Our observations indicated that D. crenata was the most frequently used woody marker among the 50 marker species in the region (Table 1). However, its presence probability was spatially heterogeneous, and the other five subordinate species (P. villosa, E. japonicus, Cam. sinensis, M. bombycis, and Cel. sinensis) were abundant in areas where D. crenata was less dominant (Figure 3). Significant spatial segregation of the 28 marker species that were observed more than twice suggested that boundary marker usage is diverse at a regional scale. Some of the pattern of segregation among the six main species seems to relate to plateau units. The clearest pattern was found for P. villosa, which is not used on Higashi-Ibaraki, Namegata, or Kashima Plateaus. In addition, the other five species each had a core distribution area on a specific plateau. According to [Fujita (2002]), ethno-regions of Ibaraki Prefecture were roughly divided into three groups based on the differences of dialects, the names of the same goods, and festivals and ceremonies. The northern part was segregated by the Naka River (Naka Plateau and further north), the southwestern part was segregated by the Kinu River or Kokai River (e.g., Yuki and Sashima Plateau), and the rest comprised the southeastern part. This segregation pattern is fairly consistent with the core distribution areas of some isolated woody plants, such as Cam. sinensis and E. japonicus. This suggests that topographically segregated areas share the same cultural histories, and isolated woody plant marker are representative of these cultures.
Although only the six species illustrated in Figure 3 had a distribution probability gradient of more than 5% at our sampling scale and intensity, minor marker usage may be heterogeneous at much finer local scales. Moreover, two managers in Tsukuba city and Ishioka city said that P. villosa was also frequently used along woodland boundaries. This may indicate that species selection of woodland boundary markers also has regional variation, as we found for upland field boundary markers.
Multiple uses may have contributed to the marker species selection in our study region. For example, the ritual use of D. crenata in burial ceremonies and tea leaf production from Cam. sinensis were supported by upland field boundary markers. According to Uehara ([1961a]), a walking stick made of D. crenata was also used in the burial ceremony in the Kita-Saku area, Nagano Prefecture, central Japan. Cam. sinensis was used as a marker most frequently in the southwestern part of the prefecture, on Sashima Plateau (Figure 3). In Ibaraki Prefecture, tea production on this plateau has been famous since the Edo era ([Imai 1974]). Cam. sinensis used to be planted both as linear hedges along farmland boundaries to protect wheat from wind damage and as isolated trees to mark farmland boundaries in the region (Yamazaki et al. ). According to one manager, such linear hedges and isolated trees had been used for small-scale tea production until about 15 years ago. Therefore, this historical local agricultural practice was reflected in the present composition of marker species.
[Ebisawa (1996, 2000]) reported on the lane-side tree compositions at two locations in Shiga Prefecture; they found 2837 individuals representing 63 species (including six unidentified groups) on 173 paddy lanes in Nagahama city and 1001 individuals representing 59 species (including two unidentified groups) on 113 paddy lanes in Youkaichi city. Such a high diversity of lane-side trees species in the two localities would be related to the functions of the trees. The lane-side trees in Shiga Prefecture had several uses, such as Hasagi (drying rice after harvest using lane-side trees as pillars), harvesting fruit and materials for craftwork, and use as a landscape plant ([Ebisawa 1982]). To serve the various purposes, such a wide variety of trees were expected to be maintained at the local scale. In contrast, the farmland markers in upland fields in our study region have not been used directly in harvesting processes, such as Hasagi. As indicated by our interviews, at present woody plant markers serve only the limited role of marking farmland boundaries. This situation suggests the vulnerability of marker maintenance in the future because artificial markers are widely available as substitutes.
As stated by eight managers, species selection for farmland boundary markers is likely primarily based on the ability of the plants to withstand periodic trimming to maintain them at a small size suitable for marking upland field boundaries. According to our interviews, one D. crenata individual was maintained for at least 66 years and many other individuals of this species have survived for approximately half a century. Similarly, most individuals of the other species noted in our interviews were also maintained for approximately 50 years. These findings suggest that individuals of the main marker species that we observed can commonly persist for decades under repetitive trimming on upland field boundaries. Such persistence would be an important characteristic in marker selection. Moreover, the degree of difficulty of propagation also would be important for species selection. If vegetative propagation is easy, neighboring markers can be the source of new markers. Although most managers did not introduce markers themselves, the two managers who did so restored their markers by division of D. crenata and by planting a cutting of E. japonicus. Euonymus japonicus is known to be easily propagated from cuttings (Uehara [1961b]). Therefore, at least these two species would be suitable for vegetative propagation in an open upland field environment.
Our study revealed the spatial segregation of boundary marker usage and the management status of these markers in Ibaraki Prefecture. The regionally diverse marker trees have an ethno-botanical value because they are characteristic features of the flat upland field landscape, which is a form of cultural heritage in the prefecture. Moreover, according to our interviews, alternative materials made of plastic or concrete are easily available. However, traditional woody markers can withstand repetitive damage during farming better than modern plastic materials. It is important that people throughout the region recognize these values of woody markers.
Marker preservation needs to be planned based on the regional variation of species usage and cultural traditions. The topographically segregated patterns of tree usage shown in Figure 3 provide beneficial information for selecting the core plateaus for marker species preservation. As noted by [Fujita (2002]), some ethno-regions should be recognized in advance. For example, preserving woody markers in certain areas on Naka, Higashi-Ibaraki, Kashima, Tsukuba, and Sashima Plateaus should achieve the preservation of the basic variation of marker tree usage in the region. As a similar problem has been discussed by Fukamachi et al. (), knowledge of marker maintenance has not been handed down well to the present managers in our study region, especially in terms of how to introduce each plant. Four managers did not know the correct name of their markers. In order to motivate the preservation of woody markers, it is important to identify other benefits they provide that would be of value to the farm managers.
To manage the remnant marker trees effectively in a context of modernization of agriculture, other possible functions for the plants should be evaluated in further research. For example, hedgerows contribute to the conservation of a variety of organisms (Barr et al. ). Moreover, sparse isolated trees maintained as paddock trees in Australia provide foraging sites and serve as stepping stones, thus enhancing landscape connectivity for a variety of bird species (Fischer & Lindenmayer [2002a, 2002b]). Considering such ecological roles of noncrop woody plants in rural landscapes, the influence of marker selection and management on wild animal populations should be evaluated in our study region. Although our interviews revealed the importance of cultural aspects of the markers, such information for minor species remains scarce. Therefore, more intensive interviewing to learn about the marker usage in the region should be conducted in the future, which would lead to a deeper understanding of the spatial segregation and traditional cultural uses of the boundary markers.