Climate Smart Farming
#3 Climate Change and Nut Trees
There are good reasons for growing nuts if you have space, water, a suitable climate and good soil conditions. Nuts are a welcome addition to a healthy diet. Nut trees are long lived and frequently deep rooted. They remove carbon from the atmosphere and can provide a comparatively secure carbon store both above and below ground. Their root systems can seek out moisture and nutrients from much lower than many other plants. They are often better able to survive dry periods. They can usually draw water and nutrients from deep in the soil and improve nutrient cycling and reduce pollution of rivers, lakes and wetlands. There is increasing demand for many nuts and nut products in Australia and globally.
Deciduous nut trees such as walnuts, almonds, hazelnuts, and chestnuts need three main things. They need some heat, so a nice warm summer. They need chill to go dormant and support spring bud burst and pollination, so a cool winter. And they need water to grow, flower, and set fruit. For good nut yields all three - water, warmth, and winter chill hours – are needed in sufficient quantities and at the right times.
From these needs we can identify some of the climate change risks to nut trees - rising temperatures, rainfall changes and change in seasonal conditions. The level of risk varies with species and variety of tree, site conditions and management practices. In some cases, the main risk is to the yield of nuts from the tree and in other cases to risk is broader - to the tree’s growth and sometimes to the tree’s survival. Combinations of extremes can have worse impacts. Lack of water combined with hot, windy conditions will more quickly result in wilting. High temperatures combined with water stress and/or nutrient deficiency risks the trees growth and reproduction declining as a result of photoinhibition - a persistent decrease in the efficiency of solar energy conversion into photosynthesis in combination with a decreased overall capacity for photosynthesis.
There are a range of actions nut growers can take to reduce climate change risks to nut trees - both in the planning and managing of trees and orchards. Several species of deciduous nuts have evolved in regions with cold winters and hot, often dry, summers.These species can have a higher resilience to climate change and help in establishing microclimates that can support other species and improve the particular site for people living and working there.
Temperature increases
Nut trees need enough warmth (expressed as growing degree days) for nuts to mature and ripen. Climate change is most likely to increase the growing degree days available and the ranges for some species are moving towards the poles and increasing in altitude.
But nut trees will suffer if temperatures are too high. The temperature increases that are occurring with climate change in Australia increase the risk of heat stress for most types of nut tree, although some species and varieties can cope with higher temperatures better than others.
The transpiration potential of many trees will start to lower when temperatures start to rise above 30 degrees C. This means that the tree slows in its uptake of water and nutrients from roots and distribution of these through its trunk, limbs and to its leaves.
Once temperatures start to climb above 35 degrees C complete shutdown may occur. The tree’s heat threshold has been reached. It closes its stomata to conserve water and transpiration stops. The length of time that temperatures are above the heat threshold is critical. For every hour a plant is shut down, it can take two hours to recover. During a heat wave, transpiration may become severely limited as the plant does not have sufficient time to recover overnight before the heat of the next day begins.
High heat events also increase water stress, resulting in more irrigation demand. High temperatures during certain times can also burn reproductive tissues, reducing nut set. Temperatures in the high 30’s can also lead to sunburn of trunks and nut kernels and kernels can shrivel and darken.
Many deciduous trees are flowering and producing leaves earlier because of climate change. In areas which experience spring frosts such as the Bega Valley a frost after budding can seriously impact nut production.
Chilling requirements
Deciduous trees have a period of dormancy in winter each year. In Australia, this is generally from about 1 May to 1 September. Dormancy protects the trees from frost damage. The timing for the break of dormancy is determined by the tree’s ability to sense temperature, both the amount of chill that has occurred and the spring warming. If trees break dormancy too early the tree will start its annual growth cycle early and flowering and leaf budding will be frost damaged. If they break dormancy too late the annual growth cycle will not be completed before the following winter.
Each hour under 7.2 degrees C is called winter chill hour. Without sufficient winter chill, flower budding and pollination is disrupted and the tree does not produce good yields of fruits and nuts. Research in Australia has shown that decline in chill hours is likely to impact nut and fruit production across many sites in southern Australia. More southerly and higher altitude locations may be less affected. Figure 1 shows predicted reduction in chill hours as a result of climate change.
Figure 1: Decrease in chill hours across Southern Australia and New Zealand with climate change. (Adapted from: Luedeling et al, 2011)
Species and varieties of deciduous nut or fruit trees have their its own specific requirement for winter chill hours. The chill hours for walnuts for example can range from 500 to 1500 hrs, depending on the variety. The current average generally experienced in the Bega Valley is adequate for almonds, chestnuts, walnuts and hazelnuts. Each site, however, has its own microclimate and thus specific chill hours and expected degree of reduction in chill hours with climate change. North west facing slopes, will for example experience lower chill hours and river flats higher chill hours. In planning to plant nut trees it is important to consider the likely decline in chill hours with climate change, the specific site characteristics and the specific requirement of species and variety of tree.
Water needs
While many nut trees can withstand some dry periods, enough water is needed to establish young trees, to support the tree’s annual growth cycle, to help the tree manage both frost and high temperatures and to ensure good nut crops. Water needs increase when temperatures and wind strengths are high. Drought will affect the leaf area, both through loss of leaves and through failure of new leaf buds to develop. The plant will be less efficient at photosynthesis. A period of drought has a carryover effect from one growing season to the next. Under extreme water deficits trees will wilt and loose leaves. Prolonged periods of wilting lead to tree death.
In areas prone to long dry periods, such as the Bega Valley, access to enough stored water and some form of irrigation is required. Water is needed in the growing season, and particularly as nuts develop and fill. To achieve good nut yields, almonds are likely to need about 10 ML water / hectare during the growing season (October to May), chestnuts about 6 ML and walnuts up to 10 ML (including rainfall). Before planting nut trees, consider whether you have enough water to establish young trees, to maintain trees, to help them manage through hot and windy periods, and, once your trees are mature, to maintain high enough yields in dry periods or whether you will need to sacrifice yields. Trees will also need water late in the growing season to help prepare them for the next growing season. To support mature walnut trees, for example, for overwintering and growing in the next season, weekly water of about 2 to 3 cm will generally be required in March/April and 1 to 2 cm per week in April/May.
Rainfall at the wrong times, such as during harvest in autumn is also a problem. It can increase the disease risk and reduce nut quality, result in waterlogging if the orchard is not well enough drained, leach mobile nutrients beneath the root zone of the trees and worsen soil erosion.
How can we adapt our nut growing to climate risks?
In planning new plantings consider the risks of increases in temperatures, strong winds, long dry periods and heavy rainfall. These risks will vary from site to site. Try to identify the best locations on your farm to plant. North west slopes will be hotter and dry out more quickly. They will also need more protection from the northwesterly winds which are increasing in strength. A lower site near a river may be more susceptible to late spring frosts. Trees will grow more easily in deep, fertile soil with plenty of organic matter, but you can improve the quality of your soil with organic fertilisers, compost and mulches. How easy will it be to get water to the trees at your planting location? Is there work you can do on your land to slow water flows, capture more water for your trees’ roots and, at the same time, ensure good drainage? Some nut trees, for example, walnuts, grow very large eventually. They will need enough space and to be placed so as not to impact on other plants.
Make sure you will have sufficient stored water to give your young trees a good 20 litres each per fortnight throughout the growing season (September to May) for their first couple of years, noting that in some growing seasons, such as 2019, rainfall may be almost non-existent. Talk to other people growing nuts in similar climates and look for varieties which are more resistant to heat and drought stress, require less chill hours and are less likely to form flower buds early in spring when frost is likely. Investigate both old and new varieties for resilience to climate change impacts. Breeding new varieties of deciduous nut trees takes time, but research is underway. Some of the old varieties are very hardy and there is increasing interest across the globe in identifying those varieties which are more likely to withstand climate change.
Once you have chosen planting sites, make sure you have all the materials you need for successful planting: holes of sufficient depth and width, good drainage, mulch, protection from animals, good planting medium, water, and frost and wind protection if needed.
If you already have nut trees, consider what you can do to help them cope with climate change. If temperatures are high and days clear, consider whitewashing trunks to prevent sunburn. Do your trees need a windbreak? Are you conserving water by mulching, making sure evaporation is reduced and by drip irrigating under mulch? Do you need extra water storage for the longer and hotter dry periods? There are different management practices for specific species and varieties that can help build the tree’s resilience. Talk to other growers and research horticultural and nut growing websites for information on your trees. For example, a healthier, deeper root system can be encouraged for mature walnut trees by delaying watering in spring for 25-35 days after bud burst. Walnut yields will, however, be reduced if the trees experience water stress in late January and February. To support walnut trees for overwintering and next season, weekly water of 2 to 3 cm will generally be adequate for mature trees in March/April and 1 to 2 cm per week will meet the need in April/May.
Resources and extra reading
Afrey, P., 2016 The Essential Guide to Everything you Need to Know about Growing Walnuts – Juglans regia
Website Link
NSW Department of Primary Industries, 2016 An objective basis for temperate nut industries’ expansion
Website Link
Australian Nut Industry Association: Climate Change: Australian Nuts
Website Link
Luedeling, E., Girvetz, E.H., Semenov, M.A. and Brown, P.H., 2011. Climate change affects winter chill for temperate fruit and nut trees. PloS one, 6(5), p.e20155.
Website Link
Cone, M 2019 In a Race Against Warming, Growers Try to Outsmart Climate Change
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