Browsing by Author "Hinckley, Thomas M"

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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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    Water relations: Soil fertility, and plant nutrient composition of a pygmy oak ecosystem
    (1980) Reich, Peter B; Hinckley, Thomas M
    The water and nutrient relations of Buzzard's Roost, an unusual pygmy oak ecosystem in southwest Missouri, USA, were studied in 1976 and 1977 in an attempt both to characterize the ecophysiology of the individuals and find the causative agents for the existence of the pygmy forest. The stunted trees were mostly blackjack oak (Quercus marilandica) with some northern red oak (Q. rubra) and a few black oak (Q. velutina). In addition to being stunted, the trees were gnarled and twisted and had miniature leaves and acorns. Results of soil analysis showed the pygmy forest soil to be very acidic (pHw = 4.6), with very low levels of Ca and Mg (0.25 and 0.12 meq/100 g, respectively) and very high levels of Al (600 ppm). Adjacent non-pygmy forest soils did not display these characteristics. Foliar analysis of blackjack, northern red, and black oak showed Ca and Mg to be much lower in pygmy foliage than in non-pygmy foliage, at three times during the growing season. Diurnal and seasonal patterns of xylem pressure potential, leaf conductance, and soil moisture content illustrated the development of very severe tree water deficits at Buzzard's Roost. Predawn and midday xylem pressure potentials declined to as low as -3610 and -4200 kPa, respectively, accompanied by complete daytime stomatal closure. However, water stresses in nearby non-pygmy oaks were greater than at two of three pygmy oak sites, implying that water stress was not the major factor or causative agent in the stunting process. The evidence suggests that the very low nutrient levels in the soil, especially of Ca and Mg, plus the very high levels of Al, may be deficient (and/or toxic) for normal tree growth and development, and possibly responsible for the existence of the pygmy oak forest. Also, the xeric site characteristics add another severe stress to this ecosystem.