A Test of the Function of Drip Tips 1

DRIP TIPS, OR THE AcuM1w.m APICES OF LEAVES, are a common feature of understory plants in the humid tropics (Richards 1996). Perhaps the most persistent hypothesis for their function is that they facilitate the rapid channeling of water from leaves, which decreases the drying time for the leaf surface (Stahl 1893, Lightbody 1985). Retention of water on leaves may be favorable for the colonization or growth of epiphylls (e.5, lichens and bryophytes) and fungi (Richards 1996). These organisms may reduce the photosynthetic potential of leaves through shading or necrosis (Coley & Kursar 1996). Water more rapidly flushed from leaves may also help to dislodge debris, such as dead plant material or soil, the presence of which could shade leaves and provide favorable sites for colonization by epiphylls or fungi. These ideas have been tested a few times (Dean & Smith 1978, Williamson 1981, Lightbody 1985, Monge-Najera & Blanco 1995), but rarely with real (non-artificial) leaves under natural conditions. We experimentally studied the role of drip tips on leaves of saplings in the tropical forest understory. Specifically, we tested the hypotheses that drip tips reduce the occurrence of fungi, epiphylls, and debris on leaf surfaces. We also tested the hypothesis that drip tips reduce the volume of water retained on leaves. We examined the function of drip tips between 23 November and 2 December 1998, during the rainy season in tropical wet forest at the La Selva Biological Field Station in the Caribbean lowlands of Costa Rica. La Selva receives an average of 3962 mm of rain annually (Sanford et al. 1994); rainfall during the study was over twice the average (T. Brenes, pers. comm.). We used multiple species of saplings, including Faramea parv$ora (Rubiaceae), Cassipourea elliptica (Rhizophoraceae), Eugenia casipurea (Myrtaceae), Ocotea meziana (Lauraceae), Rourea sp. (Connaraceae), Protium sp. (Burseraceae), Zygia gzgantifolia (Fabaceae), and Stemmadenia robinsonia (Apocynaceae). To help isolate the effects of drip tips from other variations in leaf morphology, we chose species that had glabrous, similarly sized leaves with entire margins and similar drip tip length. All individuals included in the study occurred in the same habitat: older secondary, closed-canopy forest. Epiphylls were present on the older leaves of all study plants. O n 23 November, we selected 28 saplings that were 1.0-1.5 m tall and chose three of the youngest, filly expanded leaves within each that had no visible signs of fungi, epiphylls, or debris and were relatively horizontal to the surface of the ground. To reduce unseen propagules and debris on leaves, each leaf was wiped on its upper surface with a soft cloth. Each leaf was then assigned one of three treatments: (1) unmanipulated; (2) apices of leaves trimmed to leave the drip tip shape intact, as a control for effects of cutting (“modified drip tips”); or (3) drip tips removed, leaving smooth, rounded leaf apices (“cutoff drip tips”). Each plant received all three treatments, for a total of 84 leaves in the experiment. On 1 and 2 December, using a headlamp to improve visibility, we measured fungi (including only what were probably hngal fruiting bodies; see below), epiphylls (lichens and bryophytes), and debris (particles of humus and leaf litter) occurring on leaf surfaces. To estimate occurrence, we placed a transparent plastic sheet with a 5X 5-mm grid on each leaf and counted the number of squares containing epiphylls, fungi, or debris. Total leaf area was recorded as the total number of squares that were at least half filled with leaf surface. Percent occurrence of each category was calculated by multiplying the proportion of squares containing fungi, epiphylls, or debris by 100. We also compared the amount of water retained on leaves among treatments, using three leaves (one from each treatment) from each of seven plants (N = 21 leaves). Leaves were removed from their branches, and petioles were embedded in plasticine so that the leaf surfaces were horizontal. We dripped 5 ml of water on dry leaf surfaces for 20 to 30 sec using a pipette, and collected all the water that dripped off the leaf in a 10-ml graduated cylinder. We waited at least 20 sec for the last drop to fall from each leaf. Volume of water remaining on the leaf was calculated as 5 ml minus the volume of water recovered in the graduated cylinder.