Respiratory temperature responses of tropical conifers differ with leaf morphology

1. Photosynthetic traits suggest that shade tolerance may explain the contrasting success of two conifer taxa, Podocarpaceae and Pinaceae, in tropical forests. Needle-leaved species from Pinus (Pinaceae) are generally absent from tropi- cal forests, whereas Pinus krempfii, a flat-leaved pine, and numerous flat-leaved Podocarpaceae are abundant. Respiration (R) traits may provide additional insight into the drivers of the contrasting success of needle- and flat-leaved conifers in tropical forests. 2. We measured the short-term respiratory temperature (RT) response between 10 and 50°C and foliar morphological traits of three needle- and seven flat-leaved conifer species coexisting in a tropical montane forest in the Central Highlands of Vietnam containing notable conifer diversity. We fit a lognormal polynomial model to each RT curve and extracted the following three parameters: a (basal R), and b and c (together describing the shape of the response). 3. Needle-leaved species (Pinus kesiya, Pinus dalatensis and Dacrydium elatum) had higher rates of area-based R at 25°C (R25-area) as well as higher area-based mod- elled basal respiration (a) than flat-leaved species (P. krempfii, Podocarpus neriifolius, Dacrycarpus imbricatus, Nageia nana, Taxus wallichiana, Keteeleria evelyniana and Fokienia hodginsii). No significant differences were found between needle- and flat-leaved species in mass-based R25 (R25-mass) or in the shape of the RT response (b and c); however, interspecific differences in R25-mass, R at nighttime temperature extremes (R4.1 and R20.6) and leaf traits were apparent. 4. Differences in R25-area and a suggest that needle-leaved foliage may be more en- ergetically costly to maintain than flat-leaved foliage, providing new insight and additional support for the hypothesis that shade tolerance is an important driver of Podocarpaceae success and Pinaceae absence in the majority of tropical forests. 5. Interspecific differences in R25-mass and leaf traits highlight that varying ecological strategies are employed by conifers to coexist and survive in the Central Highland