Giroldo A B, Scariot A, Hoffmann W A
Departamento de Ecologia, Universidade de Brasília, Caixa Postal 04457, Brasília, DF, 70919-970, Brazil.
Embrapa Recursos Genéticos e Biotecnologia, Laboratório de Ecologia e Conservação, Caixa Postal 02372, Brasília, DF, 70770-900, Brazil.
Oecologia. 2017 Oct;185(2):281-291. doi: 10.1007/s00442-017-3930-4. Epub 2017 Aug 24.
Over the past 10 million years, tropical savanna environments have selected for small growth forms within woody plant lineages. The result has been the evolution of subshrubs (geoxyles), presumably as an adaptation to frequent fire. To evaluate the traits associated with the shift from tree to subshrub growth forms, we compared seed biomass, germination, survival, resprouting, biomass allocation, and photosynthesis between congeneric trees and subshrubs, and quantified phylogenetic conservatism. Despite large differences in adult morphology between trees and subshrub species, the differences are modest in seedlings, and most of the variation in traits was explained by genus, indicating considerable phylogenic conservatism. Regardless, tree seedlings invested more heavily in aboveground growth, compared to subshrubs, which is consistent with the adult strategy of savanna trees, which depend on a large resistant-fire stem. Subshrub seedlings also invest in greater non-structural carbohydrate reserves, likely as an adaptation to the high fire frequencies typical of tropical savannas. The modest differences as seedlings suggest that selective pressures during early development may not have contributed substantially to the evolution of the subshrub growth form and that the distinct allocation and life history must arise later in life. This is consistent with the interpretation that the subshrub growth form arose as a life-history strategy in which maturity is reached at a small stem size, allowing them to reproduce despite repeated fire-induced topkill. The convergent evolution of subshrubs within multiple tree lineages reaffirms the importance of fire in the origin and diversification of the flora of mesic savannas.
在过去的1000万年里，热带稀树草原环境促使木本植物谱系中出现小型生长形态。结果是亚灌木（地茎植物）的进化，大概是为了适应频繁的火灾。为了评估与从树木生长形态向亚灌木生长形态转变相关的性状，我们比较了同属树木和亚灌木之间的种子生物量、发芽率、存活率、再发芽能力、生物量分配和光合作用，并对系统发育保守性进行了量化。尽管树木和亚灌木物种在成年形态上存在很大差异，但在幼苗阶段差异较小，而且大多数性状变异是由属来解释的，这表明存在相当程度的系统发育保守性。尽管如此，与亚灌木相比，树木幼苗在地上部分生长的投入更多，这与稀树草原树木依靠粗壮的抗火树干的成年策略一致。亚灌木幼苗也积累了更多的非结构性碳水化合物储备，这可能是对热带稀树草原典型的高火灾频率的一种适应。幼苗阶段的适度差异表明，早期发育过程中的选择压力可能对亚灌木生长形态的进化没有太大贡献，而且独特的分配方式和生活史一定是在生命后期出现的。这与以下解释一致，即亚灌木生长形态是一种生活史策略，在较小的茎干尺寸时达到成熟，使其能够在反复的火灾导致地上部分死亡后仍能繁殖。多个树木谱系中亚灌木的趋同进化再次证明了火灾在中生稀树草原植物区系起源和多样化中的重要性。
