Lianas as structural parasites

Abstract

The “structural parasite” strategy is explored for lianas (woody vines). Since lianas grow upon host plants which provide mechanical support, fewer mechanical support cells are required in the stems and roots. Lianas have long and narrow stems with low Huber values (xylem area per distal leaf area) and long and wide vessels with great conductive efficiency. However, when scaled to the length of the stems, liana vessels are “only average” in mean diameter. Lianas have greater hydraulic diameters than in similar length free-standing stems due to greater variance in vessel diameter. Liana stems tend to be more vulnerable to dysfunction by water stress or freeze/thaw than in free-standing plants, but the stems of lianas often have many narrow vessels and/or tracheids surrounding the wide vessels, providing alternate transport pathways in the event of xylem dysfunction. The leaf-specific conductivity values of lianas are not exceptional; pressure gradients may be similar to those of free-standing plants, which could pose hydraulic limitations since liana stems are often greater than 100 m in length. Root pressures are quite common in lianas, and many lianas are deep rooted. Lianas are effective at sprouting from the base and at rooting from fallen stems that contact the soil and, in many species of lianas, variant secondary growth facilitates repair of mechanically damaged stems. Linked to the structural parasite strategy, lianas are most abundant in seasonally dry tropical forests where there is low risk of freeze/thaw events and where deep roots may be advantageous.