biodiversity across the forest cycle in ash and sitka spruce plantations: effects of irish forestry...

Biodiversity across the forest cycle in ash and Sitka spruce plantations: effects of Irish forestry on spider communities

Anne Oxbrough, Tom Gittings, Paul Giller, John O’Halloran

Dept Zoology, Ecology and Plant Science, University College Cork

Xysticus cristatus

©Ed. Nieuwenhuys. Copyright 1996-2002

BIOFOREST

• BIOFOREST aims: • Investigate the effect of afforestation on the biodiversity of

three major taxanomic groups in Ireland

» Plants

» Invertebrates

» Birds

Why use spiders as an indicator group?

Enoplognatha ovata©Ed Nieuwenhuyys. Copyright 1996 – 2002.

1. Abundant

2. Positively influenced by vegetation structure1. Prey2. Web attachment3. Hiding places for active hunters4. Protecton from predators5. Suitable microclimates

3. Adopt many ecological strategies

4. Found in all layers of vegetation

5. Occupy a strategic position food webs

6. Taxonomically well known

Spiders:

Aims of study:

• To investigate changes in spider communities over the forest cycle

• To investigate the difference in spider communities in Sitka spruce and ash

• To identify indicators of spider biodiversity

Study sites

• Sitka spruce and ash - Conifer and broadleaf

- Widely planted

• A variety of age classes

Pitfall traps

Pitfalls

• Plastic cups (9cm depth by 7cm diameter)

• Antifreeze and drainage holes

• Five pitfalls per plot

• Five plots per site

• Plots at least 50m apart

• 2-3 weeks in the ground

• Three changes during the season

Habitat measures

• Cover abundance of plant structure

• Cover of deadwood

• Soil samples– Organic content

– Moisture content

• Litter depth and cover

Results

Linyphia triangularis

©Ed Nieuwenhuyys. Copyright 1996 – 2002

Results:

• 18730 individuals collected in 139 species

• Species classified by habitat preference: • 15 forest habitat specialists• 19 open habitat specialists

Mean species richness of spiders across the forest cycle. Bar indicate SE

0

5

10

15

20

25

Prethicket Thicket Closed maturing

Re-opening

MaturePrethicket

Pole Closedmaturing

Semi-Mature

Mature

Sitka spruce Ash

Species

richness

Species richness

Species richness of habitat specialists. Bars indicate SEOpen specialists

Forest specialists0

1

2

3

4

5

6

0

1

2

3

4

5

6

Prethicket Thicket Closed maturing

Re-opening

Mature

Sitka spruce Ash

Prethicket Pole Closed maturing

Semi-mature

Mature

Species assemblages• Cluster analysis indicated 5 groups:

1) Prethicket mix

2) Prethicket ash

3) Mature ash

4) Closed canopy spruce

5) Thicket/Mature open-spruce

• Ordination revealed similar groups as cluster analysis

Species NMS of sites with environmental variables

NMS of sites using habitat variables

Mean species richness among cluster groups. Bars indicate SE

0

5

10

15

20

25

Prethicket mix

Prethicket ash

Mature ash

Closed Canopy spruce

Thicket/Mature open-spruce

Species richness

Mean species richness of habitat specialists among cluster groups. Bars indicate SE

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Open specialists

Forest specialists

Speciesrichness

Prethicket mix

Prethicket ash

Mature ash

Closedcanopy spruce

Thicket/Mature open-spruce

Trends in vegetation structure among cluster groups. Bars indicate SE

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

Percentagecover

Ground vegetation Lower field layer

Upper field layer Canopy

Pre-thicketmix

Pre-thicket ash

Mature ash

Closedcanopyspruce

Thicket/Matureopen-spruce

Pre-thicketmix

Pre-thicketash

Mature ash

Closed canopy spruce

Thicket/Matureopen-spruce

Correlations of total S and environmental variables

Cluster group Environmental variable

Pearson correlation

p (2-tailed)

Prethicket mix (n=20)

Organic content 0.57 0.009

Lower field layer 0.45 0.05

Prethicket ash (n=34)

Soil cover -0.33 0.06

Lower field layer 0.40 0.02

Dead wood cover -0.50 0.005

Closed canopy spruce (n=29)

Soil cover -0.47 0.06

Thicket/mature open-spruce (n=44)

Lower field layer 0.26 0.06

Correlations of open specialist S and environmental variables

Cluster group Environmental variable

Pearson correlation

p (2-tailed)

Prethicket ash (n=34)

Deadwood cover -0.47 0.004

Soil cover -0.47 0.004

Closed canopy spruce (n=29)

Canopy cover -0.31 0.04

Thicket /mature

Open-spruce (n=44)

Ground vegetation

-0.32 0.04

Correlations of forest specialist S and environmental variables

Cluster group Environmental variable

Pearson correlation

p (2-tailed)

Pure prethicket ash (n= 34)

Twig cover 0.34 0.05

Ground vegetation 0.36 0.04

Leaf litter cover 0.54 0.001

Soil cover 0.45 0.007

Mature ash (n=16) Lower field layer -0.58 0.02

Closed canopy spruce (n= 59)

Upper field layer -0.27 0.03

Thicket/mature open-spruce (n=44)

Twig cover 0.46 0.002

Ground vegetation 0.45 0.002

Upper field layer -0.48 <0.001

Discussion

Meta mengei

©Ed Nieuwenhuyys. Copyright 1996 – 2002

Changes over the forest cycle• Decrease in overall S in both ash and spruce:

– Decrease in open species– Increase in forest species

Early stages• The pre-thicket sites have highest S and the highest S

of open associated species

• Greatest cover of lower field layer vegetation– Prey availability– More web attachment points – Hiding places for active predators – Microclimate – stable moisture levels

Effects of canopy closure

• Decrease in lower field layer vegetation

- Reduced light

• Increase in ground vegetation typical of forests habitats

• Effects on spiders: - Overall S and open species S is reduced

- Forest species benefit from ground vegetation

Reopening of the canopy• Mechanisms of reopening

– Thinning– Wind throw event– Disease

• Outcomes of reopening

– Increase in lower field layer– Open species recolonise– Thinning allows coexistence of both forest and open specialists

– Thicket/mature open-spruce

• Highest total S of all the cluster groups

Differences between ash and spruce

• Prethicket ash and spruce do not form such a distinct group from each other as mature sites

• Minimal effect of trees

• Preplanting habitat type

• Soil differences

Mature sitesMature ash distinct in assemblage structure and low S:

• Leaf litter

– Ash and spruce - equally high litter cover

– Ash - low litter depth

– Spruce - high litter depth

– Litter dwelling forest species

• Field layer cover:

– Both spruce and ash have high field layer cover

– Spruce: grass, ferns, brambles

– Ash: Ivy dominated

– Structure of ivy

– Finely tuned vegetation measurements required

Structural indicators of species richness

• Lower field layer cover:

• Important determinant of total spider species richness

• Diversification of habitat structure

• Evident in more open sites with high species richness

• Effect of Canopy cover and upper field layer:

• Negative effect on lower field layer vegetation

• Allows colonisation of forest ground vegetation

• Increases S of forest spiders which are positively correlated with ground vegetation

• Forest associated variables• Such as:

– Needle litter – Leaf litter– Deadwood – Litter depth

• Overall negative effect on total and open species

richness

• However it must be noted that forest specialists are

positively affected by these variables

Conclusions

• Sitka spruce and ash have different spider assemblages which change over the forest cycle:

• Canopy closure has profound effects on spider communities

• Species richness in spiders is strongly influenced by vegetation structure

• Sites with a more open canopy contain a more complex vegetation structure

• Forest species must not be overlooked: • Open and forest species show opposite trends over the forest cycle

• Paucity of natural woodlands in Ireland  

• Plantations could be a potentially very important habitat for these species 

• Balance between factors affecting open and forest species in management

• Only ground dwelling spiders studied

• Real data and structural indicators

Acknowledgements

• Many thanks to Myles Nolan, Bob Johnston and Peter Merrett for help with identification and verification of difficult specimens

• Thanks to Maire Buckley and Noreen Burke for help with sample sorting and field work

• Many thanks to Coillte for the use of their forests during this survey