Climate Change in the Orchard: Later Frosts, Earlier Harvests?

An open-access research paper – “60 Years on”- Effects of Climatic Change on Tree Phenology – A Case Study Using Pome Fruit¹Kunz A, Blanke M. “60 Years on” — Effects of Climatic Change on Tree Phenology — A Case Study Using Pome Fruit. Horticulturae. 2022; 8(2):110. https://doi.org/10.3390/horticulturae8020110 – published in Horticulturae in January 2022 has identified some of the likely consequences of the past 60 years of climate change, using data collected from a research orchard in Klein-Altendorf, near Bonn in western Germany.

The statistical analysis by Achim Kunz and Michael Blanke, both of the University of Bonn, assesses data collected over the 60 year period 1956 to 2017. Some of their findings are rather concerning, with potential implications for commercial apple growers, but also for small-scale home or allotment orchardists, in Europe and maybe the Northern hemisphere in general.

Perhaps the most headline-worthy trend that the researchers identified is the increase in average annual temperature – in this particular part of Germany – from 8.6°C in 1958 to 10.3°C in 2017, a rise of 1.7°C. Across the “vegetation period”, or in layman’s terms the growing season, the increase in average annual temperature was a slightly lower 1.5°C, but either figure ought to provide pause for thought²Organisations such as the IPCC have consistently warned that limiting global temperature increases to 1.5°C above pre-industrial levels is the key to ameliorating the potentially disastrous effects of adverse climate change. But the study suggests that a 1.5°C rise has already occurred – in this particular part of Germany, at least – since the late 1950s. And the data is already 5 years out of date, with four of the five hottest years on record having occurred since 2017.. They also identified a clear distinction between the years before and after 1983, with the clear majority of the years in the latter range resulting in average temperatures above the long-term average for the site of 9.5°C.

According to the study, a more specific effect of the generally, but not necessarily consistently, warmer winters that we’re now experiencing that could have a direct impact on fruit growers, is:

"The combination of stronger increase in winter temperatures (by +1.2 °C) than in the summer (air +1.0 °C) with advanced bud break and −0.3 °C lower minimum temperatures in April during flowering resulted a continued risk of a late frost, as experienced in 2017. The strongest climate change effect, i.e., 11–14 days advanced flowering (in apple and pear) highly correlated (R2 −0.7) with the March/April temperature. "

In layman’s terms again: warmer temperatures towards the end of winter are resulting in earlier fruit blossom emergence. But all it takes is a shift in atmospheric pressure systems, pulling down cold Arctic air and sending temperatures plunging, to bring about a sudden, late frost; a blossom-killer.

This is exactly what happened in our neck of the woods last year (2021) in both April (as per the photo at the top of the post) and May; wiping out the majority of our plum, pear and early apple blossom and contributing to a distinctly reduced harvest³The fact that 2020 was a bumper year for fruit would have had a compounding effect, with the trees lacking sufficient surplus energy to initiate next year’s fruit buds at the same time as the additional in-year fruit was developing, resulting in a biennial bearing pattern, which I talked about back in July 2021. But when the amount of blossom is already affected by a biennial pattern, the last thing you need is a blossom-killing frost to finish them off..

The study also identified that the fruit trees in the study were, on average, maturing and ripening around 4-12 days earlier in 2017 than in 1957, with results varying between the four cultivars studied. This might also have implications for commercial growers, who may need to adjust their harvest and storage requirements. But as the largest growers no doubt use a wide range of automated assessment methods to determine the optimum time to pick their fruit, it probably won’t cause any major problems.

Personally, I suspect it may also accelerate the trend towards commercial growers investing in newer, modern cultivars that have been bred to provide later blossom and a shorter period to harvest quality, and this might come at the expense of some traditional, heritage varieties. Which means even more of our favourite old apples may disappear from supermarket shelves, to be replaced by the sort of sweetly bland, but dependably robust modern types that are already proliferating.

It would just be the continuation of an already ongoing trend, of course, but one that results in the vast majority of apple consumers only ever getting to see, buy and taste a very small group of very similar fruit. So it will be even more important for small-scale growers and orchardists to continue to plant, nurture and enjoy the fruits of heritage trees, in order to keep the old varieties alive for future generations to enjoy.

As for the impact of climate change on smaller-scale growers, I suppose we might need to be aware that the information on fruiting times gleaned from books written before the early 1980s could now be a little out-of-sync. For instance, there might be some ‘October’ apples that ripen towards the end of September these days. Again though, that sort of thing shouldn’t cause any major problems as long as we keep a close eye on our trees and step in to harvest their fruit before it all drops of its own accord and becomes too badly damaged to be stored.

I suspect it’s the increased risk of late, blossom-killing frosts that we should perhaps be more concerned about. But whatever scale we’re growing at there’s probably not a lot we can do to prevent that sort of damage from occurring when adverse weather conditions to strike, although Mathew Olson has recently posted a few frost-protection suggestions on Russ Metge’s Simply Fruit Trees blog. But for most of us it will be a case of crossing our fingers, attempting to appease any weather gods we personally believe in, and hoping for the frosts to be over before the blossom comes out.

How about you? Have you noticed a definite change in the pattern of your trees’ blossoming, fruiting or maturity times? Have you had a chance to read the 60 Years On… report, and if so, do your conclusions differ from mine? Please do feel free to leave any comments below, I’d be very interested to hear what you think.

Footnotes

• 1
  Kunz A, Blanke M. “60 Years on” — Effects of Climatic Change on Tree Phenology — A Case Study Using Pome Fruit. Horticulturae. 2022; 8(2):110. https://doi.org/10.3390/horticulturae8020110
• 2
  Organisations such as the IPCC have consistently warned that limiting global temperature increases to 1.5°C above pre-industrial levels is the key to ameliorating the potentially disastrous effects of adverse climate change. But the study suggests that a 1.5°C rise has already occurred – in this particular part of Germany, at least – since the late 1950s. And the data is already 5 years out of date, with four of the five hottest years on record having occurred since 2017.
• 3
  The fact that 2020 was a bumper year for fruit would have had a compounding effect, with the trees lacking sufficient surplus energy to initiate next year’s fruit buds at the same time as the additional in-year fruit was developing, resulting in a biennial bearing pattern, which I talked about back in July 2021. But when the amount of blossom is already affected by a biennial pattern, the last thing you need is a blossom-killing frost to finish them off.