Variation in city tree ecophysiological characteristics under changing temperatures

Background/Question/Methods

Climate models are predicting that the world will become warmer and understanding how individual tree species cope with increased temperatures will be important for making predictions of species survival.  City trees are valuable resources as they include trees from a wide variety of habitats under regular watering regimes with only temperatures changing.  A tree’s ability to acclimate to changing temperatures may be controlled by plasticity in leaf shape and stomatal conductance.  We hypothesized species with wider environmental distributions would show greater plasticity than those with narrower distributions and that those individuals in warmer areas would have more highly dissected leaves and reduced stomatal conductance levels.  For this study we focused on 10 species growing across three sites distributed along a coastal to inland gradient in southern California, USA.  Selected trees had the requirement of being found in vegetated areas with regular watering in at least two of the three sites.  A minimum of five specimens from each species were selected at each site.  Three recently mature, sun-exposed leaves were collected from each tree, scanned and the Dissection Index calculated.  Stomatal conductance of leaves will be measured during peak growth periods.

Results/Conclusions

Initial measurements indicate that of the species which have been measured two, Platanus racemosa (p = 0.0265) and Schinus terebinthifolius ( p < 0.001) show increasing levels of leaf dissection across the temperature gradient while one, Jacaranda mimosifolia, no difference was detected (p > 0.05) in part because of a high degree of variability across the gradient.  The species with the highest increases in dissection, P. racemosa, originated in limited riparian environments while the species with lower dissection levels, S. terebinthifolius, came from a wider environmental distribution.  These finds support a hypothesis of increasing dissection with increasing heat, but do not support the hypothesis that natural habitat distributions may be linked to plasticity.  The high degree of variability in J. mimosifolia, which is considered invasive in tropical areas of the United States, may suggest that this species may survive by producing a wide range of individual capable of surviving in different habitats.