A Test of the Climate Hypothesis for Divaricate Plants

To test the hypothesis that the divaricate habit protects internal leaves from cold and dry conditions, and raises leaf temperatures on cold sunny days, thermocouples were attached to internal and external leaves on divaricate plants. For Coprosma propinqua plants at Cass, inland Canterbury, there were consistent differences in temperature between leaves, but these were not simply related to their canopy position (interval vs. external). While leaf temperatures fell as low as -9.5 °C on frosty nights, internal leaves were not consistently warmer (between 0.28 °C warmer and 0.04 °C colder) than external leaves. Consistent differences in temperature between leaves were not clearly related to their position or degree of exposure. Internal leaves near the top of the canopy were often close in temperature to external leaves at the same height. Specific humidity was higher inside the bush than outside, but again the effect was small. On calm, sunny winter days, internal leaves were usually slightly cooler than external leaves. For Hoheria angustifolia on the Port Hills, there was only a slight temperature difference between internal and external leaves (0.3 to 0.5 °C). Overall, the temperature and humidity data give little support to the climate hypothesis. However, there was more damage (apparently due to frost) to Coprosma propinqua leaves on the outside of bushes over winter 1989, suggesting that the biological significance of the divaricate habit may be greater than these physical data would indicate.