Mating in Single Oaks

Sometimes one encounters a single, isolated individual tree, thriving in a private collection as the only representative of its species. The proud owner admires and perhaps even pampers it. But little thought is given to it besides the esthetic, botanical or horticultural merit of such a plant.

It is this case of a single individual that I would like to consider here, specifically the isolation of the species and the significance and biological interest of the reproductive barriers that prevent it from crossing with other species.

Oaks are wind-pollinated trees with male and female flowers on the same tree. But in spite of this fact they set fruit usually only when another individual of the same species exists in the vicinity and can serve as a pollen donor. The phenomenon is known as self-incompatibility and the plants distinguished as such are defined by geneticists as outcrossers. However in some rare cases this pattern is not true and according to a recent study[i] a few cases (3.5%) of self-pollination give rise to viable acorns.

Outcrossing is a well-known phenomenon, familiar in agricultural practice since antiquity and found later in many other wind-pollinated arboreal (date palms) and insect-pollinated herbaceous plants (irises).

In nature, for example in Southern Europe, closely related trees often grow together in the same or nearly the same habitat. If, like in oaks, the reproductive barriers that isolate the different species and maintain their distinctive morphological features are weak, the result is the development of a whole series of intergrading forms defined by science as a hybrid swarm – the result of a powerful evolutionary process known as introgression. A recent publication[i]considers all these aspects in an experimental context.

In gardens and plant collections, (and also rarely in nature!) actually two different pollinating scenarios can be described: that of a single specimen of oak in an environment devoid of oaks and another one of an isolated plant in a collection of other oak species. I have experienced both cases.

The first type is encountered with foreign, introduced oak trees in Israel. They were in the past handed out to gardeners to test their suitability for horticultural practice in different parts of Israel. A suitable case for this situation is a single, sexually mature specimen of the evergreen Californian Quercus agrifolia Née growing at Kibbutz Horshim in the coastal plain of Israel, north east of Tel Aviv, since 1985.

The second type of case is the single, sexually mature, deciduous plant of Q.pontica K. Koch cultivated at the Jerusalem Botanical Garden. In both cases the characteristic outcrossing mating system of oaks produces two profoundly different results:

a) The Q. agrifolia in the coastal plain of Israel is exposed to only three types of pollen donor: the native evergreen Kermes oak (Quercus coccifera subsp. calliprinos (Webb) Holmboe), the cork oak (Q. suber L.) and the holm oak (Q. ilex L.) – all characteristic and ancient natives of the Mediterranean region and not native to California. But in spite of this fact the tree is heavily loaded with perfectly viable acorns.

b) Contrarily, the tree in the Botanical Garden grows in the immediate close proximity of a significant number of mature trees from different deciduous European species – but is absolutely barren of any ripe acorns! Though it usually flowers, any pollinated female flowers are aborted and shed in the course of the summer.

These two examples can be interpreted in different ways, but there can be no doubt that their significance and interest is much greater than what the casual observer might think. For example, let us consider the case of the Californian and the native Israeli or Mediterranean species: all evolved under very different geological historical scenarios. If they are compatible it means they share not only the same chromosome number but much more in their molecular makeup. Given that all three Israeli species represent a very ancient element (probably Miocene) in the circum-Mediterranean Flora, this makes the elucidation of the phylogenetic background most interesting in its implications.

The first scenario that comes to mind is one where all of the four species share the same chromosome type and the same type of pollen grain. The ability of the Israeli species to pollinate and fertilize the Californian species – and to a lesser degree the cork oak – while holm oak remains sterile and isolated, might indicate their very ancient origin from times before the breakup of the ancient single continent, when their distribution comprised all the Northern Hemisphere.

Other mating patterns could be described, but all would need critical experimental verification.

To sum up: watch the mating of your isolated species – they could tell a fascinating story.

Editor's note: The author now considers that the tree of Q. agrifolia may be self-compatible and could have pollinated itself.