FACILITATION BY PLANTS
Plants change the immediate environment around them, affecting other plants that grow in their vicinity. Can we use this phenomenon to our advantage?
Can some plants help other plants grow?
Plants that have a positive effect on their neighbors are called ‘facilitators’ by ecologists, or ‘nurse trees’ by foresters.
In our case, they may help by improving the harsh environment of eroded cattle pasture. Some ways they could do this include offering shade, providing mulch that improves soil nutrients or texture, secreting compounds into the root environment, providing helpful fungi, maintaining humidity during the dry season, and confusing plant enemies.
Knowing the biology of different plants helps us guess how one plant might help our target seedlings, and we design our experiments around those guesses.
Facilitation by legumes
Our first large experiment, ‘Bloques’93’, addressed the question whether legumes can help valuable target trees grow better in these degraded soils than when target trees go it alone. Legume roots have nodules containing bacteria that ‘fix’ nitrogen, thereby increasing the availability of nutritious forms of nitrogen to the plant and, indirectly, to the soil surrounding the legume.
Our experimental method was to mix different species of legume herbs, shrubs, and trees within plots containing seedlings of the tropical hardwood tree Terminalia amazonia. The photo below left shows Project Administrator Eduer Sandi climbing to collect Terminalia seeds to raise in our nursery.
We discovered that only one of the several species of legumes that we tried, a tree Inga edulis, could even grow in our soils and therefore potentially help the seedlings of Terminalia that we planted. See the page on fungi for a possible explanation why this legume tree could survive. Photos 2 through 4 in the panel above show Inga: its growth form, gigantic 'bean' pods that we collected, and nodules in the roots of a seedling.
The exciting result was that mixing Inga with Terminalia resulted in significantly better growth of Terminalia. Furthermore, this facilitation emerged when the trees were only four years old and accelerated through time. Meantime, the positive effect of chemical fertilization in the same experiment diminished and disappeared. See the video interview on the HOME page for a demonstration of this experiment.
We performed several related studies within this large experiment, specifically:
- Initial soil analyses: We established baseline soil characteristics in the 45 large plots of the experiment before planting seedlings two months after we excluded cattle. See soil corer in the photo at left in the panel below. We air-dry samples (next photo) before taking them to the lab at the University of Costa Rica for analysis. As we expected, soils in the overgrazed pastures were acid and nutrient-poor, lacked humus and other organic matter, and contained toxic forms of aluminum.
- Subsequent soil analyses: We tested whether the growing trees could improve the soil faster than natural improvement of soil in unplanted control plots that experienced only growth of weeds and pasture grasses (third photo, above). We found that over eight years, topsoil had already improved in all plots, a recuperation rate astoundingly fast relative to soil regeneration in areas outside the wet tropics.
- Nitrogen analyses of soil and Terminalia leaves: We tested the idea that Inga facilitated Terminalia by improving its nitrogen nutrition. In fact, in plots where Inga grew well, nutritious forms of nitrogen were higher in soils under neighboring Terminalia saplings than under control saplings without Inga. Most important, nitrogen content of Terminalia leaves was higher than in control saplings without Inga and the trees looked healthier (fourth photo, above).
- Competition between the two kinds of trees for light: Once the Terminalia saplings were large and facilitation by Inga was clear, we did a subsequent experiment within the plots containing mixed Inga and Terminalia: Would removing (killing) the Inga now stimulate Terminalia growth by removing competition with the now-large Ingas for light?
Several publications emerged from this work, three in the journal Forestry Ecology & Management.
Facilitation by Vochysia
The results of Ensayo1994 and Ensayo1996 showed that two species in the genus Vochysia (right) were the native trees that did the best, after the non-native pine. Vochysia can store vast amounts of aluminum from the soil in its tissues without suffering damage, even though some of the forms of aluminum are usually toxic to plants.
We did several experiments testing whether Vochysia might detoxify the soil, thereby facilitating growth of tree seedlings planted nearby. Also, the mechanism by which these trees take up aluminum might free up the nutrient phosphorous in the soil, yielding a fertilizer effect.
We tested this concept in three experiments, established in 2004, 2006, and 2008:
1. Facilitation of ron ron: In an experiment in 2004-2009 (right), we tested whether planting the beautiful hardwood ron ron (Astronium graveolens) mixed with Vochysia would improve the survival of this desirable species, which usually dies in our experiments. We used pine and Inga seedlings as controls for the effect of Vochysia. It appears that all three trees may have facilitated survival of ron ron. However, we need more time for this result because ron ron is so slow-growing.
2. Facilitation of cedro maria: An experiment in 2008-2010 tested mixtures of Vochysia with cedro maria (Calophyllum brasiliense), a valuable species that had low survival but good growth in those individuals that did survive in Ensayo'94. No significant facilitation was seen after two years. Ongoing.
3. Facilitation of four species using Ensayo'94: Once the results of Ensayo'94 were clear, we used that experiment to test whether mature Vochysia could facilitate the growth of four hardwood species planted under their canopies. These four valuable species grow poorly or die when planted in abandoned pasture. Perhaps they need the presence of pioneer trees first, since all four species are characteristic of mature forest in this region, so their seedlings likely thrive in shady conditions of established forest.
Their seeds of some of these rare trees are hard to find but we planted some under the established trees of Vochysia and under pines as a control. The numbers were small so the results are uncertain, but it seemed that one of the most valuable, “cristobal” (Platymiscium), did better under the pine, not the Vochysia!
We want to follow up with future experiments.
Site preparation by potential facilitators
In natural regeneration without human intervention, facilitation often happens when the first ‘pioneer’ species take a few years to change the environment before other species can then colonize and thrive. Therefore, we wanted to try preparing an area with several potential facilitator species for two or three years before planting the valuable target tree.
We also expanded our concept to include a long-lived grass, Vetiver, that according to a study by the National Academy of Science, is reported by farmers to slow erosion by forming terraces when planted in lines along slopes.
Site Preparation by trees and terracing: This most recent experiment, planted in 2012, is really two experiments, one testing facilitation by potential nurse trees, and one testing facilitation by Vetiver grass that causes terraces to form. Our idea is that tree seedlings may grow better on terraces because erosion is slowed so topsoil can regenerate and accumulate, providing better soil nutrients, moisture, and texture. R.A.I.N.'s report letter in 2012 (scroll down to list of years) explains the concept of this experiment
The valuable target tree is maria (Calophyllum). In one area we planted plots with a legume shrub calliandra, the legume tree Inga, and the non-legume pilon (Hyeronima) as site-preparers and potential facilitators for maria. Photos below show the eroded hillside at the beginning of establishment of the experiment, seedlings of pilon and Inga hauled up from our nursery, and measurement of planting spots within a plot
In an adjacent area we planted lines of Vetiver along the contours of the steep slope. Photo strip below shows contour measuring, collection of Vetiver plugs to plant, and the final planted slope.
We will allow enough time for the facilitators to grow large enough to affect the soils and for terraces to form between the lines of Vetiver before planting the target seedling, maria.
In control plots we planted the maria without site preparation. For site preparation to be worthwhile economically for farmers, growth of the target species should be better in the long run despite the delay in planting on the conditioned site. Thus this experiment is long-term and will require perhaps 10-20 years for definitive results!