Effect of rodents on acorn dispersal and survival of the Liaodong oak (Quercus liaotungensis Koidz.)

Abstract

We used a new method of labeling seeds with small pieces of coded tin-tags to investigate the effect of rodent on the seed fate of Liaodong oak (Quercus liaotungensis) in a mountainous area (40°00′N, 115°30′E) of Beijing, China. In the fall of 1999 and 2000, a total of 1994 and 660 acorns were released and then seed loss was checked. Both sides of the 50 m wide transect were extensively checked to locate the tagged seeds or their fragments. The results indicated that Apodemus speciosus, Rattus confucianus and A. agrarius were potential Liaodong oak acorn predators. Nearly all acorns were consumed by small rodents within 10 days after acorn placement, demonstrating that seed predation was very heavy and that rodent predation contributes to the poor seed regeneration of the oak. The dispersal distances of most seeds were within 20 m. The dispersal distance of the buried seeds was the longest and it was in favor of seed survival and seedling recruitment. Rodents preferred to eat tagged seeds under dense shrub. This is mostly related to predation risk in different micro-environments. Rodents tended to bury the tagged seeds under dense shrub. This burial place of rodent does not favor the seedling recruitment of the oak. The rodents preferred to move seeds to the horizontal and down directions of a slope and this tendency would save energy for rodents. The seeding regeneration rate of tagged acorns were very low in 2 years. Rodent predation is the causative factor of low regeneration of Liaodong oak in the study region.

Introduction

The propagules of plants are exposed to high risk of attack by natural predators such as insects, vertebrates and fungi (Janzen, 1970). Seed mortality due to predation by animals may affect plant fitness, population structure and dynamics, community structure and maintenance of species diversity (Janzen, 1970, Grubb, 1977, Clark and Clark, 1984, Schupp, 1988b, Schupp, 1990, Willson and Whelan, 1990). In forest ecosystems, rodents are recognized as an important factor affecting acorn fate (Shaw, 1968a, Shaw, 1968b; Kanazawa and Nishikata, 1976; Kikuzawa, 1988; Herrera, 1995). In most situations, a large proportion of dispersed acorns by rodents are consumed, hence very few acorns are able to become seedlings (Sork, 1984, Miyaki and Kikuzawa, 1988, Herrera, 1995). However, rodents can also disperse some acorns and bury a few of them. Seed dispersal is recognized as key phase affecting plant regeneration, species survival and distribution (Howe and Smallwood, 1982). Seed dispersal affects fitness of parent and offspring plants through its effects on seed density, the dispersal distance from the parent tree, and the habitat where seeds arrive. Differences in dispersal may influence early seed and seedling survival by affecting incidence of predation or attack by pathogens, the habitat to which seeds are dispersed, as well as the types of plants with which the developing plant will compete (Schupp, 1988a, Schupp, 1993, Jordano, 1992, Willson, 1992).

Although seed dispersal by animals is one of the most important mechanisms in the ecology and evolution of mutualistic systems (Bronstein, 1994), we still have very limited knowledge about seed dispersal systems. One of the reasons why research progress is slow is the difficulty in tracking the fate of all the seeds that are dispersed (Sork, 1984). A disadvantage of present techniques is that tagged seeds are not distinguishable from each other. Zhang and Wang (2001a) developed a new method of labeling seeds with small pieces of tin-tags. The tin-tagged seeds are coded and then identified easily from each other. This method makes it possible to determine the distribution of individual seeds dispersed by animals and the ultimate fates of those seeds.

Quercus liaotungensis is one of the common climax tree species in warm temperate broad-leaved deciduous forests in northern China and Q. liaotungensis forests are typical vegetation in the mountains of Beijing (Chen, 1992). Because of extensive human disturbances, primary Liaodong oak forests no long exist and only small patches of secondary Liaodong oak forests or shrubs are scattered among shrubs or grasses (Chen, 1997). The acorn of Q. liaotungensis is ovoid, 0.9–1.8 cm wide, 1.4–2.3 cm long and 0.6–3.4 g in mass (Wang and Ma, 2001). The oaks (Quercus sp.) can be grouped into two subgenera which are White oak (WO) (Lepidobalanus) and Red oak (RO) (Erythrobalanus). Liaodong oak (Q. liaotungensis) belongs to the WO. Since Liaodong oak acorns are nutritious, they are excellent food resources for small mammals and birds, especially for small rodents (Wang et al., 1999). The natural regeneration of Quercus is lacking in the field in many woodlands (Cho and Boerner, 1991). The regeneration mainly depends on the sprout and the growth of the coppices is bad. In the study region, the natural seeding regeneration rate of Liaodong oak is less than 0.1% (Zhang, unpublished). It is very hard to see the seedlings of Liaodong oak even after the acorn mast-year. Previous studies showed that rodents eat and disperse a large part of Quercus acorns in temperate forests (Shaw, 1968b, Kikuzawa, 1988, Iida, 1996). So it is necessary to understand effect of rodents on acorn dispersal and survival of the Liaodong oak for improving its natural regeneration.

The purpose of this study was to investigate the effect of rodents on the fate of Liaodong oak acorns. Specially, we addressed the following subjects: (1) acorns fate in situ; (2) dispersed acorns fate; (3) dispersal distance and patterns of the tagged seeds; (4) micro-environments where seeds are dispersed or buried; (5) survival of the tagged seeds.