



^' 



Figure 2-2. Invasion of a 

 waste area by annual weeds: 

 Iva xanthifolia, Kochia 

 scoparia, Bromus tectorum, 

 Chenopodium sp., Poly- 

 gonum aviculare, and Lac- 

 tuca scariola — a pioneer 

 stage in secondary succes- 

 sion. June, Colorado. 



does not grow as well as C. rangiferina in the shade of the taller 

 shrub, Betula glandulosa (Figure 2-1). 



The formation of weed groupings in early stages of succession 

 in cultivated or abandoned fields is often closely related to en- 

 vironmental conditions. ^^•^*'° Man has undoubtedly influenced 

 the formation or modification of communities for a long time, and 

 this continues now at an accelerated rate. The maqui vegetation 

 of the Mediterranean region, the shiblyak of the Balkans, and the 

 carbon scrub in Central America are apparently the result of 

 man's interference.'* The entire chaparral community in Califor- 

 nia seems to be in a state of flux, with variations of Adenostoma 

 fasciculatum, hybrid manzanitas, hybrid oaks, and Ceanothus spp. in 

 an ecologically disturbed habitat; but the extent to which it is 

 man-made is not clear,^ and further study by analytical methods, 

 unbiased by preconceived dogmas, is required. 



Seven basic ecological processes are involved in the formation 

 of new groupings and in the modification of existing ones: migra- 



Figure 2-3. Aggregation by 

 growth of seedlings of Pinus 

 ponderosa in an opening in 

 the forest, showing a dense 

 stand with excessive compe- 

 tition. Lassen National For- 

 est, California. (U. S. Forest 

 Service.) 



