During lunch hour last Wednesday at the Urban Flower Field, passers-by and lunch-goers peered curiously from their blue chairs as Hunter, Kristen and Liz gave the site a small make-over. We carefully trimmed away Canola from tall stalks of sunflowers and delicate perennials. The main goal of this undertaking was to assess the biomass of one of our 8 wildflower species, but there is so much more to the story!
Our project is aimed at evaluating the value of biodiversity (the variety of life within a particular area) in an urban phytoremediation research experiment. Understanding the ecological impacts of biodiversity is notoriously difficult to quantify and has been a major research objective for decades. Biodiversity is important for humans and the environment because it can boost ecosystem productivity and provide natural services such as water purification and soil stabilization. Global biodiversity, however, is facing severe threats. Human activity, such as habitat destruction, has caused a 1,000-fold increase in species extinction rate.
We are testing to see if biodiversity can increase net primary production, or the amount of plant matter produced in a given area. All plants compete for resources such as soil nutrients, water and sunlight. For example, sunflowers invest a lot of their resources in producing tall stalks with big leaves and big flowers while canola plants focus on producing seeds in small pods at shorter heights. The sunflower and the canola fill different niches. The interspecies competition between canola and sunflowers is indirect because they have different growth strategies. Two sunflower plants growing at close proximity, on the other hand, will employ the same strategies to acquire the same resources and will therefore experience intraspecific competition. This reasoning led our hypothesis that planting different species in mixtures may generate greater amounts of plant matter and subsequently hyper-accumulate more contaminants to effectively remediate soil.
At the beginning of our experiment each of our 96 plots received 16 grams of seed. Some plots received 16 grams of one species and others received 8 grams from 2 species, 4 grams from 4 species, or 2 grams of each of the 8 species in the experiment. Seed yield, or the amount of biomass produced in relation to the amount of seeds we planted, is one of the main ways to quantify biomass.
The figure above depicts the biomass to seed planting density ratio. A high biomass to seed planting ratio occurs if a large amount of plant biomass is generated from a small amount of seeds, while a low ratio indicates that the seeds were not as productive. While there is a general increase in biomass to seed density ratio as more species are planted, the 8 species plots produced a significantly larger amount of plant biomass per seed than the other mixtures (as indicated by the “A” above the 8 species box).
This data set makes us excited about collecting the rest of our samples. This type of biodiversity data coupled with our soil sampling data set may provide additional evidence to support our hypothesis. This type of data is meaningful in quantifying the value of biodiversity in both urban ecosystems and ecology in general.