HARPER: VEGETATION OF THE PINUS TAEDA BELT 57 
heavier, so that we will assume the average weight to be 37 pounds, 
and that of the woody increment I,110 pounds per acre. 
The percentage of ash in the wood is 0.26 in Pinus Taeda, but 
more in most of the other trees, say 0.32 on the average, which 
would make over three pounds of mineral matter taken each year 
from the soil by the wood alone. As the inorganic constituents 
of plants are chiefly concentrated in leaves and bark, and the 
herbs, which are renewed from the ground up every year, probably 
take as much from the soil per acre per year in such open forests 
as the trees do, we may safely multiply the last figure by forty or 
fifty, making something like 150 pounds per acre (equivalent to a 
layer of soil about 0.0005 inch deep) for the annual draft on the soil. 
This is less than is taken from the soil by the average culti- 
vated crop, even without fertilization, and it all goes back to 
the soil in the course of time, except what is removed by lumbermen 
and grazing animals or carried away by streams. Fire, though it 
destroys the humus and thus dissipates the nitrogen, at the same 
time accelerates the return of the mineral substances to the soil, and 
thus enables forests of the pine-barren type to do a large business 
on a small capital, so to speak. The gradual erosion of the whole 
surface, continually exposing deeper layers of soil, is probably 
sufficient to counteract the leaching and keep the soils from becom- 
ing steadily poorer. 
The amount of water evaporated by average vegetation in 
moderately humid regions is said to be about 700 pounds to one 
pound of dry vegetable tissue, and if the total amount of vegetable 
matter produced in a year is four times the amount of new wood 
estimated above, or 4,440 pounds per acre, the water required 
for the region in question would be 3,108,000 pounds per acre, 
equivalent to about 16 inches of rain, or less than half as much 
as would evaporate from open water in that climate.* 
The foregoing estimates are only crude approximations, but 
probably lie somewhere between half and double the correct 
figures; and when such a method is applied with skill and patience 
to the vegetation of different regions, Or different habitats in the 
same region, it ought to yield some extremely significant results. 
’ COLLEGE Point, NEw YORK 
pa eee ae ete 
* See Transeau, Am. Nat. 39: 885; Rep- Mich. Acad. Sci. 7: 74- 1905. 
