Independence of stem and leaf hydraulic traits in six Euphorbiaceae tree species with contrasting leaf phenology

Jun-Wen Chen 0 1 2 Qiang Zhang 0 1 2 Xiao-Shuang Li 0 1 2 Kun-Fang Cao 0 1 2 0 J.-W. Chen College of Agronomy and Biotechnology, Yunnan Agricultural University , 650201 Kunming, Yunnan, People's Republic of China 1 J.-W. Chen (&) Q. Zhang X.-S. Li K.-F. Cao (&) Kunming Division , Xishuangbanna Tropical Botanical Garden, The Chinese Academy of Sciences , 88 Xuefu Road, 650223 Kunming, Yunnan, People's Republic of China 2 Q. Zhang College of Life Science, South China Normal University , 510631 Guangzhou, Guangdong, People's Republic of China Hydraulic traits and hydraulic-related structural properties were examined in three deciduous (Hevea brasiliensis, Macaranga denticulate, and BischoWa javanica) and three evergreen (Drypetes indica, Aleurites moluccana, and Codiaeum variegatum) Euphorbiaceae tree species from a seasonally tropical forest in south-western China. Xylem water potential at 50% loss of stem hydraulic conductivity (P50stem) was more negative in the evergreen tree, but leaf water potential at 50% loss of leaf hydraulic conductivity (P50leaf) did not function as P50stem did. Furthermore, P50stem was more negative than P50leaf in the evergreen tree; contrarily, this pattern was not observed in the deciduous tree. Leaf hydraulic conductivity overlapped considerably, but stem hydraulic conductivity diverged between the evergreen and deciduous tree. Correspondingly, structural properties of leaves overlapped substantially; however, structural properties of stem diverged markedly. Consequently, leaf and stem hydraulic traits were closely correlated with leaf and stem structural properties, respectively. Additionally, stem hydraulic eYciency was signiWcantly correlated with stem hydraulic resistance to embolism; nevertheless, such a hydraulic pattern was not found in leaf hydraulics. Thus, these results suggest: (1) that the evergreen and deciduous tree mainly diverge in stem hydraulics, but not in leaf hydraulics, (2) that regardless of leaf or stem, their hydraulic traits result primarily from structural properties, and not from leaf phenology, (3) that leaves are more vulnerable to drought-induced embolism than stem in the evergreen tree, but not always in the deciduous tree and (4) that there exists a trade-oV between hydraulic eYciency and safety for stem hydraulics, but not for leaf hydraulics. - The issue of divergences in functional trait, ecoclimatic adaptation, and global distribution for deciduous and evergreen tree species has long interested physiologists, ecologists, and biogeographer. Many researches have conWrmed that seasonal drought can favor deciduous leaves, and that infertile soils can favor long-lived evergreen leaves (Aerts 1995; Salleo et al. 1997; Eamus 1999; Givnish 2002), but the deciduous and evergreen tree species can co-exist in most habitats (Sobrado 1991; Quigley and Platt 2003). The comparison between the deciduous and evergreen tree species in the same habitats has been widely made. It has been generally accepted that, compared with the evergreen tree species, the deciduous tree species have higher photosynthetic rate, higher relative growth rate, and lower leaf mass per area (Reich et al. 1991; Villar et al. 1995; Cornelissen et al. 1996). On the other hand, lower hydraulic conductivity and lower hydraulic vulnerability have been observed in stems of the evergreen tree species (Choat et al. 2003, 2005; Chen et al. 2009). In contrast, there has also been evidence of the overlap and convergence in stem hydraulic traits for tree species with contrasting phenology (Bucci et al. 2004; Maherali et al. 2004; Choat et al. 2007). Thus, it seems likely that leaf lifespan aVects photosynthetic characteristics, but stem hydraulic traits may be independent of leaf phenology. Nowadays, most studies on hydraulics of tree species have concentrated on property of roots and stems (Brodribb et al. 2003; Maherali et al. 2006; Choat et al. 2007), but relatively few have elucidated leaf hydraulics due to methodological barrier. Nevertheless, recent work has shown that resistance to water transport in leaves accounts for 3080% of the total hydraulic resistance of the whole-plant water transport pathway (Becker et al. 1999; Sack et al. 2002), even though the water transport pathway in leaves represents a very small fraction of that in the whole plant. Furthermore, compared with the stems, leaves appear to be more sensitive to cavitations, often losing a substantial fraction of their hydraulic conductivity under non-extreme conditions (Brodribb and Holbrook 2004; WoodruV et al. 2007; Hao et al. 2008). However, leaf life-span has never been taken into account when the comparison of leaf and stem hydraulic vulnerability is made. Fortunately, many studies have shown that stem hydraulic traits of tree species are closely correlated with wood structure (Hacke et al. 2001; Rosner et al. 2007; Hao et al. 2008). Accordingly, it has been proposed that the divergence in stem hydraulic property for plants with contrasting phenology may result from the diVerence in stem functional structure regardless of leaf phenology (Chen et al. 2009). Likewise, Sack and Frole (2006) also found that leaf structural diversity was related to hydraulic capacity in tropical rain forest tree, implying that leaf hydraulic traits may depend on leaf structure, and thus may be independent of stem structure. To our knowledge, the comparison of stem hydraulic traits between the deciduous and evergreen tree species has been made, but the comparison of leaf hydraulic traits is yet unavailable. In the present study, leaf and stem hydraulic traits and their hydraulic-related functional structure were examined in six tree species with contrasting phenology, and the correlation analysis between hydraulic traits and functional structural properties was also made. The six tree species cooccur in a seasonally tropical forest in south-western China. All of the six tree species belong to a single family (Euphorbiaceae), and thus the experimental results might largely rule out the impact of diVerences in phylogeny. Our working hypothesis was that there may be substantial diVerences in hydraulic traits between the evergreen and deciduous tree species, but hydraulic traits may be only related to functional structure regardless of leaf phenology, and thus that leaf hydraulic traits may be independent of stem hydraulic traits. Materials and methods Study site and plant materials The study was carried out at Xishuangbanna Tropical Botanical Garden (2141 N, 10125 E, and 570 m a.s.l.), the Chinese Academy of Sciences in south-western Yunnan, China. The mean annual air temperature is about 21.7C and the annual precipitation is about 1,560 mm. However, it has a strong seasonality that more than 85% of precipitation occurs between the months of May and October, thereby resulting in a pronounced rainy season from May to October, and a well-deWned dry season from November to April. All measurements in (...truncated)