Plant-plant interactions can be competitive, mutualistic, or even parasitic. Most often when we think of plants interacting in the same space we think competition, especially when that space has been disturbed and destroyed leaving very little behind. However, there is research that suggests that plants can facilitate growth of nearby plants and actually work together under higher levels of abiotic stress (Anthelme, Gómez-Aparicio, & Montúfar 2014; Stachowicz 2001; Aerts, et al. 2006). I dug through 8 articles that talk about these facilitative plant-plant interactions in fragmented habitats. Results from the research tells us that for the most part nurse plants can be an extremely helpful tool in the restoration process, but there are many factors that come into play that can affect whether the outcome is positive or negative.
NURSE EXPECTATIONS
Nurse plants can have major impacts on biodiversity and community structure through positive plant-plant interactions according to all 8 articles. They serve important functions (as you can see in the chart below) within environments that have experienced lots of stress or have been disturbed by human inflicted habitat fragmentation.
From the research that is currently available, we can read about all of the benefits like: reducing soil erosion, protecting seedlings, reducing herbivory, and providing lots of shade in the face of more extreme weather patterns (Stachowicz 2001; Howe & Miriti 2004). For instance, if the nurse plant is very popular with the nearby pollinators, some of those pollinators just might stop by the little guys and further help the growth of the facilitated species. 1 of the 8 articles reported that there was "high community structure" in the patches where nurse plants were implemented, and the results showed a positive relationship in patches with a focal nurse plant species than in those without (Nuñez et al. 1999). The study also showed a steady increase in diversity over time (Nuñez et al. 1999).
It has been stated in two of the studies, that the shade from the the nurse plant provides lots of shade especially in very dry conditions where seeds have shelter from the sun and can have a greater chance for germination (Valladares & Gianoli 2007; Howe & Miriti 2004). Anthelme, Gómez-Aparicio, & Montúfar report on results from experiments in the Andean tropical forest they saw that seedlings left without a nurse plant or artificial shade were all dead within 10 months. But the results from nurse tussock grass caused reduced soil temperature and composition which made for soil more suitable to grow in; the nurse grass also caused reduced stress from lack of water and reduced stress from herbivory (Anthelme, Gómez-Aparicio, & Montúfar 2014).
Introducing a focal nurse plant changes soil composition, which can lead to richer soil and increase biodiversity over time (Nuñez et al. 1999). Restoring these patches could increase connectivity and promote more spread of plant species and increase biodiversity. However, one study did find that facilitative effects may not be long term, because the positive effects began to fade with time (Anthelme, Gómez-Aparicio, & Montúfar 2014).
FACILITATION VS. COMPETITION
So, we can see from these articles that nurse plants can have a major impact on a fragmented ecosystem. Now, we will look at potential downfalls of the nurse plant. While facilitation is a fascinating display of plant-plant interactions, it can sometimes be too much of a good thing. You can be an extreme coupon-er (is that a word?) but at some point you are just spending way too much money on toilet paper, deli-meat, and jello than you actually need, and the cost outweighs the benefits. The 200 packs of chocolate pudding that you purchased for your family of 5 to snack on will most likely expire before it can be consumed, and that is not an example of resourcefulness, that is wasteful.
In fact, 2 of the 8 articles I reviewed found that in some experiments, nurse facilitation was not advantageous at all (Galindo, et al. 2017; Padilla & Pugnaire 2006). Some of these cases reported negative effects after introducing nurse plants. 1 of the 8 articles reported that nurse plants placed can out-compete native species once both plants have grown to a stable size (Galindo, et al. 2017). Padilla and Pugnaire found in their research that nurse plants in fragmented habitats were advantageous for the facilitated plant in "optimal conditions," but when a drought came, all the plants died (Padilla & Pugnaire 2006). They also discovered that in fertile soils, the nurse plants depleted the land of its nutrients and there was less biomass (Padilla & Pugnaire 2006). From studies like this, we can see that there may not be any advantages from nurse plants in restoring habitats.
TAKEAWAYS
To find a one size fits all answer to the problem surrounding habitat fragmentation would be wonderful, but sadly it is not realistic. There are far too many plant responses in addition to the varying conditions of each ecosystem to take into account; so, it is best to combine different techniques for the most productive outcome. It is so imperative that "caveats regarding species and site characteristics" are taken into account because they could have a major impact on the productivity and restoration of a fragmented habitat (Padilla & Pugnaire 2006).
WORKS CITED
Aerts, Raf, Wouter Maes, Eva November, Aklilu Negussie, Martin Hermy, and Bart Muys. "Restoring dry Afromontane forest using bird and nurse plant effects: Direct sowing of Olea europaea ssp. cuspidata seeds." Forest Ecology and Management 230.1-3 (2006): 23-31.
Anthelme, Fabien, Lorena Gómez-Aparicio, and Rommel Montúfar. "Nurse-based restoration of degraded tropical forests with tussock grasses: experimental support from the Andean cloud forest." Journal of Applied Ecology 51.6 (2014): 1534-543.
Galindo, Víctor, Zoraida Calle, Julian Chará, and Inge Armbrecht. "Facilitation by pioneer shrubs for the ecological restoration of riparian forests in the Central Andes of Colombia." Restoration Ecology (2017).
Howe, Henry F., and Maria N. Miriti. "When Seed Dispersal Matters." BioScience 54.7 (2004): 651.
Nuñez, Cecilia I., Marcelo A. Aizen, and Cecilia Ezcurra. "Species associations and nurse plant effects in patches of high-Andean vegetation." Journal of Vegetation Science 10.3 (1999): 357-64.
Padilla, Francisco M., and Francisco I. Pugnaire. "The role of nurse plants in the restoration of degraded environments." Frontiers in Ecology and the Environment 4.4 (2006): 196-202.
Stachowicz, John J. "Mutualism, Facilitation, and the Structure of Ecological Communities." BioScience 51.3 (2001): 235.
Valladares, Fernando, and Ernesto Gianoli. "How Much Ecology Do We Need to Know to Restore Mediterranean Ecosystems?" Restoration Ecology 15.3 (2007): 363-68.
Picture 1: http://rainforests.mongabay.com/facts/rainforest-facts.html
Picture 2: https://www.researchgate.net/figure/265053296_fig1_Figure-1-A-conceptual-framework-for-the-abiotic-and-biotic-mechanisms-of-nurse-plant
