Impacts of Climate Change on Pastures
Plant response to temperature
Rates of photosynthesis in the plant increase as temperatures rise to an optimum level. Beyond this, photosynthesis and growth rates are hampered. C4 grasses are better adapted to higher temperatures (29-30°C) than C3 grasses (20-25°C).
Some natural adaptation and selection may occur in pasture populations to adjust for temperature changes over time, however this is not currently well understood for NZ grown species.
Plant response to water availability
Water deficits caused by dry weather conditions reduce the photosynthetic capacity of plants and therefore impact on plant growth potential. A lack of water sends a message to the water regulators in a plant’s leaves, known as stomata, to close. This process retains plant water but reduces significantly the plants ability to respire and therefore to produce energy via photosynthesis for further growth. Both C3 and C4 grasses are sensitive to this process.
Prolonged water excesses such as waterlogged soils or flooding also impact on a plants ability to photosynthesis and grow.
Plant response to increased atmospheric carbon dioxide
Elevated CO2 levels have the effect of stimulating photosynthesis and therefore plant growth, both above and below ground. It is estimated that a potential increase of 4-33% of gross canopy photosynthesis may be achieved in NZ pastures if future atmospheric CO2 levels rise. A limiting factor here is that this growth is optimised between 14.5 and 18.5°C. Above 18.5°C however, the negative effects of increasing temperatures counters the positive effect of increased CO2 on plant growth.
The plants ability to respond to extra CO2 also relies on N being available for plants, suggesting that an increase in N applications may be required in the future for some pastures. Nutrient poor soils will not be able to take as much advantage of CO2 responses as soils with adequate nutrients.
Pasture quality and composition responses
Pastures may be susceptible to earlier flowering and a more rapid senescence resulting in a loss of quality earlier in summer than is currently experienced.
Legumes may increase in number in pastures from warmer temperatures and elevated CO2. This would increase the nutritive value, crude protein levels and ME.
Should a considerable shift from C3 to C4 grasses occur along with an expected increase in weeds within the pasture composition a negative impact on overall nutritive value may occur. While C4 grasses and weeds do better under higher temperatures they have a reduced protein content and are less digestible and lower ME than C3 plants.
Pests and diseases are likely to be more prevalent under predicted climate changes. This includes faster lifecycles and wider geographic spread. Responses to heat or excessive moisture stress may also increase plant susceptibility to disease.