The Root of the Slope Stabilization Problem

The practice of stabilizing the soil of slopes with vegetation has occurred for a significant period of time.  The soil on slopes lacking vegetation is often shifting and otherwise moving slowly downwards by gravity, rainfall, wind, and various other factors.  Erosion that occurs may lead to further instability and massive movement of soil towards the bottom of the slope.  While the effects of this are more or less inconsequential in remote locations of forested areas, when near human populations it can cause severe damage to property, roads, and lives.  However, with the addition of vegetation, the unstable slopes become less likely to shift due to heavy rainfall or other factors of erosion.  This is because the plants’ roots take up water from the soil, preventing the accumulation of excess water, and anchor the soil so that it acts as a unit rather than individual pieces.  Although we know this basic information, one question still remains: which root characteristics provide the most stability for the soil on these unstable slopes?

Mohammed Saifuddin and Normaniza Osman set out to find the answer to this question.  In their paper, “Evaluation of hydro-mechanical properties and root architecture of plants for soul reinforcement,” they compared the roots of three different legumes.  They focused on two properties of the roots: the hydro-mechanical characteristics (water uptake) and the root architecture (underlying structure).  They determined that the species Leucaena leucocephala (of the three species studied) was the most effective for use in soil reinforcement.  This species has a long taproot, the main vertical section of the root, and lateral roots, growing from the taproot, that extend horizontally while staying close to the surface soil.  These roots have a high tensile strength (the amount of stress that can be withstood before breaking) which also provides strength to the soil, reducing the chances of movement.  The increased number of fine lateral roots and elongated root improves the uptake of water, reducing the amount left in the soil.  With this information, they concluded that L. leucocephala can be planted on unstable slopes to reduce the amount of erosion.

This picture taken from the paper (Saifuddin et al. 2014) indicates the root structures of the three species that they observed.  The first species pictured in this table has the properties that they deemed to be the best at soil reinforcement.

Link to paper: http://eds.b.ebscohost.com/ehost/detail/detail?sid=1280345c-ee2e-4fe1-b391-ab45b0013f19%40sessionmgr115&vid=0&hid=113&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=aph&AN=98388258

Even though the one species that they found to have the most desirable root system may not be the best species to grow on all unstable slopes, this study provides a set of root characteristics to look for when selecting plants for soil reinforcement.  By planting species with a long taproot system that are also native to the area in question, the number of slope failures can be decreased by a good margin.  By extension, this also means that the area can be preserved in its current state and that there would be fewer instances of destruction in human inhabited areas.

In light of this article, it may be interesting to see how trees are able to provide structure to slopes that have collapsed in the past in order to prevent further collapses.  It would also be interesting to see just how many of these plants need to be placed in an area to reduce soil movement and if there is a threshold at which there would be no more benefit in increasing the number of plants.

References

 Saifuddin, M. and O. Normaniza. 2014. Evaluation of hydro-mechanical properties and root architecture of plants for soil reinforcement. Current Science. 5:845-852

How to Keep Your P out of the Water

The phosphorus cycle may not be the first nutrient cycle we learn about when the content is first introduced to us in middle-school science class, but it is becoming more and more relevant in our lives. Phosphorus moves through the air, soil, and water; it is eroded out of rocks and released into the atmosphere by factories. Perhaps one of the biggest issues caused by phosphorus is runoff from fertilizer used on farms, which is not healthy if it gets in our water supply. However, this problem in the phosphorus cycle has a potential solution: another stage of the phosphorus cycle! Plants absorb phosphorus form the soil and use it to grow, so why can’t we use this as a natural way to get rid of phosphorus we don’t want just rolling around out there. Further, what kinds of plants can help with this and what kind of plants don’t really need that much phosphorus at all?

Depiction of the Phosphorus cycle. Could C3 plants be more effective at taking
up Phosphorus? Retrieved from http://swroc.cfans.umn.edu/

                I looked at an article by Halsted and Lynch that focused on how plants respond to different levels phosphorus in the soil; in particular, comparing the plants of the C3 and C4 photosystems. The authors grew a number of different species - including C3 and C4 plants as well as monocots and dicots – and grew them in a sand/alumina medium at varying phosphorus treatment levels from stable to phosphorus stress (low levels). They measured each plant’s height, stem width, and carbon dioxide exchange rate (CER), as well as number of branches (for dicots only) and height of highest tiller (for monocots).

                As we would expect, all species grew less under phosphorus stress. However, it turns out that C4 plants’ CER was less effected in situations with low phosphorus, meaning that they are not as reliant on phosphorus availability in soil as C3. This could mean that C3 plants have to take up more phosphorus out of the soil to survive.

                So how can we use this research for our own good? Well, conservation societies have already started working on a solution known as “buffer strips”. These patches of land filled with native plant species are positioned along rivers between main farming sites to absorb the phosphorus before it gets into the water supply. Thanks to the Halsted and Lynch paper, we now know that C3 plants are more phosphorus dependent. This could spark further research to see if C3 plants can actually make buffer strips more effective by absorbing more phosphorus. Research like this can help protect our water supply from dangerous algal blooms; thus, keeping water potable and protecting native fish populations. It can give us good, clean water. And it is a solution we need because we need agriculture, but we need water more.

This is what will happen if you P in the Pool! Algal blooms kill fish and make the water unsafe and they occur if you let too much phosphorus get in the water. Also, it looks and smells gross. Retrieved from http://www.circleofblue.org/

References:

Halsted, M. and J. Lynch. 1995. Phosphorus responses of C3 and C4 species. Journal of Experimental Botany 47: 497-505.

No Pain, No Gain…In Species Diversity

Close your eyes and imagine it is 200 hundred years ago. What do you think about? Perhaps a horse and buggy heading down Main Street? Maybe even sepia tone, old timey photos? Something that may or may not have come to mind is the change in the Midwestern landscape. In the early 1800s, Iowa was made up of mostly tallgrass prairie with an abundance of animals and diverse vegetation. Today, the countryside no longer resembles the scenic prairie, as the vast majority of the land has been converted into cropland. Before you begin to cry, don’t worry! All is not lost. Ecologists have been working to restore the tallgrass prairie at different sites in Iowa. Interestingly, while the restorations have been successful, it has been difficult to increase the diversity of vegetative species to the level of diversity achieved by the remnant native prairie. I’m particularly curious about how a disturbance, such as soil compaction, affects the diversity of species. Given the lack of research that has been done, I secretly (and now not so secretly) wonder if this could be the key that unlocks greater diversity in restored prairie.

A visual comparison of Iowa land cover in (A) the mid-1800s and (B) as of 2001.
Photo credit: http://armi.usgs.gov/story/story.php?contentid=1630

 

I acted on my detective impulses and found a paper that could shed light on the effect of soil compaction on the tallgrass prairie via bison wallows. The paper is entitled, “Potential Impacts of Bison Wallows on a Restored Tallgrass Prairie Community” by Miller et al., 2014. The researchers sought to investigate the effects of bison wallows on “weedy” versus “non-weedy” vegetation. Weedy plants have been defined in previous literature as more tolerant to disturbance, whereas non-weedy plants have been defined as less tolerant to disturbance. Bison wallows are a natural disturbance that leave large “bald patches”, as a result of the large animal rolling around on the ground and compact the soil in the process. The research was conducted in Jasper County Iowa at the Neal Smith Wildlife Refuge where there are a heard of Bison present. They randomly sampled 7 wallows and 7 non-wallows along a transect, while controlling for elevation, proximity to each other, and proximity to nearby perennials. Of particular interest to me is their findings related to the vegetation inside versus outside the wallows. Ultimately, the authors found that the results support the presence of a disturbance gradient. The gradient favored weedy species near disturbance and vice versa. Given that the adjacent prairie tended to have lower weedy plant cover, it is possible that the soil compaction of the bison wallow enables weedy-er species to flourish. On the flip side, the adjacent prairie enabled the success of the non-weedy species. This suggests that different degrees of soil compaction are necessary to have both weedy and non-weedy species thrive. 

A bison in the process of leaving a wallow.
Photo credit: Mary Jane Gibson

This raises certain questions about prairie restoration. Specifically, how feasible is larger scale soil compaction and would it help increase diversity enough to justify the cost? How important is increasing prairie diversity?! Should we even try restore the prairie to how it was 200 years ago?!! The larger questions are current conundrums to not only me, but also all restoration ecologists today. Restoration ecology is challenging in that even if the goal is to restore the land back to the way it was hundreds of years ago or more, it simply may not be possible. What factors contribute to the possible impossibility? Of course, humans are culpable for disrupting and modifying certain ecosystems, but this is only one piece of the puzzle. In fact, there are many factors that can contribute to changing landscapes, such as cyclical climate change and natural extinction of various species. In attempting to convert cropland back into tallgrass prairie, the impossibility of increasing species diversity exists, even if we know all there is to know about restored prairie. Similarly, we have ethical dillemas to consider as well, in thinking about how much humans should alter current ecosystems to try to achieve restoration goals. There may come a time when we need accept failure of restoration to historical standards and perhaps modify ecological restoration goals.    

 

Reference

Miller, Kimran, et al. "Potential Impacts of Bison Wallows on a Restored Tallgrass Prairie Community." Proceedings of the North American Prairie Conference. Vol. 23. 2014.

Expanding Killer Whale Distribution in the Arctic with Melting Ice

          As the world’s oceans warm from global climate change, marine organisms have to adjust to altered habitat conditions and subsequent changes in population dynamics. As temperate waters warm, certain species are able to move further towards the poles where suitable conditions now exist. With the invasion of species into new waters, previously established marine populations are in threat of being outcompeted for resources or becoming prey to unfamiliar predators. These consequences of climate change are apparent in Artic regions, where rising temperatures are decreasing ice coverage, allowing normally ice-avoiding species to disperse further North than in previous decades. As high latitudes are expected to show the most pronounced effects of climate change, scientists are beginning to study how organisms and ecosystems are changing in these increasingly ice-free areas.

One species benefiting from decreased ice coverage in Artic waters is Orcinus orca, or the killer whale. Though found almost globally, killer whale populations differ from one another with their specific environmental adaptations, dietary habits, and geographic range, resulting in what are called different ‘ecotypes.’ Residential ecotypes exclusively prey on fish, while the transient ecotypes, or ‘visiting’ populations, will only prey on marine mammals such as seal, sea birds, and other whales. Though these transient ecotypes hunt along ice edges, sometimes even flipping small ice floats to overturn resting seal, they tend to avoid the larger, heavier ice surfaces.

Two killer whales about to flip over an ice float a seal is resting on. (http://e360.yale.edu/images/features/antarctica_killer_whales_seal_e360.jpg)

Because of this ‘ice-avoidance’ behavior, killer whale distribution is usually restricted geographically and seasonally, dependent upon the surface ice cycle; though sea ice is prevalent year-round in some areas, surface ice generally breaks apart or melts in August, and reforms by November. However, with increasing temperatures, this melt-reform cycle no longer occurs in some areas, decreasing habitat availability to ice-adapted marine mammals and potentially increasing accessibility to species such as the killer whale. As killer whales are an apex predator, or a predator at the very top of the food chain, their expanded distribution in the Arctic may have a significant impact on pre-existing population dynamics. It is therefore important to see whether sea ice reduction really is redistributing the transient ecotypes, a question Jeff Higdon and Steven Ferguson addressed in their paper ‘Loss of Arctic sea ice causing punctuated change in sightings of killer whales (Orcinus orca) over the past century’ (2009.)

In their study, the authors compiled historic data on July sea ice concentration in the Canadian Arctic from 1900 to 2006 and reported killer whale sightings in the Hudson Bay region for the same time period. They found that, since 1900, the number of killer whale sightings per decade increased exponentially while median ice concentration over time showed a significantly declining trend. In sum, they found that ice concentration in the Hudson Strait was negatively correlated with the number of killer whale sightings reported per decade, as well as with the range of sightings. This means that with receding ice coverage in the Hudson Strait, killer whales are no longer spatially restricted, seen more frequently in areas further west than ever before.

Median ice concentration from 1902 to 2004 taken from three areas of the Hudson Strait (Western, Central, and Eastern.) All three areas show a significant decline in ice concentration.

Due to its apex status, the killer whale’s increased range poses as a top-down ecosystem force. In other words, its recent expansion in places like the Hudson Strait will result in a significant increase of predation on whales and seals, which will decrease their predation on other species. Since sea ice is predicted to decrease even more within the next few decades, the killer whale’s distribution will most likely continue to expand, impeding upon the habitats of other apex predators such as the polar bear, who rely on high ice concentrations. Higdon and Ferguson conclude by predicting that with this expanded distribution, killer whales will most likely replace polar bears as the dominant predator of the Arctic, changing the marine mammal community in this environment.

With global climate change, the distribution and abundance of organisms will change drastically. This study is just one example of how a predator’s range expansion has the potential to drastically change an entire ecosystem’s community dynamics, and similar shifts should be expected in the future. It has been projected that within the next 50 years, surface ice coverage in the Arctic may eventually be non-existent, making it even more pertinent to study the potential effects this will have upon species’ presence and abundance. Though it now seems inevitable that species such as the killer whale will continue moving North, learning of the consequences now may better prepare us for future management of fisheries that will be impacted as well as form potential conservation plans for species in these rapidly changing environments.

Reference:

Higdon, J.W, Ferguson, S.H. Loss of Arctic sea ice causing punctuated change in sightings of killer whales (Orcinus orca) over the past century. 2009. Ecological Applications

19(5): 1365-1375