Window on the future: fruit and nut growers can adapt to climate change
Winter chill: two words that carry grave implications for many growers of temperate fruit and nut trees in warm regions in the face of our changing climate. But according to World Agroforestry Centre scientist Eike Luedeling, there is much that can be done to help growers adapt.
Great taste and good looks are not the only things apples, peaches, apricots and pistachios have in common: they also undergo winter dormancy, a particular adaptation to the cold winters found in their native, temperate climates. This dormancy protects the trees’ sensitive tissues against winter frost damage until it gets warmer. Even when grown in warmer climates, temperate tree species undergo a period of dormancy, during which all visible growth halts and physiological processes are slowed or suspended.
Breaking out of this period each spring requires the tree to have been exposed to a certain amount of cold conditions, or winter chill—a requirement that is crucial to producing economically viable yields.
Scientists, however, are predicting that in the face of warmer temperatures due to climate change, winter chill requirements will become harder to meet in many important temperate-fruit and -nut-producing areas, including southern and northern Africa, Australia, southern Europe and South America.
Recognizing the critical link between winter chill and fruit and nut production, several approaches to modeling chill have been developed – including the Chilling Hours model, the Utah model and the Dynamic model - and are currently employed by growers. But are these models sufficient to inform climate adaptation planning?
According to Eike Luedeling, a climate change scientist with the World Agroforestry Centre (ICRAF) and author of a recent review published in the journal Scientia Horticulturae, some of the common approaches to modeling winter chill are inaccurate, especially when applied in warm growing regions.
In particular, despite its popularity with growers, a range of studies point to inaccuracy in the Chilling Hours model and cast serious doubt on its value for climate change projections. And while the Dynamic model appears to be the most accurate approach at present, it has seen little uptake by practitioners due to its complexity.
“Assuming that one of these models is ‘correct’, chilling estimates by a grower using the wrong model could be off by a factor of more than 2, in extreme years” says Luedeling. This means a grower could overestimate winter chill by more than double the actual amount – or grossly underestimate it. “Selecting cultivars based on an inaccurate model is a risky gamble that could result in growers introducing trees that are a very poor match for future conditions,” he adds.
Luedeling says one way to ensure future productivity in the absence of a full understanding of tree dormancy and accurate climate models is the use of climate analogue analysis. This new approach to adaptation planning capitalizes on the fact that the future projected climates for a given location can be found today in another location.
For example, the climate projected for a particular growing region in South Africa for the year 2030 might be found today in Argentina or the United States. These analogous locations offer growers a window on the future that can inform adaptation planning as their climate shifts. Farmers in a particular growing region could use an analogous location both to identify appropriate future species and cultivars and to test-drive potential species and cultivars in a ‘future’ climate.
In 2011, Luedeling and co-authors provided the first global-scale analysis of the effects of climate change on the available winter chill for temperate fruit and nut trees using the best model available at present, the Dynamic Model. Their results, published in the online journal PLoS ONE, predicted substantial reductions in winter chill for most warm growing regions, with particularly drastic implications for 21st-century production in North Africa, South Africa, the Southern United States and Southern China, among others.
Growers of temperate fruit and nut trees typically use a number of strategies to get around unseasonably warm conditions: introducing cultivars that require less chill, manipulating orchard microclimates through shading and/or targeted irrigation, artificially inducing tree dormancy through defoliation, and applying rest-breaking chemicals that promote budding. Replacing orchards with better-suited cultivars is often the last resort, as it represents a significant economic investment for growers due to high establishment costs and the long-term nature of orchard production. Growers, therefore, must carefully select trees that are suitable for their location from the outset.
Luedeling says the challenge to accurately model winter chill lies primarily in the need to better understand tree physiology.
“Despite 200-plus years of scientific interest, our understanding of tree dormancy is still far from complete. Without a basis in physiological processes, the accuracy of chilling models – and especially their ability to extrapolate to future climatic conditions – remains limited.”
When it comes to choosing the best cultivars for future climates, it is unlikely that growers can afford to play a waiting game while science develops models that can more accurately inform adaptation planning. In the interim, climate analogue analysis could prove extremely useful in selecting cultivars that will carry on producing good yields into future, warmer climates,
“For complex agricultural systems, searching for future climates among present-day locations and extracting adaptation lessons from such sites may be the most promising strategy for ensuring that production remains viable in a climatically changing future,” says Luedeling.
Read the full article: Climate change impacts on winter chill for temperate fruit and nut production: A review.
Growing Temperate Fruit Trees in Kenya by J. Griesbach