Plant Community Diversity After 

 Herbicide Control of Spotted 

 Knapweed 



Peter M. Rice 

 Donald J. Bedunah 

 Clinton E. Carlson 



E^RODUCTION 



Spotted knapweed (Centaurea maculosa Lam.) has 

 aggressively invaded extensive areas of rangelands 

 and forest sites with open overstories at low to mid 

 elevations in the Northern Rockies (Losensky 1987). 

 The weed had infested more than 7 million acres 

 in Montana and adjoining States and Provinces by 

 1988 (Lacey 1989). As knapweed increases, cover 

 of more desirable but less competitive grasses and 

 forbs is significantly reduced, sometimes as much 

 as 60 to 90 percent (Baker and others 1979; Bucher 

 1984; Harris and Cranston 1979). Spotted knap- 

 weed causes reduced vigor of native plant popula- 

 tions, less plant diversity on infested sites, and 

 economic losses because of reduced hvestock produc- 

 tion. It also possibly impacts wildlife populations. 

 Spotted knapweed can expand slowly into natural 

 grassland sites that are undisturbed by human or 

 hvestock activities, cause declines in plant commu- 

 nity diversity (Forcella and Harvey 1983; Tyser and 

 Key 1988), and invade forested sites disturbed by 

 timber harvesting, thinning, and livestock grazing. 



Spotted knapweed is susceptible to low rates of 

 certain herbicides. Picloram, clopyralid, and 2,4-D 

 have high efficacy when apphed properly. But biolo- 

 gists and land managers are concerned about the ef- 

 fect of herbicides on nontarget plant species. Will 

 native forbs be eliminated and grass "monocultures" 

 be created by herbicide treatments? What is the re- 

 sponse of natural bunchgrass and serai forest com- 

 munities following herbicide control of spotted knap- 

 weed infestations? Our study addresses these 

 practical questions. Prehminary results suggest 

 that herbicides are a feasible vegetation manage- 

 ment option for land managers concerned about 

 noxious weed invasions and biological diversity. 



METHODS 



In Missoula County in west-central Montana, re- 

 searchers conducted a pilot study to estimate species 

 diversity changes at two sites where herbicides had 

 been apphed in 1985 to control spotted knapweed. 



In 1988 the pilot diversity data were collected — 3 

 years postspray. Those data were then used to plan 

 an indepth formal community response study at four 

 new sites in the same county. 



Pilot Diversity Study 



The 1988 measiu-ements fi'om the two pUot study 

 sites showed moderate knapweed cover values of 27 

 and 30 percent. The Fort Missoula site — a bluebunch 

 wheatgrass {Agropyron spicatum) series habitat 

 type — had low overall plant diversity in addition to 

 knapweed dominance because of a long history of se- 

 vere disturbance by livestock and human activities. 

 The Lolo site was dominated by spotted knapweed, 

 but the plant community was much more diverse 

 and representative of a rough fescue (Festuca scab- 

 rella Torr.) potential natural commiinity. Herbi- 

 cides had been apphed to repUcated test plots in the 

 spring of 1985. Picloram (Tordon 22K) and clopyra- 

 hd (Stinger or Transhne) alone at a rate of 0.25 lb 

 acid equivalent (a.e.) per acre were used at both 

 sites, and a mixture of Tordon + Stinger at 0.125 lb 

 a.e. per acre each was applied at the Lolo site. Her- 

 bicide treatments and check plots were replicated 

 three times at the Lolo site and twice at Fort Missoiala. 

 Livestock grazing was excluded, but not wildlife ac- 

 cess. Commvinity response was measured in the 

 1988 cool season (May and June) using a cover micro- 

 plot method (Hann and Jensen 1987). 



Formal Community Response Study 



For the formal community diversity study we 

 chose four sites with low to moderate spotted knap- 

 weed infestations, but also still with diverse plant 

 communities. The average spotted knapweed abso- 

 lute cover for the 1988 warm season was: site 1 (21 

 percent), site 2 (7 percent), site 3 (10 percent), and 

 site 4 (3 percent). Disturbed sites with 60 to 80 per- 

 cent spotted knapweed cover are not uncommon in 

 western Montana. Below-average soil moisture and 

 precipitation also limited plant growth dming the 



1 



