Isohydric versus Anisohydric Tree Species: Do They Need Different Irrigation Strategies to Maximize Growth during Production?

 Isohydric woody plants regulate xylem water potentials under levels of moderate soil moisture, or moderate vapor pressure deficits (vpd) by progressive closure of stomata.  In contrast, anisohydric species maintain high transpiration rates that track vpd and are generally independent of soil moisture content until there is little water left to withdraw.  This results in very negative xylem water potentials below those that completely close stomata of isohydric species.  Pieces of data drawn from the past 23 years for one isohydric (Ulmus parvifolia ‘Allee’)  and one anisohydric  (Quercus viriginiana) species of each classification is assembled to highlight differences between these two types of trees in terms of transpiration, stomata conductance and changes in water use efficiency.  How these differences affect expanding shoot elongation and the need for different irrigation strategies to maximize growth while conserving irrigation water will be discussed.