Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of Bern, Bern, 3013, Switzerland.
EPHE Department of Palaeoclimatology and Marine Palaeoenvironments, PSL Research University, UMR CNRS 5805 EPOC Université de Bordeaux, Pessac, 33615, France.
Ecology. 2019 Nov;100(11):e02833. doi: 10.1002/ecy.2833. Epub 2019 Aug 20.
Mediterranean rear-edge populations of Betula, located at the southwestern Eurasian margin of the distribution range, represent unique reservoirs of genetic diversity. However, increasing densities of wild ungulates, enhanced dryness, and wildfires threaten their future persistence. A historical perspective on the past responses of these relict populations to changing herbivory, fire occurrence and climatic conditions may contribute to assessing their future responses under comparable scenarios. We have reconstructed vegetation and disturbance (grazing, fire) history in the Cabañeros National Park (central-southern Spain) using the paleoecological records of two small mires. We particularly focused on the historical range of variation in disturbance regimes, and the dynamics of rear-edge Betula populations and herbivore densities. Changes in water availability, probably related to the North Atlantic Oscillation (NAO) index, and land-use history have played a crucial role in vegetation shifts. Our data suggest that heathlands (mainly Erica arborea and E. scoparia) and Quercus woodlands dominated during dry phases while Sphagnum bogs and Betula stands expanded during wet periods. Betula populations survived past moderately dry periods but were unable to cope with enhanced land use, particularly increasing livestock raising since ~1,100-900 cal. yr BP (850-1,050 CE), and eventually underwent local extinction. High herbivore densities not only contributed to the Betula demise but also caused the retreat of Sphagnum bogs. Ungulate densities further rose at ~200-100 cal. yr BP (1750-1850 CE) associated with the historically documented intensification of land use around the Ecclesiastical Confiscation. However, herbivory reached truly unprecedented values only during the last decades, following rural depopulation and subsequent promotion of big game hunting. For the first time in temperate and Mediterranean Europe, we have used the abundances of fossil dung fungal spores to assess quantitatively that current high herbivore densities exceed the historical range of variation. In contrast, present fire activity lies within the range of variation of the last millennia, with fires (mainly human-set) mostly occurring during dry periods. Our paleodata highlight the need of controlling the densities of wild ungulates to preserve ecosystem composition and functioning. We also urge to restore Betula populations in suitable habitats where they mostly disappeared because of excessive human activities.
地中海边缘种群位于欧亚大陆分布范围的西南边缘，是遗传多样性的独特储备库。然而，野生动物密度的增加、干燥程度的提高以及野火威胁着它们的未来生存。从历史角度研究这些遗留种群对不断变化的食草作用、火灾发生和气候条件的过去反应，可能有助于在类似情况下评估它们的未来反应。我们使用两个小型沼泽的古生态学记录，重建了西班牙中部-南部卡瓦尼略斯国家公园（Cabañeros National Park）的植被和干扰（放牧、火灾）历史。我们特别关注干扰机制的历史范围变化，以及边缘贝氏种群和食草动物密度的动态。水供应的变化，可能与北大西洋涛动（North Atlantic Oscillation，NAO）指数有关，以及土地利用历史，在植被变化中发挥了关键作用。我们的数据表明，石南灌丛（主要是 Erica arborea 和 E. scoparia）和栎树林在干旱时期占主导地位，而泥炭藓沼泽和桦树种群在湿润时期扩张。贝氏种群在过去的适度干旱时期幸存下来，但无法应对日益增强的土地利用，特别是自大约 1100-900 年前（公元 850-1050 年）以来，牲畜养殖的增加，最终导致局部灭绝。高食草动物密度不仅导致了桦树的灭绝，还导致了泥炭藓沼泽的退缩。大约 200-100 年前（公元 1750-1850 年），随着围绕教会没收的土地利用的历史记录的加剧，食草动物的密度进一步上升。然而，仅在过去几十年中，随着农村人口减少和随后提倡大型猎物狩猎，食草动物的数量才达到真正前所未有的水平。在温带和地中海欧洲，我们首次使用化石粪便真菌孢子的丰度来定量评估当前高食草动物密度超过了历史范围。相比之下，目前的火灾活动处于过去几千年的范围之内，火灾（主要是人为引发）主要发生在干燥时期。我们的古数据强调了控制野生动物密度的必要性，以保护生态系统的组成和功能。我们还敦促在适宜的栖息地恢复桦树种群，因为它们主要由于过度的人类活动而消失。
