title: Becoming less tolerant with age: sugar maple, shade, and ontogeny.
abstract: Although shade tolerance is often assumed to be a fixed trait, recent work suggests ontogenetic changes in the light requirements of tree species. We determined the influence of gas exchange, biomass distribution, and self-shading on ontogenetic variation in the instantaneous aboveground carbon balance of Acer saccharum. We quantified the aboveground biomass distributions of 18 juveniles varying in height and growing in low light in a temperate forest understory in Minnesota, USA. Gas exchange rates of leaf and stem tissues were measured, and the crown architecture of each individual was quantified. The YPLANT program was used to estimate the self-shaded fraction of each crown and to model net leaf-level carbon gain. Leaf respiration and photosynthesis per gram of leaf tissue increased with plant size. In contrast, stem respiration rates per gram of stem tissue declined, reflecting a shift in the distribution of stem diameter sizes from smaller (with higher respiration) to larger diameter classes. However, these trends were outweighed by ontogenetic increases in self-shading (which reduces the net photosynthesis realized) and stem mass fraction (which increases the proportion of purely respiratory tissue) in terms of influence on net carbon exchange. As a result, net carbon gain per gram of aboveground plant tissue declined with increasing plant size, and the instantaneous aboveground light compensation point increased. When estimates of root respiration were included to model whole-plant carbon gain and light compensation points, relationships with plant size were even more pronounced. Our findings show how an interplay of gas exchange, self-shading, and biomass distribution shapes ontogenetic changes in shade tolerance.
authors: Department of Plant Biological Sciences, University of Minnesota, 1445 Gortner Avenue, St. Paul, MN, 55108, USA. senda002@umn.edu.|Department of Biological Sciences, Macquarie University, Sydney, NSW, 2109, Australia. senda002@umn.edu.|Biology Department, University of Minnesota, Morris, MN, 56267, USA. senda002@umn.edu.
authors: Kerrie M
authors: KM
authors: Sendall
authors: Department of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand.
authors: Christopher H
authors: CH
authors: Lusk
authors: Department of Forest Resources, University of Minnesota, 1530 Cleveland Avenue N, St. Paul, MN, 55108, USA.|Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, NSW, 2753, Australia.
authors: Peter B
authors: PB
authors: Reich
mesh_terms: D031002:Acer / Q000254:growth & development* / Q000378:metabolism; D018533:Biomass; D002244:Carbon / Q000032:analysis / Q000378:metabolism*; D065928:Forests; D008027:Light; D008910:Minnesota; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000254:growth & development; D018517:Plant Roots / Q000254:growth & development; D018547:Plant Stems / Q000254:growth & development
chemical_list: D002244:Carbon
keywords: Acer saccharum; Carbon balance; Crown architecture; Dark respiration; Light compensation point