Response Mechanism Of Desert Plant Allometric Exponents And Spatial Patterns To A Precipitation Gradient

Plant allometry is the quantitiative relationship between growth and allocation. In response to numerous selection pressures and constraints, plants evolve many allometric patterns, which can in turn help explain many behaviors of plant individuals and poulations, such as interactions between neighboring plants. The spatial pattern of plant individuals observed in a plant community at any point in time is the product of many processes in its past, both biotic and abiotic. Plant spatial patterns determine the direction and strength of plant interactions, and consequently influence the allometric relationships at different scales. Therefore, the analysis of allometric growth patterns and spatial patterns of plant individuals would facilitate the comprehension of the role of both biotic interactoins and abiotic variables influencing the dynamics and structure of plant communities. Howevere, allometric growth patterns and spatial patterns of plant individuals in the desert plant communities have yet to be explicitly investigated, in particular their response to the environmental gradient.To this end, the present study analyzes the allometric growth patterns and spatial patterns of plant individuals in differnt desert plant communities along a precipitation gradient (i.e., Jingtai, Zhangye, Jiuquan, Jiayuguan, Guazhou and Dunhuang), in an attempt to explore how plant allometric relationships and plant spatial patterns respond to the precipitation gradient. The study results will guide us to better manage the natural resources of the desert region of northwest China, to restore the degraded vegetation and to acheive a sustainabel use of natural resources. The main resulsts and conclusions obtained in the present study run as follows:(1) The scaling exponent of the relationship between plant height (H) and basal stem diameter (D) approximates 1.0 for each of the three desert herbaceous species, as predicted by the geometric self-similarity model. While the H-D scaling exponents do not differ significantly among the five desert shrub species (ranging from 0.55 to 0.79), they do not exclusively follow any of the three models (i.e., geometric, elastic and stress self-simiarity model). Nevertheless, the observed scaling exponents for the five shrub species are closer to the predicted value by the elastic self-similarity model, compared to the other two models. The higher scaling exponent for herbaceous species than shrub species may be attributed to the differnce among them in stem anatomy and physical properties. In addition, the intercepts of the H-D scaling relationship are found to decrease with decreasing precipitation, suggesting that as plants are subjected to a more severe water stress, plants are confronted with higher mechanical and physiological constraints, resulting in a lower growth rate in height relative to the horizontal expasion of the stem and crown.(2) For Reaumuria soongorica growing in Zhangye, standing leaf biomass (ML) is found to scale as the 0.86 power of standing stem biomass (Ms) and as the 0.80 power of standing root biomass(MR), and standing MS is found to scale as the 0.93 power of standing MR, i.e. ML∝MS0.86, ML∝MR0.80 and MS∝MR0.93, respectively. Furthermore, standing aboveground biomass (which equils the sum of ML and MS) scales nearly isometrically with respect to standing belowground biomass (i.e. root biomass MR) for all three shrub species from differnt field samplin sites, i.e., R. soongorica in Zhangye, Ajania fruticulosa in Jiayuguan and Salsola passerina in Jiayuguan (scaling exponents are 0.87,1.14 and 0.90, respectively). All these observed scaling exponents are indistinguishable from those predicted the allometric theory, suggesting that the same scaling exponent governing vegetative organ biomass partitioning may hold true within and across species comparisons.(3) Based on the geometry of space filling by growing plants, an allometric model is numerically derived to predict the relationship among the three scaling exponents, i.e., mean plant biomass vs. population density (M-N), plant height vs. crown diameter (H-Dc) and mean nearest neighbor distance vs crown diameter (di-Dc). The model predicts that the M-N scaling exponent y will vary with the H-D scaling exponentβand the di-DC scaling exponent 8, asγ=-(2+β)/28. The prediction of this model is verified by statistically comparing the numerical values of predicted scaling exponents with those observed for the collected dataset along a precipitation gradient. For all desert populations at four field sampling sites along the precipitation gradient, the M-N scaling exponents significantly deviate from both-3/2 and-4/3. Specifically, with decreasing precipitation, y increases significantly from-1.01 at Zhangye field site to-0.21 at Dunhuang field site,βtends to decrease from 0.83 at Zhangye field site to 0.61 at Guazhou field site, and 8 increases significantly from 1.13 at Zhangye field site to 1.54 at Guazhou field site. The observed y values are shown to be close to the expected values by the allometric model and the two observed valuesβandδ, except for populations at Zhangye field site. These results verify the predictions of the allometric model and support the hypothesis that variation of allometric exponentsβandδin response to environmental gradients will yield the M-N scaling exponent different from both-3/2 and-4/3.(4) For both S. passerina population at Jingtai field site and R. soongorica population at Zhangye site, shrub stems are significantly clumped at small scales up to 3m and 1.1m, respectively. When shrubs are analyzed separately in three predetermined size classes of increasing canopy diameter, for both populations there is a variation tendency in the spatial patttern from significantly strongly clumped in the small-sized plants, to weakly clumped in the median-sized plants, and to almost competely random in the large-sized plants. These results indicate that the spatial structure of the whole population is primarily determined by that of the small-sized individuals, and that with increasing plant size, a prograssive shift of the spatial pattern will occur from clumping in small-sized individuals to randomness or regularity in large-sized individuals, probably due to increasing intensity of intraspecific competition for soil water or other limiting resources. The spatial correlation analysis reveals that small-, medium-and large-sized plants are distrubuted independently of one another in the S. passerina population. This may suggest that small-sized individuals are aggregated in the canopy gap, rather than in the vicinity of or under the canopies of large-sized mother individuals, likely due to the asymetric competition between small-and large-sized conspecifics. In contrast, in the R. soongorica population, small-sized individuals are found to be spatially strongly aggregated with medium-sized individuals and weakly aggregated with large-sized individuals at small scales up to 1.1m. This is most likely to be caused by limited seed dispersal and/or vegetative recruitment, as well as a net positive effect of large mother individuals on seedling emergence and early growth. The difference between two populations in the spatial association of small-and medium-or large-sized individuals may imply that apart from the inherent differences between the two species in eco-physiological traits, facilitative effects of large mother individuals on seedling emergence and growth become more important as habitats becomes less favorable for plant growth and survival. All three dominant shrub species in the community at Jiayuguan field site are randomly distributed at both the whole population level and the size-class level. Furthermore, small-sized individuals are shown to be distributed independently of both medium-and large-sized individuals of the same or the other species. The lack of either aggregation or regularity for each species and each size class, as well as either attraction or repulsion between small-and large-sized conspecific or heterospecific individuals may stem from the strong intraspecific and/or interspecific competition in its past, or from the overgrazing, or from both.