Journal of Ecology ( IF 5.3 ) Pub Date : 2024-07-12 , DOI: 10.1111/1365-2745.14366 J. L. Godlee 1 , C. M. Ryan 1 , A. Siampale 2 , K. G. Dexter 1, 3
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1 INTRODUCTION
Foliage forms the primary interface between the vegetated land surface, the atmosphere and incoming solar radiation (Gu et al., 2003; Penuelas et al., 2009). Seasonal cycles of foliage production (leaf phenology) play an important role in regulating global carbon, water and nitrogen cycles (Garonna et al., 2016). Vegetation indices derived from remote sensing data, used as a proxy for foliage display (land surface phenology), have been used to constrain estimates of primary productivity in terrestrial biosphere models (Bloom et al., 2016; Helman, 2018), and to characterise the functional response of vegetation to various climate drivers (Richardson et al., 2013). Previous studies have identified environmental drivers of land surface phenology (Adole et al., 2019; Guan et al., 2014), but we lack understanding of how the floristic diversity and structure of the vegetation itself mediates these relationships (Pau et al., 2011; Whitley et al., 2017).
At continental scale, patterns of land surface phenology can be explained adequately using only climatic factors, namely precipitation, diurnal temperature oscillation and photoperiod (Adole et al., 2018b, 2019; Guan et al., 2014), but significant local variation exists within biomes in the timing of foliage display that cannot be attributed solely to abiotic environment (Stöckli et al., 2011). Additionally, we lack a mechanistic understanding of the biotic controls on land surface phenology, which hampers our ability to make informed predictions of change in phenology under global environmental change. It has been repeatedly suggested that the diversity, composition and structure of plant communities plays a role in determining ecosystem response to abiotic cues driving patterns of land surface phenology (Adole et al., 2018a; Fuller, 1999; Jeganathan et al., 2014), owing to differences in phenological strategy among species, but studies on this are lacking (Ma et al., 2022). Indeed, ground observations find wide variation in temporal patterns of foliage display among plant communities within a given biome (Seyednasrollah et al., 2019), but this variation is rarely represented in predictive models of biosphere-atmosphere exchanges (Pavlick et al., 2013; Scheiter et al., 2013).
Across the miombo woodlands of southern Africa, the largest woodland formation in the region (White, 1983), seasonal oscillations in water availability drive strong seasonal cycles of foliage display (Chidumayo, 2001; Dahlin et al., 2016). Within miombo woodlands, the phenomenon of pre-rain green-up, where trees produce leaves in advance of seasonal rains, serves as a striking example of phenological response to seasonal rainfall patterns. While pre-rain green-up requires heavy investment in hydraulic architecture to access deep groundwater and to produce embolism resistant hydraulic systems, it may provide competitive benefits, allowing immediate access to early rainfall and the associated release of soil nutrients prior to grass flushing (February & Higgins, 2016; Ryan et al., 2017). Detarioid species, slower growing with robust leaves, dense wood, and deep root systems (Timberlake & Calvert, 1993; Zhou et al., 2020), frequently green-up before the rainy season has commenced. They may also retain their leaves for longer after the end of the rainy season, though the mechanisms of this behaviour are unclear (Giraldo & Holbrook, 2011; Kushwaha et al., 2011). Other species may only begin to produce foliage during the rainy season, creating a dense flush of low cost leaves during the mid-season peak of growth and dropping their leaves earlier as soil water content drops towards the end of the rainy season (Lasky et al., 2016). While species that green-up early gain a competitive advantage from having fully emerged leaves once the rainy season starts, they may be forced to prematurely drop their leaves to avoid excess water loss and maintain a positive carbon balance if seasonal rains occur much later than normal (Vinya et al., 2018), an occurrence which is becoming more common (Wainwright et al., 2021). It has been suggested that variation in phenological strategy among tree species is one mechanism by which increased species diversity increases resilience to, and maximises productivity in, water-limited woodland ecosystems (Morellato et al., 2016; Stan & Sanchez-Azofeifa, 2019).
Variation in seasonal patterns of foliage display affects broader ecosystem processes. Woodlands with a longer period of foliage display support a greater diversity and abundance of wildlife, particularly birds, but also browsing mammals and invertebrates (Cole et al., 2015; de Araujo et al., 2017; Morellato et al., 2016; Ogutu et al., 2013). As climate change reduces rainy season length and increases the intensity of drought events in African water-limited woodlands (Cook et al., 2020; Gore et al., 2019), woodlands with a diverse tree community may provide refugia for many animal species (Bale et al., 2002). The period of green-up is a key time for invertebrate reproduction (Prather et al., 2012) and herbivore browsing activity (Morellato et al., 2016; Velasque & Del-Claro, 2016). Pre-rain green-up provides a valuable source of moisture and nutrients for browsing herbivores before the rainy season (Makhado et al., 2018), and can moderate the understorey microclimate, increasing humidity, reducing UV exposure, moderating diurnal oscillations in temperature, and reducing ecophysiological stress which otherwise can lead to increased plant mortality during the dry season. Thus, understanding what drives variation in seasonal patterns of foliage display in tropical deciduous woodlands can provide valuable information on their vulnerability to climate change, and therefore better information to predict their future composition, extent and function.
In this study we investigate how tree species diversity and structure influence land surface phenology in seasonally dry tropical woodlands. We focus specifically on the lag between green-up/senescence and the onset/end of the rainy season, the magnitude of foliage display within the growing season, and the overall length of the growing season. We hypothesise that: (H1) sites with greater species diversity will exhibit a longer growing season due to a higher diversity of phenological strategies; (H2) in sites with greater species diversity the start of the growing season will occur earlier with respect to the onset of the rainy season due to an increased likelihood of containing species which can green-up early; (H3) sites with larger trees will have a longer growing season, as large trees can better access deep groundwater reserves outside of the rainy season; (H4) sites dominated by detarioid species will experience earlier pre-rain green-up.
中文翻译：
树种多样性驱动季节性干燥热带林地的地表物候
1 简介
树叶形成了植被陆地表面、大气和入射太阳辐射之间的主要界面（Gu 等人， 2003 年；Penuelas 等人， 2009 年）。叶子生产的季节性周期（叶子物候）在调节全球碳、水和氮循环中发挥着重要作用（Garonna 等， 2016 ）。源自遥感数据的植被指数，用作树叶显示（地表物候学）的代表，已被用来限制陆地生物圈模型中初级生产力的估计（Bloom等人， 2016年；Helman， 2018年），并表征植被对各种气候驱动因素的功能响应（Richardson 等， 2013 ）。先前的研究已经确定了地表物候的环境驱动因素（Adole et al., 2019 ；Guan et al., 2014 ），但我们缺乏对植物区系多样性和植被本身结构如何调节这些关系的了解（Pau et al., 2011 ；惠特利等人， 2017 ）。
在大陆尺度上，仅用降水、昼夜温差和光周期等气候因素就可以充分解释地表物候模式(Adole等， 2018b ， 2019 ；Guan等， 2014 )，但大陆内部存在显着的局部变化。生物群落在叶子展示时间上的影响不能仅仅归因于非生物环境（Stöckli et al., 2011 ）。此外，我们缺乏对生物对地表物候的控制机制的理解，这妨碍了我们对全球环境变化下物候变化做出明智预测的能力。人们一再提出，植物群落的多样性、组成和结构在决定生态系统对驱动地表物候模式的非生物线索的响应方面发挥着重要作用（Adole等人， 2018a ；Fuller， 1999 ；Jeganathan等人， 2014 ） ，由于物种间物候策略的差异，但缺乏这方面的研究(Ma et al., 2022 )。事实上，地面观测发现给定生物群落内植物群落的叶子展示时间模式存在很大差异（Seyednasrollah 等， 2019 ），但这种变化很少在生物圈-大气交换的预测模型中得到体现（Pavlick 等， 2013 ）沙伊特等人， 2013 ）。
南部非洲的 miombo 林地是该地区最大的林地（White， 1983 ），可用水量的季节性波动驱动了树叶展示的强烈季节性周期（Chidumayo， 2001 ；Dahlin 等人， 2016 ）。在 miombo 林地中，雨前绿化现象（即树木在季节性降雨之前长出叶子）是物候对季节性降雨模式响应的一个引人注目的例子。虽然雨前绿化需要对水力建筑进行大量投资，以获取深层地下水并生产抗栓塞水力系统，但它可以提供竞争优势，允许在冲草之前立即获得早期降雨并释放土壤养分（2 月） ＆希金斯， 2016 ；瑞安等人， 2017 ）。 Detarioid 物种生长较慢，叶子粗壮，木质茂密，根系深（Timberlake & Calvert， 1993 ；Zhou 等人， 2020 ），经常在雨季开始前变绿。它们也可能在雨季结束后将叶子保留更长时间，尽管这种行为的机制尚不清楚（Giraldo & Holbrook， 2011 ；Kushwaha 等， 2011 ）。其他物种可能只在雨季开始长出叶子，在生长高峰期产生密集的低成本叶子，并在雨季结束时随着土壤含水量下降而提前落叶（Lasky 等人） .， 2016 ）。 虽然早绿的物种因在雨季开始后完全长出叶子而获得竞争优势，但如果季节性降雨比正常情况晚得多，它们可能会被迫过早落叶以避免过多的水分流失并保持正碳平衡（Vinya 等人， 2018 ），这种情况正变得越来越常见（Wainwright 等人， 2021 ）。有人认为，树种之间物候策略的变化是一种机制，通过这种机制，物种多样性的增加可以增强水资源有限的林地生态系统的恢复力并最大限度地提高其生产力（Morellato 等人， 2016 年；Stan 和 Sanchez-Azofeifa， 2019 年） 。
树叶展示的季节性模式的变化影响更广泛的生态系统过程。树叶展示时间较长的林地支持野生动物的多样性和丰富性，特别是鸟类，但也有食肉哺乳动物和无脊椎动物（Cole 等人， 2015 年；de Araujo 等人， 2017 年；Morellato 等人， 2016 年；Ogutu等人， 2013 ）。随着气候变化缩短了非洲水资源有限的林地的雨季长度并增加了干旱事件的强度（Cook等人， 2020年；Gore等人， 2019年），具有多样化树木群落的林地可能为许多动物物种提供庇护所（贝尔等人， 2002 ）。返绿期是无脊椎动物繁殖（Prather et al., 2012 ）和食草动物觅食活动（Morellato et al., 2016 ；Velasque & Del-Claro, 2016 ）的关键时期。雨前绿化为雨季前的食草动物提供了宝贵的水分和营养来源（Makhado et al., 2018 ），并且可以调节林下小气候，增加湿度，减少紫外线照射，调节昼夜温度波动，减少生态生理应激，否则会导致旱季植物死亡率增加。因此，了解热带落叶林地树叶展示季节模式变化的驱动因素可以提供有关其对气候变化的脆弱性的宝贵信息，从而提供更好的信息来预测其未来的组成、范围和功能。
在这项研究中，我们研究了树种多样性和结构如何影响季节性干燥热带林地的地表物候。我们特别关注绿化/衰老与雨季开始/结束之间的滞后、生长季节内树叶展示的大小以及生长季节的总体长度。我们假设： (H 1 ) 物种多样性较高的地点将由于物候策略的多样性较高而表现出更长的生长季节； (H 2 ) 在物种多样性较高的地点，由于含有可以提早变绿的物种的可能性增加，生长季节的开始将比雨季的开始更早； (H 3 ) 拥有较大树木的地点将有更长的生长季节，因为大树木可以在雨季之外更好地获取深层地下水储备； (H 4 ) 以月桂类植物为主的地点将经历较早的雨前绿化。
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