Interactions between deer and vegetation in Southern Ontario: Monitoring and restoration of overgrazed plant communities in Rondeau and Pinery Provincial Parks

Executive Summary: 1. In 1994 the Science Technology Transfer Unit of the Ontario Ministry of Natural Resources initiated a 5-year project in collaboration with the Department of Biology, York University, Toronto, to monitor and assess southern Ontario forests affected by deer herbivory. 2. In 1995, we obtained the Cornell Ecology Detrended Correspondence Analysis (DECORANA) and Canonical Correspondence Analysis (CANOCO) computer programmes. These allowed multivariate analysis of changes in the herbaceous plant communities at Rondeau and Pinery Provincial Parks, as well as the relationship between these two parks and 8 other study locations. We have analysed Rondeau data for 1995 and compared 1992 to 1995. This period covered a 1993 herd reduction. Plant community composition changed significantly in grazed areas from 1992 to 1995, but did not change significantly over the same period in deer exclosures built in 1991. We concluded that we either missed a very rapid change that occurred in exclosures from 1991 to 1992 (we have the data to check this) or the overall composition of intensely grazed plant communities in Rondeau is very slow to change to pre-overgrazed conditions. In 1995, the plant community composition was significantly different between the Bennett and Gardiner exclosures, built in 1978, and the grazed areas and also the 1991 exclosures. We suggest that these differences may be due in part to the different shade conditions in these habitats. We also suggest that intense deer grazing has radically altered subcanopy light conditions, and has subsequently affected the recovery of herbaceous communities. We will be testing this hypothesis in 1997. If it is valid then herbaceous (understorey) plant communities may only recover when overstorey woody plant communities recover from the detrimental effects of deer grazing. 3. A range of species were found to be suitable indicators of grazing pressure and vegetation recovery at both Rondeau and Pinery Parks. Plants that survived deer grazing at Rondeau increased in height from 1994 to 1995. However, a 1996 study of Arisaema triphyllum by Dennis (1996) suggested that increasing deer numbers may again have started to have an adverse affect on this species. Plant heights need to be examined in 1997 in order to confirm these trends. 4. Exotic and invasive species are of concern in Rondeau and Pinery because of their ability to invade and dominate disturbed areas. Alliaria petiolata, Garlic Mustard, populations have expanded significantly in Rondeau from 1994 to 1996. The effectiveness of control programmes for Garlic Mustard, which were initiated in 1994, could not be evaluated due to the unusual population dynamics of Garlic Mustard in which plants go through alternate years of high and low flowering populations. 5. Berberis thunbergii, Japanese Barberry, is an exotic shrub common in Rondeau. If a removal policy for B. thunbergii is to be adopted, then the most effective method is cutting the bushes and then painting the stumps with herbicide. 6. The structure and composition of woody plant communities at Pinery and Rondeau Parks have been altered by intensive vertebrate grazing, resulting in lower shrub densities and the presence at both parks of unpalatable woody species. Intensively grazed sites such as Rondeau and Pinery had smaller trees (lower diameters at breast height - DBHs) than other forest locations. We suspect that there may be higher mortality rates of larger trees at Rondeau and in some Pinery habitats due to increased windthrow. This process could be initiated when intense deer grazing reduces the sub-canopy shrubs and saplings to the point where recruitment of saplings to the canopy does not occur. When recruitment declines, canopy gaps will increase in size, subsequently increasing the likelihood of windthrow of larger trees. In 1996, Catherine Sykes (M.Sc. student) revisited 1980 plots in Rondeau and found that in some of them over 50% of the large trees were gone and many of the smaller trees were also missing. These data, along with lower DBHs, suggest that regeneration rates at Rondeau are not high enough to fill existing canopy gaps. In addition, the relatively low shrub densities in Bennett and Gardiner exclosures indicate that recovery is proceeding slowly. Overall, these results are alarming because they suggest that the Rondeau forest will continue to decline as long as trees are no longer recruited to the overstorey. We strongly recommend that the loss rates of large trees and rates of creation of canopy gaps be monitored at Rondeau (this is included as part of our 1997 research programme), and that the impact of altered understorey light conditions on plant communities be assessed. This will indicate whether some form of drastic management intervention might be justified to replace and/or protect large trees. 7. Available woody browse (current annual growth - CAG) was sampled in Rondeau and Pinery in the Fall of 1994 and 1995. In both parks the availability of woody browse was extremely low in 1995 (< 1 g dwt CAG /m2), with the bulk of CAG from species traditionally considered to be less palatable to deer.