The Reforestation Carbon Trade-off
A 5,000-hectare catchment is being considered for reforestation to earn carbon credits. It currently supports native grassland and shrubland, grazed by 3,000 sheep, and contains 12 km of eroding gullies feeding a reservoir that supplies a town of 40,000. The proposed plantation (fast-growing eucalypt) would cover 3,500 ha, sequester an estimated 8 t CO2-e/ha/year for 25 years, and be harvested at year 25. Establishment requires ripping, herbicide, and removal of grazing. Modelling suggests the plantation would reduce average annual runoff by 15% (from 90 mm to 76 mm) and cut sediment yield by 40%. The reservoir is already at 60% capacity and the town has had two water restrictions in five years. Two threatened grassland bird species depend on the site. Local employment would fall if grazing ends, but rise during establishment and harvest.
1. Quantify the carbon benefit and the water cost, then judge whether the trade-off is acceptable for this catchment.
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Carbon: 3,500 ha × 8 t CO2-e/ha/year × 25 years ≈ 700,000 t CO2-e over the period, or about 28,000 t/year. That is a real but time-limited benefit; after harvest, sequestration may reverse or pause, and harvested wood may be short-lived, so permanence is uncertain.
Water: runoff falls 15%, from 90 to 76 mm. Over 5,000 ha that is a loss of 14 mm × 5,000 ha = 70,000 m³... converting: 14 mm over 1 ha = 140 m³, so 5,000 ha loses ~700,000 m³/year, i.e. 0.7 GL/year. For a town of 40,000 using, say, 200 L/person/day, demand is about 2.9 GL/year; losing 0.7 GL is roughly 24% of town demand - substantial given the reservoir is at 60% and restrictions are already frequent.
Judgement: the carbon benefit is genuine but the water cost is high and lands on a stressed supply. A defensible position is to reject full-scale plantation, or restrict it to lower-yielding, non-water-stressed parts of the catchment, and prioritise gully remediation and grassland protection, which cut sediment without reducing yield as much. The decision is a values-and-evidence trade-off, not a simple carbon calculation.
2. Identify the trade-offs involving biodiversity and employment, and propose a monitoring plan to test the model predictions.
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Biodiversity: 3,500 ha of plantation replaces grassland and shrubland on which two threatened birds depend. Plantations are structurally different and generally poor habitat for grassland specialists, so local extinction risk is high even if total forest cover rises. A partial planting that retains grassland corridors and unplanted refuges reduces this risk. Employment: grazing supports a small number of ongoing jobs; plantation establishment and harvest create short-term jobs but few ongoing ones, so total local employment may fall over 25 years even if activity spikes.
Monitoring plan: (1) establish paired control and treatment sub-catchments before planting; (2) measure streamflow continuously at gauges to test the 15% runoff reduction; (3) sample suspended sediment at gully outlets to test the 40% reduction; (4) survey bird abundance and breeding success annually in planted and retained grassland; (5) measure soil carbon and biomass at fixed plots to verify the 8 t/ha/year figure; (6) track employment and water restrictions as social indicators. Pre-planting baseline data and controls are essential, because without them the model cannot be falsified and adaptive management is impossible.