A simple model suggests how to more efficiently prevent the spread of Asiatic shrub honeysuckles in temperate forests

Background/Question/Methods

Several taxa of Asiatic bush honeysuckles (Lonicera maackii, L. tatarica, L. morrowii, and L. x bella) colonize forests throughout the eastern United States.  Dense honeysuckle thickets suppress understory herbs, prevent tree replacement, degrade wildlife habitat, and support other undesirable species.  Managers are often confronted with populations that are too large to eliminate given the resources available.  Operationally, the problem of honeysuckle control has two aspects:  (1) reduction of the ecological effects on native systems, and (2) stemming the tide of recruitment in the landscape.  Because these species are bird dispersed, clearing the forest understory temporarily reduces the ecological effects, but does not stop recruitment.  Ideally, steps to reduce recruitment should precede wholesale removal.  An extensive collection of demographic data revealed that honeysuckle (L. maackii) in southwestern IL forest patches had markedly different reproductive potential depending on light availability.  Individuals in forest edges produced more flowers per node, more fruits per flower, more seeds per fruit, and bigger seeds than interior individuals.  We combined this information with the size distributions of forest interior and edge populations to compare the proportionate contributions of edge and interior shrubs in square forest fragments ranging 0.5-16.0 ha.

Results/Conclusions

Using a conservative estimate of edge depth (2 m on north and east sides; 4 m on south and west sides) the great majority of seed has its origin in the edge of small fragments (70-100% in 0.5 ha stands).  Even within large fragments, area in the edge contributed seeds in disproportion to the interior area (40-80% in 16.0 ha fragments).  This finding pivoted on two issues.  First, we assumed that shrubs < 0.5 m tall were not reproductive, so interior populations with many small individuals did not contribute to seed rain.  Secondly, we estimated potential maximum fruit production as a function of shrub surface area, which increases as the square of diameter (as does shrub volume), emphasizing large shrubs.  Interior populations with larger shrubs contributed more prominently than those dominated by smaller shrubs.  Different fecundity vis-a-vis sun and shade merely amplify these patterns.  Our recommendation for management is to first destroy edge populations and replace them with native shrubs or competitive vegetation, secondly to attack interior shrubs.  Intensive cutting of understory shrubs will temporarily reduce competitive effects and seed output, and may kill individuals in deep shade.