The influence of physiological traits on tree growth rate
The significant positive relationship between xylem hydraulic conductivity and leaf photosynthetic rate indicates that there is a hydraulic-photosynthetic coordination across the studied pine species. The coordination between xylem water transport efficiency and photosynthetic rate has been widely observed in many ecosystems and among different species (e.g. Brodribb et al., 2002; Eller et al., 2017; Hao et al., 2011; Hubbard et al., 2001). The hydraulic architecture of plants determines the efficiency of water delivery along the plant water transport pathway, and controls the gas exchange of leaf stomata and hence influences the capacity of photosynthetic carbon assimilation (Tyree & Ewers, 1991; Sperry, 2000). Therefore, high hydraulic conductance and photosynthetic rate are usually linked with fast growth rate across tree species (Fan et al., 2012; McDowell, 2011).
The significant hydraulic-photosynthetic coordination, however, did not lead to high radial growth rates in tree species having greater xylem water transport efficiency and photosynthetic assimilation rate in our study. The lack of a positive correlation between instantaneous photosynthetic rate and tree radial growth rate across the studied pine species is likely related to interspecific variation in costs related to drought resistance and post-drought damage repair in drought-prone environments. Besides the carbon allocation to growth, a considerable portion of carbohydrates assimilated through photosynthesis is invested to processes not directly related to growth, such as defense against pathogens and stress tolerance (Aaltonen, Lindén, Heinonsalo, Biasi, & Pumpanen, 2016; Adams et al., 2017; Anderegg et al., 2015; Chave et al., 2009). Drought-affected pine trees would allocate more carbon to roots and construct more drought resistant but more carbon costly xylems and are usually prone to growth decline (Aaltonen et al., 2016; Eller et al., 2017; Taeger et al., 2013). Meanwhile, after the drought events, trees also need to consume more carbon to recover from the adverse effect of drought (Chave et al., 2009; O’Grady, Mitchell, Pinkard, & Tissue, 2013). The fast-growing species usually have greater capacities of xylem water transport and leaf photosynthesis, which lead to faster instantaneous carbon assimilation rate under water-sufficient conditions (Poorter et al., 2010; Schuldt et al., 2016). However, these species are usually more vulnerable to drought-induced xylem embolism even under relatively mild drought stress and are more prone to hydraulic functional damage during drought events (Brodribb & Holbrook, 2003; Martínez-Vilalta et al., 2012). Severer drought-induced damage to the plant water-transport system in species taking the “fast strategy” will limit their post-drought photosynthetic carbon assimilation, and they may require more energy input for recovery from drought events, especially in sites with drought events of high frequency (Eller et al., 2017; Skelton, Brodribb, McAdam, & Mitchell, 2017). Such costs related to recovering from drought stress may offset the beneficial effect of fast instantaneous photosynthetic carbon assimilation rate in drought-prone environments. Therefore, it may be difficult to directly linked multi-year accumulated radial growth with hydraulic traits under the relatively harsh environment, since tree species with different growth strategies may have different inter-annual growth fluctuations and growth responses to drought events.