Photosynthesis is a fundamental process that trees perform over fluctuating environmental conditions. In cities, photosynthetic variation plays a role in tree growth and condition, in a variety of ecosystem services (e.g. shade, rainwater through-fall, and particulate deposition), in the longevity of urban trees, and in the sustainability of urban forests. Yet, tree physiological responses in urban environments are understudied, despite their importance in urban forest planning and management. Here, gas exchange and water use efficiency were compared in a series of measurements along an urban-rural gradient and in a common garden within a city, for red maple (Acer rubrum). This species is one of the most commonly planted trees in eastern and Midwestern cities in the United States and numerous horticultural varieties (cultivars) have been developed. Cultivars are bred for desirable and consistent morphologies relative to wildtype trees, but the impact of intraspecific variation and of selection for particular morphologies on physiological and ecological responses, and thus, the long-term performance of planted trees, is relatively unknown.
Results/Conclusions
Photosynthesis and water use efficiency differed significantly between mature red maples at urban and rural sites, however, differences were strongly related to tree genetic background rather than differences in environmental conditions per se. Overall, urban cultivars had higher maximum rates of photosynthesis but urban wildtype trees had higher water use efficiency. Differences between wildtypes and cultivars lessened as seasonal temperatures increased. In a common garden, intraspecific variation also played a role in the physiological responses of red maple seedlings. A cultivar with a high maximum rate of photosynthesis had lower total photosynthetic capacity, due to its morphology, because it produced new red leaves continuously but high rates of photosynthesis were only achieved by mature green leaves. The abundance of the insect pest gloomy scale (Melanaspis tenebricosa) was also significantly influenced by tree genetic background and water availability. In cities, tradeoffs between cultivars or between cultivar and wildtype trees are important to consider in creating a resilient and sustainable urban forest. Improving our understanding of how morphology influences tree physiology and ecology will ultimately enable better urban forestry and improve our predictions for how trees in urban and rural environments to respond to future environmental change.