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Baobab trees may be a proxy for measuring long-term use of land by humans. They live long, have economic benefits, and are used as shrines and markers on landscapes. Archaeologists have long suspected an association between baobabs and human settlements. , who researches early urbanism and the first migrations by farmers to the Tanzanian coast, , an archaeologist and remote sensing scientist and engineering Ph.D. candidate, and used to connect the baobabs living today to ancient Swahili sites. Their findings suggest that the Unguja Island community's protection of baobabs today is rooted in stewardship that extended deep into the past.

How did baobab trees get to Unguja Island?

, in Zanzibar, Tanzania, is part of the cultural region known as the . The Swahili Coast stretches from southern Somalia to Kenya, Tanzania, Mozambique, north-west Madagascar and the Comoros. The ancient Swahili were fishers, farmers, artisans and merchants who founded towns and cities across coastal eastern Africa from the 7th century AD onward.

The Swahili people who migrated from western and central Africa between 4,000 and 2,000 years ago, bringing ironworking technologies, pottery, and new crops and plants. One of these plants was the African baobab (Adansonia digitata). This tree originated in or , and then spread across Africa and the Indian Ocean as far as Australia.

Today, farmers do not often plant baobabs. However, they protect groves from land clearances and sustainably harvest baobab products. The bark was historically used to make rope and cloth. The leaves, seeds, and fruit are edible and highly nutritious, and the seeds can also be pressed to make soap and oil.

Baobab tree trunks can hold up to 10,000 liters of rainwater in their hollow spaces, which supplements water resources during times of drought. Because they are so large, distinctive and shady, they also function as landmarks and shrines for ritual activity. Performances take place under baobab trees and they are also sites of debate, discussion and education.

Archaeologists suspect that baobabs reached Unguja by the 7th or 8th century. This is based on the presence of baobab seeds , such as early Swahili ports. Of course, these might have come from trees elsewhere on the east African coast. But even if these seeds came from elsewhere, it is likely that the consumption and use of baobab fruit at the time would have fairly quickly led to the tree's propagation on the island.

What did your research set out to discover?

Our research team wanted to explore how resources from baobab trees helped ancient Swahili people migrate to islands, settle permanently into villages and towns, and adapt to changing political circumstances later on when they were colonized by Portugal, Oman and Britain.

In other parts of the world, archaeologists are recognizing the significance of arboriculture (tree cultivation) in the emergence of cities and their countrysides. We wanted to explore this in Africa.

Baobab trees spread through seed dispersal. This can happen through humans harvesting and consuming baobab fruit pods. So, we aimed to demonstrate that the current distribution of baobabs on the island is linked to ancient sites. This would mean we could start to theorize how ancient Swahili groups managed and used baobabs as they settled in island environments, founded towns and adapted to social change.

What did the drone imagery detect?

From 2016 to 2018, the flew drone missions across Zanzibar. These drones captured high-resolution images of the islands of and .

Baobabs are visible on these images as white circular shapes with spindly branches. We tested different ways to detect these trees in the imagery, including scrolling through 1,600 square kilometers of images and marking baobabs on the photos by hand.

We found that this method was the most time-efficient and accurate. We recorded 8,507 baobab trees across Unguja. This can now be used as a reference for future research. For example, researchers with computers could potentially use this data to more accurately and rapidly detect baobab trees in satellite imagery in other parts of the world.

The images showed that trees exist in groves around ancient port sites such as Fukuchani and Unguja Ukuu. But they are more strongly associated with sites from the 11th-18th centuries CE. This was the period when Swahili communities urbanized and rural communities became more permanent and agriculturally oriented.

We also found that baobab trees exist far from the 19th-century sites. This might be related to deforestation for plantations that began in the 19th century. It could also be related to the fact that newly arrived Omani plantation owners, practicing Ibadi Islam, opposed the (the blending of different religions and rituals such as revering ancestors) that had incorporated into their belief systems and which may have occurred .

Future research elsewhere could use our verified identification of 8,507 baobab trees to train computers to detect baobab trees automatically in satellite imagery.

How did you find that baobab trees were landscaped by humans more than 1,000 years ago?

that the present-day distribution of baobab trees on Unguja was not random. Instead, it is a legacy of long-term human land use.

In media, the image of a baobab tree is often used to signify a quintessentially "wild" African landscape. Our study demonstrates that, in fact, baobab tree distributions are not "natural" but are the result of landscape management. Managing the baobabs has continued through the centuries. Rural Swahili communities on Unguja Island today are stewards of baobab tree groves, using their products and cultivating surrounding environments in a sustainable way.

Today, products made from baobab seeds are supplement or as an ingredient in industrial soaps and cosmetics. As production intensifies, this threatens to divorce baobab cultivation from traditional land management practices. This will have negative impacts on local environments.

Baobabs are also under threat from rapid urbanization, intensive land use and climate change. Therefore our study helps establish a method to use drone photography and satellite images to map baobabs across tropical environments. We hope this will aid efforts to manage baobab trees, in collaboration with rural African communities who depend on baobabs as a resource and who continue to act as arboricultural stewards to this day.

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