Photosynthesis and leaf morphology of Liquidambar styraciflua L. under variable urban radiant energy conditions

Roger K. Kjelgren and James R. Clark

Abstract

Diminished sunlight, characteristic of urban canyons, has been suggested as potentially limiting to plant growth. This study investigated the response of sweetgum (Liquidambar styraciflua L.) to variable irradiance in a range of urban locations. Diurnal photosynthesis was measured in situ on mature trees, comparing an open site at an urban park with an urban canyon that received four hours of midday sun in midsummer. Photosynthesis for trees growing in the canyon was lower both during shaded and sunlit periods as compared with trees at the park. Photosynthesis of detached shoots in a growth chamber was greater in canyon than park foliage at low irradiance, indicating possible photosynthetic shade acclimation typical of tree species growing in the forest understory. Shoot and trunk growth and morphological characteristics were measured on L. styraciflua growing along boulevards at 15 additional urban sites and related to seasonal receipt of solar radiation. Angular elevation and orientation of buildings and trees that defined the horizon topography at each site were used in modeling the potential irradiance of global short-wave radiation. Seasonal irradiance among sites ranged from 21% in the urban core to nearly 95% in outlying residential districts of that po­tentially received under an unobstructed horizon. Shade acclimation was confirmed by dif­ferences in leaf morphology, as foliage became flatter, thinner, and more horizontally oriented at sites with lower irradiance. Photosynthetic and morphological acclimation to shade did not compensate for lower available radiant energy as both shoot and trunk growth decreased at lower-irradiance sites. Unlike the forest understory, the static light en­vironment of urban canyons may subject shade-intolerant species such as L. styraciflua to chronic, low-radiant-energy stress.

Supporting pdf: Photosynthesis and leaf morphology of Liquidambar styraciflua L. under variable urban radiant energy conditions