Browsing by Author "Walters, Michael B"
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Item Are shade tolerance, survival, and growth linked? Low light and nitrogen effects on hardwood seedlings(1996) Walters, Michael B; Reich, Peter BVariation in shade tolerance is a primary mechanism driving succession in northern deciduous forests. However, little is known about interspecific differences in the traits responsible for shade tolerance. Is shade tolerance due to the ability to grow or survive in deep shade, or both? How do plant morphology and photosynthesis relate to growth in shade? Is low light the sole critical stress determining differences in "shade tolerance" or do below ground resources interact with low light to affect growth and survival? In this study we address these questions for seedlings of Betula papyrifera Marsh., Betula alleghaniensis Britton, Ostrya virginiana (Mill.) K. Koch, Acer saccharum Marsh., and Quercus rubra L. grown for 2 yr in outdoor shade houses in a complete factorial of low light (2 and 8% open sky) and nitrogen (forest soil and forest soil plus 200 kg N.ha-'.yr-'). For these seedlings we examined effects of light and nitrogen on the interrelationships among survival, growth, and shade tolerance and explored the physiological bases of shade tolerance by examining the relationship of plant morphology and photosynthesis to growth. Nitrogen amendments did not have a significant effect on any plant trait at either light level. In 8% light, growth and survival were highest for shade-intolerant Betula papyrifera and mid-tolerant Betula alleghaniensis, lower for shade-tolerant Ostrya and Acer, and lowest for disturbance-adapted Quercus. In 2% light, species rankings reversed as Ostrya and Acer had higher growth and survival than the other species. Second-year survival was strongly related to 1st-yr growth (P < 0.001), whereas relationships with 1st-yr plant mass and 1styr absolute growth rates were weak. Therefore, survival of shade-tolerant species at 2% light was related to their maintenance of positive growth, whereas intolerant species had growth near zero and high rates of mortality. In both 2 and 8% light photosynthetic rates on mass (but not area) bases and the proportion of the plant in leaves (leaf area ratio and leaf mass ratio) were positively related to growth. Greater rates of growth and survival for shade-tolerant species in very low light, and for intolerant species in higher light, suggest that there is a species-based trade-off between maximizing growth in high light and minimizing the light compensation point for growth. This trade-off may be an important mechanism driving forest community dynamics in northern hardwood forests.Item Below-ground resources limit seedling growth in forest understories but do not alter biomass distribution(2003) Machado, Jose-Luis; Walters, Michael B; Reich, Peter BWe examined the long-term growth, morphology, biomass distribution, and survival response of seedlings of five conifer species varying widely in shade tolerance to an increase in soil resources in shaded forest understories in northern Wisconsin, USA. In a 4-year experiment, trenching treatment was used to increase soil resource supply to 1-year old seedlings planted across a range of low light microenvironments. Specifically we asked: (1) does an increase in soil resources alter whole-plant growth, biomass distribution patterns, and survival in lowlight, and (2) do species differ in their responses to increasing soil resources? Increased soil resource availability increased height (+11% on average) and dry mass (+23% on average) of all species regardless of light availability. However, there was no evidence that trenching affected biomass distribution for any species, as has been previously reported. Survivorship after four years was positively related to the species' reported shade tolerance ranking and was unaltered by trenching. We concluded that soil resource availability (supply-demand) can limit tree seedling growth in deeply shaded forest understories, but that increased resource availability of the magnitude caused by trenching does not favor tolerant rather than intolerant species, or vice-versa.Item From tropics to tundra: global convergence in plant functioning(National Academy of Sciences, 1997) Reich, Peter B; Walters, Michael B; Ellsworth, David SMethylation of cytosines in the dinucleotide CpG has been shown to suppress transcription of a number of tissue-specific genes, yet the precise mechanism is not fully understood. The vertebrate globin genes were among the first examples in which an inverse correlation was shown between CpG methylation and transcription. We studied the methylation pattern of the 235-bp ρ-globin gene promoter in genomic DNA from primary chicken erythroid cells using the sodium bisulfite conversion technique and found all CpGs in the promoter to be methylated in erythroid cells from adult chickens in which the ρ-globin gene is silent but unmethylated in 5-day (primitive) embryonic red cells in which the gene is transcribed. To elucidate further the mechanism of methylation-induced silencing, an expression construct consisting of 235 bp of 5′ promoter sequence of the ρ-globin gene along with a strong 5′ erythroid enhancer driving a chloramphenicol acetyltransferase reporter gene, ρ-CAT, was transfected into primary avian erythroid cells derived from 5-day embryos. Methylation of just the 235-bp ρ-globin gene promoter fragment at every CpG resulted in a 20- to 30-fold inhibition of transcription, and this effect was not overridden by the presence of potent erythroid-specific enhancers. The ability of the 235-bp ρ-globin gene promoter to bind to a DNA Methyl Cytosine binding Protein Complex (MeCPC) was tested in electrophoretic mobility shift assays utilizing primary avian erythroid cell nuclear extract. The results were that fully methylated but not unmethylated 235-bp ρ-globin gene promoter fragment could compete efficiently for MeCPC binding. These results are a direct demonstration that site-specific methylation of a globin gene promoter at the exact CpGs that are methylated in vivo can silence transcription in homologous primary erythroid cells. Further, these data implicate binding of MeCPC to the promoter in the mechanism of silencing.Item Generality of leaf traits relationships: a test across six biomes(Ecological Society of America, 1999) Reich, Peter B; Ellsworth, David S; Walters, Michael B; Vose, James M; Gresham, Charles; Volin, John C; Bowman, William DConvergence in interspecific leaf trait relationships across diverse taxonomic groups and biomes would have important evolutionary and ecological implications. Such convergence has been hypothesized to result from trade-offs that limit the combination of plant traits for any species. Here we address this issue by testing for biome differences in the slope and intercept of interspecific relationships among leaf traits: longevity, net photosynthetic capacity (Amax), leaf diffusive conductance (Gs), specific leaf area (SLA), and nitrogen (N) status, for more than 100 species in six distinct biomes of the Americas. The six biomes were: alpine tundra–subalpine forest ecotone, cold temperate forest–prairie ecotone, montane cool temperate forest, desert shrubland, subtropical forest, and tropical rain forest. Despite large differences in climate and evolutionary history, in all biomes mass-based leaf N (Nmass), SLA, Gs, and Amax were positively related to one another and decreased with increasing leaf life span. The relationships between pairs of leaf traits exhibited similar slopes among biomes, suggesting a predictable set of scaling relationships among key leaf morphological, chemical, and metabolic traits that are replicated globally among terrestrial ecosystems regardless of biome or vegetation type. However, the intercept (i.e., the overall elevation of regression lines) of relationships between pairs of leaf traits usually differed among biomes. With increasing aridity across sites, species had greater Amax for a given level of Gs and lower SLA for any given leaf life span. Using principal components analysis, most variation among species was explained by an axis related to mass-based leaf traits (Amax, N, and SLA) while a second axis reflected climate, Gs, and other area-based leaf traits.Item Seed size, nitrogen supply, and growth rate affect tree seedling survival in deep shade(Ecological Society of America, 2000) Walters, Michael B; Reich, Peter BSpecies differences in seedling survival in deeply shaded understories (i.e., shade tolerance) may depend on both seed size and growth rates, but their relative contributions to survival and how they change with time and with variation in light and belowground resource availability is unknown. With a greenhouse experiment we investigated these relationships by examining responses of growth, growth-related morphology, survival, and their interrelationships to a range of nitrogen (3.4 × 10−9–3.4 × 10−3 mol/L N fertilizer solutions) and low light (0.6–7.3% of open sky) availabilities for young seedlings of 10 North American tree species that vary in observational shade tolerance rankings and seed size (Populus tremuloides, Betula papyrifera, Betula alleghaniensis, Acer saccharum, Larix laricina, Pinus banksiana, Pinus resinosa, Pinus strobus, Picea mariana, and Abies balsamea). Within all species, relative growth rate (RGR) and survival increased with light. RGR and survival also increased with N supply but only at the two highest light levels, and then only for the shade-intolerant, broad-leaved Populus and Betula spp.. In every species, survival was positively related to RGR. Moreover, each species differed in the relationship of survival to RGR, and these differences were related to seed mass: at any given RGR, large-seeded, shade-tolerant species had higher survival than smaller-seeded, intolerant ones. Across species, in most light and N treatments, seed mass was positively related to young seedling survival, but RGR was not. In very low light the relative benefits of greater seed mass to survival were temporary. As seedlings aged, interspecific mortality rates became more dependent on observational shade tolerance rankings than on seed size, but mortality was still unrelated to RGR. Our results indicate potentially important interactions among light, N, and species that could influence regeneration dynamics. For young seedlings in deeply shaded microsites, N supply does not matter, and only shade-tolerant species survive due, in part, to large seeds and physiological traits other than RGR. In moderate shade, RGR is greater, and survival is high for all species, except that small-seeded, broad-leaved, intolerant species have low survival and RGR at low N supply. This suggests that broad-leaved shade-intolerant species compete more effectively in moderate shade on richer soils than on poorer soils. Although we found that both seed mass and vegetative physiology influenced survival in shade, they did not covary tightly, suggesting that they are under somewhat separate selection pressures.