Climate Change Trial – Apples in a Warmer World
Unfortunately, due to the serious damage to the climate change trial facility caused by storms Dudley, Eunice, and Franklin last February, the Trustees felt that the repair costs were too great and the trial was brought to a close. Although this was earlier than hoped, the trial has yielded six years of high-quality data and given a deeper understanding of how apples are likely to be affected by a warming climate.
Purpose of the Trial
The trial was designed to investigate the effect of possible changes in the climate on a range of apple varieties. To achieve this, 20 varieties were planted to include ones of interest to commercial growers, gardeners, and some with particular characteristics from the Collection at Brogdale, such as very early or late flowering and harvest times. One-third of the orchard was kept at ambient temperatures, one-third was in a tunnel 2oC warmer and the other third in a tunnel 4oC warmer. Recording was carried out for six years (2017-22), one PhD thesis has been written based on the findings in 2017-19, and another is in preparation covering the later years.
Earlier flowering and harvest. Over the last 40 years or so the increase in temperatures has led to flowering times being around 17 days earlier. The
trial has shown that we can expect a further 1.5oC rise to advance flowering by another 17 days, and harvest times to move more into August than September as is usual at the moment.
More variable yields. The trial has shown very clearly that warmer temperatures affect different varieties in different ways. In some varieties flowering times were advanced by a lot more, but the time from flowering to harvest was consistent across the different temperature
regimes in the trial. In others the flowering time was similar but the time from flowering to harvest in the warmer tunnels was shortened significantly. Another important finding is that in varieties that have a tendency to biennial bearing (a large crop one year followed by a much smaller crop in the next year, then a large crop again) the amount of flower produced after a warmer winter was reduced to the point that yields were lower and the swings between a large crop and a small one became more pronounced.
Fruit quality. The second PhD study overseen by the Trust looked specifically at the effect of the different temperature regimes on the eating and keeping qualities of the fruit of Gala. The results show that warmer temperatures can lead to reduced fruit crispness, sweetness and keeping quality.
Pest and Disease.
All through the seven years of the trial it has been noted that the pest woolly aphid thrived in the conditions in the two warmer tunnels, especially in the +2oC regime. It was also noted that in the warmer tunnels, the trees produced more growth which in turn makes them more susceptible to leaf diseases such as mildew and scab.
Implications. The trial has shown that, if climate predictions are correct, growing fruit is going to be more challenging in the future. Unfortunately, having temperatures similar to the South of France is only part of the picture as light levels will not become the same as in the Mediterranean. Early flowering times expose blossom to a greater risk of frost damage, so growing fruit in frosty areas will be more difficult and more frost protection may become necessary. Increased growth will mean that techniques such as delayed pruning during the growing season and root pruning to reduce tree vigour may need to be employed. In varieties with a biennial bearing habit, such as Cox and Golden Delicious, more attention to thinning both of fruits and flowers in the ‘on’ year will become important. As fruit quality can be impaired in warm years it will be much more important to harvest quickly at the correct time and get the fruit into ideal cool storage conditions as soon as possible after picking. More attention to pest and disease levels will be required to ensure that they do not increase to a point when the tree and the crop are adversely affected.
A further article on the Trial from Professor Richard Ellis – one of our Trustees:
The NFCT’s long-term ‘Apples in a Warmer World’ ® project ends…
The ‘Apples in a Warmer World’ ® field work ended in late 2022 and the experimental structure is being taken down (for further use elsewhere) over this winter (22/23). Here we summarise the study to date and explain why it has ended.
The National Fruit Collections Trust’s long-term ‘Apples in a Warmer World’ ® project assessing the impact of climate change (ambient or warmer temperature combined with ambient, more, or less rainfall) on apple production began with work to graft the scions of the varieties of interest on a common root stock (M9) and interstem (Golden Delicious) in 2012; it grew to a total of 20 contrasting varieties. In addition to the substantial funding from NFCT to establish and maintain the 0.7 ha research facility, several sponsors have funded the research thereby University of Reading PhD students Tobias Lane and Adam Peter. Tobias Lane was awarded his PhD in November 2022 and Adam Peter expects to submit his PhD Thesis at the end of this year. Both had their research disrupted severely by the COVID-19 pandemic but were successful in maintaining the experiment throughout and in their research studies. We congratulate Tobias and Adam on their achievements and thank the University of Reading, Worshipful Company of Fruiterers, Perry Foundation,
Elizabeth Creak Trust, and the Finnis Scott Foundation for their generous grants to support
Tobias and Adam.
Tobias Lane started the research once the facility was established with a control season in 2017 (under plastic, but otherwise “ambient” conditions in all tunnels) with modified environments (ambient, plus 2°C or plus 4°C combined factorially with ambient, plus or minus 20% of actual rainfall) throughout 2018 and 2019. (The warming was passive [i.e., driven by solar radiation] and so the plus 2°C or plus 4°C designations are nominal and refer to the maximum warming). Adam Peter continued the research, expanding the range of trees from 15 to 20 varieties, in the modified environments in 2020 and 2021.
Over winter 2021/22, NFCT was replacing the original polythene covers (at a cost far greater than had been applied in 2017). However, the research facility suffered colossal, unprecedented damage from the sequence of storms Dudley, Eunice, and Franklin between 17 and 21 February 2022. The facility was made safe and damaged infrastructure was removed by NFCT (at further considerable cost): not only was around 0.6 ha of polythene sheeting lost, but over a quarter of the steelwork was damaged beyond repair and even a few well-established apple trees uprooted (most of which were able to be replanted in situ). It was not possible to re-establish the modified environments at the time (early Spring 2022) as NFCT did not have the considerable further funds required to repair the facility.
Nonetheless, Adam Peter was able to quickly revise his experimental plans and continued the research by investigating fruit production in 2022 under ambient conditions from the almost one thousand trees that had experienced the contrasting modified environments. Over the period 2018-2021. In other words, to consider what was the legacy effect of those different environments in a single ambient environment thereafter. This objective was developed from preliminary observations of the long-term study (and one that has complicated the analyses considerably) that the effects of modified environments in one year may not only affect yield in that year but also in the subsequent year; and moreover, this “carryover” effect appears to vary greatly amongst the varieties.
Over the past year, the Trustees have engaged in many long discussions with potential partners (from fruit producers’ organizations to the retail sector) who might be able to support NFCT’s ‘Apples in a Warmer World’ ® project to achieve the 10-year study originally envisaged. All were supportive of the project, and interested in the results, but not able to commit to the scale of funding necessary to repair the tunnel structure, reclad it, and meet future running costs. Moreover, Adam Peter is approaching the conclusion of his studies (the last fruit were harvested in early November 2022; he is now engaged on analyses of the considerable data) and initial applications to sponsors for a third PhD student on the project were unsuccessful.
After much consideration, the Trustees therefore decided to end the long-term study and
clear the site (a requirement of planning permission). This work is now underway and NFCT
is doing the utmost to ensure the assets are reused wherever possible (to realise their value
and minimise costs and waste).
Nevertheless, the research is continuing with the data from four years’ results under
modified environments (harvest years 2018-2021 inclusive) and two further years in ambient
(2017 and 2022). This 4-year period of the effect of continuous modified environments on
apple trees is, we believe, around 12 times longer than any previous such study. The
research with the many years of results obtained and preparing the reports thereof will
continue for at least the next 12-18 months. Moreover, the database of results and
environmental data will be useful to researchers long after that.
The Trustees are confident that the study will prove useful to commercial and amateur
apple producers alike. Provisional results have been communicated by Tobias and Adam in
several fora (such as the National Fruit Show) to date. The Trustees will expand the
dissemination of results once Adam’s PhD studies have concluded.
In the meantime, we can already report that the decision to study apple production in
ambient conditions in 2022 was very worthwhile. Preliminary analyses show that the
differences in environment amongst treatments in previous years affected yield in the
common environment in 2022 substantially: average yield of all 20 varieties in the study’s
ambient tunnel area in 2022 was around 13kg/tree, but only 7.0-7.5 kg/tree (i.e., yield
reduced by about 40-45%) for those maintained at warmer temperatures in 2018-2021. This
not only confirms that inter-annual variation in fruit yield was greater, on average, with
warming; but also that apple yield is affected by the environment in previous years as well
as the current year; and points to effects on the tendency to alternate bearing. Adam is now
investigating these effects on the individual varieties in the study.
Future Direction of the Trust
Understandably the focus of the Trustees for the past 10 years or so has been on the establishment and maintenance of the Apples in a Warmer World trial facility and the supervision of the two PhD students who have studied in detail how apples are affected by warmer temperatures. However, before embarking on any new initiatives, the Trustees have to ensure that the site is cleared to a satisfactory standard and that the work of the Apples in a Warmer World project will continue for another year or more in order to oversee the successful completion of the second PhD thesis and to carry out the important task of disseminating the valuable conclusions from the trial on the potential consequences of warmer temperatures for UK apple production to professional growers, gardeners, and scientists.