W Harris; RE Beever; S Parkes; R Webster; S Scheele
Height growth was measured for 4 years in uniform garden environments at Auckland (36°53′S, 174°43′E), Lincoln, Canterbury (43°38′S, 172°29′E), and Invermay, Otago (45°51′S, 170°23′E) for 28 populations of Cordyline australis from wild sites covering 12° of latitude. Diameters of trunk bases were measured at the end of this period. Populations differed significantly in their final heights and in their growth rates in cool and warm seasons of the year. Generally, northern populations grew best at the Auckland garden site, especially in the warm season. Trunks of southern and higher altitudes were thicker and tree-height to trunk-diameter ratios declined the higher the latitude and altitude of population origin. Variation of population trunk diameter was related to phenotypic variation of stem diameter of wild populations. Consideration of the adaptive significance of these results suggests that more rapid and seasonally continuous height growth could confer a competitive advantage in warmer northern environments; in the south, photoperiodic restriction of winter growth could avoid cold damage. Sturdier trees from higher latitudes or altitudes could be more resilient to physical damage. Applications of the results to adaptation of plants to New Zealand environments for conservation and production purposes, to climate change, and to the health of C. australisare indicated.