4O BULLETIN 1294, U. S. DEPARTMENT OF AGRICULTURE 
The results of the process have been very clearly worked out in 
parts of the Mother Lode region along the lower edge of the 
western yellow-pine forest. ‘The process of retrogession was partic- 
ularly rapid in this region, not only because of the unfavorable climatic 
factors, but because of the indiscriminate cutting and burning of the 
forest following the discovery of gold. There are still to be seen in 
this region the most striking contrasts of excellent second-growth 
western yellow pine in the closest proximity to nontimber producing 
areas occupied only by chamise, the characteristic plant of the true 
chaparral’ type (7). Illustrations showing the original forested lands 
contrasted with present conditions, are abundant proof that the ulti- 
mate results of continued mistreatment of forest lands are the same 
in California as elsewhere. The varioussteps in the retrogression from 
standing timber to chaparral can be found onthe ground within acom- 
paratively restricted scope. Although the extent of this retrogression 
is not known exactly, the retreat of the western yellow pine from the 
lower to the upper foothills in certain portions of the Sierra region 
has certainly amounted to several miles within the past half century, 
and for the State as a whole an area of many hundreds of square 
miles has been taken from the timber-producing zone. 
In the deterioration of the site as the result of fire, the first step is 
the destruction of the organic material and microorganisms in the 
humus of the upper soil layer. After the destruction of the vegeta- 
tive cover, erosion begins and the surface soil is carried away. Fi- 
nally, the leaching of the important soluble organic salts in the soil 
reduces fertility to a minimum (20). The moisture-holding capacity 
of the soil is also reduced and the more drought-resisting plants take 
possession of the soil. 
Whether a particular area subjected to many fires becomes a brush 
field or remains in timber depends on a number of factors, some of 
which are purely accidental. Probably the most important factors 
determining the local distribution of brush fields are topographic. 
Brush fields are far more common on upper slopes or ridges, because, 
as already explained, fires traveling ixphall become more destructive 
as they go and the timber at higher elevations is exposed to a devas- 
tating heat. A second factor of major importance is the direction of 
slope, or aspect. A careful examination of the local distribution of 
brush fields shows that the relative proportion of brush areas on various 
aspects is approximately as follows: South, 100; west, 75; east, 55; 
and north, 30. Aspect is not only important as it influences the 
intensity and severity of fires, but also as it effects regeneration on 
the drier and hotter slopes. The largest brush fields are found on 
broad slopes with an absence of topographic interference to the sweep 
of Me uiliies winds. 
f a present-day surface fire is examined in detail, it is found that 
by no means all the surface within the exterior boundaries of the 
burn was actually covered by the fire. Minor natural barriers of vari- 
ous sorts, such as outcrops of rocks, moist spots, patches of nonin- 
flammable material, and certain plants such as squaw carpet (Ceano- 
thus prostratus) that resist slow, creeping fires, unite to make it possi- 
ble for a certain amount of reproduction, even small seedlings, to 
survive fires. Other influences affecting the completeness of a burn 
10 Chaparral is the term applied to the brush cover growing on lands which are incapable of support- 
ing a commercial forest for the time being. 
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