LAUSR

Plant respiration can acclimate to changing environmental conditions and vary between species as well as biome-types, though belowground respiration responses to on-going climate warming is not well-understood. Understanding the thermal acclimation capacity of root respiration (Rroot) in relation to increasing temperatures is therefore critical in elucidating a key uncertainty in plant function in response to warming. However, the degree of temperature acclimation of Rroot in rainforest trees and how root chemical and morphological traits are related to acclimation is unknown. Here we investigated the extent to which respiration of fine roots (≤ 2 mm) of four tropical and four warm-temperate rainforest tree seedlings differed in response to warmer growth temperatures (control and + 6°C), including temperature sensitivity (Q10) and the degree of acclimation of Rroot. Regardless of biome-type, we found no consistent pattern in the short-term temperature responses of Rroot to elevated growth temperature: a significant reduction in the temperature response of Rroot to +6°C treatment was only observed for a tropical species, Cryptocarya mackinnoniana, whereas the other seven species had either some stimulation or no alteration. Across species, Rroot was positively correlated with root tissue nitrogen concentration (mg g-1), while Q10 was positively correlated with root tissue density (g cm-3). Warming increased root tissue density by 20.8%, but did not alter root nitrogen across species. We conclude that thermal acclimation capacity of Rroot to warming is species-specific, and suggest that root tissue density is a useful predictor of Rroot and its thermal responses of rainforest tree seedlings.