The anticipated impacts of climate change for many arable crops depends entirely on the region and the flexibility of cropping system that is operated. Temperatures increases and rainfall events increases annually in the western parts of the country and decreasing in eastern parts will prove to require some farmer adaption. Frost risk will decrease overall however other extreme weather events such as storms are likely to increase in intensity and frequency.
Using modelling, it is expected that by 2040, and without any adaptation by management, the following yields for would increase:
Whereas, yields would be either maintained or decrease slightly for other crops such as potatoes, peas and silage maze. The baseline results in the simulations highlight the importance of water supply as a major driver of yield for both wheat and potato.
Yield is strongly influenced by temperature, as temperature regulates important physiological process.
Changes in the temperature mean and variability will influence growth and development physiological processes which operate within a temperature envelope:, ' 'cardinal temperatures' that are specific to each process.
Cardinal temperatures are the temperature thresholds: the base, low and upper optimum and the ceiling temperatures.
Increased temperatures will likely shorten crop cycle lengths (germination through to maturity) however, this can be countered by faster canopy expansion and photosynthesis from higher temperatures and CO2 levels when water and nutrients are not limiting.
Temperatures exceeding optimum will be detrimental to reproduction and growth, and extreme temperature (45-55°C) may be fatal for the plant. These temperature occur on unshaded soil surfaces.
The heat stress impacts reproductive processes e.g., through reducing flower setting, pollen tube development impairment and quantities. Most crops are sensitive to heat stress within 1-2 weeks of flowering.
Temperatures exceeding 30°C in wheat and 35°C in soybean for example will exceed optimal levels for plant development.
In the future there will be a lesser risk of frost and fewer cold days, though this will impact those crops that require a level of exposure to cold temperatures (vernalisation). A reduced yield will be achieved due to factors such as a low flower bud initiation will result for some cultivars.
Paradoxically, climate change can increase the risk of frost damage, while the increase in mean temperature reduces the risk of a frost, the changes in temperature variance increases damage risk as faster growth and earlier planting can expose plants to frost during times of greater temperature variability.
From a grain perspective, a combination of high temperatures and water and nutrient stress may have an impact on grain proteins and their composition.
Drought risk is expected to increase for important cropping areas of New Zealand.
Low rainfall will cause water stress in plants. This will limit germination, as well as canopy expansion and will increase leaf senescence, resulting in a significant reduction in the plants ability to make energy to grow. Extreme cases may result in complete crop failure.
It is expected that inter-seasonal rainfall variability will increase. An change in storm patterns may result in more intense rainfall events spaced at greater periods. These events increase the risk of runoff, deep percolation, erosion and loss of nutrients. While there will be soil water recharge, any increase in time between precipitation events increases the risk of water deficits.
Cereal crop yield can be characterised by its yield components:
- No. of ears / unit area
- No. grains / ear
- Grain weight
As these happen sequentially the timing of water stress determines which component is affected.
Early water stress affects the number of ears per unit area, where as late stress affects the grains per ear and grain weight.
Excessive rainfall will cause water overload for both plants and their soil environments. Depending on the nature of flooding or waterlogging of soils, issues such as reduced oxygen to root systems, soil and nutrient erosion and leaching. An increased risk of crop diseases may also result.
Excess or unseasonal rainfall can cause yield and quality loss through seed shedding, longer times (and cost) for seed drying, as well as flooding.
Extreme storm events
The intensity and frequency of extreme storm events are likely to increase with climate change due to increased ocean evaporation rates.
Strong winds can cause significant damage to crops such as crop lodging, seed shedding and harvest losses from rowed crops. This will compromise yield and quality particularly in cereals and seed crops.
CO2 levels are expected to rise.
Elevated CO2 levels have the effect of stimulating photosynthesis and therefore plant growth, both above and below ground. Increased levels of CO2 will likely effect both growth and transpiration rates within broad acre crops.
Increased levels of CO2 to optimal levels can enhance growth within many crops in New Zealand such as barley, oats, ryegrass, wheat, potato. This enhancement will however, be crop and cultivar specific. Utilising this high CO2 environment requires the crops to be not limited in either water or nutrients.
Climate change is likely to influence the levels and frequency of pest and disease damage that occurs within crops. Due to temperature and CO2 increases, crops will develop quicker providing the opportunity for pests to infest crops at more favourable times creating earlier infestations and space for a greater number of pest lifecycles/generations within the cropping cycle. Milder winters and large early spring pest populations may enable pests to overwinter more successfully.