5.4.3.1 New findings since TAR

New Knowledge: Plant community structure is modified by elevated CO₂ and climate change.

Grasslands consisting of fast-growing, often short lived species, are sensitive to CO₂ and climate change, with the impacts related to the stability and resilience of plant communities (Mitchell and Csillag, 2001). Experiments support the concept of rapid changes in species composition and diversity under climate change. For instance, in a Mediterranean annual grassland after three years of experimental manipulation, plant diversity decreased with elevated CO₂ and nitrogen deposition, increased with elevated precipitation and showed no significant effect from warming (Zavaleta et al., 2003). Diversity responses to both single and combined global change treatments were driven mainly by significant gains and losses of forb^[1] species (Zavaleta et al., 2003). Elevated CO₂ influences plant species composition partly through changes in the pattern of seedling recruitment (Edwards et al., 2001). For sown mixtures, the TAR indicated that elevated CO₂ increased legume development. This finding has been confirmed (Luscher et al., 2005) and extended to temperate semi-natural grasslands using free air CO₂ enrichment (Teyssonneyre et al., 2002; Ross et al., 2004). Other factors such as low phosphorus availability and low herbage use (Teyssonneyre et al., 2002) may, however, prevent this increase in legumes under high CO₂.

How to extrapolate these findings is still unclear. A recent simulation of 1,350 European plant species based on plant species distribution envelopes predicted that half of these species will become classified as ‘vulnerable’ or ‘endangered’ by the year 2080 due to rising temperature and changes in precipitation (Thuiller et al., 2005) (see Chapter 4). Nevertheless, such empirical model predictions have low confidence as they do not capture the complex interactions with management factors (e.g., grazing, cutting and fertiliser supply).

New Knowledge: Changes in forage quality and grazing behaviour are confirmed.

Animal requirements for crude proteins from pasture range from 7 to 8% of ingested dry matter for animals at maintenance up to 24 % for the highest-producing dairy cows. In conditions of very low N status, possible reductions in crude proteins under elevated CO₂ may put a system into a sub-maintenance level for animal performance (Milchunas et al., 2005). An increase in the legume content of swards may nevertheless compensate for the decline in protein content of the non-fixing plant species (Allard et al., 2003; Picon-Cochard et al., 2004). The decline under elevated CO₂ (Polley et al., 2003) of C₄ grasses, which are a less nutritious food resource than C₃ (Ehleringer et al., 2002), may also compensate for the reduced protein content under elevated CO₂. Yet the opposite is expected under associated temperature increases (see Section 5.4.1.2).

Large areas of upland Britain are already colonised by relatively unpalatable plant species such as bracken, matt grass and tor grass. At elevated CO₂ further changes may be expected in the dominance of these species, which could have detrimental effects on the nutritional value of extensive grasslands to grazing animals (Defra, 2000).

New Knowledge: Thermal stress reduces productivity, conception rates and is potentially life-threatening to livestock.

The TAR indicated the negative role of heat stress for productivity. Because ingestion of food and feed is directly related to heat production, any decline in feed intake and/or energy density of the diet will reduce the amount of heat that needs to be dissipated by the animal. Mader and Davis (2004) confirm that the onset of a thermal challenge often results in declines in physical activity with associated declines in eating and grazing (for ruminants and other herbivores) activity. New models of animal energetics and nutrition (Parsons et al., 2001) have shown that high temperatures put a ceiling on dairy milk yield irrespective of feed intake. In the tropics, this ceiling reaches between half and one-third of the potential of the modern (Friesians) cow breeds. The energy deficit of this genotype will exceed that normally associated with the start of lactation, and decrease cow fertility, fitness and longevity (King et al., 2005).

Increases in air temperature and/or humidity have the potential to affect conception rates of domestic animals not adapted to those conditions. This is particularly the case for cattle, in which the primary breeding season occurs in the spring and summer months. Amundson et al. (2005) reported declines in conception rates of cattle (Bos taurus) for temperatures above 23.4°C and at high thermal heat index.

Production-response models for growing confined swine and beef cattle, and milk-producing dairy cattle, based on predicted climate outputs from GCM scenarios, have been developed by Frank et al. (2001). Across the entire USA, the percentage decrease in confined swine, beef and dairy milk production for the 2050 scenario averaged 1.2%, 2.0% and 2.2%, respectively, using the CGC (version 1) model and 0.9%, 0.7% and 2.1%, respectively, using the HadCM2 model.

New Knowledge: Increased climate variability and droughts may lead to livestock loss.

The impact on animal productivity due to increased variability in weather patterns will likely be far greater than effects associated with the average change in climatic conditions. Lack of prior conditioning to weather events most often results in catastrophic losses in confined cattle feedlots (Hahn et al., 2001), with economic losses from reduced cattle performance exceeding those associated with cattle death losses by several-fold (Mader, 2003).

Many of the world’s rangelands are affected by ENSO events. The TAR identified that these events are likely to intensify with climate change, with subsequent changes in vegetation and water availability (Gitay et al., 2001). In dry regions, there are risks that severe vegetation degeneration leads to positive feedbacks between soil degradation and reduced vegetation and rainfall, with corresponding loss of pastoral areas and farmlands (Zheng et al., 2002).

A number of studies in Africa (see Table 5.3) and in Mongolia (Batima, 2003) show a strong relationship between drought and animal death. Projected increased temperature, combined with reduced precipitation in some regions (e.g., Southern Africa) would lead to increased loss of domestic herbivores during extreme events in drought-prone areas. With increased heat stress in the future, water requirements for livestock will increase significantly compared with current conditions, so that overgrazing near watering points is likely to expand (Batima et al., 2005).