Competition Among Grasses Along a Nitrogen Gradient: Initial Conditions and Mechanisms of Competition
Competition Among Grasses Along a Nitrogen Gradient: Initial Conditions and Mechanisms of Competition
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DOI：
10.2307/2937180
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发表时间：
1993-02
影响因子：
6.1
通讯作者：
D. Wedin;D. Tilman
中科院分区：
文献类型：
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作者：
D. Wedin;D. Tilman
4种多年生牧草(草地早熟禾、冰草、黑麦草和裂褶草)在单作栽培和氮素梯度下的两两竞争小区中种植了5年。坡度包括从100%沙地到100%黑土的地块，加上接受额外氮肥的地块。为了考察初始条件对种间竞争长期结果的影响，创造了三种竞争情况：种子与种子竞争(两个物种同时种植)，种子入侵(每个物种作为种子添加到另一个物种的一年生单层中)，以及营养入侵(隔开两个物种相邻的单层培养的分隔物，在1年后被移除)。测定了可提取的土壤NO3和NH4，以检验单一栽培土壤有效N浓度的物种差异(即N的R*，Tilman 1982)是否可以预测竞争的长期结果。到第5年，裂褶菌在土壤混合坡度(混合土而不是添加N区)上对早熟禾和冰草的生物量都发生了移位或极大的降低，这与启动条件的大范围无关。在这些土壤上，单株裂褶菌的土壤NO_3和NH_4浓度均显著低于POA或冰草。同样，冰草在第5年取代或大大降低了冰草的生物量。冰草的N03-和NO3+NH4+浓度显著低于冰草，但NH4+浓度不低于冰草。相反，POA和冰草之间的竞争没有发生竞争取代，最初的差异持续了5年以上。这两种单细胞在NO3浓度上没有差异，但在NH4+和NO3+NH4+上有所不同。因此，物种消耗第三号土壤的能力的差异成功地预测了所有四个物种对在土壤混合坡度上的竞争结果。如果资源抢占或不对称竞争是竞争的机制，初始条件就会影响竞争的长期结果。相反，这些结果支持R*(即资源减少)的土壤N竞争模型。在增施氮肥的小区中，五倍子在与早熟禾和冰草的竞争中都降低了生物量。然而，无论是冰草还是早熟禾，在追氮小区的竞争中似乎都没有优势。对于这三个树种对，由于产量增加和凋落物积累，添加氮小区的5年竞争结果极大地降低了光能利用率，这取决于初始条件。在第四对中，Agrostis在所有的竞争处理中都被Agropyron取代了。因此，我们不能否认这样的假设，即资源抢占(即不对称竞争)在轻竞争中很重要。
We grew four perennial grass species (Poapratensis, Agropyron repens, Agros- tis scabra, and Schizachyrium scoparium) for 5 yr in monocultures and in pairwise com- petition plots on an experimental nitrogen gradient. The gradient contained plots ranging from 100% sand to 100% black soil, plus plots that received additional N fertilizer. To examine the impact of initial conditions on the long-term outcome of interspecific com- petition, three competitive situations were created: seed vs. seed competition (both species planted simultaneously), seed invasions (each species added as seed to year-old monocul- tures of the other), and vegetative invasions (dividers separating adjacent monocultures of two species removed after 1 yr). Extractable soil NO3 and NH4, were measured to test if species differences in the concentration of available soil N in monoculture (i.e., R* for N, Tilman 1982) could predict the long-term outcome of competition. By year 5, Schizachyrium displaced or greatly reduced the biomass of both Poa and Agropyron on the soil mixture gradient (the mixed soils but not the added-N plots) inde- pendent of the wide range of starting conditions. On these soils, Schizachyrium monocul- tures had significantly lower soil concentrations of both NO3 and NH4, than either Poa or Agropyron monocultures. Similarly, Agropyron displaced or greatly reduced the biomass of Agrostis by year 5. Agropyron monocultures had significantly lower concentrations of N03- and NO3 + NH4+, but not NH4+, than Agrostis monocultures. In contrast, no competitive displacement occurred in competition between Poa and Agropyron, and initial differences persisted over 5 yr. Monocultures of these two species did not differ in NO3 concentration, but did differ for NH4+ and NO3 + NH4+. Thus, species differences in ability to deplete soil NO3 successfully predicted the outcome of competition for all four species pairs on the soil mixture gradient. If resource preemption or asymmetric compe- tition had been the mechanism of competition, initial conditions would have affected the long-term outcome of competition. Rather, these results support the R* (i.e., resource reduction) model for competition for soil N. In the added-N fertilizer plots, Schizachyrium had decreased biomass in competition with both Poa and Agropyron. However, neither Agropyron nor Poa appeared to have an advantage when they competed with each other in the added-N plots. For these three species pairs, the 5-yr results of competition in the added-N plots, which had greatly reduced light availability because of increased production and litter accumulation, depended on initial conditions. In the fourth pair, Agrostis was displaced by Agropyron in all competition treatments in the added-N plots. Thus, we cannot reject the hypothesis that resource preemption (i.e., asymmetric competition) is important in light competition.