Temperate oceanic treelines - Low temperature effects on photosynthesis and growth

Abstract

Altitudinal treelines form where tree growth is limited by low growing season temperatures. However, exactly what aspects of temperature are critical remains unclear. Temperate New Zealand treelines are at a lower altitude than in comparable temperate regions elsewhere. Past studies have shown them to be warmer, and suggested that New Zealand montane trees are not capable of growing at cool temperatures.
A detailed study at six sites showed that New Zealand treelines are not anomalously warm, but instead are within the global range of growing season soil temperature at treeline. The thermal environment in summer did not differ between abrupt and gradual treelines, but winters were much colder at the former. The consistency of mean daily minimum air temperature during the growing season at 20 oceanic treelines across the New Zealand archipelago suggests that thermal thresholds to tree growth are better described by minimum temperature, rather than often-used mean temperature. Extreme freezing temperatures are unlikely to control treeline position through dieback of adult trees, as the frost tolerance of trees of all species studied was in excess of the extreme minimum temperatures experienced at the New Zealand treeline. Overall, the proposition that an absence of hardy taxa in New Zealand has resulted in low treelines appears incorrect.
In controlled environment experiments, the response of photosynthesis and growth to growing season temperatures differed between the six treeline species. Three species showed evidence of reduced growth activities at low temperature (carbon sink limitation), whereas one species showed reduced growth at low temperature because of limited carbon acquisition (carbon source limitation). Different measures of sink- and source activities provided support for both hypotheses for two other species. These results highlight species-specificity in the temperature response of photosynthesis and growth as well as the complexity in the interpretation of carbon sink and source limitation hypotheses.
A combined approach in the field will be required to untangle the processes and thresholds that interact to determine tree growth and treeline positions.