Restoration after invasion by Rhamnus cathartica has become an important issue due to its potential to modify soil moisture and available nitrogen levels (Heneghan et al. 2005). One study suggests that mulch application to invaded soil, after removal of Rhamnus cathartica, reduces re-invasion (Heneghan, unpublished). Our experiment observes soil amendment impacts on R. cathartica growth in the field and the greenhouse and seed germination compared to native Rhamnus alnifolia in soil amendments.
At Whippoorwill Farm, Mettawa, IL, cathartica saplings and seedlings were counted in three restoration plots with R. cathartica mulch, commercial mulch, and no soil amendment.
288 cathartica saplings from Mary Mix McDonald Woods, Glencoe, IL, were weighed and planted in three soil types. Saplings were watered daily at the Chicago Botanic Garden, greenhouse. Sapling height and leaf number were taken at intervals.
Soil samples were analyzed for gravimetric moisture, NO3 and NH4 before and after planting using a 2M KCl extraction and colorimetric method with a HACH DR 5000 Spectrophotometer and reagents (Robertson et al. 1999).
128 seeds each of the non-native R. cathartica from Lake County, IL and native R. alnifolia from Chippewa National Forest, MN were planted in three soil treatments. Seedlings were counted.
Field experiment: Reinvasion reduced in mulch treatments. The type of mulch had no significant impact.
Greenhouse Experiments: Leaf number, biomass and proportion of belowground biomass were greater in no mulch treatments than in mulch treatments. At harvest, no mulch saplings had grown significantly taller than those in mulch treatments. Soil Nitrate (NO3) increased in all treatments while Ammonium (NH4) decreased in mulch treatments. This mechanism will be subject to future investigation.
R. cathartica seeds germinated in all soil treatments but no alnifolia seeds have germinated. Germination was highest in no mulch soils and lowest in buckthorn mulch soils. This rapid germination of cathartica may contribute to its success. The lack of germination of R. alnifolia suggests the possible need of additional treatment to break dormancy.
Our results indicate that the addition of mulch reduces overall growth of Rhamnus cathartica saplings and seedlings in the field. Mulch addition reduces cathartica seed germination and sapling growth in the greenhouse. This growth reduction is often, not always, is greater when cathartica mulch is used. Mulch addition may result in the reduction of cathartica’s growth and survival. We suggest further investigation of high C soil amendments to reduce reinvasion of Rhamnus cathartica in restoration.