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Arctic Treeline is the transition from the boreal forest to the treeless tundra and may be determined by growing season temperatures. The physiological mechanisms involved in determining the relationship between the physical and biological environment and the location of treeline are not fully understood. In Northern Alaska, we studied the relationship between temperature and leaf respiration in 36 white spruce ( Picea glauca ) trees, sampling both the upper and lower canopy, to test two research hypotheses. The first hypothesis is that upper canopy leaves, which are more directly coupled to the atmosphere, will experience more challenging environmental conditions and thus have higher respiration rates to facilitate metabolic function. The second hypothesis is that saplings [stems that are 5–10cm DBH (diameter at breast height)] will have higher respiration rates than trees (stems ≥10cm DBH) since saplings represent the transition from seedlings growing in the more favorable aerodynamic boundary layer, to trees which are fully coupled to the atmosphere but of sufficient size to persist. Respiration did not change with canopy position, however respiration at 25°C was 42% higher in saplings compared to trees (3.43±0.19 vs. 2.41±0.14μmolm −2 s −1 ). Furthermore, there were significant differences in the temperature response of respiration, and seedlings reached their maximum respiration rates at 59°C, more than two degrees higher than trees. Our results demonstrate that the respiratory characteristics of white spruce saplings at treeline impose a significant carbon cost that may contribute to their lack of perseverance beyond treeline. In the absence of thermal acclimation, the rate of leaf respiration could increase by 57% by the end of the century, posing further challenges to the ecology of this massive ecotone. 

Abstract Photosynthetic traits suggest that shade tolerance may explain the contrasting success of two conifer taxa, Podocarpaceae and Pinaceae, in tropical forests. Needle‐leaved species fromPinus(Pinaceae) are generally absent from tropical forests, whereasPinus 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.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(basalR), andbandc(together describing the shape of the response).Needle‐leaved species (Pinus kesiya,Pinus dalatensisandDacrydium elatum) had higher rates of area‐basedRat 25°C (R_25‐area) as well as higher area‐based modelled basal respiration (a) than flat‐leaved species (P. krempfii,Podocarpus neriifolius,Dacrycarpus imbricatus,Nageia nana,Taxus wallichiana,Keteeleria evelynianaandFokienia hodginsii). No significant differences were found between needle‐ and flat‐leaved species in mass‐basedR₂₅(R_25‐mass) or in the shape of the RT response (bandc); however, interspecific differences inR_25‐mass,Rat nighttime temperature extremes (R_4.1andR_20.6) and leaf traits were apparent.Differences inR_25‐areaandasuggest that needle‐leaved foliage may be more energetically 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.Interspecific differences inR_25‐massand leaf traits highlight that varying ecological strategies are employed by conifers to coexist and survive in the Central Highlands of Vietnam. Ultimately, these data further our understanding of current conifer biogeographical distributions and underscore the need for additional studies to elucidate the effects of extreme temperature events on the continued survival of conifers in this unique forest. A freePlain Language Summarycan be found within the Supporting Information of this article.