The European Holly Tree – a plant with a long Christmas tradition
The European holly tree (Ilex aquifolium L.) is a small to medium-sized evergreen plant, averaging approximately 10m in height [1] . The primary distinctive features of holly include its red berries and dark green glossy leaves containing spines.
The holly tree has a relatively wide distribution, and is native to western Europe, where its range spans from Spain to western Norway. Holly is also found in north-west Africa in the Atlas Mountains, western Balkan Peninsula, as well as northern Turkey [1] [2] . That said, holly has become a regular invader of Pacific Northwest forests and is known in Seattle’s city parks, for example, as the most abundant non-native tree species [2] . It was introduced to the region in 1869 as an ornamental plant and continues to be grown commercially. It is thought to have become naturalised in the 1950s.
Due to its festive appearance, holly has long been associated with Christmas and the festive season in many parts of the world. Consequently, the holly tree is widely cultivated commercially for ornaments. Such a tradition dates back centuries to even before the beginnings of Christianity in past societies, where evergreens were seen as a symbol of underlying life [3] . These traditions have stood the test of time and persist to this day, even if some of the links remain unknown to us [4] .
Other festive plants used for Christmas ornaments in the past have included laurel, rosemary, and yew, however holly, ivy, and mistletoe have been proven as the most popular festive ornaments over time [3] . With Christmas just around the corner, here are some fun facts about the holly tree regarding its ecology, distribution, threats, and association with Christmas!
Holly is a dioecious plant, meaning unlike most species, each individual plant is either a male or a female. For pollination to occur, a male and a female need to be in relative proximity to each other. Just one male plant has the capability of pollinating several female plants. In general, all female holly trees produce berries, male plants do not [5] . Another way to differentiate between male and female plants is to observe their white- coloured flowers. Male plants generally have more prominent stamens than the females.
The holly tree is an important source of food to approximately thirty species of invertebrates [7] , one of which includes the Holly Blue butterfly (Celastrina argiolus). The Holly Blue is widespread across Europe, as it is in Ireland [8] . This butterfly is unique in Britain and Ireland as it alternates between host plants between the first and second generation [4] . The first brood generally lays its eggs on the flower buds of holly. This typically occurs between late April and early June. The larvae of the Holly Blue feed on the flower buds, berries, and terminal leaves [9] . The second generation uses ivy (Hedera sp.) between late July and early September [8] . Upon hatching, the caterpillars eat the flower buds of the holly, as well as its young, green berries..
Berries emerge from female holly plants during the winter. Upon ripening, the red berries make a great food source for birds including blackbirds, redwings, and thrushes [10] . In addition, it may also provide a suitable habitat for bird species to nest, as they can use the spiny leaves of the holly tree for protection. For this reason, holly also provides a suitable refuge for hedgehogs and other small mammal species during hibernation season. Holly blue butterfly (Celastrina argiolus). (Source: Butterfly Conservation).
Upon eating the berries, birds aid greatly in the dispersal of holly trees. Birds disperse the stone food through their droppings. Seed dispersal by animals is well-known as an important ecosystem function as it determines the dynamics of plant communities [11] .
References:
Guerrero, Hue, N., Caudullo, G., de Rigo, D., 2016. Ilex aquifolium in Europe: distribution, habitat, usage and threats. In: San-Miguel-Ayanz, J., de Rigo, D., Caudullo, G., Houston Durrant, T., Mauri, A. (Eds.), European Atlas of Forest Tree Species. Publ. Off. EU, Luxembourg, pp. e011fbc+
Stokes, D.L., Church, E.D., Cronkright, D.M., Lopez, S. (2014). Pictures of an Invasion: English Holly (Ilex aquifolium) in a Semi-Natural Pacific Northwest Forest. Northwest Science, 88, 75-93.
Drury, S. (1987). Customs and Beliefs Associated with Christmas Evergreens: A Preliminary Survey. Folklore, 98, 194-199.
Box, J. (1995). The festive ecology of holly, ivy and mistletoe. British Wildlife, 7, 69-74.
Gardening Know How. (2022). How to Tell the Difference Between a Male and Female Holly Bush. Available at: Identifying Male And Female Holly Bushes (gardeningknowhow.com) [Accessed 26 November 2022]
Gaston, K.J., Genney, D.R., Thurlow, M., Hartley, S.E. (2004). The geographical range structure of the holly leaf-miner. IV. Effects of variation in host-plant quality. Journal of Animal Ecology, 73, 911-924. Blackbird feeding on holly berries. (Source: Discover Wildlife).
Discover Wildlife. (2022). Holly guide: why it has leaves in winter, and which plants have berries. Available at: Holly guide | BBC Wildlife Magazine | Discover Wildlife [Accessed 03 December 2022]
Aldwell, B., Nash, D.W. (2005). The Holly Blue Butterfly Celastrinus argiolus (L.) in Co Dublin. The Irish Naturalists’ Journal, 28, 120-122.
Hayward, A., Wright, C. (2021). The genome sequence of the holly blue, Celastrina argiolus (Linnaeus, 1758). Wellcome Open Research, 6, 340.
Bradbury, K. (2014). Holly: the festive berry. Available at: Holly: the festive berry | Wildlife | The Guardian [Accessed 03 December 2022]
Garcia, D., Zamora, R., Amico, G.C. (2009). Birds as Suppliers of Seed Dispersal in Temperate Ecosystems: Conservation Guidelines from Real-World Landscapes. Conservation Biology, 24, 1070- 1079.
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Article by Stephanie Corkery,
Cork Nature Network
