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​Weed Risks

Increasing weed species distribution in forests may result in lower stand productivity due to competition for soil and water resources, as weed species growing under enhanced CO2 concentrations will be more productive.

Changes in temperature and rainfall patterns may result in colonisation of warm-adapted weed species to new locations as climatic zones shift, but restrict pests that inhabit cooler areas.

Some weeds will respond more strongly to increases in CO2 concentration and temperature than other plants and may become invasive and dominant in the future. This includes ‘sleeper weeds’ – those that are present in New Zealand and relatively benign under current climate but which could become significant under climate change scenarios. There is currently not enough information to identify particular species with certainty.

An increase in the productivity of weeds and the establishment of new invasive weeds can increase their competitive effectiveness and affect tree survival and growth – especially during establishment and early growth of plantations. This has implications for weed control. Weed species will also have changed interactions with insects, pollinators, pathogens and disease, which could also affect spread and growth.

There are available tools for modelling the impacts of weed competition such as Vegetation Manager (VMAN). This is a decision support tool which models weed-free tree growth, weed growth and weed response to vegetation management treatments.

The weed growth models can be calibrated to any real data for specific site types and allows the cost-benefit of single treatments or treatment regimes to be evaluated using the time gained from the treatment as the benefit. The model is sensitive to competition between the early growth patterns of trees and weeds where small variations in the early weed or tree growth can result in substantial changes in subsequent competition. VMAN allows the sensitivity of tree growth to be explored but relating the results to a particular stand at establishment may be difficult.


The distribution of Buddleja davidii is projected using the climatic niche model Climex, using the A1B (medium emissions and A2 (high emissions) scenarios.

The current projected distribution is across most of the North and South Island as suitable for Buddleja, with unsuitable regions in high altitude part of the South Island.

The potential distribution increased under future climate scenarios with large regional variation, particularly in the high country areas of the Southern Alps. The likely areas of invasion are in the east and southern regions of the South island; and also where there is increased climatic suitability and disturbance typically associated with forest management.

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