Giller, Paul S

Gittings, Tom

O'Halloran, John

Oxbrough, Anne

Ground dwelling spider communities were investigated in Sitka spruce and ash plantations across the forest cycle. The impact of tree age and species on spider community structure and diversity was largely due to the contrasting nature and structure of ground vegetation and litter. Recommendations for enhancing biodiversity in forestry are suggested

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Spiders communities as bio-indicators in Irish plantation forests Anne Oxbrough, Tom Gittings, Paul Giller, John O’Halloran Bioforest Project, Dept Zoology, Ecology and Plant Science, University College Cork Meta mengei ©Ed. Nieuwenhuys. Copyright 1996-2002 Why identify indicators in plantations ? • Sustainable forest management  • Managing for biodiversity • Structural indicators – Environmental correlates with target taxanomic group – Potential in sustainable forest management – Implemented by non-specialists   • Abundant  • Positively influenced by vegetation structure - Prey - Web attachment - Hiding places for active hunters - Protecton from predators - Suitable microclimates  • Found in all layers of vegetation  • Occupy a strategic position food webs  • Taxonomically well known                 Why use spiders as an indicator group?   Aims of study Spider communities : • Change over forest cycle   • Differ between Sitka spruce and ash • Identify indicators of spider biodiversity  Study sites  • 32 sites across ireland • Sitka spruce and ash            - Conifer and broadleaf               - Widely planted  • Sites allocated into groups by structural features - Mean distance between trees - DBH - Tree height - Tree cover       Pitfall traps Sampling regime   • 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  •  Litter depth and cover   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. Bars indicate SE 0510152025Prethicket Thicket Closed  maturing Re- opening Mature Prethicket  Pole Closed maturing Semi- Mature Mature Species richness Sitka spruceAshSpecies richness Species richness of habitat specialists.  Bars indicate SE Open specialists Forest specialists 01234560123456Prethicket Thicket Closed  maturing Re- opening Mature Prethicket Pole Closed  maturing Semi- mature Mature Sitka spruceAshSpecies assemblages • Cluster analysis indicated 5 groups:    1) Young mix (n=20)      2) Young ash (n=34)      3) Mature ash (n=16)      4) Closed canopy spruce (n=29)      5) Open-spruce (n=44) (n = no. of plots)  • Ordination revealed similar groups as cluster analysis    Lower field layer Leaf litter Organic content Cluster groups   Young mix         Young ash  Mature ash         Closed-canopy         spruce  Open spruce NMS ordination of plots Axis 2  (r2 = 0.20) Axis 1 (r2 = 0.61) Deadwood  Twig Ground  Vegetation Litter  depth Axes 1 scores from NMS ordinations of both species and habitat data -1.5-1-0.500.511.5-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5Young spruce/ash Young ash Mature ash Closed  canopy spruce Open spruceAxis 1 Scores of Environmental NMSAxis 1 Scores of Species NMSSpecies NMS Axis 1 r2 value = 0.61 Habitat NMS Axis 1 r2 value = 0.65  Pearson correlation,  r  =0.66 (p= <0.001) Correlation of total species richness and environmental variables Cluster group Environmental variable Pearson (r) Organic content 0.57** Young mix (n=20) Lower field layer 0.45* Soil cover -0.33* Lower field layer 0.40* Young ash (n=34) Deadwood cover -0.50*** Closed-canopy spruce (n=29) Soil cover -0.47* Open spruce (n=44) Lower field layer 0.26*   *p = <0.05; **p = <0.01; *** p = <0.005 Correlation of open specialist species richness and environmental variables Cluster group Environmental variable Pearson (r) Deadwood cover -0.47*** Young ash (n=34) Soil cover -0.47*** Closed-canopy spruce (n=29) Canopy cover -0.31* Open spruce (n=44) Ground vegetation -0.32*   *p = <0.05; **p = <0.01; *** p = <0.005 Correlation of forest specialist species richness and environmental variables Cluster group Environmental variable Pearson (r) Twig cover 0.34* Ground vegetation 0.36* Leaf litter cover 0.54*** Young ash (n=34)  Soil cover 0.45** Mature ash (n=16) Lower field layer -0.58* Closed-canopy spruce (n=29) Upper field layer -0.27* Twig cover 0.46*** Ground vegetation 0.45*** Open spruce (n=44) Upper field layer -0.48***   * p = <0.05; ** p = <0.01; *** p = <0.005 Discussion Changes over the forest cycle • Decrease in overall S in both ash and spruce • Decrease in open species • Increase in forest species  Early stages  • Pre-thicket has highest S •  Also highest S of open specialists  • Highest cover of lower field layer vegetation – More web attachment points  – Hiding places for active predators  – Prey availability  Pre-canopy closure,  Sitka spruce Effects of canopy closure  • Decrease in lower field layer vegetation  - Reduced light • Increase in forest associated variables - Litter layers - Dead wood • Effects on spiders: - Overall S and open species richness is reduced - Forest species benefit from litter layers    Closed-canopy    Sitka spruce Reopening of the canopy • Mechanisms of reopening – Thinning – Wind throw event – Disease  • Outcomes of reopening  – Early on: • Typical forest ground vegetation – After successive thins: • Increase in lower field layer • Open species recolonise – Thinning allows coexistence of both forest and  open specialists • Open spruce cluster group   Re-opening canopy Sitka spruce Mature plantation Sitka spruce 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 ash distinct   • Litter cover – Ash and spruce  - equally high litter cover – BUT varying litter depths: •  Ash •  Spruce – Litter dwelling forest species • Field layer cover: – Both spruce and ash have high field layer cover – Spruce: grass, ferns, brambles –  Ash: Ivy dominated –  Less structurally diverse     Structural indicators of Spider biodiversity • Lower field layer cover:  • Important determinant of total spider species richness   • Diversification of habitat structure  • Evident in more open sites with high species richness  • Canopy cover and upper field layer: • Negative effect on lower field layer vegetation  • Allows colonisation of forest ground vegetation  • Benefits forest specialists  • Forest associated variables • Such as:  – Needle litter  – Leaf litter – Deadwood  – Litter depth • Overall negative effect on total and open species richness • Positive affect on forest specialists 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 potentially be an      important habitat for these species   • Balance between factors affecting open and forest species in management  • Real data and  structural indicators  Acknowledgements • Myles Nolan, Bob Johnston and  Peter Merrett for help with identification of difficult specimens  • Maire Buckley and Noreen Burke for help in the field  • Thanks to Ed Nieuwenhuys for use of spider photographs (www.xs4all.nl/~ednieuw/ ) • Bioforest project for funding the research (//bioforest.ucc.ie/)  • Thanks to COFORD for providing funding to attend the BES meeting 

Spider communities as bioindicators in Irish plantation forests

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Spider communities as bioindicators in Irish plantation forests

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