My graduation research on trees and arthropods in South Africa

This week I will tell you all about another step I had on my road to Africa. To finish my bachelor I had to do a graduation research, a thesis or dissertation. Different countries, different names. I wanted to find my own placement to do my research and, of course, preferably in Africa. After many many emails and a lot of time later I found Grootbos foundation connected to Grootbos private nature reserve, located in Western Cape, South Africa. I was welcome to come and conduct my graduation research in the reserve.

Grootbos means ‘big forest’ which is a bit funny, as it is located in the fynbos. Which is an ecosystem that generally does not have trees. But it is named after the beautiful Milkwood forests, a new habitat that occurs within the fynbos. These are find in the valley’s where they are safe from fires and where there is more water available. And that was also the topic of my research! There is fairly little known about the Western Cape Milkwood Forest. It is an endangered vegetation type, with a fragmented distribution range. Besides that, it is also under threat of development. Today I will tell you about some of the most interesting things I did in this graduation research.

Graduation research

So, for my university, I had to do a graduating research and thesis writing for 1 semester, 30 credits. In these 5 months or 22 weeks, I had to go and do fieldwork in a tropical country, analyze my data and finish the writing of the report. My thesis started in September and needed to be handed in the first week of January. As I was required to go to the tropics, it is pretty tight time-wise. Therefore I agreed with my supervisor that I would finish my research proposal already the summer before I would go on fieldwork.

This meant that I wrote my complete research proposal while I was travelling through Australia! But I was really determined to finish everything in that one semester so it made it all worth it. Thanks to finishing my research proposal on forehand I could already leave the first week in September to South Africa. And when I arrived there I could dive straight into preparing the fieldwork. No time to waist!

Grootbos Private Nature Reserve

As the name already gives away, Grootbos is a privately owned nature reserve. The owner decided he wanted to give back to the community and nature in and around the reserve and started the Grootbos Foundation. This foundation has conservation and a community side. The conservation team primarily focus on the amazing biodiversity of the fynbos, both flora and the fauna. And each year everybody jumps in to help with the controlled fires to stimulate the fynbos, as this is a fire-driven biome. Unfortunately, I missed those days 🙁

So where my, temporarily, colleagues were focusing on the fynbos. I was going to focus on the ancient Milkwood forest that was present in the valleys of the reserve. As my main interest is the trees, they offered me a graduation research opportunity to help them understand the biodiversity of this amazing forest type. Using the composition of the tree species and the assemblage of the arthropods.

Milkwood Forest

There are five patches of Milkwood forest present within the reserve, but due to my limited time available, we choose three patches. They were called; ‘Garden Lodge, ‘Forest Lodge’ and ‘Witvoetskloof’ being 18, 17.4 and 3 hectares in size respectively. We choose these patches because it gave a nice resemble of the variance, Garden Lodge was big in size and pristine, while Forest Lodge was large in size as well, but affected by a major fire that went through the reserve in 2006. While Witvoetskloof is small and isolated, it is also located close by an underground spring.

Tree composition

The first part of my graduation research was determining the tree composition in each of the forest patches. I did this by using plots, in each forest patch I placed 15 plots, divided into 3 clusters. I decided on the location of the clusters in such a way that all variation within the forest is taken into account. From the edge of the forest to the interior. Each plot was 20 by 20 meter and within this plot, I took the data from each tree which had a diameter from more than 5 cm. This is measured at 1.30 and also called ‘diameter breast height (DBH). So for every tree, I wrote down; the name of the species, DBH and the coordinates. And at the spot that most representable for the entire plot I took the canopy cover in percentage.

With this data, I was able to calculate a lot of different things. The richness of tree species per plot and per forest patch, as well for the beta-diversity, evenness, Shannon-Wiener and Simpson index. These are all different kind of methods to analyze the diversity of a certain area, in this case from the different forest patches in my study area. Besides that, I also calculated some more forestry-related things, such as the mean DBH. This gives me an indication of the age of the forest patch. As well for the number of trees per hectare, this gives an indication of the age as well as structure. And last I calculated the Basal Area in M2 per hectare, the amount of timber present. This again indicates age and structure.

Arthropod assemblage

The second part of my graduate research was looking at the arthropod assemblage. I was hoping that the arthropods could give an indication of the health of the forest. By linking it to the information about the tree composition and the known information about specific groups of animals. To collect the arthropods I used a collection of leaf litter in combination with berlese funnels.

In each plot, I collected leaf litter from a 30×30 cm area, directly under the place where I collected the data about the canopy cover. As this might be influencing the arthropod assemblage as well. I brought this back to the lab at the office and used a construction with heat to extract the arthropods from the leaf litter. Most of the arthropods, moths and spiders could react differently, do not like the heat. They crawl away from it, down into the funnel from where they fall in my jar. They are unable to escape the jar and from there I could examine them to order level.

With the data about which arthropods I caught were, I was able to calculate a lot of different things again. I could calculate all the different biodiversity indexes, as mentioned in ‘tree composition’ and the average tree composition. Besides that, I also analyzed them as possible ‘bioindicators’, as some species react differently to disturbance. For example,e the big fire that occurred in 2006.

Results

The results turned out rather different for both of the parts. First the tree compositions, this biodiversity was the highest in Witvoetskloof. This was for every earlier mentioned method. This forest patch has the highest biodiversity in tree species because it is located by an underground spring. Due to the dry months in the Western Cape, there are times of water shortage. This limits the growth of a lot of the tree species that you are possible to find in ‘Aframontane’ forests. These forests are located in the same area but on wetter areas. Due to the presence of this spring, there were certain of the Afromontane species that were able to grow in Witvoetskloof!

For the arthropods, I found the opposite results. Witvoetskloof showed the lowest biodiversity for the arthropods, while Garden Lodge shows the highest. This has to do with the small size and the isolation of Witvoetskloof. These arthropods are very small animals and have difficulties moving over great distances. They simply never reached the area. Garden Lodge and Forest Lodge were located next to each other, so the arthropods have the opportunity to move between those areas. But Forest Lodge still showed much lower biodiversity than Garden Lodge. This reduction in biodiversity can be explained due to the damage from the fire in Forest Lodge.

Bioindicators

As mentioned earlier, I also used the arthropods as potential bioindicators. Unfortunately, I did not catch enough individuals to be able to find them. The only significant difference I was able to find was a higher percentage of predator arthropods in Forest Lodge compared to the other ones. An increased amount of predators, or specifically spiders, might indicate disturbance. Which fits with the results we found about the tree composition and the knowledge about the fire.

This was a very short story about what I have been doing for my graduation research, with very limited information about the results. I can tell you much more about this, but than this blog post would get 3 times as long. If there is anything you are interested in, let me know! I would love to explain to you more about my amazing time in the Milkwood forests.

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