Context: Deer (Cervidae) populations are increasing in many global regions, leading to concerns about their impacts on temperate forests. Advancing evidence-based management requires a detailed understanding of the dietary habits of deer and how these are shaped by resource availability.

Methodology: We studied the diet of fallow deer (Dama dama) in North Wales (United Kingdom), using faecal DNA metabarcoding. Samples were collected monthly from three woodlands during 2019–2021. Tree surveys and seasonal ground flora surveys were conducted in these woodlands and seven additional woodlands. Preference analyses were used to assess the consumption of plant taxa relative to their availability.

Results: The fallow deer consumed high proportions of bramble (Rubus fruticosus agg.) across the seasons, especially in the winter months. Diet diversity was significantly lower in winter compared to the other seasons, suggesting that the deer were bulk foraging on a widely available, predictable resource to conserve energy during winter. Grasses did not form a major component of the diet in any season. The preference analysis showed that spatially clustered woody taxa (e.g. Betula sp., Corylus sp. and Fraxinus sp.) occurred less often than expected in the diet, while widespread woody species occurred in the diet more often than expected (e.g. Rosa sp., Prunus sp. and Quercus sp.).

Practical implication: The expansion of deer populations in the United Kingdom has occurred alongside the recovery and maturation of degraded or planted forests since the middle of the 20th century. Despite reduced light availability in these closed-canopy forests and increased herbivory pressure, bramble has remained a dominant understory plant compared to other less herbivory-tolerant plant species. Perhaps as a consequence, bramble has become the winter survival resource for this fallow deer population, remaining a prominent dietary component throughout the year. With increasing disturbance from extreme weather and tree diseases leading to a more open canopy structure, bramble cover is set to increase in European forests, which could support further expansion of deer populations. As we work to expand tree cover and enhance forest resilience and biodiversity, we should seek to understand the dynamic interactions of increasing deer populations with rapidly changing treescapes.