Water Dispersal Potential of Fruits of Dirca palustris L. (Thymeleaceae)

Abstract

Dispersal of propagules by water, or hydrochory, is a primary or secondary dispersal mode for many plant species that live in or near aquatic habitats. Buoyancy of seeds and fruits is a key attribute in hydrochory; the longer a propagule remains afloat, the greater the likelihood that it will be carried some distance from its source by flowing or pulsing water or by wind-generated surface currents (Shimamura et al. 2007). Studies of hydrochory in plants often initially focus on quantifying the buoyancy of propagules in laboratory settings to assess the water dispersal potential of a species (e.g., Williamson and Costa 2000). Plants that use hydrochory for propagule dispersal often occur in habitats that are flooded at seasonal or daily intervals (Schneider and Sharitz 1998, Williamson and Costa 2000, Moegenburg 2002, Shimamura et al. 2007). In many hydrochorous species, especially those of seasonally inundated habitats, fruit production often peaks during flooding events, which maximizes the potential for dispersal (Williamson and Costa 2000, Moegenburg 2002). Eastern leatherwood or wicopy, Dirca palustris L. (Thymeleaceae), is a shrub that occurs in rich, mesic forests in eastern North America, especially along streams (Nevling 1962, Ward and Horn 1998, Williams and Moriarity 1998). On the Allegheny High Plateau of northwestern Pennsylvania, D. palustris is among the earliest of species to flower in the region, usually in early to mid-April (Williams 2004). The fruit, an ellipsoid, greenishyellow drupe 8 to 12 mm in length, matures in May and is dispersed from shrubs during June. Little is known about the dispersal ecology of the fleshy fruits of D. palustris. Fruits are thought to be dispersed by frugivorous birds and mammals, gravity, and water (Ward and Horn 1998). In this note, I examine the potential for hydrochory in D. palustris based on field observations of shrub spatial patterning and laboratory studies of fruit and seed buoyancy. In the small stream valleys where Dirca palustris occurs in northwestern Pennsylvania (Williams and Moriarity 1998, Williams 2004), peak flows and potential for flooding typically occur after snowmelt in March to early April. Relative to hydrochory, these seasonal high flows are out of synchrony with the June fruiting season of the species, which occurs when most regional streams are beginning to reach base flow. The classic coupling of high flow events and propagule dispersal often seen in ‘‘typical’’ hydrochorous plants is lacking for D. palustris in small stream valleys of the region and likely elsewhere in its range. Our examination of the distribution of individuals of a Dirca palustris population along Sibbald Run (Forest County, Pennsylvania), in the Allegheny National Forest, suggests that proximity of shrubs to the stream could enhance the potential for hydrochory in this species during base flow periods. The population hub of D. palustris in this valley consists of over 50 individuals located along approximately 100 m of stream [see Williams and Moriarity (1998) and Williams (2004) for a description of the study area]. In early April 2000, we measured the distance of all D. palustris individuals (n 5 54) from the stream and noted their flowering status (flowering, non-flowering). We found that 55.6% of individuals in this population occurred within 2 m of the stream bank (Figure 1) and that nearly all individuals (97.0%) were in flower. Much of the potential source of fruits in this *email address: chuckvt89@gmail.com Present address: Western Pennsylvania Conservancy, 40W. Main Street, Ridgway, Pennsylvania 15853. Received 25 August, 2008; Accepted 17 January, 2009. CASTANEA 74(4): 372–375. DECEMBER 2009