184 
(D, %) 
20 
D=32¢e0-3!15L , 68e ~0:0433L 
O 
OD Rae Si le aGrZOm2428 
<| Lignin Content [L.g (100g) !] 
vitro Dry Matter Digestibility 
Fig. 13. Correlation of in vitro digestibility with 
lignin content of vascular plants. @, shrubs; O, 
sedges; @, grasses; 0), herbs; A , commercial 
pellets. 
Biomass estimates for July, August, and 
September (Table 12) are in general agreement 
with previous intensive studies at Barrow, Alaska 
(Tieszen 1972). Peak live biomass is noted in the 
last week of July, and biomass declines marked- 
ly in most communities after this date. Chemical 
analyses are not complete; hence, no trend with 
age in the degree of lignification can be shown. 
However, preliminary evidence, again at Barrow, 
indicates an increase in lignin and a decrease in 
soluble (cell constituents) components of the 
plant during August (B. H. McCown, L. L. Ties- 
zen, and P. W. Flanagan, pers. comm.). At the 
start of the growing season, the effective produc- 
tivity of herbivores is limited by the available 
biomass and the rate of biomass increase. At the 
end of the season (early to mid-August), produc- 
tivity is also limited by the nutrient content of 
the available herbage. 
The amount of energy which is harvested by 
caribou and which becomes available for produc- 
tion depends on the maintenance energy require- 
ment of the animal. In turn, maintenance 
energy requirement is highly dependent on the 
activity pattern of the grazing animal. Insect 
harassment can increase substantially the daily 
heat production, the amount of the latter de- 
pending on the duration and intensity of harass- 
ment, the speed of movement, and the distance 
moved by the caribou. Following the present 
study, we are using simulation modeling (model 
ACTIVE) to give estimates of energy costs of 
insect harassment. Until this study is complete, 
we cannot calculate the loss in production (i.e., 
in potential milk production, growth, and fat- 
tening), which could be attributed to insect 
harassment in the 1972 and 1973 field seasons. 
Growth and fattening can only occur once the 
maintenance energy requirements have been 
reached. We have calculated that for mature 
caribou, net energy of growth and fattening may 
be available for only a limited time period, i.e., 
the month of July (Table 15). During July 
weather conditions favor mild to severe insect 
harassment 20% of the time. Hence, the poten- 
tial productivity of a resident herd already sub- 
jected to a limited period of positive energy 
balance may be further limited by insect harass- 
ment. Also, during this period (July), the 
amount of energy diverted to growth and fatten- 
ing is highly dependent on the dry matter 
digestibility, or the metabolizable energy con- 
tent, of the forage. The mean metabolizable 
energy content of forage consumed by caribou 
was estimated at 2.2 kcal g! dry matter. This 
value compares with medium to good quality 
forage from other grassland systems. When avail- 
ability is not limited, this would support good 
animal productivity. 
It is clear from the present study that the 
Prudhoe Bay area is only moderately productive 
for caribou. Stability of the ecosystem is appar- 
ently achieved by low stocking densities, the 
latter resulting in consumption of less than 2% 
of the annual primary production (Table 17, 
Fig. 13). Thus, there is an adequate biomass 
“buffer’’ available to meet the infrequent and 
unpredictable entry of large caribou herds. The 
number of animals which apparently overwinter 
in the Prudhoe Bay area is small and is perhaps 
limited by nutritional as well as climatic factors. 
Adverse weather conditions plus lack of protec- 
tion from strong winds may place a severe 
limitation on the time available for winter graz- 
ing on the arctic coastal plain. This, combined 
with minimal lichen distribution and biomass 
(0-88 g m2), a low biomass of frozen forage of 
high digestibility, and possibly a snow cover of 
high hardness index, would suggest an area of 
poor winter habitat for caribou. When a poor 
winter habitat is combined with a limited period 
