308 



ECOLOGICAL FACTORS: 



Figure 17.4 The hydrologic cycle, emphasizing the more common relationships between water and 

 on environment. The water available to organisms in a given habitat is not simply the sum of pre- 

 cipitation and condensation. From this sum water is lost to lite by evaporation into the air, in- 

 terception by plants, runotT to other areas, adsorption or being bound to the soil, ond percolation 

 to depths beyond those penetrated by lite. (From George K. Reid, Ecology of Inland Waters ond 

 Estuaries, Reinhold Publishing Corp., New York, 1961.) 



greater temperature, wind velocity, and solar radia- 

 tion all increase water loss, and thus decrease the 

 amount of available moisture. Also of primary im- 

 portance in water availability is soil porosity. This 

 is the case because precipitation is the basic source 

 of soil moisture, which in turn is the main source of 

 plant moisture, and the physical nature of the soil 

 determines what water is available to plants. The 

 water-holding capacity and drainage of the soil are 

 instrumental in how much moisture is held in soil 

 spaces and how much is available to plants. This 

 interrelation of various factors acting on available 

 moisture is another example of the complete inter- 

 dependence of physical factors in a habitat. 



In spite of the importance of interrelations of 

 available moisture factors, rainfall alone can be used 

 to indicate the type of habitat in an area. Deserts 

 regularly exist wherever the rainfall is 10 or less 

 inches per year, because there is insufficient moisture 

 for most plants during all parts of the year. Grass- 

 lands, savannas, and open woodlands are found 

 where the rainfall is 10 to 30 inches per year. Dry 



forests are in places of 30 to 50 inches of rainfall. 

 Wet forests are in areas with over 50 inches of rainfall. 



PLANT DORMANCY 



Contrary to popular belief, plant dormancy is re- 

 lated most closely to available water and only sec- 

 ondarily to temperature. Dormancy occurs mostly 

 during the dry season. However, in forest areas 

 plants may be dormant during the winter, for even if 

 water is available temperature may be sufficiently low 

 to retard plant functions, including the takeup of 

 water. 



Dormancy adaptations are pronounced in de- 

 ciduous plants. The loss of deciduous leaves, which 

 because of their functions make tremendous demands 

 for moisture, preserves water supplies in nonde- 

 ciduous structures. When leaves are removed the 

 plant is mobilized for extreme conditions of the 

 environment, conditions that owing to plant 

 dormancy are mostly avoided. 



Most deciduous plants make ready for the next 



