合，无论是部分（在较大间隙中）还是完全（在小间隙中）（Leitold 等人，2022 年；Vepakomma 等人，2011 年）。迄今为止，绝大多数关于间隙动力学的研究都集中在间隙形成的过程上（例如Asner et al., 2013; Dalagnol et al., 2019; Goodbody et al., 2020; Goulamoussène et al., 2017; Hobi等人，2015；Reis 等人，2022；Vepakomma 等人，2010），而关于间隙闭合和不同闭合机制的研究仍然较少（例如 Blackburn 等人） Coates 等人，2000；Gorgens 等人，2023；Leitold 等人，2023； ，2011）。因此，不同间隙闭合机制（即树冠横向扩张与再生树木）的贡献以及间隙大小的调节作用仍然知之甚少。一方面，由于较高的种子投入（Masaki 等人，2019）和有利的小气候（Thom 等人，2023），我们可能预计小差距会比大差距更快地缩小。 另一方面，较大的间隙有利于早期系列物种的建立，由于光照有限，这些物种在小间隙中处于劣势，但高度生长较快（Muscolo et al., 2014）。因此，早期系列物种通常能够比耐荫的晚期系列物种更快地关闭间隙，但需要更多的光照，因此需要更大的间隙。此外，树种多样性对差距闭合率的影响仍然不确定。对于成熟树木，有证据表明物种多样性与生产力之间存在正相关关系（Madrigal-González 等，2016；Williams 等，2017），并且生态位互补性对树木生长的积极影响尤其明显。报道（Jucker 等，2015）。然而，这些影响如何转化为森林发展的再生阶段仍不清楚（Grossman 等，2017；Lang 等，2012）。
差距形成和缩小的净结果是森林变化的重要指标，但仍缺乏量化。在景观尺度上，这两个比率之间的关系决定了森林是否保持生态弹性（Senf＆Seidl，2022）——此处定义为间隙闭合率高于间隙形成率。相反，间隙形成率高于闭合率是森林冠层持续结构变化的指标，并且可能先于可能的向替代状态的重组轨迹（Seidl＆Turner，2022）。由于干扰和恢复都受到持续的全球变化的影响，研究其净结果可以作为监测森林冠层持续变化的重要手段。
遥感领域的最新发展改变了我们研究森林生态系统间隙动态的空间尺度和时间分辨率（Senf，2022）。 Landsat 等中等分辨率光学传感器可深入了解大规模森林扰动模式（Griffiths 等人，2014 年；Masaki 等人，2019 年；Senf 和 Seidl，2021 年），而来自航空图像和高分辨率卫星的高分辨率数据允许更精细尺度上间隙模式的表征（Fujita 等人，2003 年；Henbo 等人，2006 年）。然而，区分间隙动态的各个过程（例如横向树冠扩张）的细粒度分析仍然超出了当前卫星传感器的能力范围，因为它们无法很好地捕获垂直森林动态。机载激光雷达（光探测和测距）扩大了识别卫星数据之外的差距的能力（Dalagnol 等人，2019；Goodbody 等人，2020），并研究精细空间颗粒的树木恢复。激光雷达的主要优点是使用有源传感器，可提供有关森林植被 3D 结构的信息（Leitold 等人，2022 年；Senf 和 Seidl，2022 年；Stritih 等人，2023 年；Vepakomma 等人，2011 年） 。此外，重复的激光雷达采集使研究人员能够跟踪间隙随时间的发展，从而对间隙的形成和闭合进行细粒度分析（Jucker，2022；Maltamo 等人，2014）。重复获取激光雷达的可用性不断增加，处理基础设施快速发展（Roussel et al., 2020；Silva et al., 2019），使多时相激光雷达成为了解森林冠层动态的强大工具（Blackburn et al., 2014；Dalagnol）等人，2019；Gorgens 等人，2023；Leitold 等人，2022；Vepakomma 等人，2012）。
我们的目标是量化欧洲阿尔卑斯山北前山脉未管理的温带山地森林景观中树冠间隙的形成和闭合，使我们能够在没有人类干预的情况下关注自然森林动态。我们的具体目标是（i）通过对比新间隙的形成与现有间隙的扩大来了解冠层间隙形成的速率和模式，（ii）研究间隙闭合的速率和模式，将树木再生的闭合视为(iii) 评估间隙闭合的驱动因素，包括海拔、森林类型和坡向。我们假设（H1）间隙扩张比新间隙的形成更重要，因为我们研究系统的扰动范围中存在显着的扰动相互作用，例如风扰后树皮甲虫的传播（Seidl＆Rammer，2017；Senf，Pflugmacher，等人，2017）。我们进一步预计 (H2) 小差距比大差距缩小得更快，并且由于生态位互补性，针叶树和阔叶树种混交林中的差距比针叶树或阔叶树种森林中的差距缩小得更快（Jucker 等，2015）。我们还假设 (H3) 再生群的垂直向内生长是间隙闭合的主要过程 (Winter et al., 2015)，而横向树冠扩张仅在阔叶林中发挥重要作用，因为这些林具有较高的树冠可塑性 (Schröter)等人，2012）。最后，我们对比了景观尺度上的间隙形成和闭合率，根据近年来欧洲树木死亡率急剧上升的观察，假设（H4）间隙的形成速度快于其闭合速度（Patacca 等人，2017）。, 2022; Senf & Seidl，2021），以及气候变化下出现更多开放森林的更广泛预期（McDowell 等，2020）。
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