Mar 4, 2015Growers will have to adapt to changing weather patterns
The two agencies recently announced that Earth’s average surface temperature has warmed by about 1.4˚ F since 1880, and that the “majority of that warming has occurred in the past three decades.” In fact, the 10 warmest years in the instrumental record have occurred since 1998.
This long-term warming trend is “largely driven by the increase in carbon dioxide and other human emissions into the planet’s atmosphere,” according to NASA.
Often referred to as “climate change,” this trend poses “unprecedented challenges” to U.S. agriculture, according to a USDA report published in 2013.
The report, called “Climate Change and Agriculture in the United States: Effects and Adaptation,” states that climate change – increased carbon dioxide in the atmosphere, rising temperatures and changing precipitation patterns – will affect agricultural productivity through the next few decades and beyond. Effects will vary based on crop and region, but the overall picture for U.S. agriculture is not positive.
According to the report, which cites dozens of studies, climate change will affect the future production of specialty crops, including fruits and vegetables grown on both annual and perennial cropping systems. It could make things especially complicated for perennial systems, which can be in place for up to 30 years, because the cultivar selected at planting might not adapt well to long-term changes in the climate. And growers of perennial crops can’t just relocate to avoid the problem: Long re-establishment periods, nearness to processing plants, availability of labor, market accessibility and other factors make it extremely difficult to move perennial cropping systems.
Rising temperatures and longer periods of drought will affect production of annual specialty crops, too. For example, a shortage of water can lead to smaller leaves, roots and berries in strawberries, thus reducing yields. High temperatures and wind speeds also can reduce potato yields, according to the report.
The report detailed the effects of heat stress on tomatoes. Mild heat stress (1˚ C to 4˚ C above optimal growth temperature) commonly leads to moderately reduced yield. More intense stress (greater than 4˚ C above optimum) can lead to severe yield loss – including a complete failure of marketable produce. Tomatoes under heat stress struggle to produce viable pollen. The non-viable pollen does not properly pollinate flowers, causing a failure in fruit set. However, if the same stressed plants are cooled to normal temperatures for 10 days before flower pollination and returned to high heat, they are able develop fruit. During summer, high temperatures can cause sunburn damage, reduce pack-out at harvest, accelerate maturity, reduce fruit firmness and color development and decrease the suitability of fruit for storage.
According to NOAA, “regional differences in temperature are more strongly affected by weather dynamics than the global mean.” Parts of the Midwest and East Coast were unusually cool in 2014, for example, while Western states experienced their warmest year on record.
In other words, the changing climate will have different effects in different regions.
West. Climate change could make the production of irrigated crops less profitable, especially in states like California and Arizona, where water is restricted and/or expensive. If that were to happen, cultivation could shift to states where water is more available, according to the USDA report.
In addition to increasing the number and viability of insects, climate change could jeopardize biological control programs. For example, extensive biological control efforts are underway to combat the vine mealybug (VMB), a major pest of grapes in California. High VMB densities occur in the state’s northern regions, as well as the coastal regions of southern California. The distribution of VMB’s natural enemies is patchy across the different grape regions, however. Success to date has been elusive, but if biological control of VMB ever is established, climate change could adversely affect it, according to the report.
Great Lakes. Last fall, Jeffrey Andresen, a professor and climate specialist at Michigan State University (MSU), said that according to most climate projections the Great Lakes region would continue to get warmer and wetter. A warmer, wetter climate can benefit the region’s agriculture by increasing yields and crop diversity – but it also can lead to more pests and diseases and an increase in the number of extreme weather events.
In the last three decades, there’s been an “unmistakable” trend toward an earlier onset of the growing season. In many parts of Michigan, for example, the seasonal warm-up is occurring a week to a week and a half earlier than it did 30 years ago. An earlier spring extends the season, but it also increases the frequency of freeze events, which can be especially damaging to sensitive specialty crops. The risk of frost damage is “at least double” what it was 50 years ago, Andresen said.
The long-term trend has been toward more wet days and heavier precipitation per event. Compared to 50 or 60 years ago, there’s an extra 4 inches of water coming down, he said.
In some ways, more precipitation is a good thing. Average soil moisture is increasing, which means plants are less likely to run out of water than they were in the past. Most annual precipitation increases will occur in the cold season, however. In summer, when plants really need the water, precipitation levels are projected to hold steady or even decrease, Andresen said.
East. In the Northeast, the historical trend has been toward increased frequency of high-precipitation events (greater than 5 centimeters within 48 hours); this trend is expected to increase through the end of the century, according to the USDA report.
“More spring rainfall concentrated into high-precipitation events, combined with stable to modest reductions in summer and autumn rainfall and increased temperatures, leads to a projection for more short- (one- to three-month) and medium-term (three- to six-month) droughts for the region, particularly in the northern and eastern parts,” according to the report.
A combination of increased drought frequency and warmer growing-season temperatures in the Northeast will result in a greater need for water from crops. Many specialty crop producers in the region have some irrigation equipment, but most haven’t invested in enough equipment to optimize their irrigation scheduling, according to the report.
The warming trend in the Northeast will lead to an earlier arrival, more generations and higher wintertime survival of insects, which will lead to increased pesticide use. And some classes of pesticides, like pyrethroids and spinosad, are less effective in higher temperatures, according to the report.
The projected increase in summer heat stress will be detrimental to many cool temperature-adapted crops, such as apples, that dominate the Northeast agricultural economy. For many high-value horticultural crops, short-term, moderate heat stress at critical growth stages can reduce fruit quality by reducing visual or flavor quality, even when total tonnage is not reduced, according to the report.
Mid-winter warming in the Northeast can lead to early bud-burst or bloom of some perennial plants, resulting in frost damage when cold winter temperatures return. Yields will be negatively affected if the chilling requirement is not completely satisfied, because flower emergence and viability will be low. All perennial specialty crops have a winter chilling requirement ranging from 200 to 2,000 cumulative hours. A 400-hour chilling requirement will continue to be met for most of the Northeast during this century, but crops with prolonged cold requirements (1,000 or more hours) could be negatively affected, according to the report.
Other Eastern states, such as Florida, Georgia and New York, have been experiencing increased precipitation. This leads to a greater reservoir of irrigation water but an increase in extreme weather events. Severe flooding, for example, can kill plants, while less severe flooding can make them weaker. Moderate flooding of strawberries, for example, can decrease fruit yield, total leaf area and weight. Strawberries grown in excess water have lower sugar content and have been known to taste “watery.” Carefully restricting water, on the other hand, can increase sugar content and make the berries taste sweeter, according to the report.
Even storms that produce minor flooding or no flooding can damage marketable yield. Tomatoes are known to crack and split after a storm. High winds knock plants down. Storms can even reduce the yields of root crops like onions, according to the report.
For perennial specialty crop systems, the long-term solution to climate change is the development of adaptable cultivars. Typical breeding programs have needed decades to fully develop a cultivar, but new technology can shorten the development period, according to the report.
“For an individual crop, there are often cultivars with higher tolerances for stressful temperatures, water availability, light and other environmental factors, just as there are cultivars that are resistant or susceptible to certain diseases,” the report stated. “Studies of specialty crops have identified promising sources of heat-tolerant genetic material. This is important because borrowing superior stress tolerance mechanisms from overall inferior plants is a crucial way to improve the varieties that are grown every day in commercial production. At least one promising source of heat tolerance was identified when assessing cultivars of strawberry, tomato, lettuce, onion and potato. Cultivar differences in tolerance to temperature extremes appear to be greater than for any other environmental stressor.”
Growers of perennial crops have a wide assortment of other tools to help them adjust to climate change, including crop load adjustment, canopy pruning, irrigation and increased use of mechanization and automation technology, according to the report.