P. J. Hanson | PubMed Health

PubMed1986

A morphological index of Quercus seedling ontogeny for use in studies of physiology and growth.

Attempts to relate plant metabolic activity with developmental stage are often hindered by lack of an appropriate developmental index. Existing indices of morphological development are unsuitable for use with plants having a semideterminate, recurrently flushing pattern of growth as displayed by Quercus seedlings. We propose the following morphological index (QMI) to define the stages of Quercus seedling ontogeny: (1) radicle emergence; (2) epicotyl emergence from the soil; and (for each flush) (3) termination of elongation of the second internode, which corresponds with the period of most rapid stem elongation; (4) completion of elongation by all internodes, which corresponds with the period of most rapid leaf elongation; and (5) completion of elongation of the last leaf but one, which usually precedes closely the pause between one growth flush and another. The relationship between QMI and net photosynthesis by individual leaves of Quercus rubra L. seedlings was determined. Net photosynthesis increased with QMI during a flush, but at a particular QMI stage, generally decreased from one flush to the next.

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Passive nighttime warming facility for forest ecosystem research.

A nighttime warming experiment is proposed. Over the last four decades a significant rise in nighttime minimum temperature has been determined from analysis of meteorological records from a global distribution of locations. The experiment involves nighttime deployment of infrared (IR) reflecting curtains around four sides of a forest canopy and across the top of the forest to mimic the top-down warming effect of cloud cover. The curtains are deployed with cable and pulley systems mounted on a tower and scaffolding structure built around the selected forest site. The trunk space is not enclosed except as an optional manipulation. The curtains reflect long-wave radiation emitted from the forest and ground back into the forest warming the trees, litter, and soil. Excellent infrared reflection can be obtained with commercially available fabrics that have aluminum foil bonded to one side. A canopy warming of 3 to 5 degrees C is expected on cloudless nights, and on cloudy nights, a warming of 1 to 3 degrees C is anticipated relative to a control plot. The curtains are withdrawn by computer control during the day and also at night during periods with precipitation or excessive wind. Examples of hypothesized ecosystem responses to nighttime warming include: (1) increase in tree maintenance respiration (decreasing carbon reserves and ultimately tree growth), (2) increase in the length of the growing season (increasing growth), (3) increase in soil respiration, (4) increase in litter decomposition, (5) increase in mineralization of N and other nutrients from soil organic matter, (6) increase in nutrient uptake (increasing growth), and (7) increase in N immobilization in litter. Hypothesis 1 has the opposite consequence for tree growth to Hypotheses 2 and 6, and thus opposite consequences for the feedback regulation that vegetation has on net greenhouse gas releases to the atmosphere. If Hypothesis 1 is dominant, warming could lead to more warming from the additional CO(2) emissions. Site-specific meteorological, ecophysiological, and phenological measurements are obtained in the warming treatment and in a carefully selected control plot to investigate site-specific hypotheses. Measurements made on both plots for a baseline period and during the period of curtain deployment provide data to test the hypotheses statistically by the "before-after-control-impact" method applicable to unreplicated experiments. The enclosure has a modular design that can be adapted and combined with other forest-scale manipulation experiments such as free air CO(2) enrichment and throughfall displacement.

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