Quantifying The Variation In Below And Aboveground Biomass-Density Relationships Along A Moisture Gradient

The law of density regulation in plant populations/communities plays a significantly important role not only in theoretical ecology but in vegetation restoration. Here "density regulation" comprises self-thinning in populations from benign environments and non-self-thinning density regulation in stressful environments. Despite the importance of density regulation, much controversy still exists about which value its scaling exponents will take, and researchers keep arguing about the universality of -3/2 and -4/3. Recently, an increasing number of studies show that the scaling exponents of density regulation are not constant but vary with plant traits, species, site index, neighbour interaction intensity, soil fertility and moisture, etc.. In addition, it is a necessity to estimate root biomass when we study density regulation in plants, because roots also play a key role in mass-density relationships. Compared with the estimate of aboveground biomass which have standard and accurate approaches, the estimation of root biomass is relatively difficult and no standard methods for it are available so far.Laying emphasis on the influence of belowground parts on plant mass-density relationships and combining literature data analysis, field investigation, control experiment and model development, this paper aimed to explore into why and how plant aboveground mass-density relationships change along an aridity gradient. We planned to solve the following questions:1)What can be a better method of estimating root biomass? 2)How are the aboveground mass-density relationships regulated by root dimension? 3) How plants regulate their aboveground mass-density relationships through biomass allocation into roots and shoots? 4)What differences exist between above and belowground mass-density relationships and what are the causes of such differences? 5)How the interactions between neighbouring plants change along a water availability gradient? The results are as follows:(1) There were differences in root biomass allocation patterns between tropical/subtropical forest and temperate/boreal forest, between coniferous forest and broadleaf forest, and between evergreen forest and deciduous forest. Factors that were significantly correlated with root biomass included aboveground biomass, tree age, mean annual temperature and mean annual precipitation. Generally, root biomass increased as levels of these factors increased. Six equations were fitted, selective application of them may improve the accuracy of root biomass estimation.(2) Significant differences existed in ratios of root to shoot crown diameter between Guazhou, Jiayuguan and Zhangye [the mean value was 9.41 (±0.99),4.20(±0.31) and 2.67(±0.33) respectively], resulting in discrepancy in the intensity of root competition and thus in aboveground biomass-density relationships among the three sites.(3) Investigation on nine experimental sites (including Qiandao, Tianmu, Dengfeng, Lanzhou, Baiyin, Jingtai, Zhangye, Jiayuguan and Guazhou) showed that there was a power-law relationship between root:shoot ratios and aboveground biomass-density scaling exponents, suggesting that the aboveground density of plants can be regulated by their biomass allocation into above- and belowground parts.(4) Both above and belowground biomass-density scaling exponents changed with aridities, but the belowground exponents varied less than the aboveground ones. These differences may be due to differences in soil and air temperatures, in ratios of root to shoot crown diameter, and in root:shoot ratios.(5) From the separation experiment, we found that as water availability was reduced, aboveground relative interaction intensity (RII) decreased, belowground RII increased, and total RII remained almost unchanged. Such phenomena kept the same before and after the apparatus effects were considered. Moreover, negative effects occurred from the interactions between root and shoot competition, that is to say, the effects of full competition would be weaker than a mathematical combination of the separate effects of shoot competition and of root competition. The law of limiting factors can account for the negative interactions:Plants with reduced light supply due to aboveground competition are less likely to be affected by water limitation from root competition; and plants with reduced water availability due to root competition are less likely to be affected by light limitation from shoot competition.