Spekboom restoration: the rooting window hypothesis

by Alastair Potts

On a recent field trip to check up the status of the hydromulching experiments with Nicholas Galuszynski and Robbert Duker, we made some observations that have led me to propose a "rooting window hypothesis". The crux of this hypothesis is that damaged parts of spekboom branches (e.g. the base of a cutting) requires moisture soon after the damaging event to stimulate root growth. If there is not sufficient soil moisture during this rooting window period, then the cell death around the damage point inhibits or retards root growth even if there are periods of sustained increases in moisture levels after the rooting window.

What observations led to this hypothesis? 

• In the hydromulching experiment, we found spekboom branchlets with clear indications of root buds.

Small spekboom branchlets were removed by hand and sprinkled over ground that was then hydromulched. See this blog post for more information. The mulch is soaked with water and so would have provided a moist environment for many days after the hydromulching. 

These are small spekboom branchlets that were scattered in areas that did not receive any hydromulching. There is no evidence of the incipient root bud, and these base ends were crispy, dry, and probably dead. Note that these branchlets had leaves that were in a visually similar condition to the branchlets in the hydromulch treatment. 

• In a separate experiment run by Klaas Basson from the GIB, we observed amazing rings of rooting points at the cutting base.

We found a halo of spekboom planted around an Acacia karoo, part of Klaas Basson's new experimental planting efforts. I was astounded to see the ring of rooting buds. This is one of greatest potential rooting areas I have seen on a spekboom cutting (and we've been digging\pulling up a lot).

We observed these rings of rooting buds in many other cuttings that we deduced were planted at around the same time.

Another example.

So, did all cuttings in this site have this evidence of rooting?

No, in fact most looked like this... (these were planted using the standard protocol)

Note the green bark section exposed after some scraping with my thumb nail.

So, what was the difference between those cuttings with root rings and those without? 

This question had us stumped for quite a while -  we went down several avenues that ended up being dead ends (e.g. cuttings planted in shade developed root rings) - until I phoned Klaas and asked him if had watered the cutting associated with his most recent experiments. It turns out that he had! About 200 ml per cutting at the time of planting.

Another crucial point is that we're going through an extreme drought in the Eastern Cape, and this area in particular, so there has been very very little rain (~5 mm in the months since the hydromulching and these cuttings were planted).  So, the common denominator between the hydromulched fingerlings and the root ringed cuttings were that they had water immediately after planting.

This brings me back to the rooting window hypothesis. These observations suggest that damaged spekboom areas have a limited window of opportunity to develop root buds. 

This is a possible answer to a number of conundra that have been troubling me, such as why some damaged skirting does not form roots, and why some spekboom transplants do fine while others do not, even in years with good rainfall (the latter did not receive sufficient moisture in the post-damage rooting window).  

To reiterate, the hypothesis is that post-damage moisture availability is required to stimulate rooting. It is not for stabilising a plant that has been transplanted due to roots losing connectivity with the soil structure, which is the usual reason for watering after transplanting.  

The rooting window hypothesis leads to one major prediction:

• the entire growth trajectory for a cutting is determined by the root development stimulated by post-damage soil moisture in a narrow window of opportunity. Moisture availability outside of this window is of much lesser importance. 

On another note, in previous blog posts (e.g. here and here), we have argued that smaller cuttings should be used for spekboom restoration. Compare the protective seal in the two spekboom images below. 

On the left is the standard truncheon, and on the right is for a 'fingerling'. On the left image, that central section of xylem on the left is dead tissue that is an opening for pathogens etc. There is a complete seal on the 'fingerling' (note that the black-looking tissue is actually the hydromulch still sticking to the cutting). 

Using smaller 'fingerlings' potentially means:

• it is easier for the cutting to seal and keep out pathogens.
• it establishes a good root-to-shoot ratio before the plant becomes big enough to be readily observed by large herbivores (this is important to withstand browsing and not get pulled out of the ground).
• the roots will establish near the surface, which is crucial in arid environments where moisture rarely deeply penetrates the soil. 
• they have a smaller 'reserve' so they will have less chance of survival without help, or rather they cannot be treated the same as the larger truncheons which are stuck into bare ground away from any form of shelter (i.e. nearby trees).