Liana or woody vines are considered to be structural parasites because they commonly use the stems of other plants (usually trees) to ascend to the forest canopy. Lianas appear to have low water-storage capacity and are vulnerable to drought-induced cavitation due to their wide vessels and slim stems. The mechanism by which lianas balance hydraulic efficiency with safety and the role of stomatal regulation are both poorly understood.

  Dr. CHEN Yajun and his colleagues of Xishuangbanna Tropical Botanical Garden (XTBG) conducted a study to determine how tropical lianas cope with the extremely high transpirational demands prevalent in forest canopies. Specifically, they tested whether physiological regulation aided lianas in mediating diurnal water and carbon balances.

The researchers examined the stem hydraulic properties, leaf drought tolerance, diurnal variations in leaf and stem water potentials, stomatal conductance, photosynthetic rate, sap flow, and stem native percentage loss of conductivity for four liana species in a tropical forest in southwest China. They also measured the same physiological properties for five co-occurring trees (two deciduous and three evergreen trees) for the purposes of comparison with lianas.

They found that lianas tended to have higher stem sapwood-specific conductivity than did the co-occurring trees, conferring lianas with greater water transport efficiency. Thus, despite losing up to nearly half of their conductivity at midday, lianas were able to continue to transport water efficiently to the canopy during the afternoon. Physiological regulation (stomatal closure) and efficient water transport from the soil to terminal branches (high stem sapwood-specific conductivity and sap flux density) may help lianas maintain stem water potential within the safe range to avoid xylem dysfunction.

They also found that lianas tended to deploy the majority of their leaves at the top of the forest canopy, where they flourish despite the trifecta of high light intensity, temperatures, and windiness—all of which increase vapor pressure deficit and thus water stress. Therefore, physiological regulation seemed to be particularly important for lianas.

  Their results provided a potential physiological explanation of how lianas can have large vascular systems, which permit them to transport copious amounts of water, and yet still avoid catastrophic hydraulic failure and thrive in tropical forest canopies.

  The study entitled “Physiological regulation and efficient xylem water transport regulate diurnal water and carbon balances of tropical lianas” has been published online in Functional Ecology.

Contact

CHEN Yajun  Ph.D

Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan 666303, China
Tel: 86-691-8713195
E-mail: chenyj@xtbg.org.cn

Researchers attain access to forest canopy through  the use of a truck crane.

(Image by CHEN Yajun)