The topic of the toxicity of fruit and shelter tree foliage is of great importance to the small home orchardist grazing sheep amongst the trees.Unfortunately, there is very little information available.

Sheep will reach up and eat tree foliage even if the trunks and lower branches are protected from browsing, so it is useful to know the toxicity of fruit and shelter tree foliage.

Toxicity might be absolute - eat it and die - or toxic only if eaten in large amounts. Some fruit and shelter trees may be totally non-toxic and safe to browse.

A complicating factor might be seasonality - foliage toxic at one time of year may be safe at another time, say in autumn.

Sheep won't usually eat leaves or bark of some toxic trees; for example wooly nightshade, Solanum mauritianum. But this is not absolute - I have seen a ram strip and eat bark of this odorous shrubby weed tree; albeit in small amounts. More recently,  some hoggets have been observed taking the tips out of seedlings when at pencil thickness. It is possible the alkaloid (solasidine) in this plant is also implicated in sub-jaw oedema (edema) and possible gut damage. The possibility its accumulative steroidal effect has the ability to damage capillaries and cause bleeding  (either mimicking barbers pole worm damage, or exacerbating the effect) remains my conjecture.

Sheep do not necessarily 'know whats good for them'. For example, most sheep love avocado foliage - but if relatively unlimited amounts of foliage of some particular cultivars and seedlings are eaten, it can cause fluid accumulation under the jaw and around the muzzle, and presumably other non-obvious physiological damage. There may be individual differences, and it may be dose dependant, but it is clearly not 'good for them'.

When sheep have ample grass, they may eat only a little of the relatively small amounts of the foliage that is within their reach. Even if the foliage has some low (not acute) toxicity, the intake of toxins may be low. It is plausible that the sheeps liver can de-toxify browse of low toxicity, so long as the amounts eaten are in fact relatively low.

But if sheep are short of grass and are hungry, they are more likely to both make extra efforts to reach foliage, and to eat all the foliage that is available, whether they like eating it, or would ordinarily ignore it as unpalatable.

Individual variability also has to be factored in. A sheep may develop a 'taste' for bark quite 'out of the blue', attacking such unlikely trees as tamarillo when it had previously been untouched for months or years. Their interest in bark may then subside again. It is best to assume they may incisor-scrape or eat bark of any tree at any time, whether they have any previous history of interest in the bark of a given species or not.

Sheep also seem to self regulate, presumably according to their 'needs'. Even where pasture is plentidul, green foliage of Coprosma may initially be eagerly eaten, then ignored. Very often it will be returned to later in the day. In the same way, sheep turned onto fresh pasture in an orchard row of loquats will often quickly eat several dry loquat leaves before they start grazing the grass in earnest. Whether they self regulate or not, it pays not to be 'gung ho' in feeding prunings to sheep, especially to hungry sheep. Sheep don't always know best.

The following notes are from observation of half a dozen sheep that are on good pasture. No great conclusions can be drawn, as foliage intake is limited to what they can reach, which is now limited by previous browsing.

Orchard trees
Apple - foliage, twigs and bark are eagerly eaten. Small amounts constantly available have not demonstrated any toxic effects.
Peach - foliage, twigs and thinner bark is eagerly eaten. Sometime quite thick mature bark is eaten, mainly when the sap is rising. Windfall fruit are often eaten (but not always, depending on the individual, the grazing, and the variety of peach), and larger rams may crunch up and eat the stones as well. Moderate amounts of late summer and autumn prunings have not demonstrated any toxic effects. Beware of allowing sheep to eat leaves with copper spray on them. Excessive copper can be toxic to sheep.
Plum - foliage, twigs, and young bark are eagerly eaten. Small amounts constantly available (including rootstock suckers) have not demonstrated any toxic effects.
Apricot - foliage, twigs, and young bark are eagerly eaten. Small amounts constantly available have not demonstrated any toxic effects.
Cherry - while I have no information on cherry as such, I have trimmed and feed small amounts of 'Colt' cherry rootstock over summer with no sign of adverse effects. A particular species of cherry, Prunus serotina, appears in the literature associated with poisoning from the cyanogenic glucosides that are present to greater or lesser degree in most, if not all, Prunus. Whether the poisoned stock ate large quantities because they had little or no other food, or whether this species has markedly high amounts of glycosides isn't noted.
Asian pear - foliage, twigs, and young bark are eaten. Small amounts constantly available have not demonstrated any toxic effects.
Fig - new foliage eagerly eaten. The thin mature bark is sometimes stripped and eaten. Small amounts constantly available have not demonstrated any toxic effects.
Grape - foliage eaten, and terminal part of twigs. Small amounts constantly available have not demonstrated any toxic effects. Mature bark not usually chewed.
Kiwifruit - sheep are sometimes run under mature vines, so presumably mature bark is not eaten. I don't know whether the summer shoots or the foliage is palatable, and if palatable whether or not it is toxic at any intake level.
Medlar - foliage eaten, and terminal part of twigs. Small amounts constantly available have not demonstrated any toxic effects. Sometimes mature bark will be chewed, not usually in great amounts.
Citrus - foliage eaten, and terminal part of twigs. Small amounts constantly available have not demonstrated any toxic effects. On occassions mature bark will be stripped, but not usually in great amounts.
Loquat - foliage eagerly eaten. Newly senesced leaves (yellow, not yet brown) are eagerly sought. The mature bark is eagerly eaten. Small amounts constantly available have not demonstrated any toxic effects.
Feijoa - foliage not usually eaten. On rare occassions mature bark will be stripped, but not usually in great amounts.
Avocado - foliage eaten. Mature bark on large trees is chewed on rare occasions. Some sheep find the foliage very attractive, others will eat only a little. Some sheep with unlimited access to large amounts of prunings of a seedling avocado developed fluid accumulation around the muzzle and under the jaw (others with the same access didn't show symptoms - their intake relative to affected animals isn't known). Affected animals looked as though someone had given them a 'fat lip'.
Once the avocado foliage was removed, the swelling disappeared within a day or so.
The previously affected sheep (and others) continued to have continuous limited access (low foliage that grows within reach and fallen leaves) to the same 'toxic' seedling, but no further effect on animals has been observed in that pasture.
  However, a related sheep was placed in a different pasture, also containing a seedling avocado, but with only limited leaf browse within reach. This animal also showed swelling under the jaw in summer, possibly when more leaves fell in the dry. With less leaf fall in autumn, the swelling  mostly resolved. But the animal ultimately died, showing symptoms of fluid accumulation in early winter, with scouring, inability to gain weight, and finally, lassitude and death. Whether this was the result of worm challenge in a young animal, or the effect of toxicity, or a bit of both, or another totally unrelated cause, isn't known. The leaves of the seedling avocado in the paddock have a moderately strong 'anise' smell when crushed. Whether this smell is related to the toxic principle is unknown.
One report claims that some cultivars of avocado have a compound in their leaves that has this effect, whereas other cultivars either lack it entirely, or have it in such low concentrations that it has no obvious effect.
The cultivar 'Fuerte' gives off a very intense 'anise' smell when leaves are crushed. The leaves of the cultivar 'Hass' has no appreciable anise smell. 'Reed' has moderately anise smelling leaves. Avocado seedlings are variable in this character. A few have no anise smell, a few have almost 'Fuerte' intensity of anise smell. Most are intermediate.
Literature also records damage to mammary glands of unspecified animals with access to avocado foliage of unspecified cultivars.
Tamarillo - foliage not usually eaten. On occassions mature bark will be nibbled, but not usually in great amounts.  One sheep observed eating a small number of immature (green) fruit salivated voluminously when eating them, but without apparent toxic effects. According to the Duke Phytochemical database  Three glycosides are recorded as present in the tamarillo plants (malvidin-3-diglucoside, paeonidin-3-diglucoside, pelagonidin-3-diclucoside), and these are converted to the respective glucosyl-glucose compound in the fruit. Solarcarpoine is also present. No biological activity has been reported for any of these compounds.
Mountain papayas
Carica quercifolia - new foliage eaten. Small amounts constantly available have not demonstrated any toxic effects.
Carica pubescens x stipulata/parviflora/monoica/goudotiana - new foliage sometimes eaten, depending on the species makeup onf the hybrid. Sometimes the outer 'bark' of these long lived herbs is eaten, especially if well lichen covered. Small amounts constantly available have not demonstrated any toxic effects.
Psidium cattlianum - foliage eaten. Small amounts constantly available have not demonstrated any toxic effects.
Passionfruit
Passiflora edulis (purple passionfruit) - foliage not eaten
Passiflora manicata - foliage eaten. A moderate amount did not demonstrated any toxic effects.
Passiflora herbertiana - foliage eaten. Small amounts constantly available have not demonstrated any toxic effects.
Persimmon - foliage, twigs, and young bark are eagerly eaten. Small amounts constantly available have not demonstrated any toxic effects.
Casimiroa  - foliage eaten, and terminal part of twigs. Small amounts constantly available have not demonstrated any toxic effects. Thick mature bark only very rarely chewed.
Banana - uncertain, probably palatable, toxicity, if any, I don't know.
Cherimoya - mature tree foliage and bark is unpalatable. Growing tips of small volunteer seedlings are rarely eaten.
Lucuma, Lucuma obovata - foliage eaten, and terminal part of twigs. Small amounts constantly available have not demonstrated any toxic effects. A large ram that one day decided to crunch up the very large seeds on the ground under the trees showed no obvious ill health. He actively foraged for the seeds in the leaf litter, but daily intake was probably low, and discontinuous as the area is part of a rotation.
Longan - foliage eaten. Small amounts constantly available have not demonstrated any toxic effects.
Olive - young foliage eaten. Small amounts constantly available have not demonstrated any toxic effects.
Bay - sheep like fresh bay tree foliage, but as it is in the same family as avocado, it would be prudent to severely limit their intake. While they also eat the succulent tips of branches and suckers, I have not seen then attack mature bark (yet!).

Shelter trees
Casaurina - sheep sometimes eat younger growth, but don't seem to find it very palatable.
Alder - sheep sometimes eat younger growth of Italian alder, Alnus cordata, but don't seem to find it very palatable.
Cupressus - said to be toxic to lambs.
Yew (Taxus) - lethally toxic.
Bamboo - sheep eat leaves, fine green shoots, and sometimes the dry leaf sheaths.
Flax - young leaves and leaf tips are eaten on mature plants. Seedling flax are consumed in their entirety...
Tagasaste - palatable.
Chinese poplars Populus yunnanensis cvs 'Toa', 'Kawa', 'Crowsnest' - mature foliage is unpalatable. Sheep forced to eat trimmings in times of feed shortage have scoured.
Lombardy popular - fairly palatable.


Invasive weeds
Any ungrazed areas adjacent to bush will be subject to invasion by woody shrubs and trees. Birds often deposit seeds of weedy natives under shelter belts. Once above a certain size, they usually keep growing, even if their bark is nibbled. There is a temptation to leave them on the ground when they are weeded out, but there is little information on the palatability or toxicity of these species. The commenest weedy natives don't seem to show toxic effect, although their palatability seem to vary.

Mahoe - foliage, twigs, and young bark are eargerly eaten. Small amounts irregularly available have not demonstrated any toxic effects.
Coprosma robusta - foliage, softer twigs, and bark are eaten. Regrows readily when left in the paddock. Small amounts constantly available have not demonstrated any toxic effects.
Kanuka - foliage tips of young seedlings are sometimes eaten, but rather rarely.

The usual cast of introduced weeds also appear, although there is usually some information on their palatability or toxicity.

Timber trees
Eucalyptus globulus and E. maculata are the least palatable common Eucalyptus species, according to an Australian report. Presumably the sheep will not normally ingest enough oil laden leaves to do themselves harm.
Eucalyptus camaldulensis, E. nitens, and many Acacia species are recorded in Australia as severely browsed. There is no comment on animal health effects.

References
Duke Phytochemical Database
http://www.ars-grin.gov/duke/plants.html

Oelrichs, P et al. 1995. 'Isolation and identification of a compound from avocado (Persea americana) leaves which causes necrosis of the acinar epithelium of the lactating mammary gland and the myocardium.'
Natural-Toxins. Vol 3. No 5.pp 344-349