Honey Mesquite Water Relations and Gas Exchange Following Herbicide-Induced Morphological Change

Honey mesquite (Prosopis glandulosa Torr.) may maintain apical dominance after a treatment that causes partial top-kill (PTK) and leaves canopies with "stem flagging." In contrast, top-killing treatments stimulate multistemmed regrowth (i.e., basal sprouting; BSP). Because this difference may impact competition with grasses, a better understanding of physiology associated with PTK and BSP canopies compared with untreated canopies is needed. We quantified predawn leaf water potential (psi(PD)), leaf-level gas exchange rates (photosynthesis [A] and stomatal conductance [g(s)]), and whole-tree stomatal conductance (G(s)) of untreated, PTK, and BSP mesquites 2-11 yr after aerial herbicide application in a north Texas savanna. Total leaf area was nearly 4 xgreater in untreated and BSP compared with PTK trees. In a few situations where soil moisture stress was greatest, untreated mesquites exhibited more negative psi(PD) and lower leaf-level gas exchange than did PTK mesquites. BSP mesquites occasionally had greater psi(PD) and leaf-level gas exchange than untreated mesquites. Since imbalances in root-to-shoot ratios caused by PTK were largely not manifested at the leaf-physiology level, PTK mesquites likely adjusted rapidly to herbicide disturbance. When g(s) was scaled to the whole canopy, G(s) estimates were 3-4 xgreater in untreated and BSP than PTK trees. Thus, canopy leaf area was the primary driver of differences in whole-canopy G(s). It is LIKELY that transpiration would be lower in a stand of untreated mesquites compared with PTK mesquites of similar stand density and basal areas before treatment, as results from comparing untreated and PTK mesquites indicate. Partially top-killed trees that maintain apical dominance and do not resprout should compete less with grasses for water and light and facilitate species diversity and heterogeneity. (C) 2020The Society for Range Management. Published by Elsevier Inc. All rights reserved.