Using the TREGRO simulation model we examined the relative impact of asymmetric and symmetric elevations of day/night temperatures on 3 yr biomass gain of yellow-poplar (Liriodendron tulipifera L.) and loblolly pine (Pinus taeda L.). Temperature elevation scenarios used were: (1) an asymmetric 0.3°C/0.9°C increase in day/night temperature (T + 0.3/0.9) as observed since the turn of the century; (2) a symmetric 4°C elevation during both day and night periods (T + 4) as predicted by general circulation models; and (3) an asymmetric 2°C/6°C elevation of day/night temperature (T + 2/6). TREGRO incorporates temperature effects on photosynthesis, respiration, growth rate, and phenology. In both species for all temperature elevation scenarios, respiratory increases exceeded photosynthetic increases and reduced belowground growth. In yellow-poplar, belowground growth was reduced by 7.6% in the T + 0.3/0.9 scenario, whereas the T + 4 and T + 2/6 scenarios reduced belowground growth by 145% and 155%, respectively. The elevated temperature scenarios increased total tree biomass of loblolly pine, but belowground growth was reduced by 24%, 111%, and 113% by the T + 0.3/0.9, T + 4, and T + 2/6 scenarios, respectively. Reductions in root biomass at elevated temperatures may influence competitive interactions between forest tree species and susceptibility to environmental fluctuations. For. Sci. 46(2):248-257.
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