44 LIGHT IN RELATION TO TREE GROWTH. 
If, for instance, 8 seconds are required to obtain the “ normal 
shade ” in full sunlight and 27 seconds are required when the insolator 
is held in the shadow of the body, then the intensity of the total light 
(direct plus diffused hght) 7t=1=0.125; the intensity of the dif- 
fused light 7d=;;—0.037, and the intensity of the direct light 7D= 
[t—Id=0.125—0.037=0.088. Since the body of the observer ab- 
sorbs a portion of the diffused light, this method gives correct values 
for the total light, but does not give correct values for the diffused 
light. In order to minimize the error for diffused light it is sug- 
gested that the insolator be held as far from the body as possible 
and in such a position as to receive only the shadow cast by the head. 
WIESNER’S INVESTIGATIONS. 
By means of the chemical method Wiesner (1907) determined the 
average minimum light intensities of a comparatively large number 
of forest trees of the temperate and tropical climates, as follows: 
Average | Average 
Species. ; light in Species. light in- 
tensities. tensities. 
Boxus sempervirens (bOX).-..---.------ 11/108 || Populus alba (white poplar)............ 1/15 
Fagus silvatica (beech)......-..-.------ 1/60 || Populus nigra (black poplar). = 227.25. 1/1i 
Bsc hippocastanum (horse chest- | Pinus laricio (Austrian pine).........-_- 1/11 
Seatac yh RCO AA ge es 1/37 || Betula verrucosa (birch) ..-.----.-...--- 1/9 
Garpinus betulus (hornbeam).......... 1/56 || Liriodencron tulipifera (tulip tree)..-_. 17.5 
Acer platanoides (Norway maple)...--- 1/55 || Populus deltoides (cottonwood)......_. 1/6 
Acer campestre (field maple)........-.. 1/48 || Fraxinus excelsior (ash).............-.- 1/5.8 
Acer negundo (box elder)....----.---.- 1/28 | arix. decidua, (larch) eseee na eee 1/5 
Quercus. pedunculata (oak).............| 1/26 || Corylus avellana (hazel)...............- 1/3 
Ailanthus glandulosa (tree of heaven). . 1/22 Prunus spinosa (cherry).........------- 1/3 
Thuja occidentalis (arborvite) -......-- 1/20 
1 Measurements of light intensities below 1/80 are unsafe. 
This scale of tolerance corresponds very closely to the empirical 
scales that have been worked out by foresters, with the exception 
perhaps of ash, whose minimum light requirement is given as greater 
than that of birch, which is scarcely correct. 
Of special interest are the minimum light requirements found by 
Wiesner (1905:77-150) for some of our native trees and shrubs and 
European species commonly grown here 
| Minimum 
Species. Region. Elevation. light 
intensity. 
| | 
| 
Fa | contorta (incl. P. murr)..| Yellowstone Park, Wyo.....-......-.----- 6, 400 1/6 
See Se ees Ser e= GOs Sos ste Es Be Ey 8,500 | 1/ 6.4-1/6.9 
Pinus “fiexilis ee RI Ae tS Se SEO GOL 4 eee. oR aS eae ee ogee ees 6,000 | 1/8 -1/9 
Picea Parryana. cece <c 6 taeee (0 (1 eRe tele a eet a See eee er 8,000 | 1/60 -1/62 
BO BAe Sees hee ee oe Salt Lake City, Ogden, Utah...........-..- 4,000 | 1/64 -1/70 
Pseudotsuga taxifolia ...-.- .-| Yellowstone Park, WiyOsst S225 os eee 5, 500-6, 000 1/20 
Juniperus virginiana.....-.-.--.|----- (a (see eee ens) en ter akn se ee a a 6,000 | 1/5 -1/7 
Juniperus communis nana....--)...-. dO se ais ee ace hs Fe 6,00@-8,000 | 1/9 -1/9.6 
ACOT PROTEINS a ee ee eee ee GO. osc ses ea ee 6,000 1/30 
Acer saccharinum L..........-..| | Niagara Falls (600 ft.) and Pocatello, Idaho.| 4,000 } 1/25 -1/40 
