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

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

Aim and scope

The effect of deer on the environment not only depends on the number of deer present in a locality at a given point in time, but also on the length of time deer have been present in that area.

 To understand deer effect on the vegetation we therefore had to estimate:
  • past browsing pressure
  • the length of time deer have been present
To do so, we developed the tools needed to study the signatures left by current and past browsing on trees and shrubs and used them to estimate the time since deer colonised different islands.		 young deer feeding on salal
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Where and when

Between 1997 and 2003, Bruno Vila and co-workers collected "deer signatures" on the vegetation of the southern half of the archipelago, and more particularly on a set of small islands situated in Laskeek Bay. 
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How
We used dendrochronology, which infers the age of a tree, a shrub or of an event in the plant's life from the analysis of the variation in ring growth and shape on transversal cuts of trunks or stems.

We used the following signatures.

Effects on tree rings

Ring series contain historical and ecological information that is related to competition, release, fire, climate, insect attacks, or any other stress the tree may have encountered, like severe browsing.
dendrochronology

 

By comparing growth patterns in trees and shrub it is possible to determine how long deer have been present in a area.

  • When a stress, like excessive browsing affects a tree, growth is slowed and the rings are compressed and narrow. 
    • The stress of heavy deer browsing  also results in a  "bonsai" shape in young trees.
  • When the stress is removed, or when the apical bud of the tree becomes inaccessible to deer, growth is released, as witnessed by wider rings.
    •  A tree that escapes deer browsing resumes normal growth.
 bonsaied spruce

By using this historical record, it is possible to look back in time to estimate when deer began to affect different tree species on different parts of Haida Gwaii. 

shape modification

Fraying scars

Fraying scars are caused by deer when they rub their antlers on the trunk of a tree.
fraying scar
 Fraying scars are highly visible signatures that can be analysed on  transversal cuts of trunks and stems.

By counting the number of rings in the healing ridges on both sides of a scar, it is possible to date it, In our case, the analysis of all the scars collected in a locality allows to estimate the date deer reached that locality.		 fraying scar on transversal cut

Distribution of age-classes in shrub stems

effect in time of deer on shrubs

We studied long lived shrubs that have continuous stem replacement. In the absence of disturbance, stem replacement compensates the death of older stems.
But when deer browse the replacement sprouts, this process is stopped:
  • stems tall enough to escape deer will continue to grow and age beyond the usual age of replacement
  • numerous and heavily browsed regenerating sprouts will accumulate at the base of the shrub
  • old stems will progressively die, and the number of stems per individual shrub decrease resulting in the death of the plant.
Dendrochronology will allow to analyze the lenght of time since stem regeneration has stopped and will provide an estimate of the length of time the understory shrubs have been exposed to high browsing pressure.

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Main Results

Deer impact on tree regeneration

Analysis of radial growth allowed us to quantify the delay in tree regeneration that resulted from deer browsing.
Trees regeneration

Sitka spruce reaches the browse line above which the tree grows beyond the reach of deer in:
  • 5 years in the absence of deer
  • 13 years in the presence of deer (an 8 year delay)
For Western hemlock, reaching the browse line takes:
  • 5 years in the absence of deer
  • 19 years in the presence of deer (a 14 year delay)
Western redcedar does not regenerate in mature frests when deer are present.
The difference between species can be explained by the presence / absence of physical defences (sharp needles for Sitka Spruce).

Estimating time since deer colonization

By fraying scars
oldest scars The oldest scars allowed us to identify 2 classes of islands:
  • Islands colonised by deer for less than 20 years (at the time of the study), like South Skedans
  • Islands colonised by deer for more than 50 years (at the time of the study), like Reef.

Changes in the number of scars produced in different periods of time on these islands are indicatve of changes in deer populations.
fraying damage

By stem age structure
stem age structures When deer arrive on an island and reach a certain level of browsing pressure, then, stem regeneration is stopped.
Missing age classes correspond to periods of heavy browsing pressure.
On this example, we estimate that impact of browsing has been strong for a least a period of 30 years before the study.
 

Conclusion
The information at hand suggests that deer colonization of Haida Gwaii was rapid.

Introduced in 1878 in the north of the archipelago, deer had reached its southern tip before 1950.

Population build up seems to have been rapid all over the eastern side of the archipelago.  Most islands show signs of heavy browsing pressure as early as the middle of the 20th century.
deer spreading

In Laskeek Bay, a few small islands have not been colonized.
A group of small islands seems to have been colonised only during the last quarter of the 20th century. They offer an intermediate situation between islands without deer and islands with deer for over 50 years at the time of the study.
Laskeek Bay

This particular history of deer colonisation in Laskeek Bay provides a unique "natural laboratory" to study how time since deer colonization is related to the abundance and diversity of plants and animals.
Dendrochronology also showed that tree regeneration was delayed by over 10 years in presence of deer, that tree shape was modified and that recruitment of redcedar and understory shrubs was stopped under current borwsing pressure. browsed spruce

Scientific Publications
  • Bruno Vila, Fréderic Guibal, 2001, Assessing browsing influence in forest understory using dendrochronology on Haida Gwaii archipelago (British Columbia, Canada), Dendrochronologia 19 (1) - 2001: 139 – 151– PDF

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