1 INTRODUCTION

Successful management of fire-prone woody ecosystems is challenging and requires knowledge of the spatial arrangement of the trees (Das et al., 2008; Larson & Churchill, 2012; Meddens et al., 2018; Meyer et al., 2008) and how the tree distribution patterns influence the nature and consequences of subsequent fires (Frelich et al., 1998; Stephens et al., 2008; Turner, 1989). Tree aggregation, or clumping, is an especially common distribution pattern (Armesto et al., 1986; Condit et al., 2000; Larson & Churchill, 2012; Peterson, 2020), particularly in environments experiencing periodic fires (Bacelar et al., 2014; Moustakas, 2015; Staver et al., 2019; Tamjidi & Lutz, 2020). Clumping can provide trees a variety of benefits, including refuge from physical stress (Baumeister & Callaway, 2006; Tirado & Pugnaire, 2005), reduced susceptibility to enemies (Pineda et al., 2010) and suppression of a strong competitor (Lutz et al., 2014).

Open tree landscapes, such as savannas, parklands, brushlands, barrens, scrub and woodlands, are notably dynamic ecosystems, in which intensity, frequency and patterns of fire can determine whether the landscape is maintained or if it begins to transition into a more open or closed landscape (Hanberry et al., 2018). According to the stress-gradient hypothesis, the role of facilitation becomes increasingly important in conjunction with increasing stress (Bertness & Callaway, 1994). Fires, particularly repeated fires, represent a major type of stress for most trees. If positive neighbour interactions result in the increased survival of clumped trees during fires, then this would promote the perpetuation of the open tree landscape.

As emphasized by Brooker and Callaghan (1998), in cases where disturbances are episodic, detecting positive interactions among neighbours is only possible through long-term studies. This is especially true for studies trying to determine whether positive interactions in response to disturbances remain consistent over time. To conduct such research, a study would require a temporal scale long enough for many disturbances to occur, along with a spatial scale large enough to include many trees and tree neighbourhoods. We are not aware of any such study.

The primary purpose of this study was to determine the nature of neighbour interactions among trees in an open oak landscape experiencing frequent fires, and the extent to which these interactions are sustained over time. We were able to do this using data obtained from a large grid-plot study in which trees were censused annually for 25 years, during which time the trees experienced between 9 and 11 fires. The long-term observations allowed us to test how local tree density affects growth and survival of individuals in the face of repeated fires, and, secondarily, the extent to which these effects might be mediated by soil nitrogen and access to water.

中文翻译：

---

在经常被烧毁的开放橡树景观中，邻居始终影响树木的生长和生存

1 简介

成功管理易火灾的木本生态系统具有挑战性，需要了解树木的空间排列（Das 等人，  2008 年；Larson & Churchill，  2012 年；Meddens 等人，  2018 年；Meyer 等人，  2008 年）以及如何树木分布模式影响随后火灾的性质和后果（Frelich 等人，  1998 年；斯蒂芬斯等人，  2008 年；特纳，  1989 年）。树聚集或丛集是一种特别常见的分布模式（Armesto 等人，  1986 年；Condit 等人，  2000 年；Larson & Churchill，  2012 年；彼得森，  2020 年）)，特别是在经历周期性火灾的环境中（Bacelar 等人，  2014 年；Moustakas，  2015 年；Staver 等人，  2019 年；Tamjidi & Lutz，  2020 年）。丛生可以为树木提供多种好处，包括躲避身体压力（Baumeister & Callaway,  2006 ; Tirado & Pugnaire,  2005），降低对敌人的敏感性（Pineda et al.,  2010）和压制强大的竞争对手（Lutz et al. .,  2014 年）。

开放的树木景观，如稀树草原、公园、灌木丛、贫瘠、灌木丛和林地，是特别动态的生态系统，其中火灾的强度、频率和模式可以决定景观是否得以维持，或者它是否开始转变为更开放或更开放的环境。封闭景观（Hanberry 等人，  2018 年）。根据压力梯度假设，随着压力的增加，促进的作用变得越来越重要（Bertness & Callaway，  1994）。火灾，尤其是反复火灾，是大多数树木的主要压力类型。如果积极的邻居互动导致火灾期间丛生树木的存活率增加，那么这将促进开放树木景观的延续。

正如 Brooker 和 Callaghan ( 1998 ) 所强调的，在干扰是偶发的情况下，只有通过长期研究才能检测到邻居之间的积极互动。对于试图确定响应干扰的积极相互作用是否随着时间的推移保持一致的研究尤其如此。为了进行这样的研究，一项研究需要足够长的时间尺度以发生许多干扰，以及足够大的空间尺度以包括许多树木和树木社区。我们不知道有任何此类研究。

本研究的主要目的是确定在频繁发生火灾的开放橡树景观中树木之间的邻居相互作用的性质，以及这些相互作用随着时间的推移而持续的程度。我们能够使用从大型网格图研究中获得的数据来做到这一点，在该研究中，每年对树木进行普查，为期 25 年，在此期间树木经历了 9 到 11 次火灾。长期观察使我们能够测试当地的树木密度如何在反复发生火灾时影响个体的生长和生存，其次，这些影响可能在多大程度上受到土壤氮和水的影响。