External heat-pulse method allows comparative sapflow measurements in diverse functional types in a Mediterranean-type shrubland in South Africa

Abstract

There has been limited application of sapflow technology to small-stemmed species and across co-existing functional types, restricting its use in diverse floras such as the Mediterranean-type shrubland in South Africa. Our main objective was to test whether sapflow may provide an alternative to traditional gas-exchange measurements, which would permit comparative evaluation of transpiration at a previously unattained temporal resolution. We tested miniature external heat ratio method (HRM) sapflow gauges on three co-occurring functional types with contrasting stem or culm anatomies and examined the relationship between sapflow and shoot- and leaf-level water loss in both a controlled and field environment. Our sapflow gauges captured dynamic patterns of transpiration in both settings for all three functional types. In a controlled environment the relationship between sapflow and transpiration was linear in all three species with r2 values ranging from 0.78 for Cannomois congesta Mast. (Restionaceae) to 0.96 for Protea repens (L.) L. (Proteaceae) and Erica monsoniana L.f. (Ericaceae). In the field, r2 values were lower, ranging from 0.59 for C. congesta to 0.74 for P. repens. We discuss the efficacy and potential of this methodology to cast light on patterns of community ecology in functionally diverse shrublands by capturing continuous variation in transpiration.