1 Introduction
Competition is a fundamental process affecting the structure and development of plant communities (Tilman 1982; Niklas & Hammond 2013; Iida et al. 2016; Rozendaal et al. 2020). Individuals are eliminated via inter- and intraspecific interactions, among which conspecific interactions induce intense competitive pressure, especially in the early stages of succession (Martin-Ducup, Schneider & Fournier 2016; Aussenac et al. 2019). The competitive advantages of trees involve several factors including the ability to capture soil resources and sunlight (Ford 2014; Kunstleret al. 2016), which is often closely related to different functional traits (e.g., wood density and specific leaf area) (Adleret al. 2014). Consequently, functional traits have been used to predict and explain tree performance (Poorter et al. 2018; Bongers et al. 2020), although no key trait related to tree performance has been canonically adduced (Iidaet al. 2016). However, few studies have focused on the relationship between competitive advantage and tree architecture. As both tree height and branching patterns related to light interception (Kohyama & Takada 2012), crown form, therefore, is an important factor related to tree mortality (Arellano et al. 2019). Hence, for deeply understanding tree competition strategies and mechanisms of tree mortality, it is essential to explore whether differences in tree performance are correlated with morphometric differences in crown architecture.
Multiple traits and metrics pinpoint the ecological process well (Paal, Zobel & Liira 2020). Scaling relationships provide one method to explore the relationship between tree performance and plant architecture using integrated paired architecture characteristics, e.g., the height-to-diameter (H -D ) scaling relationship (e.g., McMahon 1973; Niklas 1995). Brown et al (2004) proposed the metabolic theory of ecology (MTE), which has been widely applied to a broad range of research areas from organelles to ecosystems (Priceet al. 2012), and, at the individual and population level, yields predictions about the architecture and demography of trees (Priceet al. 2010; Loubota Panzou et al. 2018). Indeed, the tree height-to-diameter (H -D ) scaling relationship has been applied to describe the strategies both theoretically and empirically (Sumida, Miyaura & Torii 2013; Zhang et al. 2019a). Numerous research showed the H -D scaling allometric exponent varied among and within species (Feldpausch et al. 2011; Loubota Panzouet al. 2018; Mensah et al. 2018; Zhang et al.2019a). Previous studies indicate that trees can alter their architecture and thus their strategy to compete with rivals (Lintunen & Kaitaniemi 2010; Zhang et al. 2020). For example, it was widely accepted that plant density (and thus competition) can alter the numerical values of scaling exponents. Lines et al (2012) reported that plants with large neighbors are relatively tall for a given diameter. Qiu et al (2021) demonstrated the H -Dallometry exponent of Ponderosa pine increased as their neighbor competition enhanced (Qiu et al. 2021). The vaeiation ofH -D allometry in turn influences the competitiveness of trees (Poorter et al. 2003; del Río et al. 2019), since differences in trunk diameter and crown shape affect spatial occupation and light interception (Osunkoya et al. 2007). However, how tree performance (growth vigour) is linked to H -D scaling allometric exponent remains unclear, especially in terms of intraspecific interactions believed to be essential components of community and ecosystem functioning (Bolnick et al. 2011; Poorteret al. 2018). Thus, an important question is whether theH -D scaling relationship differs as a function of tree performance?
To answer this question, we determined the numerical value of the intraspecific H -D scaling exponent across different levels of tree performance, predicting on theoretical grounds that it would decrease as tree performance deteriorated. In addition, because tree architecture is determined not only by the trunk but also by first-order branches (Kunz et al. 2019), we examined tree performance in the context of the interactions among repetitive self-similar modules, in which the first-order branch is the most fundamental (Kozlowski et al. 2012; Kramer, Sillett & Carroll 2014; Loehle 2016). We focused on branch traits such as growth in length and diameter, which change along with their relative position in crown depth (Umeki & Seino 2003; Lemay, Pamerleau-Couture & Krause 2019). In light of these phenomena, ecologists have constructed branch diameter and length models across species and different life stages (Bentley et al. 2013; Dong, Liu & Bettinger 2016; Kaitaniemi, Lintunen & Sievänen 2020).Additionally, branch traits also respond to competition, and thus alter the tree competition strategy (Lintunen & Kaitaniemi 2010; Wang et al.2018). However, to the best of our knowledge, no report has linked the branch length-diameter (L -d ) scaling exponent to tree performance (defined in the context of growth vigour).
Therefore, an important second question is whether the branch L-d scaling exponent differs across tree performance and branch position? Plant invest more to crown extension can intercept more horizontal light (Xu et al.2019). We hence hypothesized that the branch L -d scaling exponent of superior trees will be numerically higher than that of inferior trees, and that the branch L -d scaling exponent will decrease as the position of branches within a crown approach ground level.
To specifically address these two questions, we determined theH -D and L -d scaling relationships of conspecific trees across different performance levels (for criteria, see Materials and Methods), and the branch scaling relationships in different canopy layers (upper, intermediate, and lower) in a high-density aerial seeding of a Masson pine (Pinusmassoniana ) forest. This uniform and almost even-age high-density forest provided an ideal living laboratory for an investigation of growth vigour because of the near homogeneity of abiotic environmental factors.