Estimating transpiration from 6‐year‐old Eucalyptus grandis trees: development of a canopy conductance model and comparison with independent sap flux measurements

Abstract

Daily patterns of stomatal conductance (gj, xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one-year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured g^ was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (gc) was determined for hourly intervals using g, measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of g^ and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly-aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter-time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of gs to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.

Forestry and Water Conservation in South Africa: History, Science and Policy

“…The creation of a forestry department 63 within the Cape Agriculture Ministry in 1881 significantly shaped the future of tree planting in the colony. A number of state-sponsored scientific programs began during the 1880s, including veterinary science, agricultural research, and irrigation.…”

“…In turn, this allowed in-field estimates of tree growth and water use, and direct estimates of water-use efficiency, 62 with the results appearing in a major series of scientific reports. 63 The availability of techniques to measure transpiration in the plant and to scale up to the canopy of the vegetation augmented measurements in experimental catchments and together these allowed the validation of techniques to measure evapotranspiration across the whole landscape. This created the capability to measure evapotranspiration directly without having to set up new catchment experiments, using the available gauged catchments as essential benchmarks.…”

“…(Cambridge University Press, 1991), 178-81; see 108-9 on natural and fortuitous biocontrol. 63 The government entity for public forestry took different forms in this period: in 1876 the Cape government formed a separate Department of Plantations and Forests as a Divison of Agriculture, from 1881 to 1891 the function formed part of the entity called the Commissioner for Crown Lands, Mines and Agriculture, and so on. We use the phrase 'department of forestry' or 'forestry department' to refer generically to any one of the forms the function took.…”

“…62 Early experiments in raingauging began to reveal the magnitude and relevance of local variations in rainfall. After resolving raingauge design and orientation, 63 the work moved on to studying the variability of measured rain, in time and space. Fifteen raingauges were installed at increasing distances from the Dwarsberg plateau, so as to represent altitudinal zones throughout the valley.…”

“…Colonial foresters restructured access to indigenous forests through a variety of means, including policing, fines, and negotiation with elected, and later appointed, headmen, while in the Transkei and other regions of communal 60 Hutchins,Transvaal Forest Report,136, tenure, permitting the teza and access to other resources by licence. 63 Foresters, however, found white and 'coloured' sawyers entering the territories from the Cape to present as great or an even greater problem; eventually, access to indigenous forest resources was limited to residents of the territories, and sale of timber to outsiders prohibited. 64 Regulations to control access to forests and to prevent fires led to the apprehension and fining of both interlopers from the Cape as well as Africans on the frontier boundaries of what is now the Eastern Cape.…”