J. Ky. Acad. Sci. 66(2): 137-138. 2005. 



NOTES 



Host Occurrence of Eastern Mistletoe (Phoraden- 

 dron leucarpum, Viscaceaej in Robertson County, 

 Kentucky — A survey of eastern mistletoe {Phomdendron 

 leucarpum) -miesieA host trees in Robertson County, Ken- 

 tucky, was conducted from 27 Dec 2002 to 6 Jan 2003. 

 All paved roads and passable gravel roads in the county 

 were traveled by vehicle. Host trees were identified and 

 tallied by species, and the number of clumps of mistletoe 

 were recorded. Voucher specimens were obtained for 

 each representative host tree species by a extendable 12 

 m fiberglass linesman pole and then mounted, labeled, 

 and deposited in the Berea College Herbarium (BEREA). 



Four mistletoe studies have been published in Ken- 

 tucky (1, 2, 3, 4). The only pubfished report for mistletoe 

 in Robertson County had been a sight record on Juglans 

 nigra by Reed and Reed (1). 



Robertson County is located in northeastern Kentucky 

 (Figure 1). With 259 km- of land area, it is the smallest 

 Kentucky county. Mount Olivet, the county seat, is cen- 

 trally located at latitude 38°31'55" N and longitude 

 84°02'14" W. Robertson County is the state's least popu- 

 lous county, having 2266 people; 287 people reside in 

 Mount Olivet according to the 2000 census (5). 



The county is located entirely in the Hills of the Blue- 

 grass ecoregion of the Interior Plateau Region of Ken- 

 tucky (6). The hills consist of upland rolling topography 

 with moderately steep slopes and broad ridges. Small ra- 

 vines form seasonally intermittent streams leading to larg- 

 er streams, e.g.. North Fork Licking River and Licking 

 River. Elevations in the county range from 170 m to 308 

 m. In Robertson County, Upper Ordovician limestone and 

 shale bedrock belong to the Clays Ferry Formation, Lex- 

 ington Limestone, Kope Formation, and Fairview For- 

 mation (7). The Eden-Nicholson-Lowell soil association is 

 found throughout the Robertson County upland terrain 

 (8). 



Braun (9) classified forest vegetation as Western Me- 

 sophytic Forest in this region of the eastern deciduous 

 forest; Kiichler (10) placed northeastern Kentucky in the 

 Quercus-Canja forest. I observed forest vegetation on up- 

 land limestone and shale hills as Quercus alba-Cartja spp. 

 forest with scattered large stands of Juniperus virginiana 

 and a riparian forest of Acer negundo-Platanus occiden- 

 talis-Acer saccharinum in alluvial areas. A majority of the 

 county is composed of pasturelands and agricultural crop- 

 lands. 



The continental climate in northeastern Kentucky is 

 characterized by cool to cold winters and warm, humid 

 summers with precipitation well distributed throughout 

 the year (11). Cfimate data, 1977-2000, are from the Fal- 

 mouth weather station in Pendleton County, 32 km west 

 of Robertson County (12). Mean annual precipitation is 

 111 cm with the lowest precipitation, 7.2 cm, in October 

 and the highest, 11.8 cm, in May and July. Mean annual 

 temperature is 11.7°C with the mean lowest temperature, 

 — 1.6°C, in January and the highest temperature, 23.8°C, 



in July. The mean growing season is 175 days with a range 

 of 132 days to 211 days. The median first fall frost is 15 

 October and the last spring frost is 24 April (12). 



Phoradendron leucarfjum was found only on 45 trees 

 from 7 host tree species in 6 families. Juglans nigra ac- 

 counted for 23 host trees (51.1%). Other host trees were 

 10 Ulmus americana (22.2%), 6 Robinia pseudoacacia 

 (13.4%), 3 Acer saccharinum (6.7%), and 1 each of Gle- 

 ditsia triacanthos, Fraxinus americana, and Primus sero- 

 tina (6.6%). All host trees were situated east of Mount 

 Olivet in the eastern half of the county (Figure 1). The 

 sparse number of host trees in Robertson County was ver)' 

 significant when compared to other recent mistletoe stud- 

 ies in central and south central Kentucky, i.e., Lexington- 

 Blue Grass Army Depot in Madison County with 1837 

 host trees (2), Rockcastle County with 3502 trees (3), and 

 Garrard County with 1740 (4). 



Low winter temperatures are the principal factor re- 

 sponsible for the sparse number of mistletoe-infested 

 trees in Robertson County, e.g., in January 1994 the low- 

 est temperature was — 34.4°C and in February 1996 it was 

 — 22.1°C. My observations of host trees in mistletoe sur- 

 veys showed extensive mistletoe die-back to the endo- 

 phytic root system when extremely low temperatures have 

 occurred. Spooner (13) found that eastern mistletoe 

 reaches its northernmost distribution range in southern 

 Ohio. He produced evidence that the main reason for the 

 northern limits of eastern mistletoe corresponded to the 

 mean minimum January temperature of -4.5°C. My ex- 

 tensive reconnaissance has shown that the incidence of 

 mistletoe is sparse not only in Robertson County, but in 

 all of the northern Kentucky counties within the Hills of 

 the Bluegrass and Outer Bluegrass ecoregions. In several 

 of these northern counties, I have observed eastern mis- 

 tletoe only once or twice from one to three host tree spe- 

 cies for the entire county, e.g., Bracken, Campbell, Lewis, 

 Gallatin, Kenton, Owen, and Pendleton counties. Eastern 

 mistletoe has not been observed in Grant County despite 

 repeated searches. 



Other mistletoe studies have documented the effects of 

 low temperatures on spread and mortality of mistletoes. 

 Garman (14) reported that severe winters destroyed much 

 eastern mistletoe in Kentucky, but mistletoe gradually re- 

 appeared under more favorable temperatures in later 

 years. Deam (15) observed the detrimental effects of low 

 temperatures on eastern mistletoe and the restriction of 

 the plant's Indiana range to the southern part of the state. 

 Lightle (16) documented that low temperatures were fac- 

 tors in injury and mortality to three Phoradendron spp. in 

 southern Arizona and New Mexico. Wagener (17) con- 

 cluded that very low temperatures restricted two Phora- 

 dendron spp. to their present distribution limits in Cali- 

 fornia and explained their absence in other parts of the 

 hosts' range. 



The dioecious characteristic of mistletoe may be anoth- 

 er important factor in the overall sparse numbers of mis- 



137 



