Patterns of leaf resistance to lodgepole pine transpiration in wyoming

TLDR: The results indicate that stomatal control of transpiration is more important towards the end of the growing season and that the degree of control varies with the soil water regime of the site.

Abstract: Leaf resistance (r_L) of lodgepole pine (Pinus contorta ssp. latifolia [Engelm. ex Wats] Critchfield) was measured with an aspirated porometer at three sites in the Medicine Bow Mountains, Wyoming. In the early part of the growing season all the sites had low leaf resistances (3—5 s/cm) and few significant changes in r_L during the day. Towards the end of the growing season, in August, significant increases in midday leaf resistance occurred at all sites. The greatest increase (to 57 s/cm) was found on the driest site. Smaller increases (to 10—14 s/cm) were measured at a dry—mesic site and a mesic site. The results indicate that stomatal control of transpiration is more important towards the end of the growing season and that the degree of control varies with the soil water regime of the site. The response of r_L to soil H_2O status appears to be nonlinear. When based pressure potential readings were above an upper threshold ( -9 bars) r_L did not increase during the day. Below a lower threshold ( -17 bars) r_L increased significantly. Between the two thresholds, changes in r_L under high light conditions are explained more readily by the interaction between vapor pressure deficit and soil H_2O status than by either factor alone.