Introduction

Heathlands and peatlands are found within temperate regions1 and represent globally important carbon stores2. Heathlands are upland or lowland habitats with acidic soils or shallow peat layers1 and peatlands contain peat soils and blanket bogs. Both ecosystems are notably dominated by Calluna vulgaris (hereon Calluna), a dwarf shrub-type heather. Other key vegetation species include purple moor grass (Molinia caerulea)3. These habitats are under threat from the increasing frequency and severity of wildfires3,4. Deposits of ash, fine sediment and organic materials also have the potential to pollute waterways and accumulate in the soil substrate5. By 2050, it is predicted that the average UK summer temperature will rise by 2.5 °C while rainfall will decrease by 16%, ultimately lowering the fuel moisture content (FMC) of fuels6,7 and increasing wildfire occurrence8.

The FMC of different Calluna fuel layers is an important factor controlling fire behaviour9. Low FMC in the Calluna canopy can result in faster rates of fire spread, and low FMC in the moss/litter layers can result in more severe fires10 leading to more consumption of organic matter11 and greater release of carbon stores. Early spring is one of the major periods of high fire risk in such habitats12. During the spring, weather conditions in upland areas can be highly variable and fluctuate below and above freezing. Cold freezing conditions and dry warm conditions both lead to low FMC and hence higher wildfire danger. Physiological drought can also impact the ability of Calluna to retain moisture; for example, frost and wind can damage the cuticles of the leaves preventing their ability to control evapotranspiration, and frozen ground can prevent root uptake of moisture, leading to below-average FMC13,14.

Previous studies have focused on improving the ability to predict FMC of live and dead Calluna vegetation layers and the associated fire risk by defining FMC thresholds for which materials can ignite. Davies and Legg9 characterised FMC thresholds in Calluna using ignition tests along FMC gradients. When the FMC in the lower canopy of Calluna was above 70% ignition tests failed, whereas at 60% fires would ignite and develop rapidly. Santana and Marrs15 developed a predictive model of the ignition of Calluna and Sphagnum moss in peat-dominated heathlands as a function of FMC and found that the FMC and proportion of dead materials within the Calluna vegetation influence the probability of ignition.

It is now well-known that during the transition from spring into summer, Calluna undergoes a “greening up” stage, where its leaf green area increases and hence so does the FMC before the summer drying starts16. However, there are very few studies investigating diurnal trends of FMC in Calluna and how these patterns change on a seasonal basis17,18.

Diurnal patterns in fire behaviour were first characterised by Beall in 193419, with further investigation into the associated diurnal patterns in FMC carried out by Van Wagner in 197720. More recent studies on diurnal FMC have typically been carried out in American forests and grassland environments21,22, showing significant diurnal patterns in moisture content. Banwell et al.21 found all forest floor litter components experienced at least a 6% change in FMC during the day, and an 11% change in FMC was the greatest range they recorded. Similarly, Livingston and Varner22 found a 4–12% variation in FMC of grassland throughout the day. This is significant considering the work by Davies and Legg3 which documented a 10% change in FMC in live Calluna signifies the difference between fires developing rapidly and failing to ignite fires. Davies18 investigated diurnal FMC from July–August but did not sample the early spring period when upland heathland and peatland fires are most frequent. There is a clear need to characterise the diurnal variation of FMC in live and dead Calluna vegetation and the underlying moss, litter and organic soil layers across the entire fire season so they can be compared to pre-defined ignition thresholds, as significant diurnal patterns could cross ignition thresholds. Diurnal patterns in FMC could also affect fire behaviour such as rate of spread. As the FMC of Calluna changes on a seasonal scale14, the diurnal pattern may also change and could be significant from a management and fire suppression perspective.

Here we examine the diurnal variation of FMC of live and dead fuel loads within a Calluna vulgaris dominated heathland and determine how the diurnal pattern in FMC varies on a seasonal basis. We also investigate whether the range in FMC throughout each sample day is linked to the range in weather (temperature and humidity) to assess the potential drivers of this diurnal variation. We expect the daily FMC measures to be higher at the earliest and latest measurement times, following a positive quadratic curve.

Methods

Site description

The North York Moors national park is an upland area in North-Eastern Yorkshire that contains some of the largest expanses of heather moorland in the UK. The habitat has been managed for centuries by rotational burning to create a mosaic of mixed-age heathland to improve grouse habitat and prevent the build-up of fuel (Fig. 1). The area chosen for this study is located on an estate in the Cleveland Hills (54° 24′ 03.4″ N 0° 55′ 44.6″ W). At an elevation of 408 m above sea level, the site is located at the top of a valley with a slope of 3.8° in a southwest direction. The site is classified as a blanket peat bog with wet heather moor23, where Calluna vulgaris is the dominant heather species.

Figure 1
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© 2022 Esri Inc.

Location of the sample site in the estate, North York Moors. Main map sources: Esri, Airbus DS, USGS, NGA, NASA, CGIAR, N Robinson, NCEAS, NLS, OS, NMA, Geodatastyrelsen, Rijkswaterstaat, GSA, Geoland, FEMA, Intermap and the GIS user community. Satellite inset map source: Esri, Maxar, Earthstar Geographics, and the GIS User Community. Created using ArcGIS Pro v 3.0.2

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Sampling procedure

Sampling was undertaken on the following seven days in 2022: 19th, 20th and 26th March; 21st April; 14th May; 15th June; 6th August. Extra samples were taken in March as this is a key fire period in these ecosystems in the UK. Herein, sample days during April–August will be referred to by the month of sampling only. Individual sampling days were selected to avoid wet antecedent conditions and to maximize diurnal humidity variations. In total, 554 samples were collected and analysed. On each day of sampling the vegetation was sampled on the hour every hour from 10:00 to 18:00 as this is the period of highest fire risk during the day and will help to inform sampling strategies. On most days only one sample was taken per hour, but on March 19th and March 26th two samples were taken per fuel by two separate individuals. Seven different vegetation layers were sampled (Fig. 2): upper live canopy, live stems, upper dead canopy, dead stems, moss, litter and organic layer (top 5 cm of organic material above mineral soil).

Figure 2
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(a) Example of how the live Calluna sprigs were separated into the upper live canopy and live stems, the dotted line representing the dividing line, (b) Sample area containing all vegetation layers.

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The sampling procedure was adapted from Norum and Miller24 and Little et al.25. Ten random sprigs of live and dead heather were collected haphazardly along a 20 m transect covering a representative area of the site. Each sprig was approximately 10–20 cm long and was taken vertically through the centre of the shrub to avoid the outer sprigs of heather that are exposed and possibly damaged by grazing sheep. The sprigs were then cut into one-inch segments, separating the top canopy and lower stems (Fig. 2a). Moss, litter and organic layers samples were collected haphazardly from five locations along the same 20 m transect. The moss was pulled from the ground and any brown and decomposing sections of moss beneath were removed from the sample. The litter was collected from the base of Calluna shrubs, ensuring not to collect any decomposing litter that may be considered the start of the organic layer. Finally, the organic material was collected after sweeping away the litter layer, and a 5 cm deep core of the organic layer was removed. All samples from each fuel layer were combined and placed in a metal tin container and taped to prevent moisture loss. Temperature and humidity measurements at the times of sampling were taken from the Blakey Ridge weather station26 which is located roughly 1.8 miles (2.8 km) from the sample site.

FMC is a measure of the amount of water in the fuel layer and is expressed as a percentage of the dry weight of that specific fuel (gravimetric moisture content). To calculate the FMC from the samples collected, the tape was removed from the tins which were then weighed to obtain the wet weight. The samples were placed in a dehydrator set to 80 °C for 48 h and then weighed again to obtain the dry weight. The FMC for each sample was then equal to:

$${\text{FMC}} = \left( {{\text{W}} - {\text{D}}} \right)/\left( {{\text{D}} - {\text{T}}} \right) \times {1}00$$

where W = wet weight, D = dry weight and T = the weight of the sample tin.

Data analysis

Due to the expected relationship between time of day and FMC, we fit linear regression models with a quadratic term, which we would expect to be positive. This statistical method has not been used to investigate diurnal patterns before. FMC was the response variable, and time of day the independent variable. We fit a single model for each vegetation layer and for each month of sampling, resulting in 49 models. Model residuals from the models were checked using the package ‘DHARMa’27, and they did not deviate from their expected distribution for any model (Supplementary Figure S1). Correlation analysis was then carried out to test whether there was a relationship between the diurnal range in FMC and the diurnal range of temperature and humidity. We used the Shapiro test to check the normality of data distribution, and used Pearson’s correlation for normally distributed data and Spearman’s correlation for non-normally distributed data. All data analysis was undertaken using R version 4.3.028.

Results

Diurnal variations in fuel moisture were observed for all seven vegetation layers in at least one month, with the majority of models showing a parabola-shaped curve (indicated by a positive quadratic sign; Table 1). Significant diurnal patterns were observed more frequently for dead Calluna than live Calluna (Table 1, shaded boxes). The moss and litter layers only showed diurnal trends towards the end of the study period, and the organic layer only had significant diurnal patterns on one sample day (March 26th; Table 1). For full summaries of diurnal models see Supplementary Table S1. Dead Calluna and surface materials (litter, moss and the organic layer) showed the largest diurnal range in FMC in March, whereas the diurnal range in FMC for live Calluna was more consistent across all sample days (Table 2; Fig. 3). Diurnal fluctuations in FMC of surface materials meant that on some sample days FMC dropped below the ignition thresholds only at certain times of day (Fig. 3).

Table 1 Regression models with a quadratic term investigating whether FMC diurnal patterns follow parabolic curve for each sample day and at each canopy layer.
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Table 2 Range between minimum and maximum FMC (%) observed during each sample day.
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Figure 3
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Diurnal and seasonal trends of FMC in the upper live canopy, live stems, upper dead canopy, dead stems, moss, litter and organic layers. The black dashed box represents defined ignition thresholds of the material as reported by Santana and Mars15 and Grau-Andrés et al.29. The lower and upper bounds of the black dashed box are the M50 (where 50% of ignition tests were successful) and the Mmax (the maximum FMC where ignition occurred). Moss thresholds = 57–80%, litter = 54–61%, organic layer = 125–150%. Ignition thresholds are not shown for live Calluna as recorded ignition thresholds (60–70%)9 are not within the axis range; all FMC are above threshold. Dead Calluna ignition thresholds are not known.

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There was no significant correlation between the diurnal range in FMC and the diurnal range in weather variables (temperature or humidity) throughout each sample day. Weakly significant correlation was observed only between the diurnal range in FMC of moss and diurnal range in humidity (0.67; P = 0.1; Table 3).

Table 3 Correlation analysis between range in FMC per sample day (max FMC—min FMC; Table 2) and range in temperature and humidity on each sample day (Supplementary Table S2) for each fuel layer.
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Discussion

Dead Calluna showed significant diurnal parabola-shaped patterns in almost every month of sampling, whereas for live Calluna, significant diurnal patterns were found in March, June and August. This supports similar work by Davies18, who found large diurnal variations in the FMC of dead Calluna from July–August and shows that diurnal patterns are a significant factor characterizing fuel moisture in dead materials. During the winter and early spring, the relative proportion of dead Calluna is at its greatest due to winter die back13,30, and the proportion of dead materials may have an important impact on wildfire behaviour, where small changes in the percentage of dead materials can have a disproportionate impact on the severity and spread of wildfires31. As a result, during the winter/spring when the proportion of dead materials is highest, the diurnal variability of dead materials' moisture content may be an important factor in wildfire danger. Although ignition thresholds are unknown for dead Calluna, low dead FMC increases the risk of fire ignition14, which suggests that ignition is more probable in dead Calluna when the diurnal FMC is at its lowest.

The upper live canopy had significant diurnal patterns in the months that wildfires are most likely; early spring (March) and mid-late summer in June and August32. As live FMC can therefore be impacted by the time of day, FMC predicting models that are calibrated with field samples likely need to take account of diurnal patterns, especially during the early spring and late summer. Significant diurnal trends in early spring could be explained in part by phenological factors such as winter damage to leaves reducing the plants’ ability to regulate water loss, meaning FMCs are correlated with diurnal evaporation30. Despite the significant diurnal trends, FMC of all live Calluna was above the recorded ignition threshold9 which means that diurnal variation is unlikely to affect ignition probability in this study. However, live FMC is linked to wildfire rate of spread14,29 which may vary depending on the time of day that a wildfire occurs. Considering diurnal patterns in live FMC is still therefore essential in wildfire management.

Different vegetation layers showed different FMC patterns depending on the time of year. The lowest FMC in the live Calluna was in March (Fig. 3), and low springtime FMCs have been recorded by other studies such as Davies et al.33. For example, it was noted that the FMC of live Calluna decreased during the winter of 2002–2003 from a usual spring value of 80% (similar to levels recorded in this study) to less than 45%30. During the winter, freezing conditions and high wind speeds can damage leaf cuticles and cause physiological drought34,35. Live FMC can further be lowered by freezing grounds locking up soil moisture and preventing root uptake14. However, an alternative explanation for large diurnal ranges and low FMC in March found in this study could be due to an increase in elevated dead fuels from winter browning (plant damage due to frost or wind in early spring13) of live materials that were not separated from live Calluna sprigs causing large variation between samples. In comparison to the live Calluna, the FMC of the dead Calluna varied less seasonally, with FMC of ~ 5% at its lowest in May to ~ 30% at its highest in March.

During the early summer when Calluna increases its green-leaf area and biomass, the FMC of the upper live canopy increased by over 50% from April–June (Fig. 3). This is a well-described phenomenon36 and is consistently shown in FMC studies16,33. The diurnal patterns of live Calluna were most significant in the late summer months after the growth spurt. However, the fire danger is likely lower during this late summer as FMC remains higher than in spring through the entire diurnal variation.

The rate of wildfire spread is influenced by FMC of the lower ground and moss/litter layers; when the FMC of the moss and litter layers are low enough to ignite, they can substantially increase available fuel loads by 13–67% which can significantly alter fire behaviour29. The FMC of the moss and litter layers had their largest diurnal ranges in March, April and August (Fig. 3; Table 2), during which FMC fluctuated above and below the ignition thresholds on some sample days. Diurnal variations in FMC could therefore affect wildfire behaviour by increasing the amount of available fuels and speeding up the rate of spread, since the litter and moss layers can dry to combustible FMCs throughout the day. Models that do not consider the hourly change in FMC will therefore underestimate the wildfire risk. This is highlighted by the fact that for some materials and sample days (March 20th for moss; April for litter), the mean daily FMC is above the ignition thresholds (Supplementary Table S3) but does drop below the threshold at some point during these days. However, on sample days where FMC of moss and litter fluctuated above and below the ignition threshold, diurnal patterns were only statistically significant in August. Despite large diurnal ranges in the moss and litter layers in early spring, the diurnal patterns were not statistically significant and the variation could have been the result of high within-plot variability in FMC.

The influence of surface fuel on wildfire behaviour, from a fuel moisture perspective, is significantly different in late summer compared to springtime as drier conditions lower the organic layer FMC to such an extent that smouldering of the peat is feasible37,38,39. We observed the organic layer moisture content fluctuating on a diurnal scale above and below the ignition thresholds defined by Grau-Andrés et al.29. However, for six of the seven sample days there was no significant parabola-shaped pattern of diurnal variation observed, likely the result of high spatial variability in organic FMC40. This suggests that diurnal patterns in FMC of the organic layer are not an important factor governing wildfire behaviour and may be more governed by seasonal weather.

We found no strong links between the range of diurnal FMC in each fuel layer and the range in weather variables (temperature or humidity), with the exception of moss where there was a very weak correlation between the diurnal ranges of FMC and humidity. Many studies have found links between weather and FMC: for example, dead Calluna is influenced strongly by the weather, likely as it can no longer regulate its moisture content by drawing in water from the ground or respiring as live plants do41, and a positive relationship between live FMC and temperature for live Calluna and other live materials has also been observed3. However, the relationship between diurnal weather patterns and the diurnal range in FMC have not been researched before. More detailed work may therefore be needed to further investigate the potential effects of day and night-time weather, as well as dew and antecedent weather conditions, on observed diurnal fluctuations in live and dead fuel moisture content.

The FMC of vegetation layers is a significant factor governing wildfire behaviour; high FMCs inhibit wildfire risk, and low FMCs increase wildfire danger and ignition probabilities10. Improving our understanding of FMC patterns in live and dead materials is crucial for developing an accurate UK fire danger rating system, yet there are no studies characterising diurnal and seasonal patterns of FMC in Calluna vegetation layers across the whole fire season. We observed significant diurnal patterns in FMC that varied seasonally. In general, diurnal patterns became more significant during the late summer period and our data suggests there might be significant diurnal patterns in live Calluna FMC during the early spring as well. The main implications of this study are that diurnal FMC in live material can impact wildfire danger during the early spring and late summer, which coincides with the two main wildfire seasons, and that diurnal fluctuations in ground materials can result in ignition risk changing throughout the day. From an FMC monitoring perspective, samplers will need to consider the time of day that samples were taken as this can influence measured FMCs and fire danger rating system models should consider the diurnal changes of FMC.