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University of Cambridge (2002)

Effects of competition and resource availability on arid-land plants (BL)

Jankju-Borzelabad, M

Titre : Effects of competition and resource availability on arid-land plants (BL)

Auteur : Jankju-Borzelabad, M

Université de soutenance : University of Cambridge

Grade : Doctor of Philosophy (PhD) 2002

Résumé
Mechanisms of competition under different environmental conditions have been described by various ecological theories, with the debate extended recently to the pulsed supply of resources, which often occurs in semi-arid regions. The work described in this thesis determined plant traits most important for competition under different concentrations (low vs. high) or mode of delivery (continuous vs. pulsed) of resources throughout vegetative growth. Experiments were carried out in two stages : firstly symmetrically, where similarly-sized seedlings of Agropyron desertorum and Panicum antidotale were grown in a response surface design ; secondly asymmetrically, where small seedlings of Panicum were introduced into pots in which Agropyron or Panicum had already been growing for seven weeks, in a target-neighbour design. The most important plant traits for competition for water were a higher leaf area ratio, stomatal conductance, resource uptake rate and rapid growth rate during resource pulses. For nitrogen, a higher rootshoot ratio (RShR), tolerance to the limited resources during inter-pulses, and spatial root plasticity to different soil depths was advantageous. Under asymmetric conditions, the C4 photosynthetic pathway, finer root system and higher morphological root plasticity increased competitive effects of Panicum during early stages of growth, whereas a larger root system, which remained active for the whole growth period, increased the competitive effect of Agropyron over later stages of growth. Under asymmetric conditions, a combination of lower competition intensity, higher resource uptake rates and higher nitrogen availability, improved the establishment of Panicum seedlings when water was pulsed, whereas a lower mean residence time of nitrogen decrease survival rate, between the pulses. Both species showed an increase in RShR under intense competitive effects from neighbours, as an adaptation to a decreasing degree of size-asymmetry. Furthermore, an increase in RShR led to a higher capacity for acquiring pulsed resources in mixed cultures, as compared to single species

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