Exclosures

History of deer colonization - Response of vegetation to deer - Response of animals to deer and squirrel - Deer biology

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Effect of browsing history on the vegetation - Role of chemical defences on deer browsing - Effects of reduced plant abundance on pollination - Effect of hunting on tree regeneration - Effect of prolonged deer population reduction on the vegetation

Effect of prolonged deer population reduction on the vegetation

Exclosures experiments - Deer cull experiments

Exclosures experiments

Aim & scope - Where & when - How - Main results - Conclusion - Scientific Publications

Aim and scope

20 exclosures were built on Graham Island in 1997 in old growth forest stands with open understorey (little shrub cover, forest floor either litter or moss covered) and where mature cedar trees were present.

Our main objectives were to assess

the survival of Western redcedar seedlings in a deer-free environment
the changes in vegetation structure and diversity over time in the absence of deer.

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Where and when

Ten sites were selected in different parts of Graham Island.

Two 5x5 meters exclosures were built on each site.

Monitoring started in 1997 and was repeated in 1998, 1999, 2000, 2001, and 2005.

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How

Western redcedar (Thuja plicata) seedlings survival

For each exclosure:

Eight 50x50 cm plots with dense cedar germination were selected, half of them inside the enclosure, half of them outside.
Seedlings were inventoried and marked in the plots at the onset of the experiment and their growth and survival were monitored periodically.

Were recorded:

total number of stems and number of browsed stems
total number of leaves and number of browsed leaves
tree height
largest leave size

Vegetation diversity

For each exclosure:

Two vegetation plots were monitored, one covered the entire area of the exclosure and the other one covered a similar area outside of the enclosure. Species present were inventoried.

Were also recorded:

the percent cover of each species (percentage of plot area that would be covered by the foliage of the considered species within the considered strata if it were virtually projected on a plane) within 8 different layers (0-0,05m; 0,05-0,15m; 0,15-0,25m; 0,25-0,5m; 0,5-1m; 1-1,5m; 1,5-2m; 2-4m).
the percentage of browsed vegetation for each species
the groundcover composition in wood, rock, root, lichen, moss, grass, litter, bare soil and water

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Main Results

A visual assessment of the vegetation inside and outside the exclosures gives us an idea about the vegetation recovery potential when released from deer browsing pressure.

Germination seems to be more dense in the inside plot (on the left) than in the outside plot (on the right) of the same exclosure after a few years of deer exclusion.

Changes in vegetation in Mayer Lake's exclosure: on the left, view of the inside in 1997 (top) and 2005 (bottom); on the right, view of the outside in 1997 (top) and in 2005 (bottom). Vegetation recovers inside the exclosure, while it remains short and browsed outside.

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Conclusion

Western redcedar seedlings survive dramatically better in the exclosures but growth is slow meaning that young trees will be totally exposed to browsing for at least one decade and possibly much more.
Outside of the exclosures most seedlings disappear and the rare ones that survive are heavily browsed and show only very limited growth.

Scientific Publications

Noemie Stroh, Christophe Baltzinger, Jean-Louis Martin, 2008, Deer prevent western redcedar (Thuya plicata) regeneration in old-growth forests of Haida Gwaii: Is there a potential for recovery?, Forest Ecology and Management 255 (2008) 3973–3979 - PDF

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