Matt Parratt, Observatree Scientific Coordinator, and colleagues at Forest Research; Michael Bell (climate), Caroline Gorton (plant pathologist), and Amirah Nigoumi (entomology), look at the effects of extreme heat and drought on trees.

On the 19th of July 2022 recorded temperatures in the UK rose above 40°C for the first time and there were two consecutive days of temperatures exceeding 38°C. Furthermore the area affected by the above average temperatures extended from Kent to Southern Scotland with a new Scottish Record high of 34.8°C set at Charterhall in the Borders. The high summer minimum temperature record was also broken, and by a considerable margin; the temperature at Shirburn Model Farm, Oxfordshire did not fall below 26.8°C for 24 hours from the morning of July 18th compared with 23.9°C from Brighton in 1990.

Despite the high temperatures of 2022 the driest summer on record for the UK was 1995, closely followed by 1976 and both of those were considerably drier than the cluster of dry summers in 2003, 2006, 2018 and 2022.

In the UK water availability in reservoirs and groundwater also relies upon rain/snow between the preceding November and April and in 2022 there was an extended dry spell between November 2021 and August 2022 – only February was wetter than the average. In this, 2022 and 1976 are very similar. Between November 2021 and August 2022 only February had higher than average rainfall.

The primary effects of drought start with wilting foliage, the first indication that evaporation of water from leaves is outstripping the ability of roots to supply it. If a water supply soon becomes available, many  plants can quickly recover. But if drought conditions persist,  then leaf scorch can follow. The condition usually presents as yellowing between leaf veins and along margins, eventually turning to brown. The ultimate stage of preventing excessive moisture loss is leaf loss and premature senescence associated with an increase in production of abscisic acid within the leaves, observed as a ‘false autumn’. Crown dieback and root death can follow, but this may be a slow process for several years after the drought event.

Scorch, early senescence, and crown dieback aren’t in themselves fatal but there are longer term affects which can have more damaging consequences and may not be seen for several years and these are described as secondary effects. Carbohydrates produced by photosynthesis are used by trees to produce proteins, fats, growth regulators and secondary metabolites so when leaf area is reduced, so does the production of these other compounds and that leads to lower growth rates, branch and fine root dieback, and a reduced capacity to heal small wounds can leave trees more open to infections such as Nectria canker and Cytospora canker which are usually associated with drought stress. Furthermore, the decrease in the ability of the tree to compartmentalise infection this leaves trees vulnerable to attacks by secondary pathogens such as Armillaria.

Secondary metabolites such as tannins, oleoresins, and alkaloids play an important role in plant defence from pests and pathogens and can initially see a rise in production but as drought persists production falls and reserves run low. Oleoresin has been associated with deterring wood boring insects so as levels decrease, so do defences, potentially increasing the risk of attacks by pests such as bark beetles, some long-horned beetles, wood wasps and various lepidopteran species.

Drought stressors on plants may cause some of their herbivorous insect predators to speed up their life cycle whilst in others their development is impeded. Several insect pest species such as moths have been found to change their feeding preferences when drought has affected their host plant, favouring the drought-stressed plants with their reduced secondary defences. As climate change creates a shift in the seasonal timing of droughts this causes a disruption to the plant host-pest relationship and subsequently a change to the entire local ecology.     

There are also pests and diseases which thrive in warmer conditions and we expect to see an increase in these.  Sooty bark disease (Cryptostoma corticale, figure 2) is one such problem where the fungus which is usually an endophyte within the tree (most commonly sycamore) becomes a pathogen as temperatures exceed 25oC for an extended period.  The profuse spore production beneath the bark which gives this disease its name is a potential respiratory risk to human health risk and so it is advised that potentially affected trees are not examined closely. Diplodia pini is disease of conifers which is more common in hot countries and this is causes shoot dieback.  There has already been an increase in cases and this trend is seen with other pathogens that may have previously been on the northern limit of their temperature range, but which are now finding summer heat more suitable for their growth optimum.

Recovery from the damage caused by prolonged drought can take a long time and it is likely we will see an increase in symptoms related to the extreme heat and drought of 2022 in the coming years.



Bettina Gutbrodt, Karsten Mody, Silvia Dorn: Oikos Volume 120, Issue 11, First published: 09 May 2011: Drought changes plant chemistry and causes contrasting responses in lepidopteran herbivores.

Karalija, E. et al. (2023): Plants strike back: Plant volatiles and their role in indirect defence against aphids: Volume 175 (1).

McCarthy, M. (2022) Guest post: A Met Office review of the UK’s record-breaking summer in 2022 [online]. Accessed 05.01.2022 at: Guest post: A Met Office review of the UK’s record-breaking summer in 2022 - Carbon Brief

Soni R, Suyal D, Goel R (ed.) Plant Protection: From Chemicals to Biologicals. Berlin, Boston: De Gruyter (2022): Srivasatava P. Chapter 13 Use of alkaloids in plant protection: p.337-352.