It doesn't look like M. alba or M. nigra as its leaves are broader than long and not so glossy, soft or sticky-smooth to the touch. The leaves are rough to the touch.

It is strikingly attractive compared to a normal mulberry found in South Africa. The twisted form and drooping umbrella of the larger tree structure is reflected in the twisting and bonzai like crookedness of even the smallest twigs.

I googled mulberry species and varieties around the world and couldn't find a match, visually. I've read that China has thousands of horticulturized varieties and most are Morus alba, listed as an invasive mulberry here. But this one doesn't seem to invade. It has made no babies anywhere in the arboretum and I've never even seen its fruit, and I've been watching it for over a year now. I think its either not a mulberry at all, or its a kind of Chinese cultivar that they brought in a long time ago (the tree is old) that has become rare in South Africa at least and the tree itself is senescent.

The tree is ancient with knotted branches leaning on the ground.