Replacing Indigenous Forest Patches in the Riparian Zone of the Upper uMngeni River
Re-establishing indigenous forest patches
“The promotion of indigenous deciduous trees for rehabilitation of clearing programmes may be important as there would be no transpiration during periods when water resources are limited”
“The total accumulated sap flow per year for the three individual A. mearnsii and E. grandis trees was 6548 and 7405 La respectively. In contrast, the indigenous species averaged 2934L a, clearly demonstrating the higher water use of the introduced species”
The above statements are just two statements extracted from the publication mentioned above. While one has to be careful not to extract just the information that proves a pre-conceived notion, having read the paper, I think these take-outs are a fair representation of the whole, albeit overly summarised. The paper displays graphs in which is demonstrated how the alien (evergreen) species have a continual high water demand during the year, whereas the deciduous indigenous species, with their winter dormancy, show a very low water demand in the dry winter months. The dry winter months are those in which the river experiences its lowest, or ‘base’ flows, and is also the time of highest irrigation demand along the river.
This additional seasonal understanding of matters goes a long way to fully comprehend the extent of the demonstrated hydrological benefits outlined in the report. This also serves in justifying the expense and difficulty of removing alien invasive species in catchments generally, but more particularly along immediate riparian zones. The applicability of this report to the projects of Upland River Conservation is leveraged even further by the fact that this two year study was done in the heart of our Upper uMngeni Super Catchment project area, on New Forest Farm.
Before having been alerted to this study, we had identified seven small areas where alien invasive species have been removed, and where we hope to establish small indigenous forest patches. There are no doubt other suitable sites as well, but these are the first seven that have been mapped.
This was prompted by the notion that having removed the tree canopy from the river banks, we may have caused a sudden warming. While the river banks were largely unshaded a hundred years ago the more recent slow ingress of the alien canopy may have mitigated slight global warming. With the sudden removal we may have caused the environment to “catch up” on that warming in a short space of time. This has made us mindful of re-establishing tall woody river bank vegetation to achieve a natural level of shading. This shading would comprise mainly tall grassland and some Nchishi , but in at least in some places it means re-forestation with indigenous species.
The sites have been carefully chosen based on observation of where forest patches do occur, and where they might have occurred before the severe alien wattle infestation, which might have displaced them. In several cases, lone indigenous trees were found left standing after the wattle had been felled. In all cases the sites are steep, south-facing slopes along the river or tributary course. These areas are cool, and remain moist enough that they tend not to burn in winter.
In fact, money has already been raised to re-establish one of the seven envisaged forest patches.
With advise from a local indigenous tree nursery, and armed with a species list from another scientific study in the same forest, we plan to first establish the forest precursor shrubs. Once these get going, they are likely to form a frost- free micro-climate and canopy in which the trees themselves have a better chance of surviving the critical first few years. Once that is achieved, we plan to plant the trees themselves. These will of course include evergreen indigenous species, in the interests of maintaining the natural biodiverse mix, so the hydrological benefits may not be as stark as those reported in our opening quotes. The initiative does however represent a return to what conditions may have been before the invasion of alien trees.
In the interim, erosion control and re-grassing has been a priority for the bulk of the cleared river bank areas. Much has been achieved, but there remains a lot to do, especially along the banks of the Furth stream. Re-grassing efforts, and any required erosion control will continue parallel to our reforestation ambitions. We are currently exploring innovative funding solutions for each of the remaining six sites.
We certainly believe that the distinct and easily identifiable outcome of a forest patch, visible on Google Earth from anywhere in the world, might have appeal to funders who would like to be able to see the fruits of their contribution. Added to that, these small forest patches are between 800 and 5000 square metres only, and are therefore not expensive to create.