Kretzschmaria deusta and the northwest European mid-Holocene Ulmus decline at Moel y Gerddi, north Wales, United Kingdom

The decline in Ulmus pollen frequencies that occurred ca. 5000 ¹⁴C years ago before present (BP) is a key biostratigraphic marker horizon in northwest European pollen diagrams, although its causes are still a subject of debate. To investigate this event further, fungal spore analyses were carried out across the Ulmus decline at Moel y Gerddi, north Wales, United Kingdom. The Ulmus decline was in three phases, with a primary decline with low cereal and Rumex pollen records as the only agricultural indicators. This was followed by a more significant decline, with general forest opening and the grassland/pasture indicator Plantago lanceolata. A third, less significant, decline was again accompanied by cereal-type pollen. Tree pollen frequencies subsequently recovered, with cereal-type pollen remaining well represented. The fungal data recorded woodland taxa and a background level of the obligate dung fungus Sporormiella, a proxy for local herbivore abundance. Sporormiella frequencies increased greatly after the main Ulmus decline, around which there were high percentages of the ascospores of the wood rot fungus Kretzschmaria deusta. The neoecology of Kretzschmaria deusta, and the behavior of its spore curve suggests the colonisation of local populations of already severely wounded trees. At this site Ulmus, and perhaps Tilia were infected at a time of markedly increased inferred herbivore concentrations. Neolithic farming techniques could have provided mortally wounded trees while enhancing livestock grazing, although the role of disease must also be considered. Elevated Kretzschmaria deusta values may be of ecological significance in the interpretation of the causes and nature of the Ulmus decline and similar forest disturbance events, and the indicator role of this fungus in forest paleoecology requires further study.