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​Increasing Productivity - Summary

These simulations highlight the potential response to increases in enhanced CO2 concentrations as a key determinant of future productivity of radiata pine within New Zealand.

The modelling work dealt with the uncertainty surrounding the response of plants to increases in CO2 concentrations by presenting simulations that bracket likely extremes.

The magnitude of growth gains under increased CO2 concentration clearly interacts with other environmental conditions. Greatest growth gains are likely to be found on warm sites with adequate nutrition, but where water is limiting, the growth response to CO2 can shift from a direct photosynthetic response to a dependence on the (more substantial) increase in water use efficency. 

As most of New Zealand has relatively fertile soils and moderate to good water availability, the effects of increasing CO2 concentrations are ubiquitous – but generally only moderate.

Greatest gains are likely on dry-land sites, located in eastern regions of both islands

Modelling Producitivty

Empirical growth models developed using datasets from historical and current growth conditions are unsuitable for predicting growth under climatic conditions that are outside the bounds of the models.

Process models are more suitable tools for future projections of productivity, as they incorporate processes, interactions and feedbacks that are affected by climatic variables of interest such as CO2 concentration, precipitation and carbon equestration.

These models can be used with greater confidence to estimate and undertake sensitivity analyses on how future conditions may influence productivity.

The CenW model (v 4.0) was used to model productivity into the future for 2040 and 2090 under different climate assumptions. The climate change data was derived from 12 climate change models, statistically downscaled to 0.05 degrees and forced by three emissions scenarios: B1 - low, A1B – mid range, and A2 – high).

CenW was run under both constant 1990 CO2 concentration and the specific CO2 concentrations corresponding to those expected by each of the emission scenarios.

 

We report here on the results from the increasing CO2 modelling.
 

Increasing Productivity - Increasing CO2

Simulations that included increasing CO2 concentrations resulted in positive impacts on wood productivity across all emission scenarios for both 2040 and 2090.

With increasing CO2 concentrations, there were productivity gains throughout the North Island, especially in the higher-elevation Central Plateau.

For the South Island, increases in wood productivity were even larger as the stimulatory effect of elevated CO2 concentration added to the positive effect of warming to lead to substantial overall productivity enhancements.
 
The climate change response was dominated by the productivity response to increases in temperature and CO2 concentrations, because expected changes in precipitation in New Zealand are generally only minor.

In areas such as the upper North Island where rainfall is expected to decrease and temperature to increase, this would lead to increasing water stress and decreased productivity if there were no changes in CO2 concentrations. However, increases in CO2 concentration are likely to avert developing soil-water shortages and lead to overall productivity enhancements – even in areas where precipitation may decrease slightly.

Productivity gains were found for over 99% of all plantations in both 2040 and 2090. On average, productivity increased 19% by 2040 and 37% by 2090. Both the average and range of these gains increased markedly from 2040 (from –5% to +35%) to 2090 (from –5% to +95%).

Very few sites were expected to experience reductions in productivity, because anticipated changes in precipitation were slight and few sites in New Zealand are likely to experience temperature increases into a range that would be detrimental for the growth of radiata pine.
 
Marginally adverse changes in temperature or rainfall, especially where they would reduce potential water availability, were negated by the effect of increasing CO2 concentration. At the same time, there were a small number of sites where beneficial changes in precipitation and temperature combined with the response to increasing CO2 concentrations to result in substantial productivity gains.

Effects of precipitation on generic tree productivity

Water limitation is a major factor that directly impacts plant growth, with increased mortality, and indirectly with increased susceptibility to, and damage from pests and fire.

Plant responses to reduced water availability relate to soil water availability, and impacts of temperature and CO2 levels on water exchange with trees.

CO2 Generic impacts  

The impact of elevated CO2 concentrations on trees is complex with well characterised effects of elevated CO2 concentrations on photosynthesis, leading to the potential for substantial growth responses. Representative results include:

  • Increasing photosynthesis under conditions without water or nutrient limitations.
  • Photosynthetic carbon gain which leads to increases in biomass.

Temperature Impacts

Most biological and chemical processes are affected by temperature with complex impacts on tree productivity involving interdependent effects on photosynthesis, respiration, transpiration, nutrition and plant development. The photosynthetic rate is generally strongly affected by temperature.
 
Plants are not expected to experience problems from a temperature increase alone, as the projected rise in temperature is too small to have a detrimental impact.

Responsiveness to increasing temperature is principally driven by the lengthening of the growing season 

Extreme and rapid temperature increases could be detrimental, resulting in extensive tissue damage, protein denaturation and mortality. 

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