Journal of Ecology ( IF 5.3 ) Pub Date : 2022-05-27 , DOI: 10.1111/1365-2745.13931 Yanjun Song 1 , Frank Sterck 1 , Ute Sass‐Klaassen 1 , Chenxuan Li 1 , Lourens Poorter 1
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1 INTRODUCTION
1.1 Effects of drought on forests
Climate change threatens forest ecosystems world-wide as an increased frequency and intensity of summer droughts (IPCC, 2013) reduces tree growth and survival (Buras et al., 2020; DeSoto et al., 2020). For example, the extreme dry summer of 2003 greatly reduced the gross primary productivity of European forests, turning those forests from a carbon sink into a carbon source (Ciais et al., 2005). The summer drought of 2018 resulted in more than 100 million m3 of wood from dead trees in Europe alone, indicating not only a large loss of living biomass and carbon sequestration potential but also of future timber supply (Nabuurs et al., 2018).
Conifers, which dominate large areas in temperate and boreal forest biomes, have drought adaptations such as needle-type leaves that reduce evapotranspiration and narrow conduits with typical margo-torus pits that allow most conifers to resist cavitation. Conifers show nevertheless strong reductions in growth and survival to droughts (Lévesque et al., 2013; Truettner et al., 2018), possibly because of hydraulic failure and carbon starvation (Adams et al., 2017). To model and predict forest responses to drought, broad-scale comparative studies are needed to quantify the growth resilience of conifer species and underlying mechanisms. Here we screen a large and phylogenetically diverse group of 20 conifer species for their growth resilience to drought. We do so by (a) comparing trees growing under similar conditions in a 50-year-old common garden experiment, (b) analysing growth resilience in terms of its underlying components (growth resistance and growth recovery; cf. Vitali et al., 2017) and (c) relating their resilience to four key hydraulic traits that are thought to govern species responses to drought.
1.2 Growth resilience is driven by resistance and recovery
Dendrochronology provides an ideal tool to track growth responses to drought over multiple decades (Camarero et al., 2018). Growth resilience is defined as the capacity of achieving pre-disturbance growth levels (Lloret et al., 2011), including two components; growth resistance and growth recovery. Growth resistance indicates the capacity to maintain growth rate during a disturbance compared to the growth prior to disturbance, and growth recovery indicates the growth rate after disturbance compared to the growth rate during disturbance (Depardieu et al., 2020). There is a trade-off between growth resistance and growth recovery (Gazol et al., 2017). Hence, species can achieve high resilience through either high resistance during a disturbance or high recovery after a disturbance, but they can also present low resilience because of low resistance and/or recovery. Conifers are known for their strong reductions in growth during drought years (e.g. DeSoto et al., 2020) but also for their quick recovery after such years (Gazol et al., 2020; Vitasse et al., 2019). Conifer species show different drought responses; for example, Abies alba and Pseudotsuga menziesii mainly rely on high resistance (Vitali et al., 2017) whereas Picea glauca and Pinus species mainly rely on high recovery (Depardieu et al., 2020; Gazol et al., 2017). The physiological mechanisms underlying such species differences are, however, poorly tested because most studies focus on few species only, or do not measure species-specific plant traits (Bréda & Badeau, 2008; Martin-Benito et al., 2017) such as hydraulic traits, that could underly differences in growth resilience.
1.3 Limitations of growth resilience studies
Many dendrochronological studies focus on synchronous stem growth reductions during specific dry ‘pointer’ years (Vitasse et al., 2019). These studies focus on a few years with reduced stem growth, but cannot compare the effects of drought dimensions on resilience, such as the frequency, timing, duration and severity of droughts. Negative effects on growth are expected when droughts are frequent, occur early in the growing season, are longer, or become severe (D'Orangeville et al., 2018; Gao et al., 2018; Güney et al., 2017). However, assessments of these multiple dimensions of droughts on resilience simultaneously are largely missing. Improved quantification of these drought components will allow for a better prediction of tree growth and forest carbon storage under climate change (D'Orangeville et al., 2018). Here we will use a water balance approach (Gao et al., 2018) to assess how the timing, duration and severity of drought affect stem growth responses of 20 conifer species.
1.4 Hydraulic traits and growth resilience
Apart from hydraulic traits, plant size can modify tree responses to drought. Large trees are more sensitive to drought (Bennett et al., 2015), although they have deeper roots to obtain water from deeper soil layers, and larger carbohydrate reserves that allow them to persist during drought and recover after drought. Perhaps large trees experience more drought stress because their crowns are more exposed to high irradiance and vapour pressure deficits leading to stomatal closure impairing carbon gain and potentially leading to carbon depletion, whereas the longer hydraulic path lengths make them more sensitive to cavitation to drought (Ryan et al., 2006).
It remains controversial whether and how hydraulic traits can affect the growth resilience of trees to drought. One of the possible reasons is that these studies have focused on broad-scale geographic patterns and species-level traits, and the observed growth responses may be potentially confounded by acclimation responses to local conditions. Here we aim to advance on previous studies, by using a common garden experiment where tree species grow under similar environmental conditions (for soil and climate), and respond to the same annual variation in drought during 11 extremely dry years. This allows us to better test for the effects of hydraulic traits on stem growth resilience than earlier studies, because confounding effects of environment and acclimation are avoided. We addressed the following questions and hypotheses:
Q1) How do growth resistance and recovery drive the growth resilience of conifer species? We expect that most species will be resilient to dry years, but in different ways. Species will achieve high resilience through either high resistance (i.e. minor stem growth reductions) during dry years or through high recovery after dry years.
Q2) How is growth resistance affected by the timing, duration and severity of a drought event? We expect trees to show stronger stem growth reductions with early, long-lasting and severe droughts, because early droughts likely hit during the period of maximum growth activity, long-lasting droughts may lead to stomatal closure and carbon starvation, whereas intense drought may lead to hydraulic failure. Additionally, all these effects may be stronger for bigger trees because the longer hydraulic path length results in earlier stomatal closure and increases the risk of hydraulic failure.
Q3) Why do conifer species differ in their growth resilience to droughts?
We predict that fast-growing species show stronger growth reductions in drought years (lower resistance) but also stronger recovery after drought years than slow-growing species, and that hydraulic safety favours growth resistance, whereas hydraulic efficiency favours fast growth and recovery.
中文翻译：
针叶树物种的生长恢复力随着早期、持久和强烈的干旱而降低，但不能用水力特性来解释
1 简介
1.1 干旱对森林的影响
气候变化威胁着全世界的森林生态系统，因为夏季干旱的频率和强度增加（IPCC， 2013 年）会降低树木的生长和存活率（Buras 等人， 2020 年；DeSoto 等人， 2020 年）。例如，2003 年极端干燥的夏季大大降低了欧洲森林的总初级生产力，使这些森林从碳汇变成了碳源（Ciais 等， 2005）。仅在欧洲，2018 年的夏季干旱就导致超过 1 亿立方米的木材来自枯死的树木，这不仅表明生物量和碳封存潜力的巨大损失，而且还表明未来木材供应的大量损失（Nabuurs 等人， 2018 年）。
针叶树在温带和寒带森林生物群系的大片区域中占主导地位，具有干旱适应能力，例如减少蒸散的针状叶子和带有典型 margo-torus 坑的狭窄管道，使大多数针叶树能够抵抗气蚀。然而，针叶树在干旱条件下的生长和存活率显着下降（Lévesque 等人， 2013 年；Truettner 等人， 2018 年），这可能是由于水力衰竭和碳饥饿（Adams 等人， 2017 年））。为了模拟和预测森林对干旱的反应，需要进行广泛的比较研究来量化针叶树物种的生长恢复力和潜在机制。在这里，我们筛选了一个庞大且系统发育多样化的 20 种针叶树物种，以了解它们对干旱的生长恢复力。我们这样做是通过 (a) 在一个 50 年的普通花园实验中比较在类似条件下生长的树木，(b) 根据其基本成分（生长阻力和生长恢复；参见 Vitali 等人， 2017）和（c）将它们的恢复力与被认为控制物种对干旱反应的四个关键水力特征联系起来。
1.2 增长韧性由阻力和复苏驱动
树木年代学提供了一个理想的工具来跟踪几十年来对干旱的生长反应（Camarero et al., 2018）。增长弹性被定义为达到干扰前增长水平的能力（Lloret 等人， 2011 年），包括两个组成部分；生长阻力和生长恢复。生长阻力是指与干扰前的生长相比，在干扰期间保持生长速率的能力，而生长恢复是指与干扰期间的生长速率相比，干扰后的生长速率（Depardieu et al., 2020）。增长阻力和增长恢复之间存在权衡（Gazol et al., 2017）。因此，物种可以通过扰动期间的高抵抗力或扰动后的高恢复来实现高恢复力，但它们也可能由于低抵抗力和/或恢复力而呈现低恢复力。针叶树以其在干旱年份的生长大幅下降而闻名（例如，DeSoto 等人， 2020 年），但也因其在干旱年份后的快速恢复而闻名（Gazol 等人， 2020 年；Vitasse 等人， 2019 年）。针叶树种表现出不同的干旱反应；例如，Abies alba和Pseudotsuga menziesii主要依靠高抗性（Vitali et al., 2017），而Picea glauca和Pinus物种主要依赖于高回收率（Depardieu 等人， 2020 年；Gazol 等人， 2017 年）。然而，这种物种差异背后的生理机制并未得到很好的测试，因为大多数研究只关注少数物种，或者不测量物种特异性植物性状 (Bréda & Badeau, 2008 ; Martin-Benito et al., 2017 )，例如水力特征，这可能会导致增长弹性的差异。
1.3 增长弹性研究的局限性
许多树木年代学研究侧重于特定干燥“指针”年份的同步茎生长减少（Vitasse 等人， 2019 年）。这些研究侧重于茎生长减少的几年，但无法比较干旱维度对恢复力的影响，例如干旱的频率、时间、持续时间和严重程度。当干旱频繁发生、发生在生长季节早期、持续时间更长或变得严重时，预计会对生长产生负面影响（D'Orangeville 等人， 2018 年；Gao 等人， 2018 年；Güney 等人， 2017 年））。然而，在很大程度上缺少同时对干旱对复原力的这些多维度的评估。改进对这些干旱成分的量化将有助于更好地预测气候变化下的树木生长和森林碳储存（D'Orangeville 等人， 2018 年）。在这里，我们将使用水平衡方法 (Gao et al., 2018 ) 来评估干旱的时间、持续时间和严重程度如何影响 20 种针叶树的茎生长反应。
1.4 水力性状和生长恢复力
除了水力特性外，植物大小还可以改变树木对干旱的反应。大树对干旱更敏感（Bennett 等人， 2015 年），尽管它们具有更深的根部以从更深的土壤层获取水分，并且具有更大的碳水化合物储备，使它们能够在干旱期间持续存在并在干旱后恢复。也许大树会经历更多的干旱压力，因为它们的树冠更容易受到高辐照度和蒸气压不足的影响，从而导致气孔关闭，从而削弱碳增益并可能导致碳耗竭，而较长的水力路径长度使它们对气蚀对干旱更加敏感（Ryan等人， 2006 年）。
水力特性是否以及如何影响树木对干旱的生长恢复力仍然存在争议。可能的原因之一是这些研究集中在大范围的地理模式和物种水平的特征上，观察到的生长反应可能会被对当地条件的适应反应所混淆。在这里，我们的目标是通过使用常见的花园实验来推进先前的研究，其中树种在相似的环境条件（土壤和气候）下生长，并在 11 个极度干旱年份应对相同的年度干旱变化。这使我们能够比早期研究更好地测试水力性状对茎生长恢复力的影响，因为避免了环境和驯化的混杂影响。我们解决了以下问题和假设：
Q1) 生长阻力和恢复如何驱动针叶树物种的生长恢复力？我们预计大多数物种将能够抵御干旱年份，但方式不同。物种将通过在干旱年份的高抗性（即较小的茎生长减少）或通过干旱年份后的高恢复来实现高复原力。
Q2) 干旱事件的时间、持续时间和严重程度如何影响生长阻力？我们预计树木会在早期、长期和严重干旱中表现出更强的茎生长减少，因为早期干旱可能在最大生长活动期间发生，长期干旱可能导致气孔关闭和碳饥饿，而强烈干旱可能导致到液压故障。此外，对于较大的树木，所有这些影响可能会更强，因为较长的水力路径长度会导致气孔提前关闭并增加水力失效的风险。
Q3) 为什么针叶树的抗旱能力不同？
我们预测，与生长缓慢的物种相比，快速生长的物种在干旱年份表现出更强的生长减少（较低的抵抗力），但干旱年份后的恢复也更强，并且水力安全有利于生长阻力，而水力效率有利于快速生长和恢复。
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