California Crops Under Climate Change

Agriculture in the United States is a multi-billion dollar industry, and California is America's most productive agricultural state in the country, producing more than 400 agricultural commodities,  collecting billions in cash receipts , and supporting  hundreds of thousands of jobs  statewide. While California agriculture includes forage, fiber, grains, legumes, vegetables, fisheries, and livestock, California is perhaps most well known for it's fruits and nuts. In fact, California produces nearly two-thirds of the nation's fruits and nuts, and is the primary or sole producer of US-grown almonds, clingstone peaches, grapes, pistachios, and walnuts. Based on data from  USDA-NASS  and  FAO , these high-value perennials collectively cover >2 million acres and generate more than $14 billion in cash receipts, comprising more than 28% of the state’s direct agricultural value.

The mismatch between when precipitation falls and when crops need water, combined with the geography of where precipitation falls relative to where crops are planted, means that  an extensive network of water storage and delivery systems  is needed to irrigate crops. The statewide system of reservoirs, canals, and ditches helps to store winter rain and spring snowmelt, and move water from the mountains and from the wetter Northern part of the state, to the drier valleys and southern agricultural lands.

 Groundwater  also plays an important role in providing irrigation water for growing California's fruits and nuts. Although unregulated pumping of groundwater in the past has threatened the sustainability of this resource, the  Sustainable Groundwater Management Act  seeks to reach a balance between pumping and recharge within 20 years.

Heat extremes can be defined in a lot of different ways. Some definitions use a fixed threshold value and any time temperatures meet or exceed that temperature, an extreme heat event is declared. Other definitions consider the impact to ecosystems and society in determining what meets the bar for "extreme." Studies examining extreme heat from a long-term or climatological perspective often use percentiles to define "extreme." This is a particularly useful definition when comparing extreme heat events between two locations with different climates. For example, a 100℉ (~38℃) July day in San Francisco may seem sweltering, but 100℉ in Bakersfield is just another summer day.

When it comes to the impact of heatwaves on California's crops, timing matters.

Although heatwaves are usually considered a summer problem, warm winter and spring temperatures can also be a problem for fruit and nut trees. For example, many of California's perennials require  exposure to cool temperatures during the winter  in order to bloom and develop correctly in the spring. The amount of winter "chill" needed can vary, depending on the crop and the variety.

When crops don't receive enough winter chill, the timing of bloom may be delayed, which can cause problems for pollination. Fruit set – the process by which flowers develop into fruit – can be reduced when there are problems with bloom and pollination, which in turn reduces crop yields. In fact, in 2015 a warm winter and a lack of chill devastated California's pistachio crop and caused more than  $180 million in crop damages .

Spring heatwaves can also limit the ability of the flowers to develop into fruit by lowering the receptivity of the flowers to pollination. And, for those flowers that do develop into fruit, high springtime temperatures can  decrease the size of the fruit  at harvest.

In the future, warming winter temperatures are expected to reduce the exposure of perennials to needed cool temperatures. This reduction in winter chill could effectively eliminate the production of some fruits and nuts in California by the end of the 21st century. For example, up to 75% of California's Central Valley may be too warm for crops that need more than 700 chill hours (i.e. hours <7.2℃) by the mid-21st century, and as much as 98% of the region may be too warm by the end of the century.

Since 2008, California has produced more than  $2.5 billion in wine grapes  on average each year, contributing to  the multi-billion dollar economic impact  of the California wine industry. Although many wine grape cultivars are well adapted to California's hot summer climate, extreme heat exposure can slow down grape development and disrupt the delicate balance between sugars and flavor compounds with the grape berries – a balance that is crucial for creating quality wine.  Temperatures above 35℃  (~95℉) have been associated with  negative effects on grape development , and temperatures between  30  ℃   and 42  ℃    have been linked to everything from inhibiting the development of flavor compounds to causing grape skins to become sunburned to making the grapes less colorful. These challenges may be of increasing concern over the coming decades as climate change is projected to increase the number of extreme heat days (days >38℃) more than three-fold in some of California's most productive wine grape growing regions.

Wine grapes can be particularly susceptible to summer heat damage because when grapevines are a little bit water stressed, the quality of the grape berries increases. For most fruits and nuts however, irrigation is a welcome relief. In fact, our current understanding for heat-tolerant crops like almonds is that the negative effects of summer extreme heat is largely a function of water stress. 

When heat stress combines with (or leads to) water stress, crop yield, size, and quality can all be affected. For example, moderate-to-severe water stress during nut development can reduce yield, size, and quality in  almond  and  pistachio . And even though some water stress can make peaches sweeter,  fruit size  might take a hit if peach trees are too thirsty. Typically, California's almond and peach growers are  advised  to provide full irrigation (100% ET) to their crops, but pistachio farmers may be able to irrigate less and still avoid heat stress.

Currently, changing when and how much irrigation water is applied is the main way that growers adapt to heat waves. However, as California growers have moved to high-efficiency irrigation systems, it has become more difficult to apply large volumes of irrigation water quickly. In addition, water in the future may be even less available than it is now due to drought or regulation, or it may be of lower quality due to  higher salinity . If quality isn't a concern, deficit irrigation – that is, giving crops less water than their evapotranspiration rates demand – could provide a way to keep crops watered and reduce extreme heat damages, even in times of limited water availability. Still, certain crops at certain stages of development don't tolerate water stress, which could limit a farmer's ability to use deficit irrigation or to prevent heat damages during drought.

Researchers have been focused on adaptive strategies like improved rootstock tolerance to water stress, and better monitoring the availability of water held in the soils in an effort to improve recommendations to farmers on when to irrigate and how much. The future of irrigation is also rooted in technology. For example,  new tools that use remote sensing  will help wine grape growers determine how much more irrigation water to apply during heatwaves. The  GRAPEx project  is a great example of on-going research conducted by USDA scientists to improve water use in perennial crop systems.

In addition to irrigation, there are a number of management strategies that California's fruit and nut farmers can enact to reduce the damages caused by extreme heat. Click through the slide show below to see a few ways growers are adapting to heat extremes.

Many popular and high-value fruit and nut crops have several different varieties, each with different climate needs and tolerances. In fruits and nuts that need a certain amount of winter chill, farmers may choose to plant varieties that require less exposure to cool winter temperatures. Likewise, some varieties of pistachios and wine grapes are known to be more heat tolerant and/or drought tolerant.

Current research efforts include work to identify rootstocks that are more salt-tolerant, and crop breeding programs that are working to develop heat tolerance and low-chill tolerance. Researchers have also noted the importance of genetic diversity and the key function of the wild relatives of our modern fruit and nut crops in ensuring that the future of agricultural biotechnology can make the most of genetic resources.

The USDA Agricultural Research Service plays a critical role in collecting, cataloguing, and preserving the genetic material of agriculturally important plants through the Germplasm Resources Information Network and  National Plant Germplasm System , including the  National Clonal Germplasm Repository for Tree Fruit, Nut Crops, and Grapes , located in Davis.

In California, the USDA California Climate Hub is actively supporting researchers working to identify best-practices for adapting some of California's high-value perennial crops to climate change, including increased heat extremes. The Hub also works with other USDA partners like NRCS to ensure that California's growers have the information and resources they need to meet the challenges that climate change poses head on so that they can keep America's fruit and nut basket full for years to come.

Plant Science (Special Issue), 2020.  Extreme heat effects on perennial crops and strategies for sustaining future production 

Bulletin of the American Meteorological Society, 2018.  The Grape Remote Sensing Atmospheric Profile and Evapotranspiration Experiment 

Agronomy, 2018.  Climate change trends and impacts on California agriculture: A detailed review