Growing season temperatures limit growth of loblolly pine (Pinus taeda L.) seedlings across a wide geographic transect

Abstract

We grew potted loblolly pine (Pinus taeda L.) seedlings from a single provenance under well watered and fertilized conditions at four locations along a 610 km north-south transect that spanned most of the species range to examine how differences in the above-ground environ- ment would affect growth rate, biomass partitioning and gas exchange characteristics. Across the transect there was an 8.7C difference in average growing season tempera- ture, and temperature proved to be the key environmental factor controlling growth rate. Biomass growth was strongly correlated with differences in mean growing sea- son temperature (R 2 = 0.97) and temperature sum (R 2 = 0.92), but not with differences in mean daily pho- tosynthetic photon flux density or mean daily vapor pressure deficit. Biomass partitioning between root and shoot was unchanged across sites. There was substantial thermal acclimation of leaf respiration, but not photosyn- thesis. In mid-summer, leaf respiration rates measured at 25C ranged from 0.2 lmol m -2 s -1 in seedlings from the warmest location to 1.1 lmol m -2 s -1 in seedlings from the coolest site. The greatest biomass growth occurred near the middle of the range, indicating that temperatures were sub- and supra-optimal at the northern and southern ends on the range, respectively. However, in the middle of the range, there was an 18% decrease in biomass increment between two sites, corresponding to 1.4C increase in mean growing season temperature. This suggests that thermal acclimation was insufficient to compensate for this rela- tively small increase in temperature.