PlantLife Volume 55.6, July 2023. Life and death of a giant swamp-forest fig tree

 

Life and death of a giant swamp-forest fig tree 

Ficus trichopoda Baker 1883

by Ricky Taylor 

In the floodplain of the stream that flows into the Mgobezeleni Estuary at Sodwana there is freshwater swamp forest. In it are huge hippopotamus figs (Ficus trichopoda). This article is about one such tree that has a canopy diameter almost twice that of the Wonderboom fig in Pretoria (figure 1).

 

Figure 1: A small part of the giant fig tree. With a canopy spread of over 100 m, surely this should be one of the ‘Champion Trees’ of South Africa (https://www.dffe.gov.za/sites/default/files/legislations/championtreesofsouthafrica_treesdeclaredprotected.pdf)

 

While mapping wetland vegetation as a component of a project funded by the Water Research Commission (Bate, Kelbe & Taylor, 2016), we found a single Ficus trichopoda tree, separate from the rest of the trees in the swamp forest, with a canopy diameter of more than 100 m. The growth form of F. trichopoda trees is characterised by aerial roots that hang from lateral branches. If these roots reach the ground they thicken and form additional trunks. The result is that there is a constant outward spreading of a multi-trunk tree. It is still a single tree as all the branches and stems are connected and would have the same genetic makeup.

In these fig swamp forests, adjacent trees usually intermingle so one cannot see which trunks belong to which trees. It is thus difficult to determine the extent of any specific tree. However, the tree described here was growing separately from other trees and had formed a circular footprint as it spread outwards (figure 2). This was a perfect opportunity for us to measure the expansion of the tree canopy over time from successive aerial photos and from this extrapolate backwards to give an estimate of when the tree started growing.

 

Figure 2: Oblique aerial photo of the giant fig we measured. Its circular canopy footprint has an average diameter of over 100 m. (Photo: 22 March 2007)

Figure 3: Google Earth image showing the tree and the positions of the measured canopy diameters that we were able to repeat going back to 1942.

The rate of this expansion was gained by measuring diameters of the canopy in two places (figure 3). We used the average radius taken from these measurements to track the growth of this remarkable tree. The measurements were taken off aerial photos (rectified to remove distortions) and from Google Earth images covering the period from 1942 to 2007. Plotting the data gave an excellent fit and we were able to show that this tree started its life well before 1900 (figure 4). We were careful not to extrapolate too much before 1900 as we do not know if a young tree has a different rate of growth or may have been subjected to unknown conditions that affected its growth.

Figure 4. Measurements of the canopy radius (in metres) of a single Ficus trichopoda taken from aerial photos as the tree grew outwards sending down prop-roots that formed new trunks. The curve fitted to these points shows that the rate of expansion has been extremely constant. Back-extrapolation of the curve indicates that the tree started growing well before 1900.

 Ficus trichopoda, the hippopotamus fig, is widely distributed, occurring in a band across equatorial Africa and in Madagascar, and extending down the eastern coastal region of South Africa (figure 8). It is a typical component of coastal freshwater swamp forests in Maputaland especially where there are deep deposits of organic matter which, over the recent millennia, have formed peat. These figs have large leaves, impressive multiple trunks and spreading branches (figures 5–7). They are an important component of the swamp forest in this area and, in places, form monospecific stands. They produce fruits that are consumed by samango and vervet monkeys and fruit-eating birds such as trumpeter hornbills, green pigeons and barbets. The fruits are also eaten by fruit bats. Like all figs, the fruits house fig-wasps. While working in this swamp we frequently saw the rare Pel’s fishing owls roosting in the upper branches of the trees. They prey on fish that live in the ponds and streams in the forest.

Figure 5: Characteristic broad leaves with prominent yellow veins on the upper surface. The veins on the lower surface are often red. Also characteristic is the large bright-red leaf sheath which protects newly growing leaves.

Figure 6: The understory of the fig forest with ferns and young trunks which have grown from aerial roots.

 

Figure 7: Fruiting body of the fig.

These trees are sensitive to fire, salinity and desiccation. We can infer from the symmetrical circular shape of the tree and its constant expansion rate this tree has not been exposed in a catastrophic way to these environmental drivers – either in the period we took our measurements or for the several decades prior to our first measurement in 1942. The tree definitely would not have been exposed to salt water flushing in this period. This is important to an estuarine ecologist as it indicates the inland boundary of the estuary did not reach this far upstream in this period.

 

 

Figure 8. Ficus trichopoda distribution (from FigWeb.org).

On 19 and 20 March 2007, extreme high seas were caused by a combination of high tides, frontal stormy weather with low bathymetric pressure and sea surges created by a far-off cyclone. These combined with the effects of human interferences at the estuary mouth and beach and possible global sea-level rise effects. These factors coincided to form the extreme high sea levels all along the KZN coast at that time (Smith et al, 2007). The high seas pushed sea water onto the Mgobezeleni floodplain and this intrusion of the salt water killed the giant Ficus trichopoda (figure 9). The photo in figure 2 was taken a couple of days after this event and the first signs of yellowing are visible in the fig tree.

 

Figure 9. Oblique aerial photo of the lower portion of the estuary after the high seas of March 2007 had caused an inflow of saline water which killed the sedge swamp and several Ficus trichopoda trees. (Photo: 28 May 2007). The salt penetrated 800 m in from the estuary mouth.

 

Although we had thought that the tree was dead, recent Google Earth images indicate that parts of the tree survived and are still living (figure 10).

 

Figure 10: Severely damaged, but not all dead! The most recent Google Earth images show that parts of the fig tree we measured were not killed. It no longer has the extensive canopy measured in 2007 (figure 3), but some of the central parts of the tree have survived. The yellow lines are the diameters we measured from the 2007 image. (Image from Google Earth)

The Forestry branch of the Department of Agriculture, Forestry and Fisheries (DAFF) maintains a register of the largest trees; the Champion Trees of South Africa (https://www.dffe.gov.za/sites/default/files/legislations/championtreesofsouthafrica_treesdeclaredprotected). An index is derived from a formula that includes measurements of height, trunk and crown diameter. The tallest trees include a Eucalyptus saligna in Magoebaskloof at 81 m and yellowwoods of the Tsitsikamma area at about 40 m tall. Based on trunk dimensions, several of the baobabs of the Limpopo area are on the list, with the largest having a trunk diameter of almost 16 m. The Wonderboom (Ficus salicifolia) in Pretoria is included for its canopy width of 56 m. The huge Ficus trichopoda trees of Maputaland are generally unknown and appear not to have been assessed. One limitation is that it is difficult to determine the extent of a single tree as each tree is generally tangled with other trees. The other difficulty is that each tree may have hundreds of trunks. As the index uses the sum of stem measurements, it would be a considerable task to measure all of them from a single tree.

Freshwater swamp forests are one of the scarcest and most threatened forest habitats in South Africa.

Other similar swamp forests in South Africa containing F. trichopoda include:

(i)     the Sihadla swamp forest at Kosi Bay;

(ii)    the Ozabeni area north of St Lucia, including the Mgobezeleni floodplain, some of the watercourses that enter the Mkhuze floodplain from the east and the Meni swamp immediately to the north-east of Lake St Lucia;

(iii)    the Mfabeni swamp on the Eastern Shores of St Lucia;

(iv)    remnants of swamp forests with F. trichopoda in the lower Mfolozi floodplain and in the Richards Bay area.

All but very small remnants of swamp forests in the Lake Sibaya catchment area have been destroyed by swamp gardening.

 

It is unlikely that the tree we measured is the biggest or oldest—it was just a single isolated tree that was easy to detect from aerial photos.

The impact of salt water on this large tree may be an early warning of sea level rise but there was the coincidence of several variables that would have affected the salt penetration. What we do know is that it has been many decades since sea water entered this far up the floodplain.

 Recently there has been a lot of clearing of swamp forests in Maputaland —including within the boundaries of the iSimangaliso World Heritage Site. This is unacceptable but shows us that in the short-term human impacts are a much greater immediate threat to swamp forests than climate change and sea-level rise. Too much of the rare swamp forests within the iSimangaliso Park have been damaged and are still being damaged, by people clearing sites for agriculture (Figures 11 a & b).

 

Figures 11 a and b: Aerial photos of swamp forests being cleared for the commercial planting of bananas as well as for small-scale subsistence farming. These patches are in the lower Mfolozi floodplain – within the boundaries of the iSimangaliso Park. (Photos Ricky Taylor, 2008)

References

Bate, G.C., Kelbe, B., Taylor, R. (2016). Mgobezeleni: linkages between hydrological and ecological drivers. Water Research Commission Project K5/2259. Final Report. Pretoria.

FigWeb.org http://figweb.org/Ficus/Subgenus_Urostigma/Section_Galoglychia/Subsection_Platyphyllae/Ficus_trichopoda.htm

https://www.dffe.gov.za/sites/default/files/legislations/championtreesofsouthafrica_treesdeclaredprotected.pdf

Smith A.M., Guastella L.A., Bundy S.C., Mather A.A. (2007). Combined marine storm and Saros spring high tide erosion events along the KwaZulu-Natal coast in March 2007. South African Journal of Science 103(7-8): 274–276.

Taylor, Ricky. (2016). Dynamics of the macrophyte vegetation of the Mgobezeleni floodplain and estuary, Northern KwaZulu-Natal. South African Journal of Botany 107: 170–178.

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The photos in this article are copyright to Ricky Taylor. The photos may be used without permission for non-commercial purposes if appropriate credit is given.

 

About the author:  Ricky Taylor is a retired nature conservation scientist. He was employed by Ezemvelo KZN Wildlife, based in the iSimangaliso Park, for most of his career. He believes that a conservation scientist must be an all-rounder. To guide the management of a natural area it is necessary to understand all the forces that have shaped its ecosystems. For estuaries these include geology, hydrology, sedimentology, biology, and human impacts.