Monitoring invasive plant populations for management efficacy: Problems and solutions

Background/Question/Methods

Monitoring vegetation to identify trends at different time and space scales to determine management strategies or outcomes can be challenging. Many agencies are requiring monitoring plans for a wide set of objectives, but implementation of their vegetation monitoring is often not accomplishing their goals. Monitoring to assess efficacy is an essential component of a non-indigenous plant species (NIS) management program, yet it often is not performed or is conducted with little attention to the quality of the data.  To evaluate treatment efficacy, we monitored plots at sites in Montana and North Carolina where Chinese lespedeza (Lespedeza cuneata (Dum. Cours.) G. Don), giant reed (Arundo donax L.), leafy spurge (Euphorbia esula L.), mimosa (Albizia julibrissin Durazz.), spotted knapweed (Centaurea stoebe L.), tree of heaven (Ailanthus altissima (Mill.) Swingle), weeping lovegrass (Eragrostis curvula (Schrad.) Nees), and yellow toadflax (Linaria vulgaris Mill.) had been previously treated with herbicide. 

Results/Conclusions

At the Montana site, mean NIS cover was reduced from 25% to 2.1%, but this was not related to treatment as cover was similarly reduced in untreated plots.  At the North Carolina site, mean NIS cover was reduced from 4.2% to 1.7%, but there were no control plots for comparison.  It is unclear whether these reductions in NIS cover are true or represent sampling error.  First, the original Montana NIS cover data were collected in categories rather than as continuous data, complicating change in cover calculations.  Second, monitoring plots had not been permanently marked at either site, making re-location of the exact monitoring points questionable.  At the North Carolina site, global positioning system (GPS) horizontal error led to as much as 36% error in plot overlap between the original and monitoring visit.  These difficulties in collecting and evaluating monitoring data highlight the need for improved methods of pre-and post-treatment monitoring.  We suggest (1) explicitly defining management goals and objectives prior to treatment, (2) determining what data need to be collected and how they will be analyzed, and how many plots to be monitored (power tests), (3) establishing permanently marked monitoring plots across a range of environments within the overall management site and (4) providing training and calibration guidance to monitoring crews to ensure consistency in measurement of continuous cover and/or density estimates.