Winter Food


Picture Gallery
Dave Rushworth

We tend to think of the warm, rainy season as a time of food production and the cooler, dry ‘winter' as the period when natural food sources are in short supply. To a great extent this is true but nature has ways of overcoming this potential problem through various adaptations in a healthy diversity of species. Food ‘production' by plants is staggered throughout the year and many animal species breed to coincide with food supplied by death and weakness in other species.

I have previously mentioned examples of vultures breeding in winter to take advantage of herbivore die-offs in peak drought periods. Large birds of prey breed early to take advantage of reduced foliage in winter and young herbivores at the onset of the rains. Jackal-berry (Diospyros mespiliformis) and Commiphora trees, among others, produce fruit for the ‘winter' months.

There are many adaptations in this respect. Of particular interest at this time of year are the winter flowering plants which produce nectar, pollen and fruit for various insects and birds. Several species of aloes have started flowering with A.arborescens providing a wonderful show along the top of the escarpment and A.marlothii and others to follow shortly.

PLANT PARASITES

At lower altitudes, including the Kruger National Park, red and yellow flowering tree parasites are particularly noticeable at the moment, and I would like to focus on this interesting group of plants. True parasites are species that obtain their nutrients solely from a host species. There are many plant parasites that feed on root stock of ptotosynthetic plants and are only seen when they thrust flowering heads above ground for pollination - rather like mycelium and ‘mushrooms'. 

Many of these flowering heads are strange looking and produce rotten smells to attract a variety of insects to assist the pollination process. The plants discussed below grow in the light on a host plant, from which they obtain their nutrients. They have green stems or produce green leaves which are photosynthetic and they therefore manufacture much of their own food. As such, they are not true parasites although they may cause certain parts of the host to die.

LORANTHACEAE - MISTLETOES

This group of plants are commonly known as mistletoes, tree parasites, ‘voëlent' or honey-suckle. They are prominent as clumps of vegetation attached to the branches of various tree species. Some produce leaves and others only have green, jointed stems. The ‘tree parasites' of Southern Africa used to all be clumped under the Family LORANTHACEAE.

This family was split to differentiate between the group with leaves and prominent flowers (the Loranthaceae) - and the group that rarely has small leaves on green, segmented stems with very small flowers (the Viscaceae). The LORANTHACEAE is a large family consisting of around 65 Genera and over 900 Species worldwide.

In South Africa, what used to be only one Genus LORANTHUS has been reclassified into 11 different genera with 37 species. Some of the genera that were changed to Tapinanthus have been given new generic names and I thank my botanist brother John for up-to-date information. The VISCACEAE consists of seven genera and 450 species worldwide.

In South Africa it is represented by a single genus VISCUM with 17 different species. Shown in the accompanying photographs are - the large, yellow flowering Pedistylis galpinii and the very common, pinky-red Plicosepalus amplexicaulis which favours the ‘knob thorn' acacia nigrescens as a host.

The Plicosepalus spreads through out the host tree by means of ‘epicortical' roots that anchor into the host with suckers - often attaching to its own roots in cases of self-parasitism. The fruit shown are of the large-leaved Agelanthus natalitius which has pinkish-white flowers earlier in the season. There is also a small, pink-flowered Helixanthera garciana (not shown) which has already flowered and fruited, for which marula is the favoured host.

The clump of Viscum combreticola shown, has minute flowers appearing and will bear fruit later in the season. As the specific name implies, it favours trees of the genus Combretum as hosts. The Wood Roses - or ‘Voëlente' - sometimes offered for sale by vendors are often used in dry arrangements. They are formed of the host material distorted by the ‘sucker' attachment of the tree parasite. The softer material of the parasite comes away leaving the harder, host-wood in flower-like shapes.

FOOD FOR BIRDS

Of general interest - this group of plants tend to flower in the ‘winter' months, with fruit ripening around the peak of the dry season. The flowers of the ‘Loranthus' group are an important source of food for sunbirds and provide an excellent attraction for the vewing of nectar feeders. Barbets and other birds are never far from the ‘tree parasites' when in fruit. Birds and certain insects assist in pollinating the flowers in which the corolla lobes separate explosively when touched, releasing a cloud of pollen to dust the forehead of the pollinator.

The style usually snaps to one side to prevent self-pollination and also to ensure that the pollen from the forehead of another visitor is smeared onto the sticky stigma. The fruit of both the ‘Loranthus' and ‘Viscum' species are relatively small and covered with a skin that turns from green to bright yellow or red as they ripen.

Birds attracted to the fruit split the skin to get at the very sticky seed covering called ‘viscin' (from which ‘bird lime' can be made). The sticky seed is rubbed off on nearby branches and thus dispersed by the feeding birds. My own observations are that the tinker barbets (among others) are the main dispersal agents in this area.

SEED DISPERSAL

For those with a deeper interest - Johann Visser, in his book "South African Parasitic Flowering Plants", gives some fascinating detail, from which I quote - (In the case of LORANTHACEAE) - ‘ Distribution of the seed is effected by birds lured by the conspicuous bright red of the mature berries.

Various fruit-eating birds have become adept at squeezing the relatively large viscin covered seeds from the fruit, and often leave the empty shell in place. The sticky seeds may be swallowed whole by larger birds to be regurgitated within a few seconds (barbets, starlings), or defaecated within minutes (bulbuls, mousebirds, thrushes and white-eyes).

Weavers and tits are sometimes observed only to peck at the fruit. The seed is not swallowed but rather smeared onto nearby branches. A major disadvantage in the dispersal of these plants is the relatively short distances over which many of the seeds are carried by birds - often, due to the very short retention time, no further than the plant on which they have fed or its host. This frequently leads to a virtual over-population of one host plant while a neighbour may be left comparatively free from infestation.

Where birds defaecate seeds, these will be carried over larger distances due to the longer retention times. Germination commences almost immediately after the seeds are deposited onto the host, providing they are exposed to light. Emerging radicles and hypocotyls grow towards dark areas such as the underside of stems, leaf axes etc. In certain instances the bark of the host stems is artificially darkened by a viscous substance leaking from the seed.'

Where birds defaecate seeds, these will be carried over larger distances due to the longer retention times. Germination commences almost immediately after the seeds are deposited onto the host, providing they are exposed to light. Emerging radicles and hypocotyls grow towards dark areas such as the underside of stems, leaf axes etc. In certain instances the bark of the host stems is artificially darkened by a viscous substance leaking from the seed.'

In the case of VISCACEAE, Johann Visser states - ‘ Viscacean fruits are eaten by a variety of birds. The ingested seeds are, however, retained for slightly longer times than Loranthacean seeds - up to between three and five minutes before regurgitation and as long as ten minutes before defaecation. Seeds have been found to be deposited on stones, fences, trees and herever birds happen to alight for distances at up to 50 metres from the fruiting plant.

Once the seed is removed from the fruit, germination will commence immediately provided it is exposed to light. There is no need for the seed to pass through the alimentary tract of a bird in order for it to germinate, as many people believe. Once germination is initiated the hypocotyls (i.e. that part of the stem between the cotyledons and the root) will elongate even in the dark. Growth of the hypocotyl is directed by a negative phototrophic as well as a negative geotrophic response.

No matter where the seed is deposited on a branch these reactions invariably direct the hypocotyl to seek the surface of the potential host. Penetration of the host's tissue and establishing contact with the conducting tissue may take up to one year. Once this is accomplished the hypocotyl is raised and connection with the remains of the seed is severed. Hypocotyls are typically dark green and apparently capable of photosynthesis, so that once the seed reserves are depleted photosynthesis in the hypocotyls may supplement the organic compounds acquired from the host.'

CONSERVATION

Tree parasites can cause some of the outer branches of trees to ‘starve' and die off but it is not a good idea to try and remove the parasites. It will just cause more damage to the host tree. The parasites are interesting plants in their own right and will produce colour and attract birds in the dry season.

If you have a particularly valuable tree that you want to protect, the best way is to trim off all the branches of the parasite and paint the cut ends to try and prevent them from sprouting again. Do not encourage the collecting of wood roses from live plants. There are many available from dead material. By denuding the veld of tree parasites you will be depleting a valuable source of winter feed for birds - which may, in turn, also vanish from our area.



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