A Severe Drought: Impact on Tree Growth, Phenology, Net Photosynthetic Rate and Water Relations

Abstract

An unusually severe drought occurred in central Missouri [USA] during the summer of 1976. The drought resulted in an average soil water potential of -26.1 bars in the upper 45 cm of soil profile in spite of the addition of 4.9 cm of irrigation water. Its effects were examined on phenology, growth and physiological processes, as well as the water relations of white oak (Quercus alba L.) and 7 other species [Acer saccharum, Q. rubra, Q. velutina, Juglans nigra, Juniperus virginiana, Gornus florida and Helianthus strumosus] in this oak-hickory forest. The drought had a dramatic impact on base (presunrise) xylem pressure potential of white oak in both irrigated (-19.6 to -34.3 bars) and nonirrigated specimens (-27.8 to -45.2 bars). Growth was reduced, die-back increased, net photosynthetic rate was depressed to near the compensation point and phenological patterns in the following year (1977) were altered. An estimation of the number of days on which stomata were closed for most of the photoperiod was compared to the number of days when mature leaves were present. The following ranking of species based on this index of stomatal control was possible (from most to least time spent with stomata closed): sunflower > flowering dogwood = black walnut > sugar maple > northern red oak > white oak = eastern red cedar = black oak. Various drought avoidance mechanisms are also presented and discussed in regard to these 8 species. The long periods of low soil water potentials and base xylem pressure potentials experienced during the drought of 1976 did not prevent the recovery of the growth processes that autumn or the subsequent spring. All study species seemed well-adapted to survival and to continued functioning during this severe drought.