Interactions between Population Dynamics and Resource Allocation
2.2 Environmental nitrogen availability
It has often been stated that nitrogen is the most limiting resource to plant growth in tall grasslands (Blair et al. 1998). Therefore it is probable that evolutionary processes have selected bunchgrasses for growth traits that respond to the low level of nitrogen in the environment, at the expense of growth processes which would be required if nitrogen were non-limiting. Hence a model that ignores the effect of nitrogen on clonal tiller growth would probably miss a fundamental component that ensures the stability of bunchgrass systems.
Daily soil nitrogen available to plants is restricted and seasonal (Knapp et al. 1998).
The maximum rate of nitrogen uptake from the soil, RNuptake, is set to 0.0 lg N (g wlr)"1 d"1 for the months July to November and then declines to zero through December, and remains zero for the remainder of the year. This period of nitrogen availability was chosen to mimic the high availability of soil nitrogen during spring and early summer relative to other parts of the year in humid grasslands growing in summer rainfall areas, because of nitrogen inputs from thunderstorm activity during this period and nitrogen from the accumulated decay of soil matter over the previous winter (Blair et al. 1998). In reality soil nitrogen availability is far less blocked and actually occurs as a series of pulses in response to environmental cues, particularly rainfall events and changes in soil water potential (Birch 1958; Campbell & Grime 1989; Blair et al. 1998).
3 ASSESSING THE PERFORMANCE OF GROWTH STRATEGIES
The multiple tiller model generates a large amount of information. In order to analyse the effects of different growth strategies it is necessary to make a list of criteria that can be used to assess the behaviour of the modelled clones. Interpreting the raw data across the entire simulation is difficult because of the number of data points. Therefore the data must be
summarised appropriately to provide relevant information. There are two components to this problem. The first component relates to the seasonal behaviour of the clone under different growth strategies. Intra-annual behaviour can be assessed by averaging daily growth values for each day of the year ('Yearday') across the years of the simulation. The second component relates to the success of a growth strategy in capturing and holding resources. This is most easily represented by averaging summed annual values across the years of the simulation. The following behaviours and parameters were considered:
1. Stability
a. Does the system self-perpetuate for the length of the simulation? Alternatively does it collapse?
b. Are there inter-annual patterns of growth (e.g. cyclicity)?
2. Average intra-annual growth patterns a. Average daily live object number
b. Average daily live root biomass (g) , live shoot biomass (g), root-to-shoot ratio c. Average daily AVC (g CHO) and AVN (g N)
3. Individual production parameters a. Number of flowering tillers
b. Average maximum blade-, sheath-, internode- and flower mass of flowering tillers c. Average annual summed net photosynthesis (g CHO)
d. Average annual summed nitrogen absorption (g N) e. Average annual summed growth (g CHO)
f. Average annual summed growth deficit (g CHO)
The model runs by initiating a clone consisting of one tiller. This tiller recruits secondary tillers and subsequently new ramets and ramet groups are formed. It takes time for the clone to occupy the system space defined by the model constraints on resources. Therefore the first five years of data are ignored for both averaging methods in order to ignore values generated by the model while the clone grows to occupy the given space. For the simulations conducted here this length of time was reasonable because most clones had filled the available space within this period.
All multiple tiller simulations conducted in this thesis were set to run for 10 000 day iterations (i.e. just over 27 years).
4 PROBLEM 1: ALLOCATION AMONG TILLERS AND RAMETS
Regulation of intra-clonal growth in bunchgrasses is poorly understood (Derner & Briske 1999). It is apparent that young tillers on individual ramets may die even during periods of active growth on perennial bunchgrasses (Tainton & Booysen 1965; Tomlinson &
O'Connor 2005). This indicates that resource allocation is disproportionate across physiologically integrated ramets.
The most probable explanation for disproportionate allocation among sinks is that passive termination may occur because restricted resources are directed towards larger sinks (Bangerth 1989) which results in a positive feedback loop that starves smaller sinks and ultimately terminates them. This process underlies what is termed apical dominance.
Bangerth's theory of auxin-induced auto-inhibition of lateral shoots proposes that the ability of a shoot to attract resources for its continued growth and development depends on its auxin output relative to other shoots (Bangerth 1989, Bangerth et al. 2000). Presumably auxin production is related to photosynthetic activity. By this means resources get directed to the most photosynthetically-active shoots and away from less active shoots and side- shoots. During periods of resource-limited growth this disproportionate allocation ensures that smaller shoots die back more than they grow, eventually killing them.
Disproportionate allocation between tillers presumably improves resource allocation efficiency across a ramet because it promotes the growth of more productive structures and therefore improves the supply of resources (Sachs, Novoplansky & Cohen 1993).
Essentially this implies that disproportionate allocation may be an adaptation for spatial heterogeneity in resource availability of sun-grown plants (Hutchings & de Kroon 1994).
However no researchers appear to have anticipated that another important role of disproportionate allocation may be to improve temporal resource allocation efficiency.
Consider proportionate allocation versus disproportionate allocation between tillers. If resource allocation is based strictly proportionately on the growth demand of individual tillers then proportionately all are equally compromised when available resources are limiting to growth, and relatively fewer will be terminated under those conditions. Hence ramet growth at all future times would still be allocated relatively inefficiently to all live tillers thus slowing the rate at which individual tillers recover after periods of critical resource limitation. Disproportionate allocation would remove less-efficient tillers more rapidly and reduce the total demands placed by the ramet for resources during resource-
limited periods, thereby increasing the rate at which individual tillers overcome resource limitation after growth-limiting periods. However it is unclear what effect the shift from proportionate to disproportionate allocation will have on total clonal growth. Specifically, does disproportionate allocation improve or reduce total clonal growth over that achieved by clones with proportionate growth? If it is the former, then we can predict that all bunchgrasses should favour disproportionate allocation regardless of environment. If it is the latter, then there is a trade-off between individual tiller growth and clonal growth, which may be associated with different growth strategies employed by bunchgrass species.
In this section the effect of proportionate and disproportionate allocation to tillers and ramets is explored using the TILLERTREE model. The following questions are asked:
1. Are there differences in the growth patterns of clones grown with proportionate allocation and disproportionate allocation?
2. Do these differences result in differences in ability to capture and utilise environmental resources?