Journal of Ecology ( IF 5.3 ) Pub Date : 2024-07-25 , DOI: 10.1111/1365-2745.14379 Jose Antonio Navarro‐Cano 1 , Marta Goberna 1 , Eduardo Pérez‐Valera 2 , Miguel Verdú 3
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1 INTRODUCTION
Fire is a key factor explaining plant diversity in many terrestrial biodiversity hotspots (Bond et al., 2005; Kelly et al., 2020). It strongly modifies plant community composition and structure, thus affecting ecosystem functioning (Keeley & Keeley, 1981; Pausas et al., 1999; Pérez-Valera et al., 2018). Fire-prone Mediterranean plant communities have a long history of evolutionary adaptation to fire (Pausas & Keeley, 2014). Their post-fire regeneration of in different ecosystems (woodlands, shrublands, grasslands) has been widely studied from a classical taxonomic perspective (see e.g. Keeley & Keeley, 1981; Pausas et al., 1999). Currently, trait-based approaches try to infer functional community patterns and link them to key ecological processes behind community assembly and ecosystem stability (Anacker et al., 2011; Clarke et al., 2015; Fyllas et al., 2020; Pausas & Verdú, 2008; Tsakalos et al., 2019). From these studies, we know that post-fire community reassembly is usually driven by shifts in the relative abundance of species rather than their total substitution (Fournier et al., 2020). Though fire induces a short-term decrease of plant community cover and biomass, taxonomic diversity tends to remain unaffected (González-De Vega et al., 2016; Gosper et al., 2012; Lloret & Vilà, 2003; Pérez-Valera et al., 2018). While surface fires allow a rapid vegetation recovery in woodlands and shrublands, crown fires have harmful effects on the post-fire regeneration of non-serotinous pine woodlands compared to Pinus halepensis or Quercus woodlands, due to their different regenerative strategies (Pausas et al., 2008). Nevertheless, as fire severity and/or frequency increase, both community composition and structure are severely affected (Clarke et al., 2015; González-De Vega et al., 2016). Under such fire regimes, plant communities become phylogenetically and phenotypically clustered, that is to say, they are composed by species more closely related than expected by chance (Pausas & Verdú, 2008). This observation responds to the overrepresentation of evolutionarily related species, which bear similar suites of traits conferring fire resistance.
Plant species have three broad functional strategies to cope with fire. On one hand, some plants have developed ‘evader’ adaptations to avoid the fire effect, such as thick barks or self-pruning ability (Romero & Ganteaume, 2020). Others have a resource conservative strategy, which is based on the evolutionary development of above- and below-ground anatomical traits, such as lignotubers, epicormic buds, rhizomes or other storage roots that confer resprouting ability (Pausas & Keeley, 2014). Finally, there is the acquisitive strategy, which is based on the ability to rapidly establish a new community after fire through traits such as seed physical dormancy, flammability, heat- and smoke-mediated germination ability (related to seed dormancy) or nitrophilous habits to cope with transitory nitrogen peaks in burnt soils (Pausas et al., 2017; Pausas & Keeley, 2014; Wan et al., 2001). Although these strategies have followed different evolutionary routes (Pausas & Verdú, 2005), some facultative species are able to exhibit some of these traits under intermediate aridity and fire intensity levels (Pausas et al., 2017). At the species level, seeders and resprouters differ in a series of above- and below-ground traits, the latter showing delayed maturity, lower resistance to xylem cavitation, larger root:shoot ratios, higher levels of root carbohydrates and lower specific root length (Jacobsen et al., 2007; Paula & Pausas, 2011; Schwilk & Ackerly, 2005). At the community level, species diversity and plant cover of seeders increase after fire and then gradually decrease, whereas resprouters are resilient to fire and become dominant after several decades in the absence of a new burning event. In fire-prone Mediterranean areas, this pattern is common to subhumid and dry broad-leaved woody and shrubby communities (Fournier et al., 2020; Parra & Moreno, 2018), whereas some pines with serotinous cones as those shaped by Pinus halepensis can persist as the dominant vegetation in the absence of new fires. All the pre-fire characteristics of the vegetation, fire severity and the post-fire management are fundamental to understand the medium and long-term recovery trajectory of these burned areas (González-De Vega et al., 2016).
Quantifying plant traits at the community level is a useful approach to depict phenotypic trajectories across environmental gradients (Bjorkman et al., 2018; Craine et al., 2001; Shen et al., 2019). This approach needs to avoid short-term monitoring to identify general patterns of community assembly (McGill et al., 2006). Moreover, studies based on a low number of traits can lead to large levels of unexplained variation if key traits are not included. Published studies work with six traits on average (reviewed by Van der Plas et al., 2020), which are typically above-ground traits. Life form, plant size, seed size, as well as morphological and physiological leaf traits are frequently used since they shape the LHS (leaf-height-seed) strategy (Díaz et al., 2016; Laughlin et al., 2010). Nevertheless, below-ground traits are key drivers of ecological functions, playing a role in soil retention and exploration, water and nutrient acquisition, and involvement in ecological interactions between plant, microbial and faunal communities (Bardgett et al., 2014; De Baets et al., 2007; Navarro-Cano et al., 2019). Below-ground traits are also essential to discern between the functional strategies that plants show to cope with fire, as explained above. The plant economics spectrum (PES) theory tries to capture all these ecological roles by integrating stem, leaf and root traits, in order to understand plant ecological strategies as response syndromes to disturbances and drivers of ecosystem functions (Reich, 2014). Thus, quantifying above- and below-ground traits concurrently is the most holistic approach to analyse community-level responses to environmental gradients, although such responses depend on the specific trait selection (Delpiano et al., 2020; Kramer-Walter et al., 2016). In fire-prone ecosystems we still do not know if the turnover of species and traits are coupled during post-fire community reassembly, and beyond, whether a synchrony exists in the reassembly of above- and below-ground traits.
Based on the assumption that some species might be functionally redundant within taxonomically over-diversified plant communities due to phenotypic overlap (Fonseca & Ganade, 2001; Rosenfeld, 2002), we hypothesised that taxonomic and phenotypic community reassembly might be decoupled after fire in Mediterranean ecosystems (Figure 1). We particularly expected that species turnover might be larger than trait turnover in post-fire trajectories. Moreover, we assumed that below-ground traits are the main determinants of plant community resilience to fire, as fire-prone communities show below-ground adaptations to cope with this disturbance (Paula & Pausas, 2011; Pausas & Keeley, 2014). Thus, we expected that below-ground traits might show the smallest turnover compared to above-ground and whole-plant traits at the community level. To test these hypotheses, we studied three 20-year post-fire chronosequences in semiarid to dry Mediterranean areas in Spain. Our previous studies in these chronosequences indicated that fire did not alter plant richness, whereas it reduced plant phylogenetic diversity—due to the promotion of evolutionarily related fire-prone species—which progressively recovered after two decades (Pérez-Valera et al., 2018). Here, we constructed a trait database for 117 species and 23 traits, including whole-plant, above-ground and below-ground traits. We aimed to: (i) quantify the post-fire trajectories of the community-weighted means of 23 traits analysed individually along the chronosequences, (ii) calculate the phenotypic diversity of burned and long-unburned plots based on whole-plant, above- and below-ground traits, (iii) test if trait reassembly simply reflects a taxonomic rearrangement or, alternatively, trait and species turnover are decoupled across post-fire trajectories and (iv) assess if above- and below-ground traits have a different relevance during post-fire community reassembly. Our results may help understand the functional basis of plant reassembly trajectories after fire, as a keystone on which both ecological theory and restoration programmes may be armed.
中文翻译：
易发生火灾的地中海植物群落的性状和物种更替的脱钩
1 简介
火灾是解释许多陆地生物多样性热点地区植物多样性的关键因素（Bond 等， 2005 ；Kelly 等， 2020 ）。它强烈改变植物群落的组成和结构，从而影响生态系统功能（Keeley & Keeley， 1981 ；Pausas 等， 1999 ；Pérez-Valera 等， 2018 ）。易发生火灾的地中海植物群落具有悠久的适应火灾的进化历史（Pausas & Keeley， 2014 ）。它们在不同生态系统（林地、灌木丛、草原）中的火灾后再生已从经典分类学的角度进行了广泛研究（参见例如Keeley & Keeley， 1981 ；Pausas等人， 1999 ）。目前，基于性状的方法试图推断功能性群落模式，并将其与群落组装和生态系统稳定性背后的关键生态过程联系起来（Anacker et al., 2011 ；Clarke et al., 2015 ；Fyllas et al., 2020 ；Pausas & Verdú） ， 2008 ；Tsakalos 等人， 2019 ）。从这些研究中，我们知道火灾后群落的重组通常是由物种相对丰度的变化驱动的，而不是它们的总体替代（Fournier et al., 2020 ）。尽管火灾会导致植物群落覆盖和生物量的短期减少，但分类多样性往往不受影响（González-De Vega et al., 2016 ; Gosper et al., 2012 ; Lloret & Vilà, 2003 ; Pérez-Valera et al. ., 2018 ). 虽然地表火灾可以使林地和灌木丛的植被快速恢复，但与黑松或栎林相比，树冠火灾对非浆松林地的火灾后再生具有有害影响，因为它们的再生策略不同（Pausas等人， 2008 ）。然而，随着火灾严重程度和/或频率的增加，群落组成和结构都受到严重影响（Clarke 等， 2015 ；González-De Vega 等， 2016 ）。在这样的火情下，植物群落在系统发育和表型上变得聚集，也就是说，它们由比偶然预期更密切相关的物种组成（Pausas＆Verdú， 2008 ）。这一观察结果回应了进化相关物种的过度代表性，这些物种具有相似的耐火性状。
植物物种具有三种广泛的应对火灾的功能策略。一方面，一些植物已经发展出“逃避”适应性来避免火灾影响，例如厚树皮或自我修剪能力（Romero＆Ganteaume， 2020 ）。其他人则采取资源保守策略，该策略基于地上和地下解剖特征的进化发展，例如木块茎、外皮芽、根茎或其他赋予再发芽能力的储存根（Pausas & Keeley， 2014 ）。最后，还有获取策略，该策略基于通过种子物理休眠、可燃性、热和烟介导的发芽能力（与种子休眠相关）或嗜硝习性等特性在火灾后快速建立新群落的能力，以应对烧毁土壤中短暂的氮峰值（Pausas 等人， 2017 年；Pausas 和 Keeley， 2014 年；Wan 等人， 2001 年）。尽管这些策略遵循不同的进化路线（Pausas＆Verdú， 2005 ），但一些兼性物种能够在中等干旱和火灾强度水平下表现出其中一些特征（Pausas等人， 2017 ）。在物种水平上，播种机和再发芽机在一系列地上和地下性状上有所不同，后者表现出成熟延迟、对木质部空蚀的抵抗力较低、根冠比较大、根碳水化合物含量较高和比根长度较低。 Jacobsen 等人， 2007 ；Paula 和 Pausas， 2011 ；Schwilk 和 Ackerly， 2005 ）。 在群落层面，播种机的物种多样性和植物覆盖度在火灾后增加，然后逐渐减少，而再发芽机对火灾具有抵抗力，并且在没有新的燃烧事件的几十年后成为主导。在易发生火灾的地中海地区，这种模式在半湿润和干燥的阔叶木本和灌木群落中很常见（Fournier et al., 2020 ；Parra & Moreno, 2018 ），而一些具有血清球果的松树（如哈勒松形状的松树）可以在没有新的火灾的情况下，它们将继续作为主要植被。植被的所有火灾前特征、火灾严重程度和火灾后管理对于了解这些烧毁地区的中长期恢复轨迹至关重要（González-De Vega 等， 2016 ）。
在群落水平上量化植物性状是描述跨环境梯度表型轨迹的有用方法（Bjorkman等人， 2018 ；Craine等人， 2001 ；Shen等人， 2019 ）。这种方法需要避免短期监测来识别群落组装的一般模式（McGill 等， 2006 ）。此外，如果不包括关键性状，基于少量性状的研究可能会导致大量无法解释的变异。已发表的研究平均涉及六种性状（由 Van der Plas 等人审查， 2020 ），这些性状通常是地上性状。生命形式、植物大小、种子大小以及形态和生理叶子性状经常被使用，因为它们塑造了 LHS（叶高种子）策略（Díaz 等， 2016 ；Laughlin 等， 2010 ）。然而，地下性状是生态功能的关键驱动因素，在土壤保持和探索、水和养分获取以及参与植物、微生物和动物群落之间的生态相互作用中发挥着作用（Bardgett 等， 2014 ；De Baets 等）等人， 2007 ；纳瓦罗-卡诺等人， 2019 ）。如上所述，地下特征对于辨别植物应对火灾的功能策略也至关重要。植物经济学谱（PES）理论试图通过整合茎、叶和根性状来捕捉所有这些生态作用，以便将植物生态策略理解为对生态系统功能干扰和驱动因素的响应综合症（Reich， 2014 ）。 因此，同时量化地上和地下性状是分析群落层面对环境梯度响应的最全面的方法，尽管这种响应取决于特定性状的选择（Delpiano 等人， 2020 ；Kramer-Walter 等人， 2016 ）。在易发生火灾的生态系统中，我们仍然不知道在火灾后群落重组过程中物种和性状的更替是否是耦合的，以及之后地上和地下性状的重组是否存在同步性。
基于这样的假设，即由于表型重叠，某些物种可能在分类过度多样化的植物群落中出现功能冗余（Fonseca & Ganade, 2001 ; Rosenfeld, 2002 ），我们假设地中海生态系统火灾后分类学和表型群落的重组可能会脱钩（图1）。我们特别预计，火灾后轨迹中的物种更替可能大于性状更替。此外，我们假设地下特征是植物群落抵御火灾的主要决定因素，因为易发生火灾的群落表现出地下适应性以应对这种干扰（Paula＆Pausas， 2011 ；Pausas＆Keeley， 2014 ）。因此，我们预计，在群落水平上，与地上和全植物性状相比，地下性状可能表现出最小的周转率。为了检验这些假设，我们研究了西班牙半干旱至干旱地中海地区的三个 20 年火灾后时间序列。我们之前对这些时间序列的研究表明，火灾并没有改变植物的丰富度，但由于进化上相关的易燃物种的促进，它降低了植物的系统发育多样性，而这种多样性在二十年后逐渐恢复（Pérez-Valera等人， 2018 ） 。在这里，我们构建了117个物种和23个性状的性状数据库，包括全株、地上和地下性状。 我们的目标是：(i) 量化沿着时间序列单独分析的 23 个性状的群落加权平均值的火灾后轨迹，(ii) 基于整个植物计算烧毁和长期未烧毁地块的表型多样性，以上和地下性状，（iii）测试性状重组是否仅仅反映了分类学的重新排列，或者性状和物种更替在火灾后的轨迹上是解耦的，以及（iv）评估地上和地下性状是否具有不同的相关性火灾后社区重新集会期间。我们的结果可能有助于理解火灾后植物重组轨迹的功能基础，作为生态理论和恢复计划的基石。
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干扰后植物群落重组的假设分类学和表型脱钩的概述（此处为火灾）。示例中根系的功能冗余产生了比根型更新更大的物种，这有助于植物群落抵御火灾的能力。在表（右）中，与 sp1 和 sp5 相比，sp2、sp3 和 sp4 具有相似的根系。
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