Baruch Zdravko, Bilbao Bibiana
Dpt. Estudios Ambientales, Universidad Simón Bolívar, Apartado 89000, Caracas 1080, Venezuela e-mail:
Oecologia. 1999 Jun;119(4):510-520. doi: 10.1007/s004420050814.
African grasses, introduced into Neotropical savannas to improve forage quality, have spread successfully and displaced native plants. To understand their competitive relationships, we compared biomass production and allocation, plant architecture and phenology, net photosynthesis (Pn), water relations, and nutrient content under fire and simulated herbivory between two C grasses, the native Trachypogon plumosus and the introduced Hyparrhenia rufa from a seasonal savanna in Venezuela. All variables were strongly influenced by the rainfall regime. Hyparrhenia produced bigger plants (in mass and size) with a large proportion of mass (>75%) allocated to leaves and culms. Its biomass production was more affected by fire than by defoliation. In contrast, Trachypogon was more affected by defoliation than by fire which promoted a flush of leaf growth even in the dry season. Fire caused up to 85% mortality in Hyparrhenia but none in Trachypogon where it increased inflorescence production. However, fire promoted abundant seed germination and fast seedling growth in Hyparrhenia, enabling it to colonize new areas. During the growing season Trachypogon had higher Pn and lower leaf water potential (Ψ) than Hyparrhenia but differences among treatments were not significant for either grass. Pn of Trachypogon ceased at a lower Ψ (-3.0 MPa) than in Hyparrhenia (-2.0 MPa), indicating its higher tolerance to water stress. During the dry season, Trachypogon leaves remained alive and retained low Pn. Leaf nutrient content was higher during the rainy season in both species. Differences in Pn could not explain the higher seasonal biomass production of Hyparrhenia. However, its water stress evasion strategy, larger biomass allocated to leaves, abundant germination and fast seedling growth appeared to be responsible for the success of Hyparrhenia as an invader of Neotropical savannas.
引入新热带稀树草原以提高牧草质量的非洲草种已成功扩散并取代了本地植物。为了解它们之间的竞争关系，我们比较了委内瑞拉季节性稀树草原上的两种C4草，即本地的羽穗草（Trachypogon plumosus）和引入的红苞茅（Hyparrhenia rufa）在火烧和模拟食草情况下的生物量生产与分配、植株结构和物候、净光合速率（Pn）、水分关系以及养分含量。所有变量都受到降雨模式的强烈影响。红苞茅植株更大（质量和尺寸方面），且大部分质量（>75%）分配给叶片和茎秆。其生物量生产受火烧的影响大于去叶处理。相比之下，羽穗草受去叶处理的影响大于火烧，火烧甚至在旱季也促进了羽穗草叶片的生长。火烧导致红苞茅高达85%的死亡率，但对羽穗草没有影响，火烧反而增加了羽穗草的花序产量。然而，火烧促进了红苞茅种子的大量萌发和幼苗的快速生长，使其能够在新区域定殖。在生长季节，羽穗草的净光合速率高于红苞茅，叶片水势（Ψ）低于红苞茅，但两种草在不同处理间的差异均不显著。羽穗草的净光合速率在较低的水势（-3.0 MPa）时停止，低于红苞茅（-2.0 MPa），这表明其对水分胁迫的耐受性更高。在旱季，羽穗草的叶片仍然存活并保持较低的净光合速率。两种草在雨季的叶片养分含量都更高。净光合速率的差异无法解释红苞茅更高的季节性生物量生产。然而，其逃避水分胁迫的策略、分配给叶片的更大生物量、大量萌发和快速的幼苗生长似乎是红苞茅成功入侵新热带稀树草原的原因。
