Natural History of Ecological Restoration

Tag: tropical dry forests

  • From Soil Seed Banks to Seedlings: Deciphering the Natural Regeneration of Tropical Dry Forests in the Americas

    From Soil Seed Banks to Seedlings: Deciphering the Natural Regeneration of Tropical Dry Forests in the Americas

    By Viviana Londoño-Lemos

    Viviana Londoño-Lemos is a Ph.D. candidate with the Powers Lab in the Plant and Microbial Biology Program at the University of Minnesota. Email: vivianalondonolemos@gmail.com or londo074@umn.edu

    The tropical dry forest is a unique ecosystem that challenges many preconceived assumptions about the tropics. Unlike the iconic tropical rainforest, dry forests are highly seasonal, marked by distinct dry and rainy seasons. In some areas, plants may experience up to six months without rainfall. As a result, the species inhabiting these forests have evolved remarkable adaptations to thrive in this challenging environment, giving rise to exceptionally diverse plant communities. However, this unique diversity is under threat, as tropical dry forests are among the most threatened lowland ecosystems in the tropics. Urban expansion, mining, monoculture agriculture, and livestock production are among the main threats to this ecosystem. The loss of these forests not only endangers biodiversity but also jeopardizes the livelihoods of many human communities that depend on them.

    Given their threatened status, tropical dry forests have become a major focus of restoration efforts across the tropics. However, these efforts often lack critical information on how these forests naturally regenerate and how abiotic environmental conditions, such as light, water, and nutrient availability, influence seed and seedling survival in this ecosystem. During my PhD, I have been studying how abiotic environmental conditions shape natural seedling regeneration in tropical dry forests across Colombia and Costa Rica. My research focuses on three components of this regeneration cycle: the formation of the soil seed bank, the environmental conditions that drive germination, and the establishment of young seedlings.

    Tropical dry forest during the dry season in the Natural National Park Santa Rosa, Guanacaste, Costa Rica. Note how many of the trees lose their leaves due to water stress. Photo by Viviana Londoño-Lemos.

    Germination and establishment: the great choices in a plant’s life

    Germination is the most consequential choice a plant makes. Because plants are sessile organisms and germination is an irreversible process, the location where a seed germinates determines the environment where the plant must grow, compete, and reproduce for the rest of their lives.

    This story begins with a seed, which is the fertilized ovule of a plant. Each seed is a highly specialized structure that consists of three basic components: a protective seed coat, the nutritious tissue, and the plant embryo. Many seeds also include specialized structures that aid dispersal and sense environmental conditions, helping them determine when conditions are suitable for germination.

    Once seeds mature, the next step is dispersal in both space and time. Spatial dispersal determines how far a seed travels from its parent, whereas temporal dispersal determines when it germinates. Thus, natural seed regeneration depends on seeds arriving in the right place at the right time. Spatial dispersal of many tropical plants, particularly those dispersed by animal vectors, has been widely studied. In contrast, temporal dispersal, encompassing the study of soil seed banks within plant communities, remains understudied. Understanding both dimensions of seed dispersal is essential for assessing the regeneration potential of plant communities, an important consideration when designing plant-based restoration strategies, because it provides information on what species need to be planted versus which may regenerate naturally.

    Soil seed banks are the storage of viable seeds in forest soil. In theory, the soil seed bank of a given plant community contains seeds representing the species present in the vegetation aboveground. But not all seeds behave the same way. Seeds from some species persist in the soil until the next germination season — typically the rainy season in tropical dry forests— after which they germinate or die (transient species). Others persist for years, decades, or in exceptional cases, even centuries (persistent species). Understanding soil seed bank strategies helps identify which species have the potential to regenerate naturally.

    Once the appropriate spatiotemporal conditions are met and a seedling emerges, the next step is its establishment, defined as the transition from reliance on maternal sources (the seed) to physiological self-sufficiency. This critical bottleneck in the plant life cycle depends on the biotic and abiotic interactions of the seedling with its new environment. Biotic interactions, including mutualistic associations, predators, pathogens, and natural enemies, are widely assumed to explain the extraordinary diversity of tropical ecosystems. However, in seasonal ecosystems such as the tropical dry forests, abiotic interactions such as water, light, and nutrient availability play an important role in seedling survival. Environmental shifts, particularly in water availability, directly influence whether a seed successfully germinates and whether the resulting seedling survives long enough to establish. Understanding these processes is essential for designing effective restoration strategies grounded in the biology of the species they aim to support.

    A seedling of Enterolobium cyclocarpum at the Horizontes Forest Research Station, Guanacaste, Costa Rica. Photo by Viviana Londoño-Lemos.

    The hidden regeneration potential of tropical dry forests

    When planning a plant-based restoration project, understanding the site’s history, particularly its historical plant communities, is essential. Even after vegetation has been lost, soil often retains biological relics, such as persistent soil seed banks, of past communities. For many tropical woody species, soil seed banks are often considered transient, since tropical forests are not typically viewed as strongly seasonal systems. However, with distinct dry and wet seasons, tropical dry forests are a striking exception.

    To determine whether tropical dry forest species rely primarily on persistent or transient soil seed banks, we conducted experiments in the Tayrona National Natural Park in Colombia and at the Horizontes Forest Research Station in Costa Rica. Our goal was to assess how long selected species can survive in the soil. Because this experimental approach can be time-consuming and challenging, we also measured key seed traits such as seed dormancy type, dispersal syndrome, seed coat thickness, mass, volume, moisture content, and embryo-to-seed ratio to identify which seed traits are related to soil seed bank persistence.

    Our first experiment, conducted in Costa Rica, aimed to determine the extent to which tropical dry forest species form persistent or transient soil seed banks and whether canopy openness influences soil seed bank formation. For this experiment, we buried mesh bags that contained seeds from 15 liana and tree species under both canopy gaps and closed forest conditions. We retrieved the bags annually over three years and tested seed viability each time. After one year, only five species exhibited a persistent soil seed bank, suggesting that most species form transient soil seed banks and do not persist beyond a single rainy season.

    To explore this result in greater detail and determine whether transient species survive only until the first rainy season, we conducted a second experiment with 40 species, using shorter intervals between packet retrievals. This experiment was carried out in both Costa Rica and Colombia, in collaboration with researchers and students at the University of Magdalena. We evaluated seed viability after 3, 6, and 13 months.

    Although data analysis is still underway, preliminary results indicate that approximately half of the species employ a transient strategy, while the remaining half are persistent. Transient species tend to disappear from the soil seed bank, either by germinating or dying, by the end of the first rainy season. Persistent species, in contrast, retain their seeds longer, and in some cases germinate only a couple of seeds per year. Some species with persistent seeds tend to survive longer in canopy gaps. As for seed traits, we have observed a tendency for transient species to be wind-dispersed, possess a thinner seed coat, and lack physical dormancy (i.e., no impermeable seed coat). On the other hand, persistent species tend to have seeds with thick coats, physical dormancy, and are more commonly dispersed by gravity (autochory). We expect that our results will help inform decisions in selecting species best suited for planting in restoration efforts. More broadly, the results highlight the importance of understanding the soil seed bank strategies of candidate species in ecological restoration projects.

    Soil seed bank experimental setting at the Tayrona National Park in Magdalena, Colombia, in partnership with the University of Magdalena. Each mesh bag contains seeds of a single species; each row represents a harvesting period (3, 6, or 13 months). Photo by Viviana Londoño-Lemos.

    Germination sensitivity to hydration and dehydration pulses in the tropical dry forest

    Due to the pronounced seasonality, the main germination cue for tropical dry forest species is the onset of the rainy season. However, during the dry season, occasional rain showers may last for a couple of days. This means that the seeds of tropical dry forest species may be exposed to unpredictable rainfall pulses that can trigger premature germination at an unfavorable time of year, placing young seedlings at high risk of mortality.

    To understand how sensitive seeds are to hydration-dehydration cycles, we designed an experimental approach under lab conditions. We selected ten tropical dry forest species with varying seed morphology (e.g., seed dispersal types and sizes) and measured imbibition curves to assess the presence of physical dormancy. The imbibition curves also allowed us to determine the rate of water uptake and the time required for the water-intake percentage to reach different levels. Using data from the imbibition curves, we designed hydration and dehydration cycle experiments and set the germination chamber temperature to match the mean temperature measured in the soil seed bank experiments. We are currently working on this experiment. We expect the results of these experiments to inform us about the germination requirements of tropical dry forest species and the extent to which these species are vulnerable to changes in their normal seasonal patterns.

    Seedling of Spondias mombin in a nursery of the University of Magdalena, Colombia. This is one of the seedlings from the soil seed bank experiment that is germinating after one year. Photo by Luciano Macías Sposito.

    Demographic response of tropical dry forest seedlings to nutrient addition

    In addition to studying germination and seed persistence, we also evaluated nutrient limitation, a factor long proposed to limit seedling establishment in tropical ecosystems. A central biogeochemical dogma is that newly formed temperate soils are primarily limited by nitrogen, whereas tropical soils are typically limited by phosphorus. Although many experiments have tested this idea, results across tropical ecosystems are inconsistent, indicating the complexity of nutrient-plant interactions in the tropics.

    To investigate nutrient limitation in tropical dry forest species, the Powers lab established a factorial fertilization experiment with nitrogen and phosphorus at the Horizontes Forest Research Station in Costa Rica. This experiment tested the responses of the adult plant community to nutrient addition. One of the main findings of this experiment was that nitrogen-fixing legumes, a dominant functional group in tropical dry forests, showed a marked increase in growth to phosphorus addition, while non-nitrogen-fixing species exhibited variable responses. However, most studies evaluating seedling responses to nutrient addition are conducted in nurseries, which may not accurately represent field conditions. To determine whether nitrogen-fixing species are limited by phosphorus and whether non-nitrogen-fixing species are limited by nitrogen during their seedling stage, we planted seedlings of ten tropical dry forest species (5 nitrogen-fixing legumes, 5 non-nitrogen-fixing) directly into the existing fertilization experimental plots. We measured their survival and growth for four years. Because field experiments introduce multiple sources of variation, we also measured other environmental variables, such as litterfall, light, soil moisture, and pH.

    After tracking these seedlings for four years, we found that fertilization treatments did not increase seedling survival. However, nitrogen-fixing species tended to produce more leaves in response to nitrogen and phosphorus fertilization than non-nitrogen-fixing species. These results highlight significant demographic variation and inherently high mortality rates characteristic of the seedling stage. They also challenge the common assumption that fertilizer addition promotes plant growth and survival in restoration projects. This study will be published this year. Stay tuned if you want to learn more about the findings.

    A seedling of Dalbergia retusa at the fertilization experiment at Horizontes Research Station, Guanacaste, Costa Rica. Photo by Viviana Londoño-Lemos.

    Some seeds for thought for the future

    Together, the chapters of my dissertation aim to deepen our understanding of how tropical dry forest species regenerate from seed. My main goal is to make this information useful for designing more effective restoration strategies for this threatened ecosystem. Across ecosystems worldwide, a key challenge is that it remains difficult to generalize about the factors that enable a seed to survive, germinate, establish, and ultimately become a mature plant. This complexity underscores the importance of continued research on seed and seedling biology, especially in ecosystems as dynamic and highly seasonal as tropical dry forests. 

    At the Powers Lab, we strive not only to advance fundamental scientific understanding but also to share our findings with the general public.  To that end, we develop science communication materials that make our findings accessible and engaging to a general audience. One example of this outreach is a forthcoming book from Missouri Botanical Garden Press, created to introduce children to the tropical dry forest and inspire curiosity about this remarkable and often overlooked ecosystem.

  • The cost-effectiveness of fertilizing and irrigating tropical dry forest seedlings in an applied nucleation project

    The cost-effectiveness of fertilizing and irrigating tropical dry forest seedlings in an applied nucleation project

    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. 

    Ten-year-old grassland where tropical dry forests seedlings are not naturally establishing (photo: Laura Toro).

    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.

    Aerial image of the 42 nucleation plots established in 2019 in El Quimbo, Colombia. The extension of the restoration intervention was 7 hectares (17 acres) (photo: Fundación Natura). 
    A close-up view of the experimental restoration plot. The orange lines represent the limits of the plot (1000 m2 = 0.25 acres), and each small dot represents a seedling planted (Photo: Fundación Natura). 

    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.

    An employee from Fundación Natura fertilizing a seedling of Vachellia farnesiana a shade-avoiding plant species common in tropical dry forests in Colombia (photo: Laura Toro). 

    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.

    Aerial image of nucleation plots in El Quimbo, Colombia in 2022. (photo: Fundación Natura).

    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.