土壤水分变化对黄土丘陵区撂荒群落共存种竞争的影响

群落共存种对限制性资源的竞争响应是群落组建、维持与演替的重要影响因素。以黄土丘陵区摞荒群落共存种为研究对象，研究不同水分条件下的竞争响应差异，以期揭示生境变化时群落演替的植物竞争机理。试验设土壤水分与混植密度两个处理，其中混植密度采用CDS （Community density series）法，含3个密度水平（衡水试验为3、12、21株/m²，变水试验为5、20、35株/m²）；土壤水分设衡水（高（75% FC）、中（60% FC）、低（45% FC）3个水平）与变水（由高水（75% FC）经50 d持续干旱至低水（45% FC））两种处理。根据共存种株高的阶段性生长与其在群落中的相对生物量等生长表现，在CDS法基础上扩展并测算了生长变化量、相对生物量和相对生长率等指标。据此，研究竞争效应和水分胁迫效应的相对影响，共存种相对竞争能力和竞争等级对土壤水分变化的响应，及高水期因竞争引起的个体大小不对称性与低水期死亡率的关系。结果表明：1）不同水分多种混植共存条件下，竞争效应对多数共存种在群落中的生长表现较水分胁迫效应影响更大；以相对生物量为指标的竞争的群落效应（4.30±0.49）较水分胁迫的群落效应（-23.56±20.38）大，且二者累加效应（4.76±0.4）大于二者单项效应之和（-19.26±20.48），即竞争+水分胁迫对多种共存群落具有协同效应。2）当水分条件发生变化时，群落共存种的竞争等级并不完全相同（P > 0.05），相应的会发生变化，且水分差别越大，共存种竞争等级的差别也越大；高水条件下群落共存种竞争所引起的群落效应最大（4.84±0.23），部分支持最大生长率理论，即限制性资源水平越高，群落共存种竞争越激烈。3）变水试验中，10种群落共存种在高水期因竞争引起的个体大小差异与低水期的死亡率呈正相关关系（P=0.015，r²=0.801），根据资源动态两阶段假说，说明试验条件下群落共存种竞争是群落组建的重要因素。

Effects of soil moisture variation on competition among co-existing species in old-field communities of the Loess Hilly Region

The competitive response of co-existing species in a community to limited resources is an important determining factor responsible for community formation, maintenance, and succession. To reveal the plant competition mechanisms of community succession owing to habitat change, we studied the competition responses of co-existing species in old-field communities of the Loess Hilly Region under various water conditions. The experiment was carried out using a change of two factor, soil moisture, and planting density. There were two treatments of soil moisture, including invariant and changed water treatments. Invariant water treatment was carried out under severe (45% FC), moderate (60% FC), and mild (75% FC) water deficit levels, and changed water treatment was begun under mild water deficit levels (75% FC) and after non-water treatment for 50 days was changed to severe water deficit levels (45% FC). There were three levels of planting density determined by the Community Density Series (CDS) method (3, 12, and 21 individuals/m² in invariant water treatment; 5, 20, and 35 individuals/m² in changed water treatment). In this study, based on the CDS method, we measured and calculated proportional growth, relative biomass, relative proportional growth, and other indexes, to show the relative effects of competition and water stress, responses of relative competition ability, and competition hierarchies to changes of soil moisture, as well as the relationship between growth variation in water-welled period and mortality rate in severe water drought period. The results showed that (1) under different water and density conditions, the competition effects on the growth performance of most co-existing species in the community were greater than the drought effects. Community competition effects (4.30±0.49) based on the relative biomass had a higher value than the community drought effects (-23.56±20.38) and the accumulative effects (4.76±0.4) were greater than the sum of the two individual effects (-19.26±20.48), which means competition and water stress had synergistic effects on the community; (2) when the water conditions changed, the competitive hierarchy of community co-existing species was not exactly the same (P > 0.05) and corresponding changes occurred. Furthermore, the larger the difference of water conditions, the greater the difference in the competitive hierarchy of community co-existing species. The value of relative community competition intensity was highest (4.84±0.23) under mild water deficit levels, which partially supported the maximum growth rate theory, and indicated that higher levels of limited resources, resulted in more intense competition between community co-existing species; (3) for the changed water experiment, there was a positive correlation between individual size variation caused by the competition in the water-welled period and mortality rate in the drought period (P=0.015, r²=0.801). According to the two-stage hypothesis of resource dynamics, it showed that the community co-existing species competition was an important factor for the community formation in this experiment.