We established our first Inga agroforest plot just over 17 months ago. Since then the seedlings have grown into 5 m tall trees, their crowns touching and shading out any potential weeds. They have captured the site meaning that it no longer needs any maintenance, allowing farmers to choose when to pollard (prune) at a time that best suits them. In the Bolivian Amazon the best time of year to sow plants is at the beginning of the wet season in October and so we are planning to return to complete this last step in the process.
There are an estimated 20-35 million ha of abandoned cattle pasture in the Amazon. For this reason we are keen to evaluate the possibility of their rehabilitation using Inga-based agroforest. We planted Las Palmas site 14 months ago and following a slow start the seedlings have reached 2 m in height, and we hope that now that their roots systems are well established they will now start to grow much more rapidly. The local farmer planted some Citrus seedlings which have not grown well at all but we hope that within a year our trees wil have reached 4-5 m in height at which time subsequent fruit-tree seedlings will fare much better as they benefit from improved soil conditions.
In many ways our most challenging site, the plot at San Jose has shown the greatest variation in growth, as you can see above. Bull-dozing means that most of the organic and weathered material was removed and the clay beneath highly compacted. The result is that roots have difficulty penetrating into the soil and water has difficulty percolating through it, resulting in anaerobic acidic conditions that may take decades to improve. Whilst our plot is disappointing compared to an abandoned slash-and-burn or pasture site, the majority of the seedlings have survived and are actively growing, albeit at a slow rate. We believe that the site could be captured within an additional couple of years. Whether this is a viable approach to rehabilitating such extremely damaged sites depends on the cost and frequency of the weeding necessary to keep invasive grasses from shading out the smaller plants.
San Antonio, our first pasture site has also been a challenge. The seedlings struggled to survive in the their first dry season but then began to grow albeit slowly compared to abandoned slash-and-burn sites. This may have been a consequence of the greater spacing between plants, compared to our abandoned pasture site (Las Palmas). We were hoping that after 2-3 years the trees would be big enough for cattle to be let back into the enclosure but this does not look likely at the moment. In addition cattle managed to get in and graze our 1-1.5 m saplings. Whilst not planned, it does suggest that cattle find Inga leaves palatable, that they do not eat the twigs and that the saplings recover quickly, producing new shoots almost immediately. Whilst too early and too small a sample to draw any conclusions, it does at least suggest that further research might reveal a role for Inga in silvo-pastoral systems.