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.