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Hero to Zero: Scotch Broom

May 21, 2012

It was December 30th, 1941, and Albert Arnst was about to tell the Northwest Scientific Association about an environmental engineering success.  For years he had been searching for an answer to the shifting sand dunes that plagued coastal Oregon: the dunes had forced the year-round dredging of harbors and channels and the relocation of roads, rail lines, and whole towns.  Yes, human and domesticated animal activity had caused the dunes to become denuded and destabilized – but it was also going to be human activity that would stabilize them.

Photo Credit: McD22, Flickr Creative Commons

The state needed a cheap solution that would work fast so that resources could be focused elsewhere.  The high winds in winter could send even wet sand whipping across the dunes at more than 50 miles per hour, shredding nearly any kind of plant.  The white sand of the dunes was virtually devoid of organic matter or nutrients and shifted rapidly, alternately burying and uncovering large swathes of the habitat. Arnst had planted some native grasses as a beginning, but he needed something that would help transition the dunes to their semi-forested natural state.

He had tried many kinds of plants to help stabilize the dunes further.  Nothing seemed to work: the conditions were too severe.  Finally, he tried Scotch Broom, Cytisus scoparius.  He described the plant’s growth and resiliency as nothing short of remarkable – in one year they could grow an average of three feet in height, and their ability to bounce back from even the most severe winter storms was miraculous (Arnst 1941).  He planted C. scoparius on the most exposed sites and within two years the plant had firmly established itself across the area. Within five years the disparate bushes had formed dense and impenetrable hedges of ten to twelve feet, providing winter storm coverage to both game animals and valuable infrastructure.  Moreover, Scotch Broom was a legume, and could fix nitrogen in the soil for the next successional wave of plantings.  Scotch Broom was the hero of the hour.

Broom Blooms in the Dunes!
Photo Credit: Breibeest, Flickr Creative Commons

Today, Scotch Broom is considered an invasive plant in the Pacific Northwest and particularly Oregon and Washington.  Millions of dollars (Oregon Department of Agriculture 2012) are spent in efforts to control its growth and spread, and environmental groups and loggers alike generally revile it.  Why did our perceptions of the plant change?  What turned C. scoparius into a bad guy?

Scotch Broom is a leguminous shrub, meaning that it can take nitrogen from the atmosphere thanks to symbiotic bacteria in its roots and then use that nitrogen for its own metabolic needs. This is often a beneficial thing – many farmers take advantage of this trait by planting legumes in crop rotation to replenish nitrogen in the soil for future crops.  To Arnst, this meant that the nitrogen C. scoparius was fixing would therefore be readily available in the soil, just like with other legume crops.  However, Scotch Broom has some secondary effects on soil chemistry that Arnst did not know about. Soil near C. scoparius shows a lower pH, lower levels of inorganic phosphorous (another nutrient essential for plants), and lower rates of organic decomposition as compared to similar soils without the presence of C. scoparius (Caldwell 2006).  As well, Scotch Broom has been shown to release alkaloids that may inhibit the germination and protein synthesis of certain plants native to the Northwest (Haubensak et al. 2004).  These traits mean that while C. scoparius may be good at withstanding hostile environments, it is also very good at creating an environment that is hostile for other plants.

The changes C. scoparius makes to soil chemistry are significant yet subtle.  What isn’t subtle about the species is its phenomenal vegetative growth.  As Arnst observed, it is capable of growing 1-2 meters high within a year of germination, and within a few more years it can form dense, impenetrable phalanx-style stands (1941).  This exerts a terrific demand on local resources and helps Scotch Broom outcompete other plant species for water, nutrients, space, and sunlight.

From a forester’s perspective, the worst thing about this plant is the combination of its rapid vegetative growth combined with its potential reproductive ability.  By its second year it can produce several hundred seeds per year, and as they mature for nearly two decades they can produce thousands of viable seeds (Bossard and Rejmánek 1994).  Few of these seeds experience predation, and all of them can lie dormant for fifty to eighty years before germinating (Bossard and Rejmánek 1994).  Scotch Broom does not tolerate shade, and so in thick cover such as that provided by dense stands of Douglas Fir (Pseudotsuga menzisii), it will not germinate.  However, once that shade is removed there are few plants that can hope to muscle out C. scoparius in the scramble for sunlight. This means that after a logging operation harvests a forest, Scotch Broom can infest a replanted harvest site and quickly shade out acres of replanted trees, costing the forestry industry millions of dollars in lost revenue and time and increased eradication  and land-clearing expenditures (ODA 2012).

Photo Credit: Sam Beebe, Flickr Creative Commons

At this point, one might wonder that if there’s so much Scotch Broom in an area, wouldn’t it be the target of more predators like herbivores and granivores?  Actually, it isn’t, likely thanks to the elevated levels of tannins and alkaloids in its tissues, which are at best unpalatable and at worst poisonous to its potential predators (Hoshovsky 1986).  In fact, Scotch Broom faces so little predatory control that state agencies are looking to introduce some predators from its native range.  Similarly, Arnst sought to help his own predicament with an introduced species.  This additive approach to ecological management is a common yet debatable one, and it is a point that will be addressed in the next blog post, which will focus on the countermeasures taken against Scotch Broom.

References:

Arnst, A. 1942. Vegetal stabilization of Oregon coastal dune areas. Northwest Science. 16:59-67.

Bossard, C.C. and M. Rejmánek. 1994. Herbivory, growth, seed production, and resprouting of an exotic invasive shrub, Cytisus scoparius. Biological Conservation. 67:193-200.

Caldwell, B.A., 2006.  Effects of invasive scotch broom on soil properties in a pacific coastal prairie soil.  Applied soil ecology: a section of Agriculture, Ecosystems, & Environment. 32:149-152.

Haubensak, K.A., C.M. D’Antonio, and J. Alexander. 2004. Effects of nitrogen-fixing shrubs in Washington and coastal California. Weed Technology. 18:1475-1479.

Hoshovsky, M. 1986. Element Stewardship Abstract for Cytisus scoparius and Genista monspessulana. The Nature Conservancy. Arlington, VA, USA.

Oregon Department of Agriculture Plant Division. 2012. Noxious Weed Control: Scotch Broom (Cytisus scoparius).  <http://www.oregon.gov/ODA/PLANT/WEEDS/profile_scotchbroom.shtml.>

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