Xylem hydraulics and non-structural carbohydrate contents in 15 temperate tree species

<p class="1CxSpFirst">In humid temperate forests, the occurrence of frequent freeze-thaw cycles (FTC) is a main factor limiting tree growth, as xylem embolism induced by FTC poses a serious threat to the hydraulic integrity of trees. A high resilience to hydraulic dysfunction involves the enhancement of embolism resistance and/or extra non-structural carbohydrate (NSC) inputs for restoration of an impaired hydraulic system. However, potentially negative implications of such NSC allocation on tree growth have not yet been explored.</p> <p class="1CxSpMiddle">At a temperate forest site of northeast China, we studied xylem hydraulics and NSC contents in relation to winter embolism resilience in 15 sympatric broadleaf tree species belonging to three genera with relatively high species richness, 6 <i>Acer</i> species, 5 <i>Betula</i> species and 4 <i>Populus</i> species.</p> <p class="1CxSpLast"><i>Acer</i> and <i>Betula</i> species had higher soluble sugar contents in the dormant season and indeed had higher hydraulic resilience to FTC induced embolism but slower stem growth. <i>Populus</i> species had higher NSC contents during the growing season and their faster stem growth was also consistent with higher hydraulic efficiency (Ks) and leaf photosynthetic rate.<span> The positive correlation between tree </span><span>trunk</span><span> radial growth rate and hydraulic conductivity suggests that</span><span> xylem water transport efficiency can be a fundamental basis for tree productivity due to a significant hydraulic-photosynthetic coordination</span><span>. The negative correlation between soluble sugar concentration in the dormant season and stem growth rate indicates that </span>metabolic carbon costs for enhancing hydraulic resilience may compromise tree growth during the growing season.</p> <p>Comparisons among <i>Acer</i>, <i>Betula</i> and <i>Populus</i> and the correlation analyses based on phylogenetic independent contrasts strongly support the existence of a trade-off between hydraulic resilience against FTC induced embolism and growth rate among sympatric tree species under humid temperate climate conditions. This trade-off has likely contributed to the sorting of temperate tree species and genera to different niches along environmental gradients with respect to freezing stress and interspecific competition.</p>