A Study on Variation in Leaf Physiognomy of City Trees in Relation to Temperature

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. We hypothesized trees species will acclimate to increased temperatures by increasing the dissection of their leaves which would be more prominent in species with wider natural distributions. In this study we focused on 7 species growing across three sites distributed along a coastal to inland temperature 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 three specimens from each species were selected at each site. From each specimen, three recently mature, sun-exposed leaves were collected from each tree, scanned and the Dissection Index calculated. 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 findings 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