[ 649 ]

Trans. Br, mycol. Soc. 85 (4), 649-653 (1985) Printed in Great Britain

CHANGES IN RELATIONSHIP BETWEEN WATER CONTENTAND WATER POTENTIAL AFTER DECAY AND ITS

SIGNIFICANCE FOR FUNGAL SUCCESSIONS

By N. J. DIXDepartment of Biological Science, The University, Stirling FK9 4LA, Scotland

Calculations from measurements ofwater content and corresponding water potentials showedthat after decay wood and leaves have higher water potentials compared with undecayedmaterial at the same water content. Improvement in water availability through decay may beone of the principal underlying forces which give direction to fungal successions on varioussubstrata.

It is acknowledged that the physical properties ofthe substratum have a profound bearing on thegrowth and survival of fungi. Water availability isan especially important growth regulator, for whilesome fungi are xerotolerant, others are dependentupon high water potentials for growth.

As plant materials decay their physical charac-teristics will change. This is particularly true withrespect to their water relations. As cell-wallpolymers are hydrolysed, the matric potential atany given water content will be expected to rise, andthis in turn will increase the availability of water tomicro-organisms growing on the substratum. Atthe same time, decayed materials will hold morewater, and this may offset the tendency which theywill have to dry at faster rates than non-decayedmaterials. In the long term such changes may makethe substratum more susceptible to colonization byfungi less tolerant of water stress. Microbialpioneer colonizers can therefore be seen aspotential modifiers of the water relations of thesubstratum and may well provide one of theprincipal underlying forces which give direction tofungal successions. Unfortunately the subject hasreceived little experimental attention and there areno quantitative data available to support thishypothesis. In an attempt to provide this informa-tion I have measured the changes that occurred inthe water relations of wood and leaves after theywere decayed by microbial pioneer colonizers.

MATERIALS AND METHODS

Dead leaves of beech (Fagus sylvatica L.) and oak(Quercus robur L.), still attached to young trees inFebruary, were collected for the experiments.

Leaves were very dry at the time of collection,appeared to have undergone little physical changesince senescence and were considered to be very

little decayed. Before use they were saturated withwater by soaking between wet filter papers for a fewdays. For each tree species three disks 1 em diamwere cut from the lamina of each of nine leaves anddisks from three leaves of the same tree speciescombined to make three samples, each containing1 disk from each leaf. Measurements of waterpotential (- bar) of the samples were obtainedusing a calibrated dewpoint microvoltmeter (modelHR 33T Wescor) with an insulated chamber. Aseries of readings was obtained over a range ofmoisture contents by allowing samples to dryslowly in air. Samples were weighed immediatelyafter each reading and the corresponding watercontent for each water potential value (ljF) calculatedas g H 20 g-1 dry weight, from the oven-dry weightobtained at the end of the experiment.

After the first measurements had been taken fromthe undecayed material, the leaves were placed onthe surface of water agar medium in Petri dishes.After 11 weeks' incubation in damp chambers,further disk samples were taken from the nowpartially decayed leaves. These were resaturatedwith water and measurements of water potentialand corresponding water content values obtained asbefore. During incubation the leaves becameheavily colonized by the indigenous microbialpopulations among which Penicillium spp. andother microfungi were prominent. Mean weightlosses from leaves at the end of this periodamounted to 27'9% for oak and 12'3% for beechwith corresponding relative densities (r.d. = goven-dry wt em'?') of 0'554 and 0'393 g cm"respectively. In total between 30 and 50 measure-ments were taken for both undecayed and decayedleaf disks.

Water potential values for a selection of watercontents for undecayed and decayed leaves werethen calculatedfrom computed regression equations

Fungal successionof log., loglo plots of water potential values (y axis)against water content (x axis).

The regression equations are given below. Aftercalculation, change sign to give answer as negativebar value.

Oak undecayed:

log y = 0'970 ± 0'026 - 1'16 ± 0'°72 log x

Oak decayed:

log y = 0'803 ± 0'038 - 0'663 ± 0'070 log x

Beech undecayed:

logy = 0'791±0'043 -1'26±o'106 log x

Beech decayed:

logy = 0·834±0·030 - 0'663 ±0'044 log x

(P = < 0'001).Water content as proportion of dry weight was thenconverted to a percentage ofthe fresh weight for thepurpose of expressing the results.

Water potential values in decayed and undecayedmaterials were compared at the same water contentper unit volume. Oven-dry weights of leaf diskswere used to calculate the amount of water presentin leaf disks at each of the water contents selectedabove and water content values converted to mgH 20 cm"" of tissue (Table 2). Volume wascalculated from the area of the disk and frommicroscope measurements of thickness, Waterpotential values were then recalculated for aselection of water contents, as mg H 20 cm", fromcomputed regression equations of loglo loglo plotsof water potential (y axis) against water content(x axis). The regression equations are given below,After calculation, change sign to give answer asnegative MPa.

Oak undecayed:

logy = 3'19±O,006-1'16±0'00310g x

Oak decayed:

logy = l'62±O'017-o'666±o'00810g x

Beech undecayed:

logy = 3'07±O'009 - 1'25 ±O'005 log x

Beech decayed:

logy = l'52±0'018 - 0'647±0'010 log x.

These procedures were repeated using thinwood samples. Shavings approx. 0' 5 mm thick wereobtained from the wood of undecayed branches ofbirch (Betula sp.) and oak (Quercus robur L.) cutfrom trees two years previously and stored dry. Thewood shavings were saturated before use and aseries of readings obtained for water content andcorresponding water potential values on five

samples of each wood species as they were allowedto dry in air, Afterwards the samples were collectedfor re-use and placed in garden soil in Petri dishes,The Petri dishes were buried in soil and thesamples left to decay for 11 weeks, On recovery,shavings were re-wetted and water content andwater potential readings taken on the samples asbefore. At the end of the decay period meanoven-dry weight loss was 35 and 53 % for oak andbirch respectively. Water potential values forundecayed and decayed woods at selected watercontents (g H 20 g-l dry wt) were then calculatedfrom regression equations as for leaves. Theregression equations are given below. After calcu-lation, change sign to give answer as negative bar,

Birch undecayed:

log y = 1'18 ±0'030 - l'14±0'134 log x

Birch decayed:

logy = l'02±O'029 - 0'911 ±O'074 log x

Oak undecayed:

logy = O'995±O'023 -1'32±O'120 log x

Oak decayed:

log y = 0,859 ± 0'023 - 0'966 ±o'o66 log x

(P = < 0'001),Water contents were converted to % fresh weightas for leaves,

Calculations were then made from the oven-dryweights of the wood shavings for one sample ofeachtree species to convert water content tomg H 20 em-a. Volume was calculated from theweight of water displaced using a density bottle,These samples showed oven-dry weight losses of38'6% for beech and 23'1 % for oak after decay,corresponding to relative densities of 0'249 g cm""and 0'384 g cm" respectively, Water potentialvalues were then recalculated for a selection ofwater contents as mg H 20 cm" tissue fromregression equations computed as for leaves, Theregression equations are given below, After calcu-lation, change sign to give answer as negative MPa.

Birch undecayed:

logy = 3'16±0'009-1'14±0'0051og x

Birch decayed

logy = 2'19±O'017-0'903±0'010 log x

Oak undecayed:

logy = 3'54±0'015 - 1'31 ±0'00710g x

Oak decayed:

logy = 2'37 ± 0'006 - 0'973±0'003 log x,

N .J. Dix

Table 1 , Comparison of water potential ( - MPa) of undecayed and decay ed leaves and wood atthe same water content

Leaves Wood

mg H .O cm-3 tissue 300 200 100 50 30 200 100 60 40 20

Oak undecayed 2'07 3'22 7"41 16'45 29 '96 3'36 8'32 16 '24 27'62 68 '49Oak decayed 0 '94 1'22 1'99 3'08 4 '33 1'35 2,65 4 '36 6 '47 12'7 1Beech undecayed 0 '94 1'56 3 '72 8'87 16'75Beech decayed 0'83 1'06 1·68 2'64 3'67Birch undecayed 3'44 7'59 13 '58 21 '56 47 '5 2Birch deca yed 1'29 2'42 3 '84 5'54 10'35

Table 2, Relationship between water potential (ljF - M Pa) and water content in undecay edand decay ed tissues

Water contentFresh wt (% ) 33 '3 3 23 '08 18'03 15'25 9'9 0 7'40mg H 20 g-l dr y wt 0 '50 0 '30 0 '22 0 '18 0'11 0 '08

LeavesOak undecayedmgH2Ocm- 3 304'1 182'5 133 '8 109 '5 66 '9 48 '7t/f 2'1 3'8 5'4 6'5 12 '1 17'5Oak deca yedmgH.Ocm- 3 276' 9 166 '1 121·8 99 '7 60 '9 44 '3t/f 1'0 1'4 1'7 2 '0 2' 7 3'4Beech undecayedmgH.Ocm- 3 201 ·6 120 '9 88 '7 72'6 44'3 32 '2t/f 1'5 2·8 4 '2 5 '4 10 '0 14'9Beech decayedmgH.O cm ? 196'4 117'8 86'4 70 '7 43 '2 31 '4t/f 1'1 1'5 1'9 2'1 2 '9 3'6

WoodBirch undecayedmgH2Ocm- 3 203'1 121 '9 89'4 73'1 44 '7t/f 3"3 6'0 8'5 10'7 18'7Birch decayedmg H.Ocm- 3 124 '7 74'8 54'9 44 '9 27 '4t/f 2'0 3'1 4'2 5'0 7 '8Oak undecayedmgH2Ocm- 3 249 '4 149 '6 109'7 89 '8 54. 8t/f 2 '5 4,8 7'3 9 '5 18 '2Oak decayedmgH.Ocm- 3 191'9 115 '1 84 '4 69'1 42 '2t/f 1'4 2'3 3'1 3 '8 6'1

RESUL TS AND DISCUSSION

Undecayed oak leaves had lower values for ljF at thesame watercontentcomparedwith beech confirmingprevious results (Dix, 1984a). Decayed leaves ofboth oak and beech had considerably higher waterpotentials compared with undecayed leave s at thesame water content (Table 1). This trend was moremarked at water contents below 100 mg H 20 em - aand was more obvious in oak than in beech becausethe latter had decayed less.

Water contents of leaves at the litter surfaceprobably seldom fall below 7 % of the fresh weightin Britain, and then only if dry weather persists inthe summer months, It is significant that the meanmoisture content of leaves gathered from litter, airdried and stored in bulk in our laboratory for 1 year,has been measured at 9'3 and 8,6 % of the freshweight for oak and beech respectively . Pitt (1975 )reported that xerotolerant Penicillium spp. makesome growth at water activities down to aboutQ·8aw (equivalent to -29'9 MPa for ljF at 15°C),

652 Fungal successionand considering that many non-xerotolerant terre- H 20 em-a (Table 2). Many agarics are secondarystrial moulds, including some leaf-inhabiting colonizers of leaf litter, invading leaves in thespecies, make slow growth at water potentials down deeper litter layers. These results may help toto about -14'5 MPa (= 0'9aw at 25°) (Griffin, explain why, for until leaves become partially1972, 1981; Magan & Lacey, 1984), it can be seen decayed and the water content increases in thethat some mould growth would be possible on buried litter, water potential values are likely toundecayed oak and beech leaves at the litter surface remain too low to allow growth.at most likely field water contents (Table 2). Quite Similar results were obtained for wood, withactive but inconspicuous growth could be expected decayed wood showing markedly higher waterat about -7'0 MPa or about 15 % moisture potentials than undecayed wood at the same watercontent, and flourishing mould communities could content (Table 1). Undecayed oak wood had a lowerbe expected to develop at water contents of about water potential than undecayed birch wood at the18 % of the fresh weight, or about 90-135 mg same water content. Undecayed leaves tended toH 20 em-a, when water potentials are higher than have higher water potentials than undecayed woodabout -6'0 MPa. That is, provided the temperature at the same water content, but the values foris high enough; the fact that persistently high undecayed birch wood and undecayed oak leaves atmoisture contents are most often likely to coincide the same water content were about the same (Tablewith low temperatures in Britain is probably the 1). Oak is a relatively dense wood but birch less so,main factor that restricts litter decay by moulds in and they probably represent a range covering manythe field in this country. temperate wood species. Water potential values of

Although differences in density will give rise to most temperate woods would therefore be expectedthe presence of different amounts of water in to be similar to these at the same percentage waterdifferent leaf species at the same water content content, and the following generalizations fromwhen water content is expressed as a percentage of Table 2 are probably valid. Some wood-rottingweight, the dead leaves of many tree species will microfungi and non-decay moulds would be able tohave densities similar to oak (r.d. = 0,608 g em-a) colonize and grow on undecayed wood at fresh-or beech (r.d, = 0'403 g em-a). Water potential weight water contents of about 15 % (700-90 mgvalues will therefore be expected to fall in or near H 20 em-a), and flourishing mould communitiesto the range for oak and beech at each percentage could be expected to develop on undecayedwater content, and these results will be generally wood at about 18 % water content (90-110 mgvalid. H 20 em-a).

Some litter-decomposing agarics have a require- The water relations of wood-rotting basidio-ment of about - 6'0 MPa for minimal growth, but mycetes are similar to their counterparts on leafmany of those so far investigated do not grow until litter. Those that have been investigated cannotljF is at least between -3 and -4 MPa (Dix, 1984b osmoregulate sufficiently well to grow at waterand unpubl.). Many litter-decomposing agarics potentials below - 6'0 MPa and several requirecould therefore not even begin to grow until the values of at least - 4 MPa before they make anyfresh weight water content in undecayed leaves detectable growth (Boddy, 1983; Dix, 1984b).exceeds 18 % or more than 90-135 mg H 20 em-a These results suggest that many would not be able(Table 2). Given that the value of ljF for optimum to make even limited growth on undecayed woodgrowth is about -1 MPa or even higher, separate unless the water content was in excess of 23 % ofcalculations from reciprocal plots of water content the fresh weight, or > 120 mg H 20 em-a (Table 2).against water potential showed that this value The figure often quoted in the literature for thewould not be reached in undecayed leaves until the water content of wood below which it is generallyfresh-weight moisture content reached 48'5 % for reckoned that it is safe from basidiomycete decayoak and 4o'0 % for beech or 572 and 268 mg is 20 % of the fresh weight. This is then in fairlyH 20 em-a respectively. Water contents of this close agreement with the figure predicted here fromorder could not be expected for any lengthy period our knowledge of the water potential requirementsin the drier wanner seasons of the year in exposed for the growth of these fungi. Optimum values oflitter. ljF for the growth of wood-rotting basidiomycetes

After a certain amount of decay, such as are at least - 1 MPa and are probably higher inmeasured here, good agaric growth could be many cases (Boddy, 1983). Separate calculationsanticipated on tree leaves with water contents as low from reciprocal plots ofwater content against wateras 10% (about 40-60 mg H 20 em-a), and near- potential showed that values ofthis order would notoptimum water potentials for growth would occur be reached in undecayed wood until the fresh-weightin partially decayed leaves with water contents of water content reached 48'7 % in oak and 57' 5 % inabout 23 % or between about 115 and 165 mg birch or at least 473'5 and 549'6 mg H 20 em-a

N.J.Dix 653respectively. After the wood had been decayed inthe soil, water potential values increase such thatlimited growth for many wood-rotting basidio-mycetes could be expected at about 18 % watercontent (55-85 mg H 20 cm") and good growthcould occur at about 23 % water content (75-115mgH20cm-3). For some the possibility existsthat they could make limited growth on partiallydecayed wood at water contents as low as about40 mg H 20 cm" (Table 2).

It is clear from these results that the capacity ofsome pioneer colonizing microfungi to osmoregu-late at low water potentials could have importantconsequences for fungal successions on exposedsubstrata with fluctuating moisture contents.Continual growth of these fungi could eventuallybring about changes in the water relations of thesubstratum such that they effectively extend therange ofmoisture contents at which fungi requiringhigher water potentials for growth can colonize it.For leaf litter and wood these changes could undercertain circumstances enormously increase theopportunities for basidiomycetes to establishthemselves on such substrata.

In the case of wood, it is interesting to speculateas to how far the further decay of the wood by thebasidiomycetes themselves influences the basidio-mycete succession. Within the relatively narrowrange of water potentials over which they grow,basidiomycetes differ in their minimum require-ments, and some wood-rotting species need higherwater potentials than others before they will grow(Dix, 1984b). This could well account for thebehaviour of species such as Pluteus cervinus,Psathyrella hydrophila and some of the Dacrymy-cetales such as Dacrymyces and Calocera spp. whosebasidiocarps appear late on rather wet, well-rotted

woods. This must remain very speculative for thetime being, since we lack definite information as totheir position in the succession (the appearance ofbasidiocarps is not always a reliable guide). Itwould be interesting to find out whether thesespecies are extremely sensitive to water stress ornot.

The late appearance of certain species in fungalsuccessions has never been adequately explained orfully investigated. These results would suggest thatthe rather neglected aspects of the changingphysical condition of a substratum probably havea very important influence.

REFERENCES

BODDY, L. (1983). Effect of temperature and waterpotential on growth rate of wood-rotting basidio-mycetes. Transactions of the British Mycological Society80, 141-149.

DIX, N. J (1984a). Moisture content and water potentialof abscissed leaves in relation to decay. Soil Biology andBiochemistry 16, 367-370.

DIX, N. J. (1984b). Minimum water potentials for growthof some liner-decomposing agarics and other basidio-mycetes. Transactions of the British Mycological Society83, 152-153.

GRIFFIN, D. M. (1972). Ecology of Soil Fungi. London.Chapman & Hall.

GRIFFIN, D. M. (1981). Water and microbial stress.Advances in Microbial Ecology S, 91-136.

MAGAN, N. & LACEY, J. (1984). Effect of temperature andpH on water relations of field and storage fungi.Transactions of the British Mycological Society 8z,71-81.

PITT, J. L. (1975). Xerophilic fungi and the spoilage offoods of plant origin. In Water Relations of Foods (ed.R. B. Duckworth), PP.273-307. London: AcademicPress.

(Received/or publication 3 January 1985)