In this report, they indicated that warmer temperatures during the crop-growing season were favorable to the cooler regions of California, but unfavorable to the arid regions. This result was consistent with national studies which showed that crop productivity increased with temperature in more northern latitudes of the United States, and decreased with increased temperatures in some of the southern regions of the Country. This may be explained by crop productivity in cooler regions benefiting from additional degree-days of warming, whereas crops in warm regions may already be at the heat threshold level . As discussed above, there are shortcomings with the simple quadratic equation approach, and crops may actually respond more strongly to increased temperature and CO2 than indicated in these studies For specialty crops such as stone fruits and grapes, water stress, temperature and the timing of precipitation can be extremely important for sustainable yields and maximizing fruit quality . However, for rain-fed crops and grazed lands, where the most productive seasons are late fall, winter and early spring, water use patterns may change markedly as a result of higher evapotranspiration . Adams et al. found that most regions and climate change scenarios for California indicated an increased demand for water over time. Also, increased precipitation did not affect water use or crop yields because many California crops are irrigated . For some crops, increased precipitation in the summer or fall would result in an increased incidence of fruit rot and decreased fruit quality. However, elevated levels of CO2 reduce crop ET,growing hydroponically primarily through a reduction in stomatal aperture, and controlled experiments that measured crop water use under elevated CO2 have shown that most crops produce similar or increased yields with less water .

Future crop water use is difficult to predict due to climate variability, increasing temperatures, and increasing CO2 concentrations. Increasing CO2 and temperatures may balance ET overall, however, water storage in the snow pack of California is predicted to decrease , which will alter the amount and timing of water available to agriculture for irrigation . As a result, California will need to cope more effectively with the constraints of its Mediterranean-type climate than it has done in the past. Even if precipitation increases, water storage will remain an important problem, and issues will arise that require that more research is devoted to understanding crop water responses, and effects of rainfall on crop quality.California agriculture can ultimately respond to the physiological impacts of climate change through cultivar selection and crop management practices designed to respond to changes in crop development. Observed cultivar variation in heat tolerance and access to germplasm from regions with higher temperatures may provide opportunities to breed better adapted cultivars for a variety of crops . Better understanding of plant physiological responses to elevated CO2 and the interacting effects of mineral nutrition, temperature, and O3 are is required to effectively guide breeding for crop performance in a changed atmosphere. Additionally, management practices such as the manipulation of planting dates and timing of thinning can be adjusted to take advantage of changes in crop development and available resources . However, adoption of new cultivars and timing of management practices will be more easily implemented for annual than perennial crops, which require more time and greater investment for cultivar development and crop establishment. Heat stress in cattle is alleviated by shade because it reduces the external radiant heat load . Cooling of the drinking water and acclimation of the animals are other useful strategies to help cattle maintain homeothermy.Selection for heat tolerance may be in conflict with maximizing high yield. In the last 50 years, metabolizable energy for milk production and heat energy have been steadily increasing. Thus, breeding for increased milk production has also changed the thermal regulatory physiology of cows .

Climate change, both within California and globally , is likely to have a significant impact upon the types, abundance and impacts of agricultural weeds, pests, and diseases. While climate change may be advantageous to some species that provide ecosystem services , such benefits will likely be offset by population increases in groups such as invasive exotics, invertebrate pests and disease causing microbes . Predicting these changes rests on better understanding of their ecophysiology and the complexity of the multi-trophic and multi-factor interactions in which they are involved. Here we review literature on agricultural weeds, pests, and disease causing microbes and how they may be impacted by climate change in the context of California agriculture. Noxious and invasive weeds infest over 20 million acres in California and are estimated to cost hundreds of millions of dollars in control expenses and lost productivity annually . Both the direct economic impacts and many of the indirect impacts of these plants such as reduced plant diversity, threatened rare and endangered species, reduced wildlife habitat and forage, altered fire frequency, increased erosion, and depleted soil moisture and nutrient levels may well be exacerbated by interactions with a changing climate . The nature of these interactions, and their variation between different commodities and growing regions, poses a serious problem for decision–maker’s response to changes in the climate of California, but are germane to achieving agricultural sustainability in California.Although increased atmospheric concentrations of CO2 may favor C3 species thereby altering competitive interactions between C3 and C4 species , higher temperatures are expected to favor plants utilizing the C4 photosynthetic pathway . Some efforts to understand these interactions have been made; for example, Tremmel and Patterson studied the growth and allocation of five weed species treated with a gradient of CO2 concentrations and two temperature regimes. Their results demonstrate that generalizations about interactions are difficult; different species and different populations within the same species showed different responses to the same treatment. Similarly, Taylor and Potvin demonstrated that even single factor experiments yield unpredictable outcomes when conducted in an ecosystem context.

In summary, though the effect of individual factors on specific functional groups is well-understood, interactions between these factors often yield unpredictable outcomes which are likely to become even less predictable in natural settings. One example of how such changes could manifest themselves in California involves experiments conducted on Hemizonia congesta, a late-season California native which is similar in phenology and in other respects to Centaurea solstitialis , a problematic Californian weed that us unpalatable except when young. Elevated atmospheric concentrations of CO2 can benefit H. congesta through increased late-season water availability ,growing strawberries hydroponically suggesting that the weed may also benefit. This may be reason for concern because many invasive plants share traits with this endemic species and because water is often limiting in hot, dry summers typical of a Mediterranean climate.Many invasive plants and agricultural weeds are expected to expand their range in response to climate change in a fashion which will likely increase their impact in California. One way to assess northern range limits of tropical and warm temperature annual species is by accumulated heat sum, measured in degree days , during the growing season . Since the number of degree days are expected to increase , new invaders and weeds may become prevalent as appropriate habitats develop and these species extend their range. It has been suggested, for example, that C4 grass weeds which are problematic in the southern U.S., may expand into higher latitudes as a result of global warming ; similar effects may be seen with elevation. Given the prolific nature of most weeds and invasive plants and their exceptional colonization capacities , these C4 grass weeds may be among the first to exhibit such range expansion. The effects of a warmer, more extreme climate, and the relatively disturbed nature of much of California, especially in the Central Valley, may predispose susceptible agricultural systems to quickly encounter new and more vigorous weeds .A complimentary contraction of southern range boundaries of weed species is not necessarily expected. It is now known that detectable adaptive divergence evolves on a time scale comparable to change in climate; within decades for herbaceous plants and within centuries or millennia for longer lived trees . Because many weeds become reproductive at an early age and are highly fecund, rapid rates of evolution will likely play a significant role in their response to climate change. While range expansions are to be expected for many species, range contractions are less likely in rapidly evolving species with significant populations already established.

Similarly should range contractions occur, it is likely that new or different weed species will fill the emerging gaps/niches. Many successful invaders and weeds such as field bindweed , giant reed , and jubata grass , reproduce primarily asexually and their populations might therefore more readily be reduced do to climate change due to their clonal nature. However, asexually reproducing clonal plants on average are not less genetically variable non-clonal plants , and thus the potential for an evolutionary response exists. There are however, large knowledge gaps regarding the evolutionary genetics of clonal plants, making any definitive conclusion difficult .California farmers contend with thousands of crop-damaging invertebrate and vertebrate pest species. As a result of adaptation to climate change, their abundance, types, and activities will likely be altered in the future . This is especially true of invertebrate pests which have rapid generation times, and as such an ability to change to a gradual shift in selection pressures, almost certainly more rapidly than their host plant species , and that of weeds . In 2002 the cost of pesticide use in California was $49.25 million . In recent years California agriculture has adopted Integrated Pest Management , an ecosystem-based strategy that focuses on long-term prevention of pests through a combination of biological control, changes in cultural practices and the use of resistant varieties, as well as chemical control when necessary , for pest management. The efficacy of these different control measures are to a certain extent determined by climate. Invertebrates cause problems such as damaging of crops, vectoring disease, contamination of food and fiber, and export and quarantine problems. Vertebrate pests transmit diseases and parasites, burrow and disturb crop plants and pastures, and damage trees resulting in sap loss and allowing infestation by harmful insects and/or pathogens. Any pest management strategy must be carefully designed, so that beneficial organisms are not negatively impacted and are able to persist. For example, many Californian farmers use IPM, including encouraging bats, burrowing owls, and kestrels on to their properties in order to help control damaging insects, rodents and other pests. Biological control agents, such as parasitoids and predators, and other beneficial species such as pollinators provide important services to agriculture ; Norris and Kogan, 2000. Impacts of a changing climate on pest species and their control are discussed here.Agriculture impact assessments do not account for all impact factors, such as potential yield losses due to changes in pest dynamics and density under climate change . While the Agricultural Assessment Group with the US Global Change Research Program considered the effect of pesticides in their model, they did not account for the effect that changing pest populations had on yield losses . This deserves further attention. For example, in a study of a pest aphid species in Britain Aphis gossypii Glover , the aphids migrated 3-6 days earlier as temperatures increased by 0.4o C over 25 yrs, which has significant implications for epidemiology of aphid vectored virus diseases in economically important crops such as barley and sugar beet . Accurate prediction of insect development and emergence are essential for effective pest management, but can be challenging as it is virtually impossible to measure the micro-environments in which pests actually live. Pest management decisions should take into consideration oquantitative information on dispersal of invertebrate pests, but such information is often lacking . Additionally, invertebrate pests are hard to detect and monitor. Farmed landscapes may need to provide opportunities for natural enemy species to disperse between habitats . However, great diversity of crops along with its own complement of pests creates logistical challenge for planning and implementing successful pest management programs, in a changing climate. This is especially true of California given its many different agricultural commodities and regions .