By Dr. Laura Toro
Laura is a Restoration Scientist at the Missouri Botanical Garden’s Center for Conservation and Sustainable Development, where her research focuses on dry tropical forest restoration (ltoro@mobot.org).
Tropical dry forests are among the most threatened ecosystems in the world. Commercial agriculture, livestock farming, and mining have damaged and reduced the extent of this ecosystem type globally. Now less than 10% of the original extension of this ecosystem persists. The conservation and restoration of tropical dry forests are often overlooked because they are not as lush and well-studied as rainforests. However, restoring tropical dry forests can ensure the survival of thousands of unique plant and animal species that only exist in this ecosystem, and the protection of food, medicine, and livelihoods for millions of people.
Colombia has been one of the few countries that has invested resources to study the diversity and function of tropical dry forests across the country to ensure the conservation and restoration of the existing remnants of tropical dry forests. Since 2013 the von Humboldt Institute has been leading most of the research initiatives, and all the knowledge compiled about tropical dry forests has motivated the creation of a biodiversity offsetting policy. This policy establishes that for every hectare of tropical dry forest that is impacted by any kind of development project, 10 hectares (25 acres) of land need to be restored and conserved.
Although our understanding of the ecology, function, and diversity of tropical dry forests has increased in the last decade, the unique characteristics of this ecosystem type including the lack of precipitation for up to 8 months, makes it challenging for seeds and seedlings to naturally establish in these forests. Therefore, restoring tropical dry forests requires extra investment, like seed purchasing, in situ seedling production, tree planting, irrigation, fertilization, and weed control. These extra steps translate into a large financial investment at the beginning of a restoration project. According to published estimates, the establishment phase (year 1) of a restoration project in a tropical dry forest can range from $105 – $25,830 ha-1 (Bodin et al. 2022).
Irrigation and fertilization are among the most expensive management practices often implemented. When seedlings are planted in previously tropical dry forest areas, they are often irrigated to extend seedling access to water, and fertilized because most tropical dry forest soils are expected to be highly degraded from previous land uses. However, it is still unknown how much irrigation and fertilization seedlings need to be able to survive and grow. To answer this question, Fundación Natura, a Colombian nonprofit organization, Enel-Emgesa, an Italian electrical company, and researchers from the University of Minnesota established an applied nucleation project (where trees are planted in small patches to serve as focal areas of recovery) in a grassland area dominated by non-native species in southwestern-central Colombia that used to be a tropical dry forest. In this area, Enel-Emgesa built a dam, and as part of their biodiversity offsetting strategy the company committed to restore ~12,000 ha of degraded tropical dry forests in the next 10 years.
The grassland, where the research project we report on here was established, covered 7 hectares (17 acres) and had little evidence of spontaneous natural regeneration, so we implemented an applied nucleation intervention. To do that, we cleared the existing vegetation of the area with machetes and scythes, tilled with a tractor to reduce soil compaction, and set up 42 hexagonal plots that had an area of 1000 m2 (0.25 acres) each. In each plot, we planted 271 seedlings belonging to 11 different plant species native to dry tropical forests of this region. Four of these species had the ability to associate with bacteria that can fix atmospheric nitrogen. The seedlings planted all received 1 kilogram of ant farm soil, 10 grams of hydrogel to extend the period of favorable soil moisture, and 50 grams of nitrogen – phosphorus – potassium (NPK) fertilizer, 25 grams of NPK, 43 grams of phosphoric rock, or no fertilizer, depending on the plot where the seedlings were planted. All seedlings were irrigated once they were planted to activate the hydrogel. The seedlings that received the irrigation treatments were watered two more times during the first month of the experiment. The control and the 50 grams of NPK without additional irrigation treatments did not receive any additional water. Once the seedlings were 6 months old, we started monitoring their survival and growth. In each plot,we recorded the height of every seedling that was still alive. Thereafter, we measured seedling height approximately every six months for two years.
Additionally, we decided to compare the costs among the different management strategies implemented. We documented the cost of seedling production, planting, fertilizers, irrigation, and monitoring. Finally, we estimated the cost-effectiveness of each treatment for any of the eleven species planted. We did that by first estimating the total cost of planting a hectare of grassland with a single tree species (a hectare is equivalent to six nuclei = 1,626 individuals) under a specific treatment, and then dividing that cost by the % survival of that species under that specific treatment after two years. The most cost-effective method was the one that yielded the lowest cost and had the highest % survival.
We found that during the first two years of the project seedling survival was on average 73% across treatments. The seedlings that received 25 grams of NPK plus irrigation treatment had the highest survival (76%), while the seedlings that were fertilized with 50 grams of NPK and did not receive additional irrigation had the lowest survival (69%). However, when we looked at the survival across species, we found that survival varied across species and treatments. The plant species with the highest survival was Ceiba pentandra (99%), while Handroanthus coralibe had the lowest survival rates (5%). In terms of costs of planting seedlings, we found that the cheapest strategy was to not fertilize or irrigate the seedlings ($7,313 per hectare, $2,961 per acre), while the most expensive strategy was 50 grams of NPK plus irrigation ($11,689 per hectare, $4,732 per acre). Finally, when we compared the cost-effectiveness of the six fertilization and irrigation strategies implemented, we found that the control treatment was the most cost-effective management practice across the eleven species planted ($19,522 per hectare, $7,903 per acre) in part due to the low costs of no additional management beyond planting and monitoring, and the modest benefits to survival of costly irrigation and fertilization practices.
Even though restoration practitioners deal with a lot of uncertainty and restoration studies still lack information about how fertilizers affect the growth of native tree species and non-native grasses, there are multiple ways to improve restoration outcomes in tropical dry forests. We encourage partnerships between scientists and restoration practitioners to test how fertilizers impact different plant species growth and survival, and to estimate the costs of this practice across different tropical dry forests. Additionally, paying close attention to the soil fertility of the area, planting species that are found locally, actively weeding during the first year of the project, and fencing the restored area to ensure herbivores do not browse the seedlings will increase seedling survival and the success of the restoration efforts. Based on our results, we suggest that restoration projects should spend less resources on irrigation and fertilization, and more on plant species selection and weed removal.
If you want to learn more about how fertilization and irrigation impacted the growth and survival of tropical dry forest seedlings, we invite you to read our recent paper in Restoration Ecology or contact Dr. Laura Toro.
Natural History of Ecological Restoration 