Doctoral Dissertations

Date of Award

8-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Louis J. Gross

Committee Members

Richard Norby, Robert Auge, Sally Horn, Michael Huston

Abstract

The hypothesis that plants face a trade-off between shade and drought tolerance is examined. Smith and Huston (1989) had used the trade-off model to predict the growth and survival of different plant functional types along gradients of light and water availability, and showed that several natural patterns of plant distribution and succession could, in principle, be explained from this hypothesized response-model. Based on a set of assumptions on plant morphological and physiological responses to drought and shade, the authors had predicted that under drier conditions plants would become less shade tolerant, and, therefore, would grow and survive better under higher light conditions. Similarly, plants would become more sensitive to drought when exposed to low light levels. The predictions were expected to hold within a population, as well as between populations. Also, it was expected that different species would show the same basic trade-off although shade tolerant ones were expected to be more susceptible to drought.

If plants cannot simultaneously acclimate to shade and drought, they are expected to be less tolerant to shading by competitors under drier conditions. However, an array of field patterns from a variety of ecosystems shows that amelioration of drought effects under a plant canopy is one of the most important mechanisms to explain positive interactions among plants. A graphical model discusses the interplay of competition and facilitation in view of the combined response of plants to light and moisture conditions.

The assumptions of the trade-off hypothesis are tested in a shadehouse experiment. Seedlings of Liriodendron tulipifera (tulip poplar) were grown under different combinations of shade (1, 5, 12, 17, 27% of ambient light) and soil water content (5-9%, 11-15%, and >20%). Physiological and growth responses were taken to test the following hypotheses: 1) leaf area ratio decreases under dry conditions 2) root/shoot dry-weight ratio increases under dry conditions, 3) whole plant light compensation point increases under dry conditions, 4) rate of carbon fixation increases with light under dry conditions, 5) the relative reduction in growth with shading increases under dry conditions. The results show that the leaf area ratio tends to decrease with photosynthetically active radiation. Under combined high irradiance and low soil water conditions, the shoot net photosynthetic rate also decreases. The estimates for the whole shoot indicate that plants under dry conditions have lower daily photosynthetic rates under high light than under low light conditions. This seems to be mainly the result of prolonged stomatal closure under combined dry and high light conditions. Thus, in this experiment, higher light levels did not compensate for the shorter period the stomata were opened. The measurements of whole shoot dark respiration rates were consistently lower for plants under dry conditions. This indicates that the reduction of the gains (daily net photosynthetic rate) under dry conditions is at least partly compensated by a reduction of the losses (dark respiration rate). Contrary to what was predicted by the trade-off hypothesis, we found that the light compensation point also tends to decrease under drier conditions. There were no evidences of an increase in the root/shoot ratio under drier conditions as predicted from the model. Overall, we do not find evidence that higher light levels compensate for the physiological and growth effects of drought in tulip poplar seedlings. Also, higher water availability did not enhance growth in the shade.

A large-scale field experiment was used to test the predictions of the trade-off hypothesis for species with different shade tolerance. In the forest area covered by the Throughfall Displacement Experiment (TDE) in Oak Ridge, TN, soil water availability is being manipulated by intercepting about 30% of the throughfall in a (80x80 m) dry- treatment plot, transporting it through a control area, and using it to irrigate a wet plot. The light conditions vary from closed understory to small gaps. In the TDE site we planted seedlings of three tree species with different shade tolerance: Liriodendron tulipifera (intolerant), Quercus alba (intermediate) and Acer saccharum (tolerant). The seedlings were planted during the late winter of 1993, and their growth and survival were followed during the next two growing seasons. According to the trade-off hypothesis, plant growth under dry conditions should increase with light availability. The results presented here indicate that this is not the case for tulip poplar and sugar maple. The growth of both species showed a unimodal response to light. At light transmittance over 25% for tulip poplar, and 15% for sugar maple, growth decreased at increasing light. In contrast, the growth of white oak, the most drought tolerant of the three species, was not significantly reduced with light. The observed decrease of growth of tulip poplar and sugar maple under high light is likely to be related to an increase in water stress. In both years mortality was highest in tulip poplar. During the much drier 1995 season, white oak mortality increased more than sugar maple's, apparently due to herbivory effects. The distribution patterns of naturally established seedlings are consistent with the experimental results.

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