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

9th IUFRO International conference on Uneven-aged Silviculture, Birmensdorf, Switzerland, 17-19 June, 2014

• 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

– 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)

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

Mixed Nothofagus forest dynamics – Hurst PhD 2014• Overall research question

• Do demographic performance trade-offs promote coexistence of red and silver beech?

• 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

The data!!

Red beech

Silver beech

Spatially mapped data allowed detailed analyses of tree growth and mortality patterns

Station creek

Pell stream

Rough creek

• 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

Simulation modelling

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

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 = 69.2m2 ha–1 (c. 80% N. fusca)

• Mean total stem density =748 stems ha–1, (c. 60% N. menziesii)

Gap size frequency distribution

Stewart et al., 1991, 2000

Simulation modelling - disturbance

Background forest dynamics Low Intermediate High

0.01 gaps/ha/year (1 gap/ha/100 years)

0.05 gaps/ha/year (1 gap/ha/20 years)

0.1 gaps/ha/year (1 gap/ha/10 years)

Disturbance frequency:

N. menziesii N. fusca

Red beech

Silver beech

Harvest frequency:Background forest dynamics High Variable

Simulation modelling - harvesting

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

Low disturbance frequency

Variable harvest frequency

• 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

Background forest dynamics

Simulation modelling - summary

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.

• 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

Constraints/ further research