Qualifying The Allometric Relationships Between Biomass And Density Within Plant Populations Of Two Species

Biomass-density (M-N) relationship has been one of the hot research topics inquantitative and theoretical ecology as it is the essential link between the dynamics ofindividuals and the macroscopic properties of the total population or community. However,both the3/2and4/3exponents of M-N relationship have been criticized on theoretical andempirical grounds, one based on the space-filling properties of canopy geometry and the otherbased on the metabolic basis of resource use. Early debate was focused on whether the scalingexponent is constant? If so, whether it is more consistent with the classic3/2Self-thinningLaw or the predictions of Metabolic Scaling Theory, or it takes values which deviate fromboth3/2and4/3. More recently, researchers focus on how the scaling exponent shifts alongenvironmental gradients and the mechanisms underlying the variations. However littleattention has been paid to what is the underlying value of the allometric coefficient (intercept)and how environment affects the it. Although effects of abiotic factors on the slope of M-Nrelationship have been well documented, it is unclear whether and how biotic factors, such asinitial sowing density, affect the intercept M-N relationship. In most of studies, M has beenfocused on shoots, and little is known about comparisons between M-N relationships forabove-and below-ground parts, as well as that for different plant organs, such as roots, stems,leaves and reproductive parts.In this thesis, I took the problems mentioned above as a starting point and try to establisha more comprehensive theoretical framework of density-dependent regulation within plantpopulations in five different aspects. Bulbous biennial Allium cepa var. proliferum and annualherb Fagopyrum esculentum were used as study species. A series of field and pot experimentshave been conducted to systematically investigate the density-dependent interactions betweenneighboring plants, and determine the individual characteristics, the dynamics of populationand modular population.In Parts two and three of this thesis, I emphasized on the regularity and mechanisms ofM-N relationships for individual plants, the above-and below-ground parts, as well asvarious organs in non-self-thinnng and self-thinning populations. In Parts four and five, Iattempted to investigate how the M-N trajectory varied with initial density and what thedifferences between M-N relationships were based on different scales (intra-population andinter-population). Two important hypothesizes based on the theory of density-dependentregulation had been proposed:(1) the self-thinning trajectory varied with initial densities;(2)there were fundamental differences between interspecific self-thinning trajectory for differentpopulations of the same species and the intraspecific self-thinning trajectory for singlepopulation, and the two reflected different biological processes. In Part six, bulbous plant wasused as study species which has robust below-ground part. I investigated how the biomassallocation pattern responded to different densities and the possible mechanisms, in order to test whether variations of the biomass allocation pattern due to different densities conformedto the predictions of the Optimal Partitioning Theory (OPT) when the target plant wassubjected to interferences from neighboring plants.The objectives of the field experiment were to (1) investigate biomass-densityrelationships at plant part level and at individual plant level for both below-and above-groundparts of plants and (2) compare size-density relationships at the levels of plant parts andindividuals. Allium cepa var. proliferum was planted at five plant densities. Results showedthat relationships between plant density and dry weight of root, bulb, leaf and sheath werefitted negative power functions with different allometric exponents for different plantmodules: bulb (–1.14)<leaf (–1.03)<root (–0.78)<sheath (–0.49). Above-andbelow-ground dry matter, as well as the whole plant, decreased significantly with increasedplant density. The allometric exponents were–0.95and–1.13,–0.98for above-ground,below-ground and individual plant dry matter, respectively. It is concluded that the effect ofdensity-dependent regulation on below-ground modules is greater than that on above-groundmodules. Below-ground parts may be more closely constrained by plant density in contrast toabove-ground parts and the whole plant. However, effects of density-dependent regulation onabove-ground parts and the whole plant were similar. Consequently, the scaling exponent forbelow-ground parts was driven by bulb biomass, and that for above-ground and the individualplant were driven by leaf biomass. The differences amongst allometric exponents at plant partlevel depended on the patterns of photosynthetic partitioning under different degree ofcompetitive stress. On the other hand, it was due to distinct competitive mechanisms for plantmodules in the population.A pot experiment was conducted to study the biomasses of plant components，such asroots, stems，leaves and reproductive parts in relationship with plant density with Fagopyrumesculentum populations．Results showed that the values of self-thinning exponents were notsignificantly different from–4/3for mean above-and below-ground biomass, and totalbiomass during the course of population development as evidence in favor of the–4/3powerlaw of self-thinning．However, the allometric relationship between leaf biomass and densityfollowed–3/2power law of self-thinning. It is suggested that there is no universal law ofbiomass-density relationship associated with plant modules．On the one hand, specificallometric relationships between plant component biomass and above-ground biomass canresult in slopes of the self-thinning lines which are different from–3/2or–4/3．On the otherhand, competition within the population affects plant development and alters the biomassallocation patterns of plants and may lead to different biomass-density relationships for plantmodules in the population.In addition, I attempted to investigate whether initial density can influence allometricrelationships and the biomass–density trajectory. By analyzing the biomass–density (B–N)allometric relationships within populations with different high densities separately, I foundthat different populations self-thinned along different self-thinning trajectory, but all slopeshad the slope of1/2. Results showed that initial density did not affect the slope of the log biomass–log density relationship, but there was a clear and significant effect on the intercept.Populations sown at higher densities had significantly more biomass at a given density ofsurvivors. Consequently, the self-thinning trajectory is not always independent of initialpopulation density. On the one hand, the position of the self-thinning trajectory is determinedin part by the biomass density: the relationship between mass and volume. Initial densitycould affect this by altering allometric growth in a way that influences architecturalcompactness. On the other hand, the intercept was affected by the mode of competition(size-symmetric competition or size-asymmetric competition). Interactions among plants andallometry are more important than internal physiological scaling mechanisms in determiningthe self-thinning trajectory of crowded stands.Furthermore, based on B–N models on different scales, I tried to test an importanthypothesis: whether the B–N relationships among populations of one species mirror thosewithin these populations. If the data for all densities and harvests are analyzed together, logB–log N relationship is linear with a slope of–0.377(close to–0.5), which is consistent withthe predictions of Metabolic Scaling Theory. If the independent variable initial density isincluded as a factor, the estimated slope of the log B–log N relationships are much steeper,and consistent with the classical self-thinning rule. Results confirmed experimentally thatinter-population scaling patterns are fundamentally different from population processes,although both constrain the other, reflecting different biological process. Interpopulationscaling patterns, even within one species, do not reflect processes within populations.Finally, the bulbous plant was used as study species to analyze effects of density on thebiomass allocation pattern of A．cepa var．proliferum. Results showed that although thecompetition occurred below-ground predominated in bulbous plant populations as plantdensity increases, the photosynthates allocated more to above-ground vegetative organs withthe cost of decreasing the photosynthates allocation to below-ground asexsual reproductiveorgan. However, the biomass allocation to root and the capacity of resource uptake remainedunchanged. It appeared that when the interference from the other plants was present, thepartitioning pattern inside plant body did not conform to the Optimal Partitioning Theory. TheOptimal Partitioning Theory was only applicable at the absence of plant competition betweenindividual plants. When the competition between plants was present, plant biomass allocationpattern was determined by the negative regulation with density.