Plant Interactions In Two Arid Communities: Implications From Spatial Patterns

Plant interactions are intrinsically spatial: since plant individuals can react only to their local environment, the spatial distributions of plant individuals have great importance for the attribute and strength of plant interactions; a relative stable spatial pattern is the result of plant interactions in the past. Clarifying the linkage between spatial patterns and plant interactions would be helpful for understanding the species coexistence mechanism and adaption strategies. The aboveground and underground patterns of a shrub-herb community (Nitraria tangutorum-Achnatherum splendens community) and a tree-grass community (Elaeagnus angustifolia-Achnatherum splendens) in northwest China investigated to analyze the inter/intra specific interactions between plants and the adaptation strategy of species to draught stress.(1) For the intra-specific interactions and aboveground structure of community, the spatial association analysis was applied to test the hypothesis that the species could be more beneficial in association with two-nurse species than with single-nurse species in the N. tangutorum-A. splendens community. The performance (frequency, abundance, and size) of the two associate species in neighbored patches (patches formed by a dominant species which were closely adjacent to the patches formed by the other dominant species), isolated patches (patches formed by one dominant species with no neighbor) and open areas was compared to analyze the individual and combined effects of the two dominant species. A. splendens and N. tangutorum appeared to have reciprocal facilitation effects when growing adjacent to one another, as evidenced by the increased size of neighbored patches over isolated ones. The individual effects of A. splendens on the associate species should be generally neutral, while the individual effects of N. tangutorum were positive to the associate species R. soongorica. In comparison to the isolated patches, there were significantly higher frequency and abundance (p<0.05), larger sizes (p<0.05), and higher co-occurrence frequency (p<0.05) of the two associate species in neighbored N. tangutorum patches. Since the neighbored patches could be influenced by both dominant species, the combined effects of A. splendens and N. tangutorum were identified as positive and over-additive.(2) For the intra-specific interactions and aboveground structure, K2 point pattern function, which could avoid the effects of environmental heterogeneity, was applied to analyze the spatial pattern and intra-specific interactions of A. splendens within a small scale (within 0.5m) in the Elaeagnus angustifolia-Achnatherum splendens community. Sixty-three A. splendens quadrats were established and studied in an arid community dominated by E. angustifolia and A. splendens in the northwest China. The quadrats were established at three density levels in three microenvironmental types and with seven replicates of each. The different responses of A. splendens were compared based on the three spatial density patterns, low, medium, and high, and three microenvironmental types, subcanopy, transitional, and open areas. The soil physicochemical properties of electricity conductance, soil organic matter, and soil bulk density, were measured in the three microenvironments to quantify the environmental stresses. The results show soil physicochemical stress increased along the subcanopy to transitional area to open area gradient. The subcanopy area had relatively low environmental stress as evidenced by low soil electrical conductivity, high soil organic matter, and low soil bulk density. A. splendens was clumped in only six of the 21 subcanopy quadrats, while in the transitional and open areas where environmental stress was high A. splendens was clumped in a small majority of the quadrats (11/21). The high environmental stress areas were defined as areas having high soil electrical conductivity, low soil organic matter, and high soil bulk density. A. splendens tended to be clumped in areas with increasing environmental stress along the subcanopy to transitional area to open area gradient. However, the spatial responses of A. splendens to environmental stress differed at the three density levels. At the low-density level, A. splendens had a clumped distribution in most quadrats at all three environmental stress levels. The clumped distribution proportions of A. splendens quadrats were 4/7,7/7, and 7/7 in subcanopy area, transitional area, and open area, respectively. In the medium density level, the frequency of A. splendens quadrats with clumped distribution increased with the environmental stress. In these medium density level areas the proportion of clumped distribution of A. splendens populations were 2/7,3/7, and 4/7 in subcanopy area, transitional area, and open area, respectively. In the high-density level, A. splendens was distributed randomly in most quadrats (except one in a transitional area) in all three environmental stress levels. Since the spatial pattern of A. splendens showed a clear tendency to be clumped along the density gradient rather than along the soil physicochemical stress gradient, this might suggest the spatial pattern of A. splendens was more influenced by density than by the stresses caused by severe soil conditions. We concluded the spatial pattern of A. splendens on a small scale responded to the environmental stresses differently based on population density. As the result, population density should be considered when analyzing the variations of spatial patterns and the occurrence of positive interactions along the stress gradient.(3) For the underground patterns, the rooting patterns of trees(E. angustifolia) and tussocks(A. splendens) along a gradient radiating from tree boles to inter-canopy area were investigated to test whether the rooting segregation patterns could vary within a community level. Both the root biomass density (RBD) of trees and tussocks decreased with the power function of the distance from tree boles to inter-canopy area. The rooting depth of trees lowered down gradually along the gradient. While the tussock roots concentrated in upper soil within a threshold distance of 1.6-2.0 times of tree crown radius, and dropped sharply to deep soil out of the threshold distance of. As a result, the root of trees and tussocks segregated within 1.6-2.0 times of tree crown radius, while occupied the same volume of soil out of the distance. So the root segregation pattern between trees and tussocks shifted at a distance threshold of 1.6-2.0 times of tree crown radius within a community level.(4) Both aboveground and underground patterns in E. angustifolia-A. splendens community were analyzed to identify the relative importance of tree canopy and root competition on the morphological traits of tussocks. Three study plots (20×20m) were respectively divided into 2×2 m quadrats, within each of which 1) canopy shading was quantified by inverse proportion of cumulative direct solar radiation (CDSR), and 2) the root effect was quantified using an empirical relationship between tree fine root density (TFRD) and relative distance to tree bases. Morphological traits were measured to represent grass size. Redundancy analysis (RDA) was conducted to examine the relative influences of grass density, CDSR and TFRD on the coefficient of variation of grass size. Results showed that no significant correlation occurred between grass density and grass size inequality. Both CDSR and TFRD had significant negative correlations with grass size inequality, suggesting that canopy shading and the presence of fine roots of trees can respectively increase and reduce grass size inequality. Canopy shading and tree fine root density played competitive roles in determining grass size inequality, where the root effect was a stronger factor than canopy shading.Summarily, the study suggested that 1) spatial patterns can be quite useful in inferring underlying intra/inter specific interactions by using appropriate hypotheses in the two arid communities 2) not only the aboveground effects, but also underground effects should be considered for inferring the ecological processes from the spatial structure analysis, and underground rooting pattern might overweigh the aboveground shading condition in influencing morphological traits of understory tussocks 3) combined with the analysis of the inequality of size, spatial pattern analysis could be more precise and informative for inferring underlying processes.