Invasive, Alien and Indigenous Trees
Certain Trees which are not indigenous to the area are often called Alien or invasive trees. We prefer to say that these trees are invasive, as no tree is Alien to Pachamama.
Currently, the Pachamama forest retreat land is occupied with two-thirds of invasive trees including several species of Pine, the Australian Acacia (Wattle) and Karri Eucalyptus. These types of trees have been imported to Southern Africa since the eighteen hundreds. Traditionally the natural forest was predominated with slow-growing trees. Therefore, exotic, fast-growing species were brought to South Africa to meet the demands for timber, mine props, charcoal and tannin (for tanneries). However, these trees ‘escaped’ from their plantations into the natural environment. Currently, about 200 plant and tree species are classified as ‘invasive’ trees. Without the presence of their natural enemies, which are mostly left behind in the country of origin, they are able to reproduce fast, survive and spread at an alarming rate across the landscape. Thereby causing personal, environmental and economic damage.
(Richardson et al., 2000a; van Wilgen et al., 2001).
It is estimated that about 90% of the Garden Route’s Fynbos vegetation is invaded by Pine trees. In addition, Australian Acacia and Eucalyptus cover 29% and 14% of the land respectively.
Listed below are several examples of environmental and economic impacts which are caused by invasive tree species:
- Reduction in, stream flow and available water;
- Loss of potentially productive land;
- Loss of grazing potential;
- Poisoning of humans and livestock (e.g. Melia azedarach and Lantana camara);
- Increased costs of fire protection and increasing damage from wildfires;
- Increased soil erosion following fires, in heavily invaded areas;
- Siltation of dams;
- Changing soil nutrient status;
- Loss of biological diversity and threat to native plant species;
- Changing the biomass of ecosystems;
- Changing habitat suitability for native animal species;
- Reduction of the borders of the indigenous forest with each fire.
- Increased wattle growth in fire zones.
We have identified some of the invasive tree species on the land of Pachamama Forest Retreat, listed below are some effects of these trees on our land.
|Increased water repellency. |
Affects soil erosion to a variable degree
|Out-competes native trees |
Dense stands limit options for fire management
Decreases stream flow
|Pinus Radiata||Fynbos||Decreases stream flow|
| Acacia Mearnsii|
| Decreases diversity of ground-living invertebrates|
Decreases stream flow
The destabilisation of stream banks
Can increase erosion, but can also used for
land stabilisation in some cases
(Wilgen et al, 2001)
Three of the main problems that arise from a monoculture of these trees in plantations, the invasion of non-native trees in the Fynbos in the Knysna region and on the land of Pachamama Forest Retreat, are further explained below.
Natural Water Cycles
Invasive trees and plants consume more water than the indigenous plants, depleting valuable underground resources. Consequently, heavily infested areas of alien vegetation can have a reduced water-runoff up to 30%. This results in the reduction of water in the river and reduces the groundwater reserves. Less water within the river means that nutrients and pollutants don’t get spread out properly throughout the ecosystem (dilution capacity?). This again can harm the water quality, affecting ecosystems, human health and crop yields. (‘Alien Vegetation Management’, n.d; Charnier et al, 2012)
In addition, invasive vegetation has an influence on the quality of water through the biomass; the ‘terrestrial litter’ such as leaves, bark, seed, flowers and twigs that they shed to the ground. The output of biomass is relatively higher compared to indigenous species. Consequently, invasive trees with a lot of nitrogen fixers such as Acacia (Wattle) or carbon can elevate the amount of this particle in the groundwater, changing the nutrient cycle in the ecosystems (Stock et al, 1992).
Although these non-native trees use much water, they catch fire like a dry bunch of matches. These are the trees which make fire roam freely through the region. Generally, trees with needles (coniferous) burn up to 10 times faster and more intense than trees with leaves (deciduous). Trees like Pine have a large amount of sap in their branches that burn very quickly. Additionally, coniferous trees often grow very close to each other. This aids the spread of wildfire and its ability to jump from one tree to the other. The rough, loose bark can act as a ladder for the fire to climb into the canopy, causing smothering and making the roots burn beneath the ground for up to two weeks, increasing the potential to flare up again with strong winds. In addition, the bark can produce burning embers that are carried ahead of the fire front(Spotting), heightening the risk for the fire to jump over to another patch of forest. When such an area of coniferous trees catches fire, it becomes extremely hot, harming the soil structure of the burned area which can result in soil erosion (Chandelier et al, 2012; “Tree Species Impact on Wildfire”, 2012).
Deciduous trees have a higher crown base height, a higher moisture content of the leaves and stems and a tight, smooth bark. Altogether preventing fire burning as quickly as with Pine trees.
We have a high percentage of invasive trees in the region. The fuel that can burn in wildfires has substantially increased. Consequently, wildfires have become more intense and much harder to control. Resulting in the Knysna fire of 2017 and the recent fires of 85.000 hectares (Tineke Kraaij et al, 2018 ).
Affection Compositional diversity animals
The number of different species in a system; the compositional diversity of animals is affected by invasive plants and trees. Invasive plants have a larger decreasing effect on native species abundance compared to native species richness (Figure 1). The severity of this effect is dependent on factors such as the extent and density of the plant invasion, the stage of invasion, the region, and the taxation group (Clusella Trullas & Gracia, 2017).
Figure 1: Percentage of comparisons performed in 42 studies that found invaded sites to have (a) positive (increased diversity), neutral or negative (decreased diversity) effects on native ectotherm species richness (n = 80 comparisons) and abundance (n = 52), and (b) the same or altered species composition (n = 36) as uninvaded sites (Clusella Trullas &Gracia, 2017, p.5)
The physical conditions of the environment are directly altered by their presence. For example through changing the light, solar radiation and temperature levels. These changes in the microclimate influence the functional diversity of animals. Additionally, native animal species are not attuned to alien trees. Therefore, the presence of these trees changes the quality and availability of the possibilities to create nests, find refuges, move, or acquire food. As a result, in an extreme case, alien species could move an ecosystem from natural heterogeneity to homogeneity if left uncontrolled over a large scale and timeframe. (Clusella Trullas &Gracia, 2017; Foxcroft, 2002).
- ‘Alien Vegetation Management’ (n.d.). Care For Nature: Alien Vegetation Management. Cape Nature. retrieved on 08-11-18 from https://www.capenature.co.za/care-for-nature/conservation-in-action/integrated-catchment-management/alien-vegetation-management/
- Charnier, J., Schachtschneider, K., Le Maitre, D.C., Ashton, P. J., van Wilgen, B. W. (2012) Review: Impacts of Invasive Alien Plants on Water Quality, with Particular Emphasis on South Africa. Water SA, 38: 345-356.
- Clusella-Trullas, S., Garcia, R. A. (2017). Impacts of Invasive Plants on Animal Diversity in South Africa; A Synthesis. Bothalia, 47(2); 1-12.
- Foxcroft, L. C. (2002). Impacts of Invasive Alien Species on Biodiversity. Invasive Alien Species, Skukuza, KNP.
- Kraaij, T., Baard, J. A., Arndt, J., Vhengani, L, van Wilgen, B. W. (2018). An assessment of climate, weather, and fuel factors influencing a large, destructive wildfire in the Knysna region, South Africa, Fire Ecology DOI: 10.1186/s42408-018-0001-0
- Richardson, D. M., Pysek, P., Rejmánek M., Barbour, M. G., Panetta, F. D. and West C. J. (2000). Naturalization and Invasion of alien Plants: Concepts and Definitions. Diversity and Distributions, 6: 93-107.
- Stock, W.D., & Allsopp, N. (1992). Functional perspectives of ecosystems. p.241-259, in: R.M. Cowling (ed). The ecology of fynbos; nutrients, fire and diversity. Cape Town, Republic of South Africa: Oxford University Press.
- “Tree Species Impact on Wildfire” (2012). How Different Tree Species Impact the Spread of Wildfire, Alberta Government.
van Wilgen, B.W., Richardson, D.M., Le Maitre, D.C., Marais, C., & Magadlela, D. (2001). The economic consequences of alien plant invasions: examples of impacts and approaches to sustainable management in South Africa. Environment, Development and Sustainability, 3: 145-168.