CORRELATION OF VEGETATION AND CLIMATE. 95 
the number of days with freezing temperature, the number of consecu- 
tive days or hours of freezing, and the absolute minimum reached. 
Any two or more of these phases may operate conjointly to influence 
a plant, and the temperature preceding a particular constellation of 
conditions may enhance the harmful effects of those conditions. In 
fact the general weather conditions accompanying or following a given 
phase of temperature may determine the full effect of the temperature. 
In those cases in which plants are killed by the action of low tem- 
peratures the most important factor to be considered is the actual 
duration of the period during which the plant is subjected to tempera- 
tures below 32°. Secondary to this are the considerations of the amount 
of precooling received by the plant, the actual minimum temperature 
to which it was taken, the condition of the soil and the atmosphere 
during the freezing, and the nature of the weather subsequent to it. 
In a previous paper* the writer has called attention to the manner in 
which the most critical phase of low temperature conditions increases 
in severity with increase of altitude. Even on the coldest winter days 
the temperature on the Desert never fails to rise above 32° during the 
mid-day. The lowest temperatures of winter are invariably accom- 
panied by a clear sky, and the days preceding and following very cold 
nights are clear. A cloudy or rainy period is always accompanied by 
more moderate temperatures, as is shown by the rarity of snow on the 
Desert itself. The longest duration of a shade temperature below 
freezing, in the ten-year records of the Desert Laboratory, is 19 hours. 
It frequently happens that a duration of 6 hours is the greatest for 
an entire winter. On ascending the mountains the length of the most 
prolonged period of freezing becomes greater until an altitude is 
reached at which there are occasional winter days when the air 
temperature does not rise above freezing. At this altitude there is 
a sudden increase in the maximum number of hours of frost from 
22 or 23 hours to a length of 40 to 45 hours. Such a sudden in- 
tensification in the duration of a critical climatic condition causes 
this condition to operate more sharply in the limitation of plant 
distribution than is the case with conditions that exhibit the usual 
form of slowly graduated change. 
No winter thermograph records have been secured in the Santa 
Catalinas, and it is therefore impossible to state the exact altitude at 
which this sudden intensification of the frost factor becomes manifest. 
It is probable that it lies at about 4,500 feet. The exposure of plants 
to insolation may often save them from the effects of an air tempera- 
ture of less than 32°, and in the case of succulents the temperature to 
which their tissue is raised during the insolation of the preceding day 
will shorten the period of freezing for them. These subsidiary matters 
* Shreve, Forrest. The Influence of Low Temperatures on the Distribution of the Giant 
Cactus. The Plant World, 14: 136-146, 1911. 
