SUMAC FAMILY 



445 



1. R. diversiloba T. & G. Poison Oak. (Fig. 223.) Erect shrnb 4 to 8 (or 

 13) feet high, or the stems climbing tree trunks by adventitious rootlets to a height 

 of 10 to 115 feet; leaflets variable, roundish to ovate, variously lobed or toothed; 

 panicles axillary, appearing with the leaves, somewhat pendulous; flesh of the 

 whitish drupe marked with black fibres. 



Foothills, valley stream-banks, river terraces, mountain slopes and mountain 

 valleys, 5 to 5000 feet, widely distributed and often abundant throughout cismon- 

 tane California. South to Lower California, north to Oregon. Mar.-Apr. 



Geog. note.— Ehus diversiloba grows in a greater number of frutescent associations than 

 any other shrub in California. In San Diego County it is associated with characteristic woody 



species of cismontane Southern 

 California, all of which are 

 wholly different from the asso- 

 ciated species in various north- 

 ern regions, as the Sierra Nevada 

 foothills or Vaca Mountains, 

 those in each case being in the 

 main different from each other 

 and yet again from the associ- 

 ated species in the case of the 

 Ecdwood belt. It has a wider 

 geographic range than any other 

 California shrub and grows under 

 a greater variety of soil condi- 

 tions than any other ; though us- 

 ually preferring good soils or rich 

 loams, it grows in blue adobe, in 

 saline soils, in gray clays, in sandy 

 flats, in heavy gravel deposits 

 and in the crevices of outcrop- 

 ping rock piles. It is also adapted 

 to a greater range of rainfall and 

 temperature than any other Cali- 

 fornia shrub and is especially re- 

 markable for its extreme shade 

 tolerance and its tolerance of in- 

 tense insolation. In common with 

 all other w'oody vegetation it is 

 absent from wide areas on the 

 inner foothills and the plains of 

 the Great Valley. Its preferred 

 habitats are moist slopes in the 

 hills ; in such situations it is sel- 

 dom lacking. It is also curious to add that of all native woody vegetation about the great Keswick 

 smelter on the upper Sacramento Eiver, the species least injured by smelter fumes was Ehus 

 diversiloba ; at a given station near the smelter it lived in spite of the conditions, while prac- 

 tically all other species were seriously injured or in the main destroyed. 



In number of individuals Poison Oak exceeds, in our judgment, any other shrub species in 

 California. While the vast colonies of Adenostoma fasciculatum or the mesa growths of Larrea 

 tridentata are impressive to the eye, Ehus diversiloba occurs over a much larger area than either. 

 It is often an unobtrusive shrub — scarcely noticed in many formations and yet the individuals 

 are really very numerous. When, for example, a Mendocino hillside wooded with Arbutus 

 menziesii, Quereus douglasii and various shrubs is cleared, the response of Poison Oak by cro-\vn- 

 sprouting tells the story of its abundance. It grows in the openly oak-wooded foothills, in 

 Douglas Fir forest, in the Eedwood belt, along river banks and "bottoms" and even in salt 

 marshes, often in chaparral and rarely in chamisal. In altitudinal distribution the species ex- 

 tends from sea-level to about 5000 feet. In the southern Sierra Nevada it grows as high as 4760 

 feet ; it is not found on the Yosemite Valley floor, but occurs in Hetch-Hetchy and on the slopes 

 of Eancheria Mt. above Hetch-Hetchy at about 4500 feet; in Butte Co. it is found as high as 3800 

 feet. A very large number of bird species feed on the seed and are a highly important factor in 

 distribution. Quail in particular are fond of the seed. Their habits often account for intensified 

 local occurrence of Ehus diversiloba. 



Biol, note.- — In outline of leaflets, shape and size, Ehus diversiloba is notoriously variable. 

 Very pronounced series of variations may be gathered from a single individual. In deep shade 



Fig. 223. Ehus diversiloba T. & G. a, flowering branch- 

 let, X % ; ft, long. sect, of pistillate fl., X 5%; c, long. sect, 

 of staminate fl., X 5%. 



