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Understory plant responses to uneven-aged forestry alternatives in northern hardwood-conifer forests Kimberly J. Smith1, William S. Keeton1, Mark J. Twery1,2, and Donald R. Tobi1 1Rubenstein School of Environment and Natural Resources, University of Vermont 2USDA Forest Service, Northeastern Research Station Structure-based forestry (Franklin et al. 2002; Keeton 2006): Managing for multiple structural conditions representative of natural successional dynamics www.dred.state.nh.us Disturbance-based forestry (Mitchell et al. 2002; Seymour et al. 2002): Developing silvicultural systems based on the scale and pattern of natural disturbances Stovall 2006 Stovall 2006 The Vermont Forest Ecosystem Management Demonstration Project (FEMDP) Structure and disturbance-based forestry practices can: Sustain a broad array of biodiversity and ecosystem functions Provide for profitable timber management Silvicultural Treatments (FEMDP) Single-tree selection (STS) and group selection (GS) Increased structural retention Modeled after the scale and pattern of natural disturbance (based on the findings of Seymour et al. 2002) Structural Complexity Enhancement (SCE) Promote accelerated development of late-successional forest characteristics: Increasing vertical and horizontal heterogeneity Elevating large snag and coarse woody debris (CWD) densities Reallocating basal area to larger diameter classes Enhance ecosystem services, including: Late-successional wildlife habitat (McGee et al. 1999) Carbon storage (Harmon et al. 1990) Riparian function (Keeton et al. 2007) FEMDP Research Monitoring indicators of biodiversity response, including: Birds Small mammals Amphibians Soil invertebrates Vegetation © Al Sheldon Previous Research Even-aged treatments Plantation forestry (Ramovs and Roberts 2005) Clearcutting (Gilliam et al. 1995; Halpern and Spies 1995; Liu and Ashton 1999) Uneven-aged treatments www.gov.ns.ca Single-tree and group selection (Jenkins and Parker 1999; Scheller and Mladenoff 2002; Kern 2006) Variable retention (Halpern et al. 2005) Experimental canopy gaps (Collins and Pickett 1988; Gray and Spies 1997) www.gov.ns.ca Hypotheses Uneven-aged, low-intensity silvicultural systems can maintain understory plant diversity and support late-successional species Retaining and enhancing stand structural complexity can increase understory plant diversity Plant responses are influenced by interactions between canopy structure, soils, and climate processes Methods Study Areas: Mount Mansfield State Forest Jericho Research Forest Paul Smith’s College Mature, northern hardwood stands with a documented history of previous timber management Experimental Design Mt. Mansfield State Forest 2 ha treatment units 0.1 ha permanent plots (overstory structure) 1 m2 (vegetation) 13 vegetation quadrats (1 m2) 4 soil subplots (2 m2) 2 m2 (soils) Data Collection Palmer Drought Severity Index (PDSI): multiple climatic parameters condensed into a single index • Temperature • Evapotranspiration • Precipitation • Soil moisture loss • Soil moisture recharge • Runoff PDSI Analysis Habitat guilds as defined by Ramovs and Roberts (2005): Early-successional Intermediate Late-successional www.nrs.fs.fed.us Response variables Diversity: Hill’s series of diversity indices (Hill 1973) Species richness Exponential Shannon Index Reciprocal Simpson Index Abundance: % cover by species Analysis of Treatment Effects Diversity and abundance: Linear mixed effects model fixed effects– treatment, site, year random effects– plots, units ANOVA models Pre- to post-harvest change of unit level means Test for differences among treatments Analyses performed for all species and by habitat guilds Analysis of Treatment Effects Compositional changes: Non-metric multidimensional scaling (NMS) interpret compositional patterns among treatment units Multi-response permutation procedure (MRPP) pre- to post-harvest differences within treatments differences among treatments before and after harvest Locally impacted species Sub-analysis of soil properties and overstory structure Five soil variables: % OM, % N, Ca, P, pH Linear mixed effects model fixed effects– treatment, site, year random effects– plots, units covariates- % OM, % N, Ca, P Pre- to post-harvest % change Soil properties Curtis’ relative density (RD) Diversity and abundance responses Explanatory variables related to ordination axes in NMS www.forestryimages.org Analysis of Moisture Stress Palmer Drought Severity Index (PDSI) Period 1 (PDSI_1) = July-September, previous year Period 2 (PDSI_2) = April-June, current year Standardized understory responses to +/- unit mean Simple linear regressions Results: Effects of Treatment Treatment*time p All Species Richness Diversity_1 Diversity_2 % cover <0.001 <0.001 0.004 <0.001 Late-Successional Species Richness Diversity_1 Diversity_2 % cover 0.002 0.024 0.047 <0.001 Intermediate Species Richness Diversity_1 Diversity_2 % cover 0.348 0.117 0.217 0.467 Early-Successional Species Richness Diversity_1 Diversity_2 % cover <0.001 <0.001 0.001 <0.001 Mixed effects model Understory responses significantly affected by treatment*time interaction Effects of Treatment: All Species 12 Control 4 8 SCE 3 Single-tree selection Diversity Richness Group selection 4 0 2 1 0 -1 -4 -1 1 2 Year Richness: p = 0.032 SCE > CON 3 4 -1 1 2 3 Year Shannon Index: p = 0.004 SCE > CON 4 Effects of Treatment: Late-successional Species 2 1.2 0.9 Diversity Richness 4 Control Group selection SCE Single-tree selection 0 -2 0.6 0.3 0 -0.3 -0.6 -4 -1 1 2 Year Richness: p = 0.012 SCE > GS 3 4 -1 1 2 3 Year Shannon Index: p = 0.009 SCE > CON 4 Effects of Treatment: Species Composition Pre-harvest CON GS SCE MRPP: A = 0.009 p = 0.320 Axis 2 (33.2%) STS Axis 1 (44.8%) Effects of Treatment: Species Composition Post-harvest CON GS SCE A = 0.026 p = 0.142 Axis 2 MRPP: (25.1%) STS Axis 1 (38.2%) Locally Extirpated Species Percent Species Lost by Treatment % Species Lost 16 12 ANOVA: 8 p = 0.07 4 0 GS STS SCE Treatment CON Locally Extirpated Species Scientific Name Actaea alba Adiantum pedatum Aralia nudicaulis Arisaema triphyllum Asarum canadense Coptis trifolia Eupatorium rugosum Lonicera canadensis Medeola virginiana Osmunda claytoniana Oxalis acetosella Panax trifolia Polygonatum pubescens Polygonum cilinode Pyrola elliptica Sambucus racemosa Smilacina racemosa Trientalis borealis Trillium erectum Viburnum alnifolium GS Treatment* SCE STS X X X X X X X X X X X X X X X X X X X X X X X Fruit type Dispersal Habitat preference perennial herb fern perennial herb perennial herb perennial herb perennial herb perennial herb shrub perennial herb fern perennial herb perennial herb perennial herb perennial herb perennial herb shrub perennial herb perennial herb perennial herb shrub berry spore drupe berry capsule follicle achene berry berry spore capsule drupe berry achene capsule berry berry capsule berry drupe biotic abiotic biotic biotic biotic biotic abiotic biotic biotic abiotic biotic biotic biotic biotic abiotic biotic biotic biotic biotic biotic late-successional late-successional late-successional late-successional late-successional late-successional intermediate late-successional late-successional late-successional late-successional late-successional late-successional intermediate late-successional early-successional late-successional late-successional late-successional late-successional CON X X X X Growth form *X indicates the species was present before harvest and absent from two or more units of this treatment type during all four post-harvest years. Effects of Soil Properties and Overstory Structure Mixed effects model: Significant effect of treatment Soil covariates not significant Exceptions: Intermediate species- % OM and % N Correlations: Unit level (coarse scale) and plot level (fine scale) Δ relative density related to Δ responses Δ soil properties generally not related to Δ responses Effects of Soil Properties and Overstory Structure Pre-harvest CON GS SCE RD: τ = 0.439 %OM: τ = -0.336 Axis 2 (33.2%) Axis 2 STS RD Ca P N OM Axis 1 (44.8%) Effects of Soil Properties and Overstory Structure Axis 21 Axis %N: Ca: -0.336 ττ==0.520 SCE STS (25.1%) %OM: %N: -0.362 ττ==0.388 GS OM Ca NP Axis 2 %OM: RD: 0.368 ττ==0.441 Post-harvest CON RD P: τ = 0.494 Axis 1 (38.2%) Effect of Moisture Stress PDSI_2 PDSI_1 (April-June of current (July-September of year) previous year) 8 Not Not related related to to understory understory responses responsesin controls or treatments GS: p = 0.012, r2 = 0.747 STS: p = 0.018, r2 = 0.703 SCE: p = 0.024, r2 = 0.602 6 4 PDSI Exception: Late-successional richness FEMDP VT FERDA NY 2 0 -2 -4 -3 -2 -1 1 2 year 3 4 Discussion Uneven-aged, low-intensity silvicultural systems with increased structural retention maintain understory plant species diversity 12 Control 4 8 SCE 3 Single-tree selection Diversity Richness Group selection 4 0 2 1 0 -1 -4 -1 1 2 3 4 Year -1 1 2 Year All Species 3 4 Discussion Post-harvest increases in diversity (Gilliam et al. 1995; Halpern and Spies 1995; Jenkins and Parker 1999) Increases in early-successional or ruderal species may mask the loss of late-successional species (Halpern and Spies 1995) Results of this study: Compositional differences not significant Pre- to post-harvest, within treatments Post-harvest, among treatments Discussion Level and spatial pattern of retention may influence loss of species following harvest Percent Species Lost by Treatment % Species Lost 16 12 ANOVA: 8 p = 0.07 4 0 GS STS SCE Treatment CON Discussion Uneven-aged, low-intensity silvicultural systems with increased structural retention support latesuccessional species 2 1.2 0.9 Diversity Richness 4 Control Group selection SCE Single-tree selection 0 -2 0.6 0.3 0 -0.3 -0.6 -4 -1 1 2 3 4 -1 Year 1 2 Year Late-successional Species 3 4 Discussion Increasing post-harvest stand structural complexity may lead to increased plant diversity Previous studies: Plant diversity increases with light availability (e.g. Brosofske 2001) Results of this study: SCE: Lower light availability Greater light heterogeneity Greater increases in diversity Stovall 2006 Discussion Retaining or enhancing stand Late-successional structural complexity northern hardwood forests may increase diversity: characterized by: light heterogeneity heterogeneous light environment microhabitat diversity retaining microsite canopy variability (Scheller and Mladenoff 2002) Discussion Understory plant responses are influenced by interactions between canopy structure, soils, and climate processes Moisture availability influences patterns of understory vegetation (Huebner et al. 1995; Hutchinson et al. 1999; Kolb and Diekmann 2004) Results of this study: Drought index correlated to late-successional richness in treatment units, but not in controls Canopy removal may increase susceptibility of understory to drought stress Discussion Percent Increase of Understory Responses in SCE Units recovery actual 60 8 FEMDP VT FERDA NY 6 40 4 PDSI % increase Post-harvest diversity increases in SCE units may be partially due to drought recovery 20 2 0 -2 -4 0 -3 -2 Richness -1 1 year Shannon Diversity 2 3 4 Simpson Diversity Discussion Understory plant responses are influenced by interactions between canopy structure, soils, and climate processes Soil nutrient availability influences patterns of understory vegetation (Kolb and Diekmann 2004; Fraterrigo et al. 2006) Harvesting overstory can affect soil nutrient availability (Johnson et al. 1997; Elliott and Knoepp 2005) Results of this study: Overstory-vegetation relationships consistently significant Soil-vegetation relationships highly variable Conclusions Uneven-aged, low-intensity silvicultural treatments with increased structural retention can maintain understory plant diversity and support late-successional species in northern hardwood-conifer forests during the initial post-harvest recovery period Treatments that enhance stand structural complexity may increase understory plant diversity by increasing the heterogeneity of light and microsite variability Level and spatial pattern of retention may be important to preserving understory plant species Plant responses are primarily influenced by changes in overstory structure Management Implications Sustainable forest management: Maintains biodiversity and ecosystem functioning Provides for timber harvest Best approaches for conserving understory plant diversity: Retain post-harvest structure Retain biological legacies (see Franklin et al. 2002) Enhance stand structural complexity Acknowledgements Vermont Monitoring Cooperative USDA McIntire-Stennis Forest Research Program USDA National Research Initiative Northeastern States Research Cooperative Other helpful folks: Field crews of 2001-2006 Alan Howard, UVM statistical counseling clinic Questions?