Searsia has dense wood, unusual for family Anacardiaceae, part 3: related genera in Kruger National Park

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continued from https://www.inaturalist.org/journal/milewski/68675-searsia-has-dense-wood-unusual-for-family-anacardiaceae-part-2#

In previous Posts in this series, I have investigated the quality of the wood of Searsia (https://en.wikipedia.org/wiki/Searsia_(plant)), relative to its family, the Anacardiaceae (https://en.wikipedia.org/wiki/Anacardiaceae).
 
Searsia is neither diverse nor common in Kruger National Park (https://en.wikipedia.org/wiki/Kruger_National_Park). However, several confamilials are indeed abundant there.

So, I turned to the detailed accounts by P van Wyk (1974), Trees of the Kruger National Park, Purnell (https://catalogue.nla.gov.au/Record/1052708 and https://www.abebooks.com/Trees-Kruger-National-Park-Wyk-P/22486254279/bd).

Van Wyk writes about Anacardiaceae on pp. 321 ff, in vol. 2. This information bears out the theme that, in general, the wood of anacardias is rather light.
 
Van Wyk (1974) states “A few of the Rhus [=Searsia] species produce excellent fodder.”

I have seldom seen searsias described as particularly palatable or nutritious. I assume that van Wyk refers mainly to Searsia leptodictya (http://pza.sanbi.org/searsia-leptodictya and https://www.inaturalist.org/taxa/593890-Searsia-leptodictya).

However, this comment bears out something I have been hinting at: that Searsia differs from the general toxicity of its family. Searsia does not have to be particularly palatable or nutritious to be somewhat attractive to herbivores. All it has to do is to refrain from being as toxic as Rhus, Toxicodendron (https://en.wikipedia.org/wiki/Toxicodendron), or Anacardium, etc.
 
Van Wyk (1974) states that Searsia leptodictya (https://www.inaturalist.org/taxa/593890-Searsia-leptodictya), which grows locally in Kruger National Park, has a single bole.
 
The wood of Sclerocarya birrea (https://www.inaturalist.org/taxa/340245-Sclerocarya-birrea) is light by southern African standards. This borne out by the the fact that borers are particularly fond of the dry wood. However, a peculiarity is that nails are difficult to drive into the dry timber, despite this lightness.

Another peculiarity of S. birrea, mentioned in a previous Post, is that the tree can recover from complete ring-barking (https://www.flickr.com/photos/andrew-on-safari/5039720206 and https://www.alamy.com/stock-photo-trunk-of-the-marula-tree-or-elephant-tree-sclerocarya-birrea-okavango-77084727.html?imageid=B027A4A5-1159-4552-B74B-1C950DDDA315&p=204841&pn=1&searchId=9349831cbcd878694284fdb850e4d316&searchtype=0 and https://dewetswild.com/2014/10/03/signs/signs-9/ and https://www.istockphoto.com/photo/bark-of-marula-tree-in-moremi-okavango-delta-botswana-gm636050222-112617183?phrase=marula%20tree).
 
The wood of Lannea discolor (https://www.inaturalist.org/taxa/588676-Lannea-discolor) and L. stuhlmannii (https://www.inaturalist.org/taxa/340118-Lannea-schweinfurthii) not only is light, but lacks heartwood. As in the case of Sclerocarya, the dry wood attracts borers. In terms of wood quality and growth rate, Lannea resembles S. birrea closely.
 
Ozoroa engleri (https://www.inaturalist.org/taxa/431394-Ozoroa-engleri) has denser wood than those of Sclerocarya or Lannea. This is consistent with the fact that Ozoroa grows slowly, compared with Sclerocarya and Lannea. However, at the centre of each stem there is a narrow, soft, corky core. "Most trees have wounds at the bases of their stems, probably caused by veld-fires, where borers enter and damage the living trees to such an extent that almost all of them are hollow."

I infer that, despite having denser wood than Sclerocarya and Lannea, Ozoroa engleri has light wood relative to that of Searsia.
 
Ozoroa paniculosa (https://www.inaturalist.org/taxa/591317-Ozoroa-paniculosa) grows faster than does O. engleri. It has wood of intermediate density between O. engleri and Sclerocarya/Lannea, and once again has a narrow core of surprisingly soft tissue at the centre of each stem. "Like the other two [members of the genus Ozoroa in Kruger National Park], this species is also injured by veldfires. Without exception, death of the tissues leads to invasion by borers. Damage by these insects is so severe that during a recent wood-gathering campaign not a single tree with a sound stem was found."

The wood of Ozoroa reticulata (https://www.inaturalist.org/taxa/441556-Ozoroa-reticulata) is similar: "Borers are so partial to this wood that not a single undamaged log has been found in the Kruger Park."
 
In summary:

Searsia is rare, in a truly arborescent growth form, in Kruger National Park.

All the arborescent anacardias of Kruger National Park, other than Searsia leptodictya, have rather light wood, attractive to borers.

There is some evidence of an inverse relationship between growth rates and wood densities. However, even the slowest-growing ozoroas here have wood light enough to be riddled by borers, once the trunk dies.

Ozoroa is typical of its family in relying on toxicity, being a virtual ‘chemical factory’.

Ozoroa and Searsia are widely sympatric in Africa. However, their anti-herbivore strategies are quite different:

• Ozoroa is typical of anacardias in relying on chemical rather than structural defences, and it has unusually corruptible wood for southern African trees. By contrast,
• Searsia can be fairly attractive to herbivores, but has extremely dense wood for a genus of Anacardiaceae.

Also please see
https://www.inaturalist.org/journal/milewski/67776-searsia-anacardiaceae-in-perspective-part-1#
https://www.inaturalist.org/journal/milewski/81419-searsia-anacardiaceae-in-perspective-part-2#

Posted on July 31, 2022 04:46 AM by milewski