Browsing by Subject "shade tolerance"

Now showing 1 - 5 of 5
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    Are shade tolerance, survival, and growth linked? Low light and nitrogen effects on hardwood seedlings
    (1996) Walters, Michael B; Reich, Peter B
    Variation 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.
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    Ecophysiological investigations of understory eastern redcedar in central Missouri
    (1983) Lassoie, James P; Dougherty, Phillip M; Reich, Peter B; Hinckley, Thomas M; Metcalf, Clifford M; Dina, Stephen J
    Eastern redcedar (Juniperus virginiana) is a sun-adapted, drought-resistant pioneer species common to pastures, abandoned fields, fence rows, and calcareous rock outcrops throughout the eastern United States. However, it is also a frequent component of the understory in mature oakhickory forests in central Missouri, where light levels are typically < 10% of full sunlight during much of the growing season. This is below eastern redcedar's reported optimum for photosynthesis. The competitive survival of understory eastern redcedar under such environmental conditions was apparently due to it being an evergreen conifer in a deciduous forest. Hence, its foliage was able to maintain a positive carbon dioxide balance throughout much of the year, with maximum net photosynthetic rates occurring during periods when the overstory was leafless. The greatest daily average net photosynthetic rates (Ph,) occurred during overstory leaf emergence when temperatures were moderate and light levels to the understory trees were annually the highest. Furthermore, since leaf temperatures and tree water deficits were relatively low at this time, daily gas exchange rates were not greatly limited by midday stomatal closure. After the overstory foliage had fully developed, understory light levels averaged -S50-800o below levels observed in early spring. Thus, photosynthesis was severely light limited during the day, resulting in Ph, that were 15-45% of the springtime maxima. The greatest daily average transpiration rates (TR) occurred during the summer due to the high evaporative demand. Increasing leaf temperatures and tree water deficits became more important by late summer, causing stomatal closure during some afternoons, which reduced Ph,, and TR to :730 and 40%, respectively, of the early summer levels. During the autumn, winter, and early spring, understory light levels were normally above the saturation point for photosynthesis. The light saturation point for an understory study tree (expressed as flux of photosynthetically active photons) was ;800 Armol m--2 s1, less than half of that reported for open-grown eastern redcedar. This relatively lower light saturation point suggested an adjustment to shade conditions. During the autumn overstory defoliation period, light levels to understory trees progressively increased, and Ph, eventually reached 80W of the springtime maximum. In contrast, TR only reached ;25% of the summer maximum, owing to relatively low evaporative demands. During the late autumn and winter, low leaf and soil temperatures combined to limit gas exchange severely. The major controlling factors seemed to be cold air temperatures directly inhibiting Ph, and cold soil temperatures indirectly producing tree water deficits due to reduced water uptake at the soil-root interface. Such conditions promoted persistent stomatal closure, resulting in Ph, near zero. However, a temporary warming trend during the winter caused an increase in Ph,, to a level -301O of the springtime maximum. Higher net photosynthetic rates probably were not possible due to the effects of low soil and air temperatures on the stomatal mechanism and on the photosynthetic apparatus.
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    Seed size, nitrogen supply, and growth rate affect tree seedling survival in deep shade
    (Ecological Society of America, 2000) Walters, Michael B; Reich, Peter B
    Species 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.
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    Temperature and leaf nitrogen affect performance of plant species at range overlap
    (Wiley, 2015) Fisichelli, Nicholas A; Stefanski, Artur; Frelich, Lee E; Reich, Peter B
    Plant growth and survival near range limits are likely sensitive to small changes in environmental conditions. Warming temperatures are causing range shifts and thus changes in species composition within range-edge ecotones; however, it is often not clear how temperature alters performance. Through an observational field study, we assessed temperature and nitrogen effects on survival and growth of co-occurring temperate (Acer saccharum) and boreal (Abies balsamea) saplings across their overlapping range limits in the Great Lakes region, USA. Across sampled ranges of soil texture, soil pH, and precipitation, it appears that temperature affects leaf nitrogen for A. saccharum near its northern range limit (R2 = 0.64), whereas there was no significant leaf N ~ temperature relationship for A. balsamea. Higher A. saccharum leaf N at warm sites was associated with increased survival and growth. Abies balsamea survival and growth were best modeled with summer temperature (negative relationship); performance at warm sites depended upon light availability, suggesting the shade-tolerance of this species near its southern range limits may be mediated by temperature. The ranges of these two tree species overlap across millions of hectares, and temperature and temperature-mediated nitrogen likely play important roles in their relative performance.
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    Untangling positive and negative biotic interactions: Views from above and below ground in a forest ecosystem
    (Ecological Society of America, 2010) Montgomery, Rebecca A; Reich, Peter B; Palik, Brian J
    In ecological communities, the outcome of plant–plant interactions represents the net effect of positive and negative interactions occurring above and below ground. Untangling these complex relationships can provide a better understanding of mechanisms that underlie plant–plant interactions and enhance our ability to predict population, community, and ecosystem effects of biotic interactions. In forested ecosystems, tree seedlings interact with established vegetation, but the mechanisms and outcomes of these interactions are not well understood. To explore such mechanisms, we manipulated above- and belowground interactions among tree seedlings, shrubs, and trees and monitored seedling survival and growth of six species (Pinus banksiana, Betula papyrifera, P. resinosa, Quercus rubra, P. strobus, and Acer rubrum) in mature pine-dominated forest in northern Minnesota, USA. The forest had a moderately open canopy and sandy soils. Understory manipulations were implemented in the forest interior and in large gaps and included removal of shrubs (no interactions), tieback of shrubs (belowground), removal of shrubs with addition of shade (aboveground), and unmanipulated shrubs (both below- and aboveground). We found that shrubs either suppressed or facilitated seedling survival and growth depending on the seedling species, source of interaction (e.g., above- or belowground), and ecological context (e.g., gap or forest interior). In general, shrubs strongly influenced survival and growth in gaps, with more modest effects in the forest interior. In gaps, the presence of shrub roots markedly decreased seedling growth and survival, supporting the idea that belowground competition may be more important in dry, nutrient-poor sites. Shrub shade effects were neutral for three species and facilitative for the other three. Facilitation was more likely for shade-tolerant species. In the forest interior, shrub shade negatively affected seedling survival for the most shade-intolerant species. For several species the net effect of shrubs masked the existence of both positive and negative interactions above and below ground. Our results highlight the complexity of plant–plant interactions, demonstrate that outcomes of these interactions vary with the nature of resource limitation and the ecophysiology of the species involved, and suggest that ecological theory that rests on particular notions of plant–plant interactions (e.g., competition) should consider simultaneous positive and negative interactions occurring above and below ground.