The Arborloo is the simplest type of EcoSan toilet. It’s rather like a short-life shallow pit latrine, but it’s specifically designed to maximize the growth of high-value trees − fruit trees, for instance, or medicinal trees (see below).

Arborloos were developed by Dr Peter Morgan and described in his 2007 book:
Toilets that Make Compost: Low-cost Sanitary Toilets that Produce Valuable Compost for Crops in an African Context.
Read chapter 3 of this book Arborloo − The Single Pit Compost Toilet. 

Also by Peter Morgan: The Arborloo Book: How to make a simple pit toilet and grow trees or make humus for the garden.

An Arborloo functions like this: a shallow pit (0.8 m dia. and 1−1.5 m deep) is dug and a coverslab and portable superstructure (made from local materials) placed over it. This Arborloo latrine is used for 6−12 months (soil, leaves and/or ash are regularly added to the pit to accelerate the composting process − ash is a good source of potassium), after which time a new pit is dug and the superstructure placed over it. Soil is added to the full pit to just above ground level and a young tree is then planted; its roots grow down into the composted excreta/soil/ leaves/ashes in the pit and, as a result, the tree grows quickly and soon provides an income for the farmer. This process is repeated until the farmer has an orchard. Schematically an Arborloo works likes this (this is Figure 2.1 of Peter Morgan's book Toilets that Make Compost; see also pictures at bottom of page):

Why are Arborloos such a good idea? Well, they're very simple and very cheap, there’s no handling of the excreta even when composted, and the trees that grow in the full pits produce high-quality crops that increase rural people's incomes, as described in the following papers: 

Rapid Assessment of CRS Experience with Arborloos in East Africa (CRS, 2010)

Low-cost Arborloo offers Ethiopians health and agriculture benefits (Waterlines, 2007). There are over 12,000 Arborloos in Ethiopia and they each cost ~USD 5−10.

Arborloo (SOIL webpage)

Waste Not, Want Not (IPS, 2007)

Ayo's Secret Garden (CRS webpage)

Food Gardens (Africa Research Bulletin, 2007)

Lessons from a low-cost ecological approach to sanitation in Malawi (WSP, 2007) Quote: … the simplest and most widely used [EcoSan system] is the “Arborloo”, which is popular particularly among women because it is relatively simple to construct.

Peter Morgan’s conference papers (2001−)
Ecological sanitation: Many approaches to a varied need (presentation at AfricaSan 2008, Durban, February 2008)

Fertilizer Use in African Agriculture: Lessons Learned and Good Practice Guidelines (World Bank, 2007) − you can now see why Arborloos are such a good idea.  

►Arborloos are excellent for use in Africa and Asia where the average farm size is only 1.6 ha (see: Small Farms: Current Status and Key Trends: Information Brief, Wye College, 2005)

Arborloos are a bit like the “fossas migratórias” (mobile latrines) developed to fertilize food trees in the Amazon rainforest of Brazil − details here.  

Ecological Toilets: Start simple and upgrade from Arborloo to VIP (by Peter Morgan, 2009)

What trees to grow?

It's best to grow high-value trees. You really need to consult a local expert (local people are pretty expert too), but here are some webpages and papers to start with:

Tree domestication programme in Africa helps families out of poverty (IFAD webpage) 

Cameroon: tree domestication boosts family income (IFAD webpage)

[IFAD = International Fund for Agricultural Development]