Jenny Hurst 1,3 , Glenn Stewart 2 , Robert Allen 1 , Susan Wiser 1 , David Norton 3

1. Simulated harvesting scenarios in old-growth mixed southern beech (Nothofagus) forest determine composition and structure Jenny Hurst1,3, Glenn Stewart2, Robert Allen1, Susan Wiser1, David Norton3 1Landcare Research, 2Lincoln University, 3University of Canterbury, New Zealand 9thIUFRO International conference on Uneven-aged Silviculture, Birmensdorf, Switzerland, 17-19 June, 2014

2. Mixed-Nothofagus forest • Relatively ‘simple’ forest type- 2 canopy dominants • Both species widespread and common • Potential for increased management of beech forest for timber N. fusca-red beech N. menziesii-silver beech

3. Previous studies mainly focussed on life-history differences at seedling or sapling life stages (Stewart & Rose 1990) • Both species regenerate via gaps (Wardle 1984, Stewart & Rose 1990) • Abundance of each species in gaps depends on gap size (Stewart et al 1991, Wiser et al 2007) • Growth rates are extremely variable (Wardle 1984, Runkle et al 1997, Peltzer et al 2005) • Canopy silver beech have greater growth releases following disturbance than canopy red beech (Hosking & Kershaw 1985, Wiser et al 2005)

4. Mixed Nothofagus forest dynamics – Hurst PhD 2014 • Overall research question • Do demographic performance trade-offs promote coexistence of red and silver beech? 1. Seedling demography • Species-specific performance amongst microhabitats e.g. substrates & light 2. Tree growth • Species-specific & ontogenetic patterns • Intra- and interspecific neighbourhood interactions 3. Tree mortality • Species-specific, spatial, & ontogenetic patterns 4. Simulation modelling • Changes in composition and structure following disturbance and harvesting

5. The data!! • Permanent plots established in 1980’s • 3 plots, 0.8−1ha in size, divided into 5×5m subplots • All trees ≥ 5 cm DBH tagged, measured & mapped • Tree mortality censuses 1986−98, 2001, 2009−10 • Plots re-measured and all trees spatially re-mapped in 2001 and 2009−10

6. Spatially mapped data allowed detailed analyses of tree growth and mortality patterns Pell stream Red beech Silver beech Station creek Rough creek

7. Simulation modelling • Spatially explicit simulation model • Parameterised using individual tree recruitment, growth and mortality functions • Used to examine consequences of disturbance and harvesting on forest structure and composition

8. Simulation model • Consists of a series of sub-models • These determine the fate of each individual plant throughout its life • Recruitment, growth, mortality • Life-history stages: trees (all stems > 5 cm dbh) • Keeps track of tree locations on a 140m x 140m plot • Starting conditions were based on current forest structure and composition across the three sampled stands

9. Starting conditions • Size-class structure for 50 starting condition stands generated randomly from permanent plot data • Example stem map for the starting conditions (140 x 140m, 1.96 ha) • Mean basal area = 69.2m2ha–1(c. 80% N. fusca) • Mean total stem density =748 stems ha–1, (c. 60% N. menziesii)

10. Gap size frequency distribution Stewart et al., 1991, 2000

11. Simulation modelling - disturbance Red beech Silver beech Disturbance frequency: Background forest dynamics High Low Intermediate 0.05 gaps/ha/year (1 gap/ha/20 years) 0.1 gaps/ha/year (1 gap/ha/10 years) 0.01 gaps/ha/year (1 gap/ha/100 years) N. menziesii N. fusca

12. Simulation modelling - harvesting Harvest frequency: Background forest dynamics High Variable Red beech Silver beech 10% basal area extracted every 10 years 10% basal area extracted when forest basal area recovers from previous harvest- on average every 130 years

13. Simulation modelling - summary • Similar basal area, stem density and size structure after 500 years under variable frequency harvest, background forest dynamics and low frequency disturbance • forest increasingly dominated by N. menziesii through time- infrequent harvesting has minimal effects on long-term trends in forest composition and structure Low disturbance frequency Background forest dynamics Variable harvest frequency

14. Caveats • Simulation parameterised over a 23-year period - does the disturbance regime captured represent average conditions for mixed-Nothofagus forests? • Large-scale disturbances would likely occur over a 500 year period- drought, insect outbreaks, earthquakes • Extrapolating the model to simulate the consequences of infrequent, larger-scale disturbances should be done with caution • because model parameterisation was undertaken from a period characterised by small-scale disturbances.

15. Constraints/ further research • Simulations did not examine the relative importance of harvest frequency vs gap size on long-term structure and composition - simulation should be adapted to address this limitation • Validate simulations against independent data (e.g. NVS) • Comparing variable frequency harvesting and low frequency disturbance may provide guidelines for appropriate return intervals for harvesting