Abstract
Frost damage is among the major limitations to reforestation and forest restoration projects worldwide. Investigations of environmental and genetic effects on frost resistance have focused on boreal and temperate tree species rather than tropical trees. Koa (Acacia koa A. Gray) is a valuable tropical hardwood tree species endemic to the Hawaiian Islands, USA. Koa occurs across a wide elevational gradient, and newly planted trees are subject to winter frost at high elevations. We sought to determine whether different koa populations show variation in freeze hardiness as a cold-tolerance mechanism, and whether exposure to hardening conditions prior to frost exposure can modify koa cold-tolerance adaptation. Seeds from 13 populations of koa (Acacia koa A. Gray) were collected across an elevational range (603–2050 m) on the Island of Hawai’i. Four-month-old seedlings grown from the 13 population seed sources were divided into control (non-acclimated) and cold-acclimated treatments, maintained at 26 °C/22 °C (day/night) or exposed to gradually decreasing temperatures to 8 °C/4 °C (day/night), respectively. After six weeks, control and cold-acclimated seedlings from each population were tested for freeze tolerance by electrolyte leakage at five test temperatures ranging from 5 °C (control) to −20 °C. Treatment effects were mainly observed at the lowest test temperatures (−15 and −20 °C). A higher index of cold damage occurred in the non-acclimated seedlings for most of the populations. Several of our higher elevation populations showed greater cold tolerance than populations from lower elevations, particularly when cold-acclimated. Our results suggest that cold acclimation may increase frost hardiness in a tropical forest tree species, and that there is likely some adaptive variation in frost tolerance among populations from different elevations. Cold acclimation could be a useful tool to prepare koa seedlings to be planted in high-elevation sites prone to freezing winter temperatures.
Highlights
The target plant concept provides a useful framework to help identify and overcome limiting factors in reforestation and forest restoration projects [1,2,3]
We accomplished this objective by measuring frost tolerance for koa seedlings representing a range of elevation zones using electrolyte leakage, which provides an effective measure of cold tolerance in forest trees [31], and has been used previously with koa [27]
To gradually expose the greenhouse-grown seedlings to changes in chilling and light regime upon transfer to the growth chambers, the temperatures were gradually lowered according to the specifications of Table 2, to final temperatures of 8 ◦ C/4 ◦ C, with the photoperiod reset to 11 h light/13 h dark, and light intensity of 200 μmol/m2 /s (Table 2)
Summary
The target plant concept provides a useful framework to help identify and overcome limiting factors in reforestation and forest restoration projects [1,2,3]. The most important limiting site factor for the restoration of koa and other native species at high-elevation sites in Hawai’i is exposure of planted seedlings to winter frost. The objective of this study was to assess the differences in freeze-tolerance responses between koa populations occurring across an elevational range on the Island of Hawai’i, as well as to determine whether exposure to hardening conditions prior to frost exposure can modify cold tolerance adaptation We accomplished this objective by measuring frost tolerance for koa seedlings representing a range of elevation zones using electrolyte leakage, which provides an effective measure of cold tolerance in forest trees [31], and has been used previously with koa [27]. Specific research questions were as follows: (1) Do koa populations from all altitudinal zones develop freeze hardiness as a mechanism to tolerate frost? (2) Do populations of koa from higher elevations show greater freeze tolerance than koa populations from lower elevations? (3) To what extent does induction of hardening via cold acclimation affect the freeze tolerance of koa?
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