Sap Flow and Water Consumption of Captain Cook Tree [Cascabela thevetia (l.) Lippold

Abstract

A two-year field study documented the diurnal and nocturnal sap flow rates and water consumption of young (YCC), adult (ACC) and mature (MCC) Captain Cook trees [Cascabela thevetia (L.) Lippold] that were invading a riparian habitat in northern Queensland. For comparison, two native trees [black tea tree (Melaleuca bracteata F. Muell.) and Moreton Bay ash (Corymbia tessellaris (F.Muell.) K.D.Hill & L.A.S.Johnson)] growing in association with Captain Cook tree were also monitored. Sap flow measurements were grouped into eight timeframes per day (early morning, late morning, early afternoon, late afternoon, early night, late night, early dawn and late dawn). Significant interactions in sap flow rate occurred between plant types, timeframes, and months. The magnitude of sap flow rate was Moreton Bay ash>YCC>ACC>black tea tree>MCC. Maximum sap flow rates tended to occur during early (1-3 pm) to mid-afternoon (4-6 pm) for all age groups of Captain Cook tree and the two native trees. Diurnal sap flow rates were significantly greater than nocturnal, and on a monthly basis sap flow rates were highest over the spring to autumn period (September-May) and lowest during winter (June–August). Significant differences in water consumption also occurred between species and months. Water consumption peak time varied between plant types with most plants peaking in January except for MCC and Moreton Bay ash trees for which peak water consumption occurred in June and July respectively. Water consumption was high across all seasons except winter. The magnitude of water consumption was Moreton Bay ash>black tea tree>YCC>ACC>MCC trees. Moreton Bay ash registered maximal monthly water consumption (4700 L) compared with minimal consumption by MCC trees (55 L). On average, Captain Cook trees used 99% and 72% less water than Moreton Bay ash and black tea trees respectively. The significantly lower water consumption by Captain Cook trees compared with Moreton Bay ash and black tea trees may be offset by high population densities. Results also suggest that knowledge of optimal sap flow timeframes may be advantageous in exploring optimal timing for application of control operations related to management of Captain Cook trees.