Consequences of architecture and resource allocation for growth dynamics of bunchgrass clones.
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Date
2005
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Abstract
In order to understand how bunchgrasses achieve dominance over other plant growth forms
and how they achieve dominance over one another in different environments, it is first
necessary to develop a detailed understanding of how their growth strategy interacts with
the resource limits of their environment. Two properties which have been studied separately
in limited detail are architecture and disproportionate resource allocation. Architecture is the
structural layout of organs and objects at different hierarchical levels. Disproportionate
resource allocation is the manner in which resources are allocated across objects at each
level of hierarchy. Clonal architecture and disproportionate resource allocation may interact
significantly to determine the growth ability of clonal plants. These interactions have not
been researched in bunchgrasses.
This thesis employs a novel simulation technique, functional-structural plant
modelling, to investigate how bunchgrasses interact with the resource constraints imposed
in humid grasslands. An appropriate functional-structural plant model, the TILLERTREE model, is developed that integrates the architectural growth of bunchgrasses with environmental resource capture and disproportionate resource allocation. Simulations are
conducted using a chosen model species Themeda triandra, and the environment is
parameterised using characteristics of the Southern Tall Grassveld, a humid grassland type
found in South Africa. Behaviour is considered at two levels, namely growth of single
ramets and growth of multiple ramets on single bunchgrass clones.
In environments with distinct growing and non-growing seasons, bunchgrasses are
subjected to severe light depletion during regrowth at the start of each growing season because of the accumulation of dead material in canopy caused by the upright, densely packed manner in which they grow. Simulations conducted here indicate that bunchgrass
tillers overcome this resource bottleneck through structural adaptations (etiolation, nonlinear
blade mass accretion, residual live photosynthetic surface) and disproportionate
resource allocation between roots and shoots of individual ramets that together increase the
temporal resource efficiency of ramets by directing more resources to shoot growth and
promoting extension of new leaves through the overlying dead canopy.
The architectural arrangement of bunchgrasses as collections of tillers and ramets
directly leads to consideration of a critical property of clonal bunchgrasses: tiller
recruitment. Tiller recruitment is a fundamental discrete process limiting the vegetative growth of bunchgrass clones. Tiller recruitment occurs when lateral buds on parent tillers
are activated to grow. The mechanism that controls bud outgrowth has not been elucidated.
Based on a literature review, it is here proposed that lateral bud outgrowth requires suitable
signals for both carbohydrate and nitrogen sufficiency. Subsequent simulations with the
model provide corroborative evidence, in that greatest clonal productivity is achieved when both signals are present. Resource allocation between live structures on clones may be distributed
proportionately in response to sink demand or disproportionately in response to relative
photosynthetic productivity. Model simulations indicate that there is a trade-off between
total clonal growth and individual tiller growth as the level of disproportionate allocation
between ramets on ramet groups and between tillers on ramets increases, because
disproportionate allocation reduces tiller population size and clonal biomass, but increases
individual tiller performance. Consequently it is proposed that different life strategies
employed by bunchgrasses, especially annual versus perennial life strategies, may follow
more proportionate and less proportionate allocation strategies respectively, because the
former favours maximal resource capture and seed production while the latter favours individual competitive ability.
Structural disintegration of clones into smaller physiologically integrated units (here termed ramet groups) that compete with one another for resources is a documented property
of bunchgrasses. Model simulations in which complete clonal integration is enforced are
unable to survive for long periods because resource bottlenecks compromise all structures
equally, preventing them from effectively overcoming resource deficits during periods when
light is restrictive to growth. Productivity during the period of survival is also reduced on
bunchgrass clones with full integration relative to clones that disintegrate because of the
inefficient allocation of resources that arises from clonal integration. This evidence
indicates that clonal disintegration allows bunchgrass clones both to increase growth
efficiency and pre-empt potential death, by promoting the survival of larger ramet groups
and removing smaller ramet groups from the system.
The discrete nature of growth in bunchgrasses and the complex population dynamics that arise from the architectural growth and the temporal resource dynamics of the environment, may explain why different bunchgrass species dominate under different environments. In the final section this idea is explored by manipulating two species tiller traits that have been shown to be associated with species distributions across non-selective in defoliation regimes, namely leaf organ growth rate and tiller size (mass or height). Simulations with these properties indicate that organ growth rate affects daily nutrient demands and therefore the rate at which tillers are terminated, but had only a small effect on
seasonal resource capture. Tiller mass size affects the size of the live tiller population where
smaller tiller clones maintain greater numbers of live tillers, which allows them to them to
sustain greater biomass over winter and therefore to store more reserves for spring
regrowth, suggesting that size may affect seasonal nitrogen capture. The greatest differences
in clonal behaviour are caused by tiller height, where clones with shorter tillers accumulate
substantially more resources than clones with taller tillers. This provides strong evidence
there is trade-off for bunchgrasses between the ability to compete for light and the ability to
compete for nitrogen, which arises from their growth architecture.
Using this evidence it is proposed that bunchgrass species will be distributed across
environments in response to the nitrogen productivity. Shorter species will dominate at low nitrogen productivity, while taller species dominate at high nitrogen productivity. Empirical evidence is provided in support of this proposal.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
Keywords
Grassland ecology--Mathematical models., Growth modelling., Resource allocation--Mathematical models., Grassland ecology--Computer simulation., Theses--Mathematics.