Exploring Holocene Changes in Palynological Richness in Northern Europe – Did Postglacial Immigration Matter?

In mid to high latitudes glacial and interglacial cycles have repeatedly changed the area available for plant growth. The speed at which plants are able to colonize areas at the onset of an interglacial is hypothesized to limit their distribution ranges even today (migrational lag). If the spread of plants would have been generally slow then plant diversity in previously glaciated areas would be expected to increase over time. We explore this hypothesis using results from six palynological investigations from two previously glaciated regions: central Sweden and north-eastern Germany. Rarefaction, slope of rank order abundance, and taxa accumulation plots were used to evaluate richness and evenness in pollen data in an attempt to separate richness from evenness. These analyses show little change in palynological richness for the northern sites throughout the Holocene. In contrast, the southern sites show an increase in richness and evenness during the early Holocene; this may be explained by the different initial conditions at the onset of the Holocene. A strong rise in palynological richness around 6000 and 1000 years ago at the southern sites can be attributed to the regional initiation of agriculture and major opening of the forest, respectively. For the northern sites there is no evidence for increased taxonomic diversity through time that could be due to delayed immigration of species.在中高纬度地区，冰期与间冰期旋回反复改变了植物可利用的生长区域。学界曾提出假说：即便在今日，植物在间冰期伊始的区域拓殖速度，仍会限制其分布范围（迁移滞后（migrational lag））。若植物的扩张速率普遍偏缓，则此前受冰川覆盖区域的植物多样性理应随时间推移而提升。 本研究借助两处此前曾被冰川覆盖的区域——瑞典中部与德国东北部——的6项孢粉学（palynology）调查结果，对上述假说展开验证。本研究采用稀疏化法（rarefaction）、多度排序斜率（slope of rank order abundance）与类群累积曲线（taxa accumulation plots），对孢粉数据中的物种丰富度与均匀度进行评估，以期将丰富度与均匀度二者区分开来。 上述分析结果显示，北部采样点的孢粉丰富度在全新世（Holocene）期间几乎未发生变化。与之相对，南部采样点的物种丰富度与均匀度在早全新世阶段均有所提升，这或许可归因于全新世伊始两地存在差异化的初始环境条件。南部采样点的孢粉丰富度在约6000年与1000年前出现显著上升，二者可分别归因于该区域的农业起源与森林的大规模开垦。北部采样点则未出现可归因于物种迁入延迟的、随时间推移而提升的分类学多样性迹象。