Theophrastus’ challenge accepted: Correlating "‘easy" plant functional traits with frost resistance in rainforest seedlings

Background/Question/Methods ‘Easy’ plant functional traits are readily measured, but may be less directly related to plant function, whereas ‘hard’ traits are closely tied to plant function, but are time-consuming or difficult to measure. Both are useful in predicting plant response to disturbance, but ‘easy’ traits are more valuable in predicting response of many species. Frost is an important disturbance due to its role in determining species distribution and the damage it can cause to susceptible species, e.g. leaf loss and plant death. Many studies have identified ‘hard’ traits associated with frost resistance (ability to withstand damage), but few have attempted this with ‘easy’ traits. A severe frost occurred in June and July 2007 on the Atherton Tablelands, Queensland, Australia and extensively damaged restoration plots of upland rainforest species, providing the opportunity to assess relationships between frost resistance and ‘easy’ traits (wood density (WD), leaf size, specific leaf area (SLA) and leaf phenology). We scored frost resistance for 589 rainforest seedlings from 20 species immediately after the frost by estimating proportion of foliage retained. Frost resistance of species was related to species mean trait values as measured on healthy individuals and also derived from the literature (for WD, leaf length and width).

Results/Conclusions Foliage retention significantly varied among species; some lost almost all foliage, others retained nearly 100%. As predicted, proportion foliage retention was significantly correlated with both measured and literature WD. Unexpectedly, proportion foliage retention was not significantly related to leaf area, leaf width, or measured leaf length, but was significantly negatively correlated with literature leaf length and SLA. When regressions were split by leaf phenology there was no relationship between leaf retention and WD or SLA among deciduous species, but there was a significant positive relationship between leaf retention and WD and a significant negative relationship between leaf retention and SLA among evergreens. This demonstrates that ‘easy’ traits can explain frost resistance in rainforest seedlings. Many of these traits are also widely available in the literature, suggesting that predictions can be made for many species and can guide future restoration projects. Frost resistance was driven by evergreen species: resistance/trait relationships were not found in deciduous species. A related study showed the opposite result: patterns of frost resilience (ability to recover) were driven by deciduous species. Collectively, these results suggest that evergreen rainforest species must be resistant to survive a severe frost, whereas highly damaged deciduous species rely on resilience to survive.