The effect of Chalara dieback and rainfall on tree-ring increment in European ash (Fraxinus excelsior)

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1 The effect of Chalara dieback and rainfall on tree-ring increment in European ash (Fraxinus excelsior) Summary During the summer of 2016 ash (Fraxinus excelsior) trees were sampled across 10 different woodlands within Suffolk and Norfolk. The tree rings of 66 ash trees have been examined. The trees were grouped into four dieback categories relating to the health of each crown. The mean growth rates were compared across the dieback categories and the influence of rainfall was also examined. Annual mean tree-ring increment was found to correlate with rainfall. The strength of this relationship is shown to reduce as dieback severity increases. The trees with the highest dieback (>75% crown dieback) have had significantly lower growth then the healthiest trees (<25% crown dieback) for the last 13 years. However, it is unknown if the reduced treering increment was solely because of the Chalara fungus Hymenoscyphus fraxineus or whether it formed part of a multitude of aspects causing stress, crown decline and reduced growth. Introduction European ash (Fraxinus excelsior) is currently experiencing crown dieback across Europe associated with the pathogenic fungus Hymenocyphus fraxineus. More commonly referred to as Chalara dieback, it was first confirmed in the UK in nursery stock in February 2012 and then confirmed in the wider woodland environment in Suffolk and Norfolk by October 2012 (Forestry Commission, 2015). However, Hendry (2013) suggests that two sites in East Anglia may have had Chalara dieback present since 2008/9. Ash growth increment has been shown to correlate with early summer rainfall (García-Suárez et al., 2009). The role of abiotic factors in causing stress in ash trees is outlined by Schumacher et al. (2007) who suggest that abiotic predisposition, such as drought, are still an important component of ash dieback. This is

2 demonstrated historically by Rackham (2014) who outlines a period of ash dieback in the East Midlands during the drought of Dendrochronology studies investigate events over time that are captured in tree rings (Speer, 2010). There have been some studies exploring dendrochronology, ash and Chalara dieback in Europe (Tulik et al., 2010; Vacek et al., 2015; Matisons et al., 2016). However, there is very limited dendrochronological data available for ash within the UK. Only two studies were found (García-Suárez et al.,2009; Rogers et al., 2014) but these did not factor in the influence of Chalara dieback. The aim of this study was to investigate the impact of Chalara dieback and rainfall on tree-ring increment in ash trees from established woodland. 2

3 Method Study area and sampling The 10 woodlands were chosen through opportunistic sampling following requests to owners in the region. These included private estates (via RFS East England), Suffolk and Norfolk Wildlife Trust woodlands, an education charity (The Green Light Trust) and Forestry Commission woodland. It took a total of ten days from 28/07/16 19/08/16. Figure 1 outlines the different woodland locations and ownership types. Figure 1 - sites sampled during the summer of

4 Trees were selected to get a variation in dieback categories. All trees were sampled from the same compartment or area of the woodland to limit the effects on growth of localised conditions. Each tree had one increment core (5.15mm diameter) extracted using a Haglof 30cm increment borer at 1m from ground level (Figure 2). The increment core was taken up to the pith (the centre of the tree formed by the terminal leader). Tree ages reported are therefore based upon the age of the tree at 1m above ground rather than germination. Figure 2 - each tree had one increment core extracted to examine the tree growth rings A thorough assessment of the health of each ash tree was also undertaken; based upon the Forestry Commission s Field Book 12 Assessment of Tree condition (Innes, 1990). Visual signs of Chalara dieback on each tree and the surrounding woodland were also noted. Photographs were also taken of each tree (Figure 3). The trees were assigned into four dieback categories based on the dieback extent visually observed (Table 1). 4

5 Figure 3 - An example of an ash tree with 75% dieback in the crown Table 1 - explanation of dieback categories Dieback Category Dieback Percentage details Crown Class <25% 1 24% Class <50% 25 49% Class <75% 50 74% Class 3 75% % Class 4 May and June rainfall significantly correlate with ash tree growth of the same year (García-Suárez et al., 2009) and provide a suitable variable to explore changes in the relationship between tree-ring increment and rainfall. Rainfall data was used from the Met Office based on historical monthly data for a meteorological station in Lowestoft, Suffolk (Met Office, 2017). This weather station was chosen as it is central between Norfolk and Suffolk. Across each dieback class a regression analysis was undertaken to determine the strength of the relationship between growth increment and rainfall. 5

6 Tree-ring analysis The 66 tree cores were stored in paper straws and air dried. These were then mounted on grooved wooden lengths and sanded using a belt sander. They were measured using a Lintab dendrometer and TSAP-Win software (Rinn, 2003) to an accuracy of 0.01mm (Figure 4). This was provided by Forest Research at their Alice Holt research station and took two days (5 th and 6 th December 2016). The last partially formed tree ring (2016 growth) was omitted from assessment. For this study, only the mean tree-ring increment from all trees for each year was used (either across all trees or within all trees within each dieback category). Although the oldest tree dated back to 1921, the plotted tree ring chronologies, based on mean growth, only begin at This was because this is the first year in which each dieback category had at least three trees sampled ensuring an appropriate mean value. Figure 4 - the microscope has a crosshair which you align at the pith before using the moving stage to guide the crosshair to the next growth ring 6

7 The tree ring chronologies from the 10 different sites were not crossdated (Speer, 2010). This was because of the limited time and resources to repeat measuring. The large sample size and use of mean growth ensures that the effect of misdating tree rings is limited. Key characteristics of sites and trees sampled Table 2 outlines some key characteristics of the ten sites sampled and the ash trees cored at each site. 7

8 Table 2 - Characteristics of the sites sampled Site ID Woodland Ownership Type No. of trees sampled Canopy Composition Ash tree characteristics Tree-ring increment details A Wildlife Trust 7 ash, English oak (Quercus robur), hazel (Corylus avellana) DBH ± SD Height ± SD Age ± SD Min Max Mean ± SD (cm) (m) (years) (mm) (mm) (mm) B Wildlife Trust 7 ash, silver birch (Betula pendula), English oak C D Educational Charity Forestry Commission 7 ash, English oak, hazel English oak, ash E Private Estate 6 English oak, ash, hornbeam (Carpinus betulus), western red cedar (Thuja plicata), Douglas fir (Pseudotsuga menziesii), hazel F Wildlife Trust 7 ash, English oak, hazel G Wildlife Trust 8 ash, English oak, hazel, hornbeam H Private Estate 6 ash, English oak, hazel I Private Estate 5 sycamore (Acer pseudoplatanus), ash, hazel, black poplar (Populus nigra), common alder (Alnus glutinosa) J Forestry Commission 6 ash, Douglas fir, English oak, western red cedar

9 Results Descriptive statistics of mean growth Figure 5 displays the mean growth chronology for each dieback category for 70 years ( ) with standard error of the mean was a period of lower growth across all the dieback categories was a period of lower growth across all the dieback categories. Following 1996 the mean tree ring increment in the 75% dieback category is consistently lower, except in 2002 where it reached similar levels to the other dieback categories. The 50 - <75% dieback category differ from those trees with under 50% dieback from Standard error bars in the trees with greater than 50% dieback become progressively smaller from 2013 onwards. 9

10 Tree-ring increment (mm) Rainfall (mm) % of average rainfall % of average rainfall % of average rainfall 39% of average rainfall 40% of average rainfall 0 Year <25% 25 - <50% 50 - <75% 75% May and June annual rainfall combined total average May and June annual rainfall combined total Figure 5 - Mean tree-ring increment chronologies across the different dieback categories and comparison with annual May, June rainfall totals from (Data.gov.uk, 2017) with standard mean. No rainfall data is available for 1959, 1960 or As of summer 2015, <25% (n = 19); 25 - <50% (n = 12); 50 - <75% (n = 15); 75% (n = 20). Increasing dieback severity demonstrated in traffic light colour system (green = <25%, yellow = 25 - <50%, orange = 50 - <75% and red = 75%) 10

11 F Statistic (* = signicant year) Dieback class statistical analysis The mean growth chronologies within Figure 5 suggest differences in tree-ring increment across ash trees assigned to different dieback categories. One-way ANOVAs show significant differences across ten recent years (Figure 6) (f = 4.397; P <0.01); 2006 (f = 2.882; P <0.05); 2007 (f = 5.443; P <0.01); 2008 (f = 4.815; P <0.01); 2009 (f = 4.338; P <0.01); 2010 (f = 2.941; P <0.05); 2011 (f = 3.613; P <0.05); 2013 (f = 9.426; P <0.01); 2014 (f = ; P <0.01); 2015 (f = ; P <0.01). The statistical analysis was undertaken from 1991 as this covers the period when Chalara dieback was present in mainland Europe * * * * * * * * * * Year Figure 6 - One-factor ANOVA analyses of ring increment and crown class run for each year ( ): data presented as per Drobyshev et al. (2007) A Post hoc Fisher Least Significant Difference test showed significant differences between different dieback categories (Table 3). Table 3 - Post hoc fisher LSD significant differences across crown classes observed Dieback class contrast Class 1 vs. Class 2 Class 1 vs. Class 3 Class 1 vs. Class 4 Class 2 vs. Class 3 Significant difference observation Never significantly different P <0.05 last three years P <0.05 last 13 years P <0.05 last year only Class 2 vs. Class 4 P < and Class 3 vs. Class 4 P < ; and

12 Rainfall and tree-ring increment regression analysis The <25% group regression indicates that rainfall has an influence on tree -ring increment in the healthiest trees (r² = 0.349). This is followed by 25 - <50% (r² = 0.190) and then 50 - <75% (r² = 0.067). The 75% group have tree-ring increments that are the least influenced by rainfall (r² = 0.017). Figure 7 demonstrates the regression analysis for the <25% group. Even though there has been reasonable rainfall in recent years ( ) there is consistently lower growth then the regression line predicts. The rainfall analysis was undertaken from 1991 as this covers the period when Chalara dieback was present in mainland Europe Figure 11 - <25% mean growth increment and rainfall regression 12

13 Discussion Tree-ring increment and rainfall The mean tree-ring increment across ash trees was shown to correlate with rainfall. This agrees with the findings of García-Suárez et al. (2009) helping to strengthen their observation that early summer rainfall (May and June) is a key factor influencing the tree-ring increment of ash. The regression analysis demonstrates a consistent inverse relationship between dieback class and the strength of rainfall as a determining factor, which decreases as dieback increases. This would suggest that the weaker trees are less able to respond to rainfall and would support the finding from Tulik et al. (2010) that weakened ash trees are less efficient in the transport of water compared with healthy trees. Nevertheless, even in the healthiest class, which demonstrated the strongest regression, there was only up to 35% of tree-ring increment being influenced by rainfall. This low relationship is probably explained by the influence of other factors that determine stem increment, such as thinning regimes, that are not explored within this study. Although the relationship between rainfall and tree-ring increment becomes less relevant as dieback increases the low rainfall in 1996 and 2010 could still have been an important factor in the dieback development of the ash trees sampled. This would agree with Schumacher et al. (2007) who suggest drought can predispose ash to Chalara dieback. Indeed, the low rainfall of 2010 could have caused crown decline in the 25 - <50% and 50 - <75% trees which began the reduction in tree-ring increment which Chalara dieback then exacerbated. This would agree with the conclusion drawn from Oliva et al. (2014) who suggest that pathogens accelerate drought induced mortality. Chalara dieback, its arrival and dieback progression within Suffolk and Norfolk The first year that the dieback categories became statistically relevant was This could indicate that a common limiting factor was influencing the different groups of ash trees from this point. It is hypothesised that Hymenoscyphus fraxineus was influencing growth of ash trees before the official identification in 2012, although there could be other explanations for reduced tree-ring increment which are not explored within 13

14 this study. The relationship between Hymenoscyphus fraxineus and other fungi species is another important aspect not investigated within this study. As suggested by Kowalski et al. (2016) Hymenoscyphus fraxineus causes early necrosis but it could be other fungi that then increase the size of necrotic lesions and therefore it may not be solely Hymenoscyphus fraxineus causing the observed reduction in tree-ring increment. Indeed, other fungi such as Diplodia mutila may form part of the dieback process (Kowalski et al., 2017). In a study by Husson et al. (2011) Hymenoscyphus species were only present in 80% of lesions suggesting that 20% could be a result of other fungi, pests or abiotic factors. Queloz et al. (2011) suggest a subtle change in climate may have favoured Hymenoscyphus fraxineus over the native Hymenoscyphus albidus resulting in the dieback outbreak. However, the current ash dieback outbreak is not explained by any noticeable European wide change in the environment (Hietala and Solheim, 2011). It would certainly be interesting to examine the relationship of population development and sporulation of Hymenoscyphus fraxineus and Hymenoscyphus albidus in response to weather anomalies, especially very high rainfall, as a high population density of Hymenoscyphus fraxineus is suggested to have caused the current dieback outbreak (Hietala and Solheim, 2011). It would also be of interest to investigate whether the high early summer rainfall of 2007 enabled the reproduction of Hymenoscyphus fraxineus, in the ash woodlands of Suffolk and Norfolk leading to the current dieback outbreak. Conclusion This study suggests that tree-ring increment could have been influenced by Chalara dieback before the official confirmation in A 13-year significant difference in mean tree-ring increment was demonstrated from in the healthiest (<25% dieback) and unhealthiest ( 75%) ash trees. However, it is unknown if the dieback was solely because of the fungus Hymenoscyphus fraxineus or whether it formed part of a multitude of aspects causing stress, resulting in crown decline and leading to reduced tree-ring increment. Reduced tree-ring increment in 75% crown dieback trees was probably initiated following low rainfall in 14

15 1996, then exacerbated by Chalara dieback. It is probable that the low rainfall of 2010 also caused drought associated stress in some ash trees which may have been intensified by Chalara dieback. The influence of climatic factors, such as rainfall anomalies, are a key factor which can limit tree growth and should form part of the analysis of how Chalara dieback will impact ash trees. It is important to remember how a tree would normally react to weather anomalies, how the pest or disease also reacts and how the combination of these factors will ultimately influence the tree. A changing climate and increasing episodes of extreme weather will surely lead to the greater potential of tree stress, and this in turn will result in the greater impact on tree growth from pests and diseases. Acknowledgements I would like to thank Gary Battell for the initial inspiration along with Jo Clark (Earth Trust, Living Ash Project) and Joan Webber (Forest Research) for nurturing the project. I am grateful to Michael Meakin, Anthony Meynall, Miles Barne, Diana Macmullen (c/o Mr and Mrs Barratt), Ben Mattock (Forestry Commission), Tom Brown (Green Light Trust) and both Suffolk and Norfolk Wildlife Trust for providing me the ash trees to sample. Thanks to Andrew Weatherall and all the forestry staff at the National School of Forestry for their support and encouragement throughout my time at the University of Cumbria. RFS Viking Bursary This research has been funded by the Royal Forestry Society's Viking Bursary. I would like to honour the late Mrs Sheila Jorgensen, a former RFS Yorkshire Division member, who requested the bursary. It is her forward thinking that has enabled me to undertake this research. 15

16 References Met Office (2017) Historical monthly data for meteorological stations. Available at: (Accessed 7th April 2017). Forestry Commission (2015a) Chalara dieback of ash (Hymenoscyphus fraxineus). Available at: (Accessed 18 th April 2017). García-Suárez, A.M., Butler, C.J. and Baillie, M.G.L. (2009) Climate signal in tree-ring chronologies in a temperate climate: a multi-species approach, Dendrochronologia, 27(3), pp Hendry, S. (2013) Chalara dieback of ash - an overview of the disease and current research. Available at: (Accessed 18 th April 2017). Hietala, A.M. and Solheim, H. (2011) Hymenoscyphus species associated with European ash, Bulletin OEPP/EPPO Bulletin, 41(1), pp Husson, C., Scala, B., Caël, O., Frey, P., Feau, N., Ioos, R. and Marçais, B. (2011) Chalara fraxinea is an invasive pathogen in France, European Journal of Plant Pathology, 130(3), pp Innes, J.L. (1990) Field Book 12: Assessment of Tree Condition. 1 st edn. London: HMSO. Kowalski, T., Kraj, W. and Bednarz, B. (2016) Fungi on stems and twigs in initial and advanced stages of dieback of European ash (Fraxinus excelsior) in Poland, European Journal of Forest Research, 135(3), pp Kowalski, T., Bilański, P. and Kraj, W. (2017) Pathogenicity of fungi associated with ash dieback towards Fraxinus excelsior, Plant Pathology, doi: /ppa Matisons, R., Inohosa, L.G. and Laiviņš, M. (2016) Pointer Years in Tree-Ring Width of European Ash with Different Crown Condition and Their Relationships with Climatic Factors in Latvia, The Journal of Latvian Academy of Sciences, 70(3), pp Oliva, J., Stenlid, J. and Martínez Vilalta, J. (2014) The effect of fungal pathogens on the water and carbon economy of trees: implications for drought induced mortality, New Phytologist, 203(4), pp Queloz, V., Grünig, C.R., Berndt, R., Kowalski, T., Sieber, T.N. and Holdenrieder, O. (2011) Cryptic speciation in Hymenoscyphus albidus, Forest Pathology, 41(2), pp Rackham, O. (2014) The Ash Tree. 1 st edn. Dorset: Little Toller Books. Rinn, F. (2003) TSAP-Win User Reference. 1 st edn. Heidelberg: Rinntech. Rogers, K. Lawrence, V. Hutchings, T.R. (2014) Determining Tree Growth in the Urban Forest (Trees, People and the Built Environment II). Available at: Paper-Rogers-01.pdf. (Accessed 18 th April 2017). Schumacher, J., Wulf, A. and Leonhard, S. (2007) First record of Chalara fraxinea T. Kowalski sp. nov. in Germany - a new agent of ash decline, Nachrichtenblatt des Deutschen Pflanzenschutzdienstes, 59(6), pp Speer, J.H. (2010) Fundamentals of tree-ring research. 1 st edn. Tucson: The University of Arizona Press. Tulik, M., Marciszewska, K. and Adamczyk, J. (2010) Diminished vessel diameter as a possible factor in the decline of European ash (Fraxinus excelsior L.), Annals of Forest Science, 67(1), p Vacek, S., Vacek, Z., Bulusek, D., Putalova, T., Sarginci, M., Schwarz, O., Srutka, P., Podrazsky, V. and Moser,W.K. (2015) European Ash (Fraxinus excelsior L.) dieback: Disintegrating forest in the mountain protected areas, Czech Republic, Austrian Journal of Forest Science, 132, pp