Thermal Microclimates of the Redgrave and Lopham Fen Nature Reserve

THERMAL MICROCLIMATES OF THE REDGRAVE AND LOPHAM FEN NATURE RESERVE A. P.

GAY

Summary T e m p e r a t u r e profiles in the Redgrave and Lopham Fen N a t u r e Reserve were investigated from 7.5cm below the soil surface to 275cm above the soil surface in three Vegetation types, both in detail on various days and in less detail simultaneously. In each Vegetation type the highest and lowest temperatures were recorded at the same position, the height of which from the ground surface was proportional to the height of the Vegetation. The highest temperatures were recorded nearer the ground in the lower Vegetation type but they were maintained for the shortest vertical distance. The data are compared with data from nearby Meteorological Stations.

T h u s at any time in the Fen there were a large ränge of temperatures available to insects depending on height above the ground and the Vegetation type. This factor may contribute to the diversity of insect species in the Fen since the Vegetation types appear as a mosaic owing to the Variation in the surface microtopography of the Fen.

Introduction Redgrave and Lopham Fen, which lies 8km west of Diss in Norfolk, is a nature reserve managed by the Suffolk Trust for N a t u r e Conservation. An important feature of the Fen is the large number of different habitats available within its boundaries. This results from its formation in a shallow-eastwest Valley between chalk hĂźls, filled with reed and sedge peats overlaid with sand in places. This leads to a contrast between the alkaline ground water and the more acid conditions on the sandy ridges. The pattern is further complicated by variations in the water table caused by peat cutting and a fuller description is given by Pope (1967).

T h e diversity of conditions gives rise to many Vegetation types in close proximity and each in turn will have its own microclimate. We decided to study the thermal microclimates in three main Vegetation types, namely Sedge, Phragmites and Acid Heath. The correspondence of these regions with those

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Comparison

361

Table 1 of Vegetation types with those used by Pope (1967).

Present Classification

Pope's Classification

Phragmites Sedge Acid H e a t h

16 Very wet vegetable debris 15 Damp vegetable debris 3 Herbage on Acid Heath areas.

habitats of the Coleoptera listed by Pope (1967) is given in Table 1. Measurements were made of temperature profiles in each Vegetation type during 24 hour periods.

Equipment and Methods Thermometers T e m p e r a t u r e s were measured by means of a Grant Instruments Electronic Thermistor Thermometer (reading from - 10째C to +50째C by 0.1째C steps). This type of thermometer has the advantage that the electrical sensors used (thermistors) are small and can be placed some distance from the rest of the instrument so minimising the disturbance of the microclimate being measured. The thermistors used were encapsulated in needle and rod probes and shielded from direct radiation from the sun by means of small cardboard shields or plastic beakers.

All probes used were individually calibrated against an accurate mercury-in-glass thermometer in a well stirred water bath and the resulting corrected values are accurate to + 0.2째C.

Location of Thermistors It has been suggested elsewhere (Geiger, 1966) that different h e i g h t s o f V e g e t a t i o n w o u l d r e s u l t in d i f f e r e n t t e m p e r a t u r e

profiles. In order to investigate this the probes were arranged on a pole at 30cm intervals from the soil surface to 270cm above the ground, with additional probes at 15 and 7.5cm. In addition unshielded probes were buried in the soil 7.5 and 15cm below the surface. A second set of recordings was carried out with probes at 270, 90, 7.5 and 0cm above the soil surface.

The sites of the observations are shown in the map (Figure 1)

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Fig 1.

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M a p of Little Fen showing location of recording sites. National grid refereuces are given.

Table 2 Description of Sites Ph ragmites Height of Vegetation Position of Water Table below surface, 7 July 1969

Sedge

A cid Heath

270 cm

95 cm

10 cm

5 cm

30 cm

60 cm

and the height of the Vegetation and depth of the water table at each site are given in Table 2.

Timing and Weather Conditions T h e recordings were all made on clear, sunny days and readings were taken every hour or one and a half hours, continuing for 24 hours. In some cases, when the weather changed it was not possible to obtain complete 24 hour sets of recordings due to rain which caused short circuiting of plugs on the thermistor thermometer.

R E D G R A V E A N D L O P H A M FEN N A T U R E RESERVE

I

24

I

Time

F i g 2. o cm,

1

6

12 hours

363

1

18

B.S.T.

T e m p e r a t u r e s in Phragmites at f o u r heights above ground level: ; 7.5 c m ,

; 90 cm,

and 270 cm,

1969, (b) 3-4 September 1969.

: (a) 30-31 Jul>

24

I 6

1 12

1— 16

Time hours B.S.T.

Fig 3.

Temperatures in Sedge at four heights, symbols as Fig. 2 14-15 August 1969, (b) 3-4 September 1969.

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T h e first set of recordings was made on different days in July a n d A u g u s t 1969 at each site. A second series of recordings was m a d e during the same day (3^4 S e p t e m b e r 1969) at all three sites, but with a restricted n u m b e r of probes at e a c h site.

Kesults and Discussion T h e r e is s o m e difficulty in reporting the observations in this type of study because of the large a m o u n t of data and the p r o b l e m of r e p r e s e n t i n g Variation in three dimensions on a page. T w o types of diagram have been used here: T h e first ( F i g u r e s 2 - 4 ) shows t h e Variation of temperature with t i m e , t h e temperature at each height in the Vegetation

being represented by a separate line. This m e t h o d is only suitable for sets of data w ith probes at relatively few heights in t h e Vegetation. If many probes a r e used t h e graph b e c o m e s confused.

T h e second m e t h o d (Figures 5 - 7 ) is an isotherm diagram.

This is a way of representing data for many heights and a c o m p l e t e day in o n e diagram. Each line (isotherm) joins p o i n t s of e q u a l t e m p e r a t u r e (similar to the c o n t o u r s on a m a p , but with t e m p e r a t u r e r e p r e s e n t e d instead of height). The o t h e r axes are height in the Vegetation and time of day. It is possible by r e f e r e n c e to an isotherm diagram to d e t e r m i n e the t e m p e r a t u r e at any point in time for any height within the limits of the axes. A horizontal line across the diagram indicates the t e m p e r a t u r e s t h r o u g h o u t the day at the height s h o w n by the point at which the line crosses the vertical axis.

A vertical line indicates the profile of t e m p e r a t u r e at the time s h o w n on the horizontal axis. When t e m p e r a t u r e s are c h a n g i n g rapidly the lines on the diagrams are close together, as in the lines near the base of Fig 7b. High and low t e m p e r a t u r e s in relation to the surrounding t e m p e r a t u r e s a p p e a r as concentric circles, such as those showing the high t e m p e r a t u r e reached at about 1400 at ground level in Fig 7b.

T h u s these d i a g r a m s are a convenient m e a n s of summarising a n d p r e s e n t i n g the main features of a microclimate.

Vegetation and Microclimate T h e Vegetation of an a r e a has an important influence on its m i c r o c l i m a t e a n d m e a s u r e m e n t s have been m a d e of Variation in t e m p e r a t u r e above the ground in Vegetation types having characteristic but different heights. Variation of t e m p e r a t u r e

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with height a b o v e the ground d e p e n d s on the height and density of the Vegetation, since the leaves of the plants (or the g r o u n d surface if there are no plants) intercept radiant energy f r o m the sun in sunlight. O n striking the leaves the sun's rays a r e c o n v e r t e d into heat ('sensible h e a t ' ) causing the t e m p e r a t u r e in that region to rise. If the conversion of radiant energy to h e a t occurs sharply, as in the sun striking bare g r o u n d , then the t e m p e r a t u r e rise will be large over a narrow height zone, just a b o v e the g r o u n d . H o w e v e r , if the ground is covered with a m a s s of Vegetation through which the sun can filter, then the t e m p e r a t u r e rise will occur over a much greater height f r o m the t o p of t h e plants to the ground. This process is well known a n d is discussed in detail by Geiger (1966) and Macfadyen (1963) and the effect of the density of Vegetation cover has also b e e n described (Landsberg, 1973).

H i g h e r m a x i m u m t e m p e r a t u r e s will be obtained on clear s u n n y days w h e n the incoming sunlight is at its most intense a n d this is the reason for the selection of this type of day for the p r e s e n t study. A t night the earth cools by loss of radiation to t h e sky a n d the opposite effects to those outlined above will o c c u r , the positions of day m a x i m u m t e m p e r a t u r e becoming the position of t h e night m i n i m u m . T h e t e m p e r a t u r e of the soil is also d e p e n d e n t u p o n the a m o u n t of sunlight but lags c o n s i d e r a b l y b e h i n d air t e m p e r a t u r e due to its much lower t h e r m a l conductivity.

T h e microclimates f o u n d in t h r e e of the Vegetation types in t h e F e n are discussed since they provide an illustration of t h e s e principles and the results suggest reasons for the d i f f e r e n t types of insects and other organisms f o u n d in the t h r e e Vegetation types.

T h e locations of the m a x i m u m t e m p e r a t u r e s will d e p e n d p a r t l y u p o n the height of the Vegetation, and this can be seen in Figs 5a, 6a a n d 7a. In the 270-cm high Phragmites Vegetation the m a x i m u m t e m p e r a t u r e occurs 210cm above the g r o u n d surface (Fig 5a). In the 95-cm high Sedge Vegetation t h e m a x i m u m t e m p e r a t u r e occurs 60cm above the ground s u r f a c e (Fig 6a). In the 10-cm high Acid H e a t h Vegetation the m a x i m u m t e m p e r a t u r e occurs between 7.5 and 30cm a b o v e t h e g r o u n d surface (Fig 7a). T h u s the higher the Vegetation the g r e a t e r the distance of the m a x i m u m t e m p e r a t u r e f r o m the g r o u n d , as s h o w n by Fig 8.

T h e actual values of the m a x i m u m t e m p e r a t u r e s reached in e a c h Vegetation type a r e related to the height of the Vegetation. In the Phragmites Vegetation the m a x i m u m

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Fig 4 .

367

T e m p e r a t u r e s in A c i d H e a t h at f o u r heights, s y m b o l s as Fig 2 (a) 2 A u g u s t 1969, ( b ) 3-4 S e p t e m b e r 1969.

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a

Time hours B.S.T.

b

Time hours B.S.T. Fig 5. Isotherm diagrams in Phragmites Vegetation at 2°C intervals, •* indieates height of Vegetation, (a) 30-31 July 1969, (b) 3-4 September 1969.

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t e m p e r a t u r e was 20째C (Fig 6a) whereas in the Acid Heath the maximum temperature was 28째C and this was maintained for a longer period (Fig 7a). Thus the maximum temperature reached was approximately inversely proportional to the height of the Vegetation. The reason for this is that in the shortest Vegetation (i.e. Acid Heath) the energy supplied by sunlight is converted to heat over the shortest distance and t h e r e f o r e produces a large temperature rise. However in the Phragmites Vegetation the sun's energy is intercepted over a considerable vertical distance as the canopy is not as dense as in the other Vegetation types and therefore the temperature is raised over a much greater depth of canopy and the maximum t e m p e r a t u r e is correspondingly low. The density of the Vegetation layer over which the light is intercepted will also be an important factor determining the maximum temperature, but this was not investigated here.

T h e time of day at which the highest temperature is reached is also related to the height of the Vegetation. In the Phragmites the maximum temperature occurred at 1800 hours (Fig 5a), in the Sedge the maximum temperature occurred at 1600 hours (Fig 6a) and in the Acid Heath the maximum t e m p e r a t u r e was reached at 1200 hours (Fig 7a). Thus the time of the maximum temperature is later the taller the Vegetation.

A t night the minimum temperature occurred at the same height above the ground surface as the maximum temperatures in the day time. The relationship is clearest in Figure 7b, for the Acid H e a t h area. Similarly the relation between the heights of the maxima and minima can be seen in the Sedge and Phragmites regions but since the maxima and minima in these regions are less extreme the effect is less clear in the isotherm diagrams, however it can readily be seen in the ' t e m p e r a t u r e at four heights' diagrams (Figures 2b, 3b and 4b for Phragmites, Sedge and Acid Heath respecively) where the line representing the height at which the highest temperature occurs in the day also passes through the lowest night temperature.

T h e isotherm diagrams show the Variation in the temperature gradients with respect to height in the three Vegetation types. T h e temperature gradients are greatest at the time when the maximum temperature is reached. In the Phragmites Vegetation (Fig 5a) the temperature gradient with respect to height is lowest and there is a maximum r채nge of temperature of 4째C over a distance of 210cm from the ground. In the Sedge Vegetation (Fig 6a) the temperature has a steeper gradient with

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respect to height and there is a ränge of 14°C over a height of 60cm from the ground. The temperature gradient with respect to height is greatest in the Acid Heath Vegetation (Fig 7a), a ränge of 12°C occurring in the first 30cm from the ground. Thus it can be seen the shorter the Vegetation the greater the temperature gradient with respect to height.

T h e isotherm diagrams also illustrate the ränge of temperatures experienced throughout a 24 hour period in different Position in the canopies. Generally the greatest change in temperature occurs just below the surface of the canopy and the least at the soil surface; for example in the Phragmites Vegetation (Fig 5a) the ränge was 12°C at 220cm and 2°C at the soil surface.

In general the microclimate found in the Fen was similar to that observed by other workers in the same Vegetation type elsewhere, for example the data of Delaney (1953) for Calluna heathland agrees with the data given here for Acid Heath Vegetation and the data given by Conway (1935) for Cladium mariscus is similar to that observed in the sedge Vegetation here.

Soil Temperatures The soil temperatures observed in the study are summarised in Table 3 from which it can be seen that the maxima and minima are less extreme than those observed for the air temperature.

Temperatures

Maximum °C Minimum °C Range °C

Table 3 15 cm below soil surface

Phragmites

Sedge

Acid Heath

15.3 14.6 0.7

14.3 13.4 0.9

15.6 14.3 1.3

The main reason for the stability of the soil temperature is the greater thermal capacity of soil than air which means that much more heat is required to raise the temperature of the soil than is required to raise the temperature of the air by the same a m o u n t . In addition less radiation reaches the soil surface because of the Screening effect of the dense Vegetation cover

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371

a

Time

hours

B.S.T.

Fig 6. I s o t h e r m d i a g r a m s in S e d g e Vegetation at 2°C intervals, •* indicates h e i g h t of Vegetation, ( a ) 14-15 A u g u s t 1969, ( b ) 3-4 S e p t e m b e r 1969.

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a

Time hours B.S.T.

b

Trme hours B.S.T. Fig 7.

I s o t h e r m diagrams in Acid H e a t h Vegetation at 2°C intervals, •* indicates height of Vegetation, (a) 2 August 1969, (b) 3-4 S e p t e m b e r 1969.

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which allows very little heat from the sun to penetrate to the soil surface. The thermal capacity of the soil is further increased by high water content. These effects explain why the smallest ränge of temperature was found in the soil having the highest water content and tallest Vegetation (Phragmites) and the highest ränge of temperature was found in the soil with the lowest water content and shortest Vegetation (Acid Heath).

Fluctuations in temperature in the soil will obviously decline with the depth in the soil.

Simultaneous measurements A i r temperatures The patterns shown by observations made on the same day (Figs 2 to 7 suffix b) were similar to those made on different days (Figs 2 to 7, suffix a). The only differences in detail are in the temperature gradients and the period for which higher temperatures were maintained. This confirms that the observed differences between microclimates of the three Vegetation types were a result of the differences in the Vegetation and did not arise because of differences in the weather on Observation dates.

£

o

o 180

0 0

90 H e i g h t of

Fig 8.

180 Vegetation

270 cm

R e l a t i o n between height o f Vegetation and height o f m a x i m u m t e m p e r a t u r e f r o m the g r o u n d in the three Vegetation types.

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Comparison of Temperatures on the Fen with Loeal Meteorological Station Recordings The nearest Meteorological Stations to the Fen are at Morley St. Botolph (24km north of the Fen) and Santon Downham (27km east of the Fen) and a summary of the relevant data from their records is included in Table 4 (courtesy of the Meteorological Office).

Comparison

Table 4 of Fen and Meteorological temperatures (°C)

Maximum temperatures D a t e (1969) Meteorological Stations Morley St. Stanton Botolph Downham 30-31 July 19.4 18.3 2 August 23.8 23.2 14-15 August 22.2 23.3 3-4 S e p t e m b e r

20.5

21.1

Minimum temperatures 30-31 July 10.0 2 August 13.8 14-15 August 14.4

5.0 14.4 14.4

3-4 S e p t e m b e r

10.6

9.4

Fen Location

Depth cm 30-31 July 2 August

Temperature

Phragmites Acid Heath Sedge [ Phragmites \ Acid Heath Sedge

20.0 29.5 29.4 24.3 26.7 24.3

Phragmites Acid H e a t h Sedge I Phragmites | \ Acid Heath Sedge

5.6 14.9* 13.5* 6.4 5.4 6.4

1

Soil temperatures

Station

at 10.00 B. S. T. 10 17.3 18.0

20 17.4 18.2

10 16.3 18.0

20 15.4 17.7

Phragmites Acid Heath

7.5 13.9 15.7

15 14.7 15.3

* T e m p e r a t u r e recorded for part of night only

The maximum temperatures in Vegetation on the Fen are almost always higher than those obtained at the Meteorological Stations and can be up to 6.3°C higher in the Sedge and Acid Heath Vegetation. The maximum temperatures in the Phragmites Vegetation on the Fen were the dosest to the Meteorological Station values.

Minimum temperatures on the Fen are generally close to those observed by the Meteorological Stations. The minimum temperature recorded at Santon Downham gave the dosest

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375

a p p r o x i m a t i o n to the Fen t e m p e r a t u r e s for the dates when r e c o r d s w e r e m a d e in the study.

T h e c o m p a r i s o n of soil t e m p e r a t u r e s shows that t e m p e r a tures in t h e F e n soils are generally below the soil t e m p e r a t u r e s at the Meteorological Stations; this is probably due to the g r e a t e r height of Vegetation in the F e n and the greater thermal capacity of the F e n soils due to water-logging. T h e changes of soil t e m p e r a t u r e with time are similar at both stations and on the F e n .

The Significance of the Microclimates T h e significance of these observations is the d e m o n s t r a t i o n of the d i f f e r e n c e s b e t w e e n the thermal microclimates of three Vegetation types on the F e n . It can be seen that in Vegetation of d i f f e r e n t heights different t e m p e r a t u r e s will exist simultaneously. This has a particularly important influence on the activities of insects in the Fen since their body t e m p e r a t u r e is largely d e p e n d e n t u p o n the surrounding air t e m p e r a t u r e . It is k n o w n that this effect of Vegetation on microclimate is a m a j o r f a c t o r influencing insect activity ( W a t e r h o u s e , 1955).

A s well as the effects of the microclimate created by the Vegetation on insects the microclimate also affects the plants.

F o r e x a m p l e , leaf t e m p e r a t u r e has an important effect on p h o t o s y n t h e s i s as has b e e n d e m o n s t r a t e d for Phragmites ( P e a r c y et al. 1972). T h e position in which the maximum day a n d m i n i m u m night t e m p e r a t u r e s occur w h e r e most of the r a d i a t i o n is i n t e r c e p t e d , has b e e n called the outer active s u r f a c e ( G e i g e r , 1966) and is probably the region of maximum p h o t o s y n t h e s i s ( M a c F a d y e n , 1963). T h e microclimate created by t h e p r e d o m i n a n t plants in each Vegetation type also affects t h e o t h e r plants growing there. For example, delicate m e s o p h y t i c b r y o p h y t e s may be able to grow at the base of the Phragmites w h e r e they avoid desiccation and are protected f r o m e x t r e m e s of t e m p e r a t u r e .

T h e relatively constant soil t e m p e r a t u r e s have an important e f f e c t on plants a n d soil organisms. T h o s e parts present below the g r o u n d are p r o t e c t e d f r o m e x t r e m e s of t e m p e r a t u r e , and in s o m e sedges this may protect the rhizomes which are sensitive to large t e m p e r a t u r e fluctuations.

T h e survey carried o u t by P o r t e r (1969) shows a strong c o r r e l a t i o n b e t w e e n the d e p t h of water table and the height of t h e Vegetation. In the F e n the ultimate control of microclimate m a y t h e r e f o r e be by the water table. Since the Fen has a very

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variable microtopography different microclimates are formed in close proximity which may explain in part its unique flora and fauna.

Acknowledgements T h e author would like to thank the Suffolk Naturalists' Society for a grant from the Charles Morley Bequest to cover the expenses of the fieldwork and the officers of the Society for making the study possible. Thanks are also due to Mr. P. J. Wanstall for advice and encouragement, to Dr. S. Prince for reading the manuscript and to D. Lane for assistance with the fieldwork.

A. P. Gay, B.Sc., Queen Mary College, University of London, Mile End Road, London, El 4NS. Present address: Welsh Plant Breeding Institute, Aberystwyth.

References Conway, V. M. (1935). Studies in the autecology of Cladium mariscus.

New Phytol. 35, 359.

Delaney, M. J. (1953). Studies in the microclimate of Calluna heathland. J. Anirn. Ecol. 22, 227.

Geiger, R. (1966). The climate near the ground. Harvard University Press.

Landsberg, J. J. (1973). Microclimate and the potential productivity of sites. Scient. Hort. 24, 126.

MacFadyen, A. (1963). Animal Ecology, Pitman, London.

Pearcy, R. W., Berry, J. A. and Bartholomew, B. (1972).

Field measurements of the gas exchange capacities of Phragmites communis under summer conditions in Death Valley. Carnegie Inst. Yrbk. 71, 161.

P o p e , R. D. (1967). A preliminary survey of the Coleoptera of Redgrave and Lopham Fens. Trans. Suffolk Naturalists' Trust. 14, 1.

Porter, B. D. (1969). Levelling survey of Lopham Little Fen and River Waveney. (Unpublished, filed at Fiatford Mill Field Centre).

Waterhouse, F. L. (1955). Microclimatological profiles in grass cover in relation to biological problems. Q. J. R. Met. Soc. 81. 63.