Dendroclimatological observations on trees at Kew and Wakehurst Place: event and pointer years
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Dendroclimatological observations
on trees at Kew and Wakehurst
Place: event and pointer years
M. C. BRIDGE,1 P. E. GASSON2 AND D. F. CUTLER2
1
Department of Geography, London Guildhall University, Old Castle Street, London, El 7NT, England
2
Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, 1 W9 3DS, England
Summary
The growth responses to varying meteorological conditions of a number of trees grown at Kew
Gardens and Wakehurst Place are investigated. Event years for each tree-ring series are identified
and then pointer years for all trees recognized and related to weather records. The use of pointer
years has highlighted the similarity in growth characteristics throughout a range of taxa in the
more extreme years. The strongest pointer year, 1958, coincides with a warm wet summer. Nar-
row rings generally correlate well with known periods of agricultural drought for the trees
grown at Kew. Response functions for oaks and hickories are also presented. Several individual
Kew trees have growth patterns which cross-match with oak site chronologies from southern
England using standard dendrochronological techniques.
Introduction
records in the world (e.g. monthly rainfall
A previous paper (Cutler et al., 1993) gave records back to 1697: Wales-Smith 1973, 1980)
details of the background and some preliminary and thus presents a rare opportunity to relate
results of the dendrochronological studies on tree growth to meteorological data from the
windblown trees from Kew and Wakehurst same site. This paper documents the first
Place following the severe storms in October attempts to relate the growth of trees from sev-
1987 and January 1990. The next step in the eral genera to these data. Wigley and Atkinson
study was to try and establish the growth (1977) presented a table of agricultural drought
responses to weather conditions of these and years ranked in severity on the basis of these
additional trees from a wide range of genera records which, although not intended to relate
that fell in the storms. Kew is relatively flat, on to tree growth, does refer to the same location
sand and gravel river terraces with a high, and may give more relevant information than
tidally-influenced water table, on the western simply precipitation and temperature data. The
edge of London. Wakehurst is undulating, with most severe agricultural droughts in rank order
a shallow, generally well-drained soil overlying were 1976, 1934, 1944, 1893, 1938, 1921 and
sandstone, in rural Sussex. Kew is fortunate in 1974; the tree ring records are related to this
having some of the longest meteorological information.
© Institute of Chartered Forester*, 1996 Forestry, Vol. 69, No. 3, 19%264 FORESTRY
The accepted method for establishing con- Pointer years are defined as years in which
nections between climate and radial growth is significant proportion of the trees exhibit an
based on the statistical comparison of growth event year. Although generally applied to a
ring chronologies (either in the form of raw group of trees of the same species, Schweingru-
ring-widths, indices, or maximum latewood ber (1990) discusses the recognition of pointer
densities) with series of meteorological data years among three species in a climatically uni-
(usually monthly rainfall and mean monthly form area in northern Switzerland. Of the 14
temperature figures). This approach has proved pointer years occurring between 1920 and 1985,
robust at enabling deduction of average cli- only two (1959 and 1976) occurred in all three
mate—growth relationships in multi-tree species. A heterogeneous group of trees, such as
chronologies, but seldom allows for the evalua- those growing at Kew and Wakehurst might be
tion of individual or extreme years (Schwein- expected to provide difficulties in identifying
gruber etal., 1990). Equally, the use of response pointer years. Schweingruber et al. (1990) make
functions based very largely on monthly meteo- the point that defining levels at which one
rological data tends to miss growth effects attributes the term 'pointer' year depends upon
which result from short-lived but severe condi- the aim of the study. The threshold value could
tions, e.g. late frosts which may damage leaf be as low as 20 per cent of all trees in timber-
buds, and short-term droughts. line bog sites, or as high as 90 per cent in a pop-
While the use of response functions has ulation of at least 20 trees in more extreme
proved extremely useful in widescale climatic semi-arid environments.
reconstruction from tree data, the lack of repli- With trees from many provenances each hav-
cation within species in the present data set ing different ecological preferences it is likely
necessitates a different approach. Some of the that some trees will thrive in conditions in
earliest dendrochronological studies, carried out which others grow less well, and the hetero-
in more extreme climates, used 'skeleton plots' geneity of the group precludes defining a thresh-
to record particularly wide or narrow rings. old limit a priori. While in many cases there is
These were then summarized in master plots the problem that individual species are not well
which recorded the major events in a group of replicated in the sample population, there is an
trees (Douglass 1939; Stokes and Smiley, 1968). opportunity here to present some data where
Huber (1951), working on oaks in Germany, none previously existed.
defined 'pointer years' as those in which 80 per Some of the data available are relatively well
cent or more trees exhibited the same trend. replicated and were considered suitable for lim-
Schweingruber etal. (1990) redefined 'event' and ited further analysis using response function
'pointer' years. techniques.
Event years are particular years in the life of The two contrasting sites, both within essen-
a tree which show marked differences in their tially the same climatic unit of south-east Eng-
morphology (especially in terms of growth ring land, allow for some limited discussion of the
width). It is often possible to employ seemingly influence of site conditions on the growth of
subjective but replicable criteria to the defini- trees. Within the sites, the position of each tree
tion of event years. For example one could sim- at Kew is known, and therefore microsite vari-
ply assess growth by eye and recognize rings ation may also be examined.
with reduced or increased increments in relation
to their immediate neighbours, perhaps employ-
ing classes such as 75 per cent difference com-
Methods
pared with the average of x previous years Details of sampling and preparation have been
growth. There are particular problems here, recorded elsewhere (Cutler etal., 1993). For this
especially near the boundaries between classes, study only trees in which two or more radii
but the method does allow rapid and repro- have been successfully crossmatched were used
ducible results in the hands of experienced for further analysis. This limits the sample size
workers (Schweingruber etal., 1990). to a total of 79 trees, 53 from Kew and 26 fromDENDROCLIMATOLOGICAL OBSERVATIONS ON TREES 265
Wakehurst Place representing a total of 44 beech tree ring series from Kew were compared
species. with individual Wakehurst oaks and published
For this study, the ring-widths had already oak chronologies for southern England using
been measured as part of the internal quality 'f-values' calculated by the program CROS
checking procedures of crossmatching. It would (Baillie and Pilcher,4973; Munro, 1984). Other
be possible therefore to calculate the variation individual trees were also considered, and this
of the ring-widths from an overall or a running information is in Table 1.
mean for each series. With great variation in the
sensitivity of the trees investigated it would
have been unreasonable to set an a priori
threshold of difference in ring-width to qualify Results and discussion
as an event year. Instead, the percentage varia- There was much variation between the individ-
tion from a 5-year running mean was calculated ual tree-ring series. The most significant event
for each eligible ring of each series, and the year in each series was always more than 30 per
largest five variations noted. Pointer years were cent different from the mean of the four rings
then sought by reference to common changes either side of it, the lowest variation occurring
among these five most significant variations in in a Carya (33 per cent), and the highest (259
each series. per cent) in a Fagus.
Next, the growth in the most severe soil The most replicated event year, and hence the
moisture deficit years as defined by Wigley and strongest pointer year, was 1958 in which 17
Atkinson (1977) was assessed. In each year, the trees exhibited a positive (wide) ring. This year
ring-width record was examined to see whether had above average precipitation in February
the growth increment in these years was notica- and during the summer, and cool spring tem-
bly smaller or larger than neighbouring years, peratures followed by a warmer than average
but no threshold value of significance was late summer. The year 1976 also shows as an
employed. event year in 17 trees, but while 16 show
For Kew trees, the location within the Gar- reduced growth, one (an elm from Wakehurst)
dens was noted to see if any association showed increased growth.
between location and growth was evident. The next ranked pointer year is 1921, with
Response functions were performed using just nine trees showing a negative event greater
data from two genera, first the oaks {Quercus) than 25 per cent and one showing a positive
for comparison with existing British oak site event. After this, 1946 is the strongest pointer
response functions (Pilcher and Gray, 1982) and year with eight trees all showing a positive
second for the hickories {Carya) the only other event.
group with good replication of samples. In gen- When the tree-ring widths were noted in the
eral, response functions are usually used for seven most severe agricultural drought years
replicates of the same species, but since these some interesting differences emerge. Taken in
were not available, we used this method for the the order of drought severity, narrow rings were
two best replicated genera. This would not nor- evident in:
mally be considered viable since one might rea-
1976 85% Kew trees (only 72% of all trees,
sonably assume that different species would
both Kew and Wakehurst)
have different physiological responses to the
1934 82% Kew trees
same external stimuli. It would certainly not be
1944 65% Kew trees
acceptable to apply the response function to any
1893 61% Kew trees
wider grouping of trees, even if, superficially,
their ring-width curves crossmatched well. The 1938 87% Kew trees (but much smaller dif-
response functions used monthly values for pre- ferences)
cipitation and temperature from 1872 to 1987 Some Kew trees showed no narrow ring, or
and were regressed against summary indexed in some cases a wider ring for 1976. These
values for the eight oaks and eight hickories. include Phellodendron chinense from Kew area
Data from the oak, hickory, chestnut and 252 (the same area as Rhus and Castanea whichTable 1: Crossmatching between the tree-ring series of groups of trees and individuals, with oak chronologies from southern England, expressed
as values of Student's t (see text)
Kew accession Kew Kew Kew Kew Bath Oxford Sotterley Gutteridge Old Savernake Buff Blickling
number hickory chestnut beech oak oak oak oak oak Park oak oak oak
Wood
oak
Kew oak (160) 4.4 8.1 10.0 5.0 6.2 6.8 5.3 4.6 t 6.1 3.2
(138) (160) (160) (152) (151) (146) (160) (160) (100) (152)
Kew hickory (138) — — 5.9 6.6 4.4 5.5 5.7 3.7 4.6 4.8 3.3 6.5 3.3
(138) (138) (138) (130) (129) (129) (138) (138) (129) (100) (130)
Kew chestnut (198) — 5.9 — 8.7 8.1 4.2 4.9 4.4 6.0 4.1 t 5.1 t
(138) (198) (160) (190) (189) (146) (173) (198) (100)
Kew beech (199) — 6.6 8.7 — 10.0 4.4 4.2 6.5 4.7 4.1 t 5.8 t
(138) (198) (160) (191) (190) (146) (173) (198) (100)
Celtis occidentalis 000-73-15602 4.0 3.7 3.5 t t 3.1 t t t t t t
(87) -n
Fraxinus 000-73-19908 4.2 3.7 4.7 3.8 O
t t r t t t t t m
americana (72)
Gleditsia 000-73-11956 4.7 3.9 3.4 3.1 4.1 t t t t t t t
triacanthos (81)
Larix decidua (87) 000-69-16265 4.0 4.0 5.4 4.5 t t 3.5 t t t t t
Khus uerniciflua 000-69-16312 4.9 8.7 7.0 7.8 3.8 5.3 4.6 t 3.4 t 4.6 3.1
(56)
Tilia x moltkei (98) 000-73-11581 5.2 5.5 7.8 4.7 3.5 t 3.8 3.4 5.3 t 3.8 t
Tilia x orbicularis (91) 445-00-44505 t 4.6 4.1 4.2 t t t t t t t t
Platanus x hispanica Kew Green 5.3 7.7 7.3 5.3 3.4 3.6 3.6 t 3.0 t 4.2 t
(94)
* = short overlap against published chronologies,
— = no overlap attempted,
t = 't' value below 3.0.
Bath and Oxford Pilcher and Baillie (1980)
Sotterley Briffa et al. (1986)
Old Park Wood Bridge, unpublished Numbers in brackets are length of series in left hand column or number of years of overlap where
Gutteridge Wood given under the V value
Savernake Briffa et al. 1986
Buff N. Holman, unpublished
Blickling Briffa et al. 1986DENDROCLIMATOLOGICAL OBSERVATIONS ON TREES 267
(a) SONDJFMAMJJASO SONDJFMAMJJASO
-fl.4 J
PPT
(b) SONDJFMAMJJASO SONDJFMAMJJASO 1 Z
9.Si I B.41 I.Z
-9.5 J -|.2J
PR. YR.
Figure 1. Response functions for (a) oaks and (b) hickories grown at Kew. These are based on monthly pre-
cipitation (PPT) and temperature (TEMP) values from 1872 to 1987, arranged from September (S) of the year
before growth through to October (O) of the year of tree-ring growth, and including the three previous years
(PR. YR.) The vertical axis represents standardized regression coefficients.
each exhibit narrow rings), two Morus alba var. correlation is significant. The two genera show
bungeana from area 412, Catalpa ovata from overall similarity, precipitation being positively
416, and Ailanthus altissima from 412. Areas correlated from April to July in oak and March
412 and 416 are adjacent, to the north of the to June in hickory, and temperature negatively
Temperate House. correlated for July in both. The graphs indicate
While 1974 was an agricultural drought year, some degree of negative correlation with tem-
the situation is complicated by several trees perature in the previous winter, but only
being damaged in September gales, but these slightly so in hickory. Since our results for a
would be expected to affect radial growth in the mixture of oak species are broadly in agreement
following year. At Kew, all the narrow pointer with those for native oaks from 16 sites in
years can be correlated with drought, whereas Britain (Pilcher and Gray, 1982), it may be
other phenomena such as late frosts are known assumed that those for the hickories also give a
to have the same effect elsewhere (Schweingru- true indication of climate—growth relations. The
ber etal., 1990). wide ring for the year 1958, identified as a pos-
The response functions for oak and hickory itive pointer year above, and the narrow rings
are shown in Figure 1. Points above the zero line in the drought years, fit well with the cli-
are positively correlated, and those below, neg- mate-growth response expected from this
atively correlated with tree ring growth. The model.
bars represent 95 per cent confidence limits, and The Wakehurst oaks do not match each other
where the bar does not cross the zero line, this very well as individuals and cannot therefore be268 FORESTRY
N
Old Park W o o d * Gutteridge
1
? kilometres ?°
Figure 2. Map to show locations of oak chronologies, Kew and Wakchurst, referred to in Table 1.
combined into a composite chronology. This is cata (Moir, 1994), growing in similar riparian
surprising in view of the widescale similarity in conditions at Hampton Court (15 km upstream
oak growth across southern England, but may of Kew) also cross-matches well with the Kew
reflect the variable topography of the site. On chronologies. The t values in Table 1 and loca-
the other hand, the Kew oaks, hickories, chest- tions in Figure 2 show no relationship between
nuts and beeches from this more uniform site the level of crossmatching and distance between
show significant matches with each other and sites at the macroscale. Within Kew, the vari-
several published native oak chronologies able responses of trees in area 252 in 1976 sug-
(Table 1 and Figure 2). Such interspecies cross- gest physiological variation between some taxa,
matching has been previously reported in oak, while the responses of taxa in areas 412 and 416
ash and alder by Groves and Hillam (1988), and may be due to specific microsite differences,
could be the result of similar physiology and/or which could include, for example, watering dur-
similar site conditions. The hickories would be ing periods of drought.
expected to agree with each other since the This study demonstrates that various levels of
three species examined have a similar distribu- sophistication can be used to investigate the cli-
tion in eastern North America, but the Kew mate-growth relationships of several species.
oaks are from a range of native distributions The use of event and pointer years identified
and their agreement is perhaps more surprising. one widely replicated year of enhanced radial
An unpublished chronology for yew, Taxus bac- growth, 1958, which shows the conditionsDENDROCLIMATOLOGICAL OBSERVATIONS ON TREES 269
favourable to tree growth at Kew. The several der Jahrringfolge. In Handbuch der Mikroskopie
negative pointer years are correlated with times V/I, S. Freund, H. (ed.). Frankfurt a. M., Umschau,
of agricultural drought. Response functions in 172-192.
two genera give a model of climate-growth Moir, A.K. 1994 The dendrochronological potential
relationships which probably holds true at this of yew {Taxus baccata): with special reference to
yew from Hampton Court Palace. BSc Thesis
site for other species whose ring width curves (unpublished). London Guildhall University.
match closely. Munro, M.A.R. 1984 An improved algorithm for
crossdating tree ring series. Tree-Ring Bull. 44,
Acknowledgements 17-28.
Pilcher, J.R. and Baillie, M.G.L. 1980 Eight modern
We would like to thank Andy Moir for technical oak chronologies from England and Scotland.
assistance and access to his unpublished yew chronol- Tree-Ring Bull. 40, 45-58.
ogy, Nigel Holman for the use of his Buff Wood oak Pilcher, J. and Gray, B. 1982 The relationships
chronology, and Gareth Owen for preparation of the between oak tree growth and climate in Britain. / .
figures. Ecol. 70, 297-304.
Schweingruber, F.H. 1990 Visual Analysis. In Meth-
ods of Dendrochronology. Cook, E. and Kairiuk-
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