Gap Partitioning among Maples (Acer) in Central New England: Shoot Architecture and Photosynthesis

Abstract

We measured shoot architecture and photosynthesis by three species of maple (Acer pensylvanicum, A. rubrum, A. saccharum) in response to understory and small canopy gaps in the mixed deciduous forests of central New England. Trees were felled to create six cleared gaps of two sizes (8 ° 12 m, 75 m2; 16 ° 24 m, 300 m2). Seedlings of the three species (2160 total, 720 per species) were transplanted into five plot locations (center plus northwest, northeast, southwest, and southeast gap edges) within all gaps and additional understory sites 1 yr before gap creation. Measurements of microclimates, architecture, photosynthetic performance and seedling survival and growth were made over 1 yr before, and 2 yr following, gap release. Architectural variation increased greatly over the 3—yr period. Striped maple (A. pensylvanicum) and red maple (A. rubrum) increased branch numbers, leaf numbers, and total leaf areas in gaps, especially large gaps, while sugar maple (A. saccharum) showed much smaller changes. Red maple tended to increase the number of leaves while leaf size decreased; striped maple increased leaf number but held leaf size constant. Diurnal patterns of photosynthesis by these species differed within and between gap and understory sites. Red maple showed higher photosynthetic rates per unit leaf area than striped and sugar maple in all site/plot combinations except the large—gap south plots, where striped maple exceeded red maple. Estimated diurnal shoot—level assimilation differentiated species more than unit area assimilation rates and also altered the rank order of performance, with striped maple > red maple > sugar maple in all microsites except the large gap north. Population—level assimilation vs. irradiance response curve exhibited a similar pattern, with red maple dominating unit area rates in most microsites. In contrast, shoot assimilation curves showed striped maple > red maple > sugar maple in all microsites except the large—gap north, where red maple > striped maple. Architectural variation among these species interacted with leaf—level assimilation rates to produce some differences among these species in shoot—level assimilation across the gap—understory microclimatic gradient. Since survival and growth patterns are usually correlated with differences in whole—plant carbon assimilation, our results suggest that there is some photosynthetic potential for gap partitioning among these three species of Acer.