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Individual study: Population recovery of wild leek Allium tricoccum following differential harvesting in the southern Appalachians of North Carolina and Tennessee, USA

Published source details

Rock J.H., Beckage B. & Gross L.J. (2004) Population recovery following differential harvesting of Allium tricoccum Ait. in the southern Appalachians. Biological Conservation, 116, 227-234

Background

In the USA, over 200 native plant species are at risk of extinction due to collection for consumptive purposes, about half of which are herbaceous forest perennials. One such plant is the wild leek Allium tricoccum, found in deciduous forests in eastern North America, which is harvested for consumption and is declining as a result. As with other perennial forest herbs, appropriate conservation management requires information on the long-term effects of varied harvesting levels on population recovery rates. In this case, the effects of differing levels of experimental harvesting on wild leek population recovery rates are described.

Action

Study sites: The study was conducted at Great Smoky Mountains National Park in the southern Appalachian mountains bordering North Carolina and Tennesse, USA. Three study sites were selected which were located in the central portion of the National Park at elevations between 960-1400 m. These were:

1) Porters Creek - located on a rocky north-facing slope (60%) adjacent to a wet-weather drainage within an old-growth rich cover forest.

2) Elkmont - located on a gently sloping (19%) seepage area in rich secondary forest.

3) Mingus Lead - located on a rocky flat, flanked by two steep (26% gradient), rocky slopes within old-growth northern hardwood.

All sites were easy to access but not visited regularly by harvesters and had an abundance of wild leeks, with no fewer than 20 plants per square metre.

Experimental removal: Fifteen 1 m² plots were established at each site (three replicates of five treatments). Four of the treatments represented different harvest levels, with either 25, 50, 75 or 100% of wild leek plants removed, with the remaining plots used as controls. Plants were harvested in their entirety (leaves, bulb and rhizome). Control plots were not manipulated. The 1 m² plots were set up in a non-linear configuration and arranged sequentially for ease of relocation. Plant removal occurred over a four year period (1989-1993)

Monitoring of plant growth: In April 1989 (before harvesting began), the maximum leaf width of the largest leaf of each plant at full leaf expansion in all 45 plots, was measured. Between 1990-1993, leaf widths of plants remaining in each of the 45 plots and population recruits were measured without further harvest. Leaf width measurements were used to estimate specific growth rates and population recovery times for each site and treatment, as leaf widths are good indicators of bulb and plant size.

Consequences

Harvesting treatments reduced average leaf widths of wild leeks over the period of 1990–1993, with the magnitude of the effect related to the degree of the initial population reduction (see Table 1, attached). Leaf widths measured four years after harvesting as little as 25% of wild leeks, were substantially lower than in the control plots, suggesting that the populations did not recover within the 4-year study period. Using the results of this study theoretical recovery times were estimated for differing harvest intensities - a population from which 25% of plants are harvested was estimated to take approximately 22 years to recover if plants were only harvested once in a 5-year period and plants of all sizes are removed (see Table 1, attached). In reality, harvesting of wild leek populations is not restricted to a one-time harvest and harvesters are more likely to collect larger plants, both of which will increase population recovery time.

Conclusion: Harvesting wild leek is not sustainable except at very modest levels. Using the results of this study to predict recovery times, by assuming that growth rates and concomitant recovery times are affected in a consistent manner by levels of harvesting, the sustainable harvest level is predicted to be 10% or less, once every 10 years.


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