TREES FOR WATER CATCHMENT AREAS 165 
With all these merits, there are difficulties in the 
cultivation of black Italian poplar as a forest tree in 
plantations, whether pure or mixed. Like all poplars, it is 
unable, after the first few years, to bear the slightest shade, 
the whole crown of foliage requiring full light; and each 
stem in consequence needs a large space for its proper 
development. Mixed with other species in a wood, black 
Italian poplar ultimately occupies an enormous area, over- 
topping and injuring the neighbouring trees. It is useless 
to plant it with other species, unless the latter is to 
constitute an underwood. It has been suggested to plant 
poplars 15 to 20 feet apart, and to fill up the intervals 
with alders; but the latter would make poor growth in 
this mixture unless the soil were good and deep. In a 
pure plantation of black Italian poplar, only the outer trees 
that are fully exposed to the light make large dimensions, 
the interior trees remaining small in diameter and of little 
use as timber. This might be obviated by severe thinnings ; 
but these would be of no saleable value, and the number of 
stems left standing would be so reduced as to yield only a 
moderate volume per acre. Large plantations of poplar are 
rarely seen in this country, and actual measurements are 
still rarer. At Benefield, Northamptonshire, 324 acres of 
strong clay land, practically useless for agriculture, were 
planted in 1887 with black Italian poplar, 538 to the acre. 
In 1916 there were 206 trees per acre surviving, which 
when felled averaged 45 feet of timber length and 24 
inches girth at breast height, the total volume being 1300 
cubic feet (quarter-girth measurement) per acre, or an 
average annual increment of 45 cubic feet per acre. 
Plantations of black Italian poplar contain a much 
smaller volume of timber than is generally expected; and 
in land where any of the vigorous conifers, such as Sitka 
spruce, Japanese larch, ete., can be grown, cultivation of 
poplars would be a failure financially. Black Italian poplar 
should be restricted to situations where it is impossible to 
establish conifer plantations; and in such cases the best 
method is a combination of meadow or pasture with poplars 
