Shift Of The Scaling Exponent At Stand Level And Its Mechanism Along Salinity And Moisture Gradients

The relationship between organism body size and abundance is an essential link between the individual- and population-level traits of species and dynamics of ecological communities, while the allometric relationship between mean biomass and density (M-N) is one of such relationships, with receiving most attention and reflecting many population traits. What is the underlying value of the scaling exponent and whether it is variable along environmental gradients are obscure questions rising by the -3/2 scaling exponent based on geometric model and the -4/3 scaling exponent based on metabolic theory of ecology which assumes that total energy flux of a population or community is constant. This study focuses on whether the scaling exponent (population regulating exponent) of M-N relationship varies along environmental gradients, how it changes if the variation is existent, and what is the mechanism underlying the variation.Little attention has been paid to the relationship between plant interactions and M-N relationship in previous studies. According to the relative interaction intensity model, we hypothesize that plant interactions determine the M-N relationship along environmental gradients. We predicted that the scaling exponent will increase with drought stress as well as the relative interaction intensity. Meanwhile, there is a conflict between two basic assumptions underlying the -3/2 self-thinning law (i.e. isometric growth and closed canopy) and the fact that plant has an allometric growth and open canopies in stressful environment. Based on a geometric model, we hypothesize that plant height-crown radius (H-r) relationship and canopy-density (C-N) relationship determine the aboveground M-N relationship. By integrating the abiotic stresses into the ecological field model, we could present a theoretical prediction to exploring the change rule of plant interactions along environmental gradients.Relative interaction intensity model showed that plant interactions covaried with the scaling exponent of M-N relationship. The geometric model showed that scaling exponent of H-r relationship and scaling exponent of C-N relationship determined the scaling exponent of M-N relationship. The advanced ecological field model showed that plant had the potential of inhibiting near-neighbor and facilitating far- neighbor, and this potential would decrease with increasing abiotic stresses.The experimental result showed that the scaling exponent of barnacle in intertidal subzone shifted from -1.37 to -0.39 when the salinity shifted from 31.1‰to 19.6‰. And previous literatures reported that it was -1.68 under 33‰salinity. The survey of vegetation showed that scaling exponent of aboveground biomass increased from -1.652 to -1.006 with drought stress. Likewise, the scaling exponent of aboveground biomass increased from -1.652 to -0.883 with altitude in mountain Tianmu. In addition, the survey also showed that the scaling exponent of height-crown radius (H-r) relationship decreased from 1.343 to 0.639 while the scaling exponent of canopy-density (C-N) relationship increased from 0.016 to 0.317, as a result of increasing drought stress. The experimental result of broad bean showed that both plant interactions and self-thinning exponent increased with salinity.According to the theoretical and empirical results, we obtained the following conclusions:1. The scaling exponent of M-N relationship in sessile organism populations was not constant along environmental gradients, but increased with abiotic stresses.2. Plant interactions determine the performance of M-N relationship. Scaling exponent of M-N relationship increased with abiotic stress because competition and facilitation vary inversely across abiotic stress gradients, with facilitation being more common in harsh environment while competition in benign environment.3. Plant height-crown radius relationship and canopy-density relationship determine M-N relationship in stressful environment.4. According to the advanced ecological field model, plant will potentially restrain the neighbor at near but facilitate the neighbor at far, and the potential will be inhibited by abiotic stress. This result presents a theoretical interpretation to the stress-gradient hypothesis.Comparing with the results in previous literatures, this study has innovations in the research of M-N relationship with varied canopy, the covariation of sessile organism interactions and scaling exponent, and the exploration of individual interactions along environmental gradients. Those results do not only have important theoretical significance to self-thinning law and population dynamics, but also present powerful scientific support to population density regulation at the field of agriculture, forest and breeding, and vegetation restoration.