Allometric Law Of Desert Herbs Under Different Nutrition Status At Different Growth Stages

Since the Metabolic Theory of Ecology(MTE)was proposed,it has been a great concern,and greatly developed and improved.Yet it has been controversial about whether there is a constant exponent.It is found that the exponent between metabolic rate and body size of small individual plants,such as herb,is 1,while the exponent of large individual,such as the adult tree,is 3/4.However,whether there is a constant metabolic exponent "1" or a dynamic change from seedlings to mature stage of herbs?This has become an important scientific problem for ecologist.Moreover,plant resource and energy metabolism and biomass allocation pattern are closely related to its habitat nutrition status.Thus,different nutrient gradients were set up for desert herbs,and plant traits of body size,photosynthesis rate and respiration rate at different growth stages were measured.The allometric relationships at different growth stages and under different nutrient gradients was studied for intra-and inter-species.The experimental analysis results show that:1)In the whole dynamic process from seedling to mature stage,the exponents of total photosynthesis rate or total respiration rate vs.total biomass and the biomass allocation pattern between above-and below-ground biomass are dynamically changed along with individual ontogeny under different nutrition gradients.The results are not consistent with the MTE prediction that scaling exponent for small plants is 1 as well as the isometric theory.The dynamic change of allometric scaling relationships among five desert herbs are different along with individual ontogeny,which show some species specificity.But for five desert species across all nutrition gradients,the exponent of total photosynthesis rate or total respiration rate vs.total biomass decreased first and then increased,and it is close to 0.75.For above-and below-ground biomass allocation,the exponent is close to 1 at early and later stage,which accords with isometric theory prediction.but at middle stage,it is more consistent with the optimal allocation theory because of more substance and energy allocated to reproductive growth.2)Under different P concentration treatments,the exponents of total photosynthesis rate or total respiration rate vs.total biomass and the biomass allocation pattern at every stage also changed dynamically under environmental stress.The analysis results are: a)all exponent of five desert herbs(except Eragrostis pilosa(Linn.)Beauv)fluctuate within a certain range with the increase of P concentration,and have no significant increase or decline trend;b)for all five desert species,the exponent of total photosynthesis rate or total respiration rate vs.total biomass decreased with the increase of the P concentration,but the exponent is close to 0.75.This isn't consistent with the MTE prediction that the exponent of small plants is 1,probably attributed to the effects of environmental stress on the metabolic rate and individual ontogeny.Meanwhile,the exponent of aboveground biomass vs.belowground biomass decreases with the increase of P concentration,this may be attributed to the P increase which promotes the plant root growth.c)for three different growth periods of desert herbs under different P concentration treatments: at early stage,it is found that the exponent of total photosynthesis rate or total respiration rate vs.total biomass and the exponent of aboveground biomass vs.belowground biomass under 0 mg/kg P treatment are greater than the exponent under 0.317 mg/kg,0.952 mg/kg and 9.520 mg/kg,and there is no significant difference among the other three treatments;at middle stage,the exponents of total photosynthesis rate or total respiration rate vs.total biomass under 0 kg/mg P treatment are the largest,and there is no similar change trend among the other three treatments.However,the exponent of aboveground biomass vs.belowground biomass increases with the increase of P concentration.at later stage,the exponent of total photosynthesis rate or total respiration rate vs.total biomass increased with the increase of P concentration.The exponent of aboveground biomass vs.belowground biomass decreased with the increase of P concentration.In addition,we found that there is no obvious rule under different N treatments.3)Under the treatment of different N and P concentrations,the relationship between aboveground biomass and total biomass,leaf biomass and total biomass of all six desert herbs show an isometric growth,while the relationship between belowground biomass and total biomass,belowground biomass and aboveground biomass show an allometric growth,whoes exponents are significantly greater than 1.The results show that the resource and energy may prefer to allocate to the underground part under the stress of N and P elements,or that the growth of plant aboveground part is inhibited under the stress of N elements.This study will provide certain foundation and direction for further study on the distribution of resource and energy under different nutrition status and the dynamic change of allometric relationship between different physiological properties of small individual as plant ontogeny.it also has some reference to improve the MTE.