THE CHAPARRAL COMMUNITY. 



25 



II. THE CHAPARRAL COMMUNITY. 



Between the great Ohio-Missouri deciduous forest complex on the 

 east and the prairies to the westward, there extends a chaparra 

 community. Indeed, this is more or less continuous from Canadl 

 to Texas. In this shrub community, tongues of which extend far into 

 the subchmax prairie, species of Symphoricarpos, Rhus, Corylus, and 

 Rosa play the role of dominants. Indeed, all but the latter form 

 thickets of greater or less extent in the moister places throughout much 

 of the prairie community or occur as more or less isolated clumps or 

 individuals held in check only by the severe root competition of the 

 prairie species (plate 16, b). They rapidly spring into dominance, 

 suppress the grasses, and form centers for further outward invasion, 

 when soil-moisture conditions are even shghtly increased above the 

 normal. This may be due in the first instance to the loosening of the 

 soil by burrowing animals, the building of a fence, or other disturbance 

 of the sod. 



In order to understand more clearly the nature of the competition 

 between scrub and grassland, as well as to determine more exactly 

 environmental conditions indicated by these phyads, a number of root 

 systems were examined along the loess hills of the Missouri River near 

 Peru, Nebraska. 



Symphorlcarpos vulgaris. — ^This species, together with its western ecological 

 equivalent, S. ocddentalis, is a very conspicuous and important shrub of the 

 chaparral community. Growing in clumps to a height of only 2 to 4 feet, its 

 shade is frequently so dense as to exclude even the very tolerant Poa pra- 

 tensis. In a well-established shrub area the latter almost invariably occurs 

 between the individual clumps. 



A long trench was dug at the edge of such a thicket and the roots of numer- 

 ous plants were examined. The larger roots arise mostly from near the base 

 of the erect shoots, but numerous smaller ones occur, especially all along the 

 underground stems (plate 13, a). The maximum depth to which the roots 

 penetrate in the loess soil is only 65 inches. Although the trench was simk to 

 a depth of about 8 feet and a part of the soil underlying the thicket at this 

 depth was removed, no roots of Symphoricarpos were found below the 65-inch 

 level. Indeed, except for a few plants of Rhus, competition for Ught above 

 ground was so intense that no other plants were present and the deeper soil 

 was free from roots of any kind. But lack of linear extent is amply recom- 

 pensed by a wonderfully well-developed absorbing surface. Perhaps the roots 

 of no other plant examined, with the exception of certain surface-feeding 

 grasses, occupy the soil more thoroughly than does the deHcate network of the 

 root branches of this shrub. 



The larger roots vary from 3 to 7 mm. in diameter. While they may come 

 off vertically, usually they pursue an oblique direction for some distance 

 (1 to 3 feet) before turning downward (plate 13, a). These reddish-brown, 

 tough, woody roots taper imiformly, frequently giving off large branches and a 

 network of finer ones, beginning just beneath the surface of the soil. Indeed, 

 the roots are profusely and minutely branched and rebranched throughout, 

 the ultimate branches being almost microscopic in size. The laterals vary 



