Abrupt Alnus population decline at the end of the first millennium CE in Europe – The event ecology, possible causes and implications

Abstract

The study, based on the examination of 70 published and unpublished pollen profiles from Poland and supplementary data from the surrounding regions, shows that an abrupt, episodic Alnus population decline at the end of the first millennium CE was a much more widespread event than has been previously reported, spanning large areas of the temperate and boreal zones in Europe. The data from Poland suggest that the decline was roughly synchronous and most likely occurred between the 9th and 10th centuries, with strong indications for the 10th century. The pollen data indicate that human impacts were not a major factor in the event. Instead, we hypothesize that one or a series of abrupt climatic shifts that caused floods and droughts at the end of the first millennium CE could have initiated this ecological disturbance, leading to a higher vulnerability of the alder trees to a pathogen outbreak. Following current observations of the decline of alder stands in Europe due to a Phytophthora outbreak, we suggest that a similar process may have occurred in the past. This study provides insight into long-term alder (mainly Alnus glutinosa) dynamics in a condition of climate change and illustrates its great resilience, enabling the natural, successful regeneration of alder stands after critical diebacks if environmental conditions improve. Our finding that the Alnus pollen decline reflects a roughly synchronous event indicates that the decline could be used as an over-regional chronostratigraphic marker for 800–1000 CE in pollen diagrams from a large part of the European Lowland.

Environmental changes and plant use during the 5th-14th centuries in medieval Gdańsk, northern Poland

“…2b, 4). These ecological processes agree well with the data on climatic changes around the 9th-10th centuries from the Medieval Climate Anomaly, including longlasting droughts (Helama et al 2009;Büntgen et al 2011) and a widespread Alnus decline most probably caused by a sequence of climatic events detrimental to alder followed by a pathogen attack (Stivrins et al 2017;Latałowa et al 2019).…”

Emerging palaeoecological frameworks for elucidating plant dynamics in response to fire and other disturbance

“…The most straightforward of these solutions is to undertake intensive sampling that provides enough temporal resolution to resolve most disturbances (e.g., Booth et al, 2012;Latałowa et al, 2019;Peglar & Birks, 1993). The utility of intensive sampling is well illustrated by a recent effort that compared low-versus highresolution palaeoecological records to elucidate the impact of disturbance on vegetation (Calder & Shuman, 2019) In existing records where uneven sampling resolution makes it difficult to identify rapid vegetation change, disentangle multiple drivers and identify interactions with disturbance, emerging statistical frameworks that account for data irregularly spaced in time could greatly improve estimation of trends, regime shifts and rates of change Seddon et al, 2014;Simpson, 2018).…”

Coastal peat‐beds and peatlands of the southern North Sea: their past, present and future

“…If unmanaged and therefore with biomass retained, fen carr communities have the ability to store large quantities of carbon, particularly when the water level is maintained close to the surface (Barthelmes, 2009; Table 1). A. glutinosa is also reported as showing long‐term resilience (the ability of a system to tolerate change while retaining essentially the same function and structure) to climate change with stands regenerating after diebacks if environmental conditions improve (Latałowa et al ., 2019). As peatland areas are expanded, greater consideration should therefore be given to converting areas into A. glutinosa woodland either by planting or allowing succession to occur.…”