Evergreen deciduous oaks a mixed bag

There is this thing called marcescence, where leaves brown off in autumn but remain on the tree. A good example is the juvenile beech (Fagus sylvatica) hedge so popular in northern Europe, another the Persian Ironwood (Parrotia persica) in my back yard. Although in the latter, only half of the leaves are still hanging around in July.

The leaf stalk near to where it is attached to the stem will typically stay alive in a marcescent tree, rather than dying as happens in trees that drop their leaves. The value of holding on to brown leaves is debated but may be to protect softer branches from grazing, or to allow a dump of mulch in spring when the trees needs a boost in nutrients, or sometimes an adaptation to coping with snow or drought.

The commonly planted English Oak, Quercus robur, is not a marcescent species. Typically we expect the leaves of this oak to yellow, then drop in June (early winter) or thereabouts. But things are not that simple. Sometimes, and some, oaks keep their leaves (of various colour) through winter and beyond.

This is what I found a few weeks ago in a small troupe of oaks growing in Ferndale Park, in Glen Iris. It was mid-July. Some trees had lost all their leaves, some had lost some, some had retained a mix of yellow and green leaves, and others were fully green leafed.

There are a few possibilities: different species (or crosses between species, or cultivars); different genetic strains within a species/hybrid/cultivar; or different growing conditions (e.g. more shade or more water). So I investigated further.

Oak species do hybridise a lot, particularly the English Oak, Quercus robur, and the Algerian Oak, Quercus canariensis. The latter, by the way, does grow naturally in Algeria, and in Portugal and southern Spain, but not (naturally) in the Canary Islands.

All the trees seemed to be about the same age but a closer look at the leaves - even those devoid (almost) of leaves had a stay yellow leaf dangling somewhere in canopy - revealed some variation in leaf shape.

Those trees without any leaves (represented by the yellow leaf near the top of this post) were a good fit for your classic English Oak. These with lots of green leaves (the tree above and the leaves just below) better match the Algerian Oak, although those leaves are not bluish underneath or finely hairy where the leaf stalks join the stem, supporting characters for this species. (I'll take another look later this year, when new leaves emerge, to see they are at all bluish or the stalks 'floccose'...)

Roger Spencer, in the Horticultural Flora of South-eastern Australia, describes the Algerian Oak as 'semi-deciduous', noting that it is 'naturally deciduous but often retaining some leaves'. Particularly in warmer areas, he writes. The English Oak is described as simply 'deciduous'.

A few other trees in the park (pictured below) had both yellow and green leaves. Spencer notes that many if not all Algerian Oaks grown in cultivation are hybrids with English Oaks, so these are possibly intermediates. It is also possible some trees are simply genetic variants of one of the two species, expressing a slightly different overwintering approach.

My first thought was that some of the trees may have kept their leaves because they were more shaded or protected from the wind but in this case I don't think that matters here. It just happens that the English Oaks with no leaves are out in the open and the Algerian Oaks with leaves are huddled together in a bit of a dip in the park.

There is variation among species. In an article on the 'physiology of the oak tree' (reported by Nick Lear at Woodlands.co.uk), ecologist Patrick Roper says that many oak trees will retain dead brown leaves until spring. But this seems to be your typical marcescence, and the brown leaves are throughout the tree or only in lower branches.

Roper says the degree of leave retention is 'strongly inherited' and holds true when a branch is grafted. He hypotheses it may be a residual expression of a shared ancestral trait with those oaks that retain leaves all year all the time (the 'live oaks' such as Holm Oak, Quercus ilex, which I'll post about in a few weeks). He also says that during mild winters, seedlings and coppicing branches can keep their green leaves until the new ones push them aside.

Back in Ferndale Park it seems the explanation is simpler. There are two species, perhaps some hybrids, and they are doing what we expect them to do in a 'warmer area'. Whether climate change and milder winters are changing the degree of leaf retention I don't know. To answer that I'll have to observe over a few more winters.

Images: First two (devoid of leaves apart from an occasional yellow leaf) are an English Oak; the next three of a nearby specimen of Algerian Oak, fully cloaked in green leaves; the last two of a presumed hybrid tree with most of its leaves intact but a mix of yellow and green.