Oak tree budding increases with increasing air temperature

Introduction

Oak trees (Quercus spp.) enter a period of dormancy during the winter season. While some evergreen species, such as Quercus ilex (the Holm Oak), retain their leaves during this period (Alderotti & Verdiani, 2023), most are deciduous and shed them.

During the spring, the tree cautiously exits this dormant state, in a process dictated predominantly by organism responses to temperature and photoperiod (the length of daily sunlight illumination received by an organism) (Basler & Körner, 2014). In a process known as ‘leafing-out’, new leaves emerge from their buds (bud burst), signifying the start of the new growing season (Polgar & Primack, 2011). The phenology of oak trees in temperate climates during this time intends to maximise the duration of growing time while balancing it against attempting to minimise, or ideally avoid altogether, damage from very low temperatures (Basler & Körner, 2014), and exact leaf-out times can vary considerably, both between and within species (Lechowicz, 1984).

Photoperiod requirements to trigger bud burst are generally less well understood than temperature, although it has been noted that the exact cue required varies considerably, and some species do not respond to photoperiod cues at all with respect to exiting dormancy (Ghelardini et al., 2010). It has been widely documented that it is temperature – in particular air temperature – that is the main environmental mediator for exiting winter dormancy and initiating bud burst in temperate trees (Linkosalo, Häkkinen & Hänninen, 2006).

Global mean temperatures are predicted to rise by between 2 and 5°C in the next 75 years, according to current climate change models (Intergovernmental Panel On Climate Change (IPCC), 2023), raising the concern that temperature-determined budding phenology could be significantly impacted. Indeed, previous studies have already documented rapid climate change-induced shifts in the phenology of many non-tree (Visser & Both, 2005) and tree species (Primack et al., 2009).

An earlier bud burst has a multitude of knock-on impacts on ecosystem processes, including plant-animal interactions (affecting organisms that feed on its foliage, those that use it as shelter from the sun, and those that use the leaf-cover to hide from predators) and carbon sequestration (Polgar & Primack, 2011). Initially it may seem that an earlier leaf-out time might be desirable in helping to counter climate change due to it providing a longer window of opportunity for photosynthesis to occur, with a 5°C increase in temperature being associated with an earlier bud burst and leaf development time, leading to an extension in the growth season of both oak and beech trees of 5 days (Didion-Gency et al., 2024). However, in spite of the prolonged growing season and increased opportunity to photosynthesise, both the height and diameter growth increments were significantly decreased in the oak tree grown in the elevated temperature environment compared to control. Extrapolated to a forest of 1,000 trees, over a time period of 50 years, the total collective tree height lost due to warming would be around 5,000 metres, which could not only have a dramatic economic impact on the construction and logging industries, but also serious negative consequences for carbon sequestration compared to a non-warmed alternative scenario (McMillan, 2024). This is even more concerning when considered alongside the myriad other challenges that climate change can impose upon plant growth, from increased disease susceptibility (Huot et al., 2017) to drier soils (Didion-Gency et al., 2024).

As such, monitoring the scale, reach and effects of climate change is of vital importance, and should early bud burst indeed be a reliable indicator of increased climatic temperature, it could serve as a relatively simple and accessible way of observing the effects of climate change. In this paper, a dataset of budding scores from oak trees at Imperial College London’s Silwood Park Campus, near Ascot, Berkshire, UK, was analysed against on-site weather recordings to assess whether or not a significant relationship between mean monthly air temperature and mean monthly bud score existed.

Methods

During the spring months of 2007 to 2023, students recorded the status of the leaf buds of a set of oak trees located around the Imperial College Silwood Park Campus. The bud status was classified according to a numeric scale, which represented the stage of leaf flushing of the tree on the day of the visit, ranging from 0 (‘no sign of green’) to 6 (‘leaves turned dark green and waxy’) (Table 1; for photographic examples see Appendix Figure A1). This was then logged along with the date of the visit, the tree’s unique identifier, the visit’s unique identifier, and the tree’s coordinates.

Data processing and analysis was performed in R. To investigate the effect of air temperature on leaf budding, mean monthly temperatures and mean monthly budding scores were calculated (Appendix Table A1), and a linear regression was performed (Table 2), the assumptions of which were confirmed to have been satisfied through examining diagnostic plots (Appendix Figure A2). This linear regression was modelled using data from all of the months for which there was both temperature and bud scoring data, which meant it included readings from across multiple different months of the year.

It was acknowledged that this method of combining data for multiple months had limitations, including the confounding by non-weather factors that were also expected to vary as you move from early to late spring, such as photoperiod (the length of daylight changes significantly from March to June, which can influence plant growth independent of weather conditions (Svystun, 2021)), biological interactions, and general changes to the wider biotic and abiotic environment. Also, weather conditions aside from temperature can vary significantly between April and May (for example precipitation). To account for these flaws, the data was then re-analysed and modelled separately for the two months that were present in both the temperature and bud score datasets for all years between 2010 and 2022: April and May.

Results & Analysis - Combined data (not separated by different months)

Warmer months have a higher mean oak budding score (Figure 1), indicating a more advanced stage of leafing-out. A significant regression equation was found (F-value=188.6, df=(1,23), p-value<0.001) with an adjusted R2 of 0.89. Mean monthly budding score was equal to ‑2.20+0.51*mean monthly temperature (°C) (Table 2). This suggests that for every 1°C increase in mean monthly temperature, mean monthly oak budding score increased by just over a half (0.51). The intercept value is not a valid value for budding score, so there are clear limits to the range of the model. It can safely be applied to the range of input temperatures used (5.42 to 16.82°C).

Results & Analysis - Separate data: April

A relationship between temperature and budding score can be noticed when both variables are plotted for the Aprils from 2010 to 2022 and overlaid (Figure 2).

Results & Analysis - Separate data: May

A relationship between temperature and budding score can be noticed when both variables are plotted for the Mays from 2010 to 2022 and overlaid (Figure 4).

Discussion

As predicted by prevailing theory in the field (Linkosalo, Häkkinen & Hänninen, 2006) and previous studies (Ghelardini et al., 2010), budding significantly increased with air temperature.

Appendix