HYDROLOGICAL PROCESSES, VOL. 4, 145-155 (1990)

PARTITIONING OF RAINFALL I N A EUCALYPT FOREST A N D PINE PLANTATION I N SOUTHEASTERN AUSTRALIA: I1

STEMFLOW A N D FACTORS AFFECTING STEMFLOW IN A DRY SCLEROPHYLL EUCALYPT FOREST A N D A PINUS

RADZATA PLANTATION

R. H. CROCKFORD AND D. P. RICHARDSON CSIRO Division of Water Resources, Canberra Laboratory, G.P.O. Box 1666, Canberra, A.C.T., 2601, Australia

ABSTRACT

Stemflow of a dry sclerophyll eucalypt forest and a nearby Pinus radiata plantation was studied on a rainfall event basis. The stemflow yields of the forests are quantified, compared, and presented on an annual basis for four years. Yields of the individual eucalypt species are compared and the tree characteristics responsible for the yield differences are discussed. The influence of event size, type, and season on stemflow are also shown. Rainfall angle is shown to have a significant effect on stemflow yield.

KEY WORDS Stemflow Rainfall partitioning Rain angle

INTRODUCTION

This is the second paper in a series dealing with the interaction of rainfall with forest canopies. The introductory paper (Crockford and Richardson, Part I) sets out the general problem, concepts, experimental design, and throughfall measurements. This paper discusses stemflow.

In many interception studies little attention is paid to stemflow. In some it is not measured and a figure is used which is hoped will suffice; in others a few trees are chosen often on the basis of access and convenience. In this study access was easy, topography was gentle, and the almost complete absence of a shrub layer made it possible to carry out a very detailed study. In addition, the growth rate of both forests was so low over the four years that little change in tree form occurred.

The proportion of rainfall reaching the ground as stemflow depends on certain characteristics of the vegetation (trees in this case), the rainfall event type, and the climate. Although stemflow may be small in terms of volume, it can be responsible for quite a large proportion of chemical input to the soil surface.

The following tree characteristics are of relevance to stemflow:

1. Crown size. The larger the crown size for a given trunk diameter (DBH; diameter at breast height), the greater the potential for stemflow yield.

2. Leafshape and orientation. If leaves are angled above the horizontal, i.e. the leaf tip is above the petiole, and have a concave shape, water lodging in the leaves can be channelled to the branches. As leaves of most dry sclerophyll eucalypt species are vertical or near vertical, their only contribution to stemflow occurs when leaf drips land on branches.

3. Branch angle. Steep branches have a greater potential for contributing to stemflow than more horizontal (Herwitz, 1987) or below horizontal branches (these make no contribution to stemflow). The angle effect also applies to smaller wood (twigs and fine twigs).

0885-6087/90/020145- 1 1%05.50 01990 by John Wiley & Sons, Ltd.

Received 23 March 1989 Revised 24 September 1989

146 R. H. CROCKFORD AND D. P. RICHARDSON

4. Flow path obstructions. The flow path is on the underside of branches. Obstructions such as detaching bark pieces or scars on this path can divert all or part of the flow to drip, which then becomes throughfall unless it lands on lower branches. The effect of such a drip point depends on whether it is on the outer end of the branch where it has little effect, or near the trunk end where the effect can be substantial.

Such obstructions can also occur on the trunk. If a trunk is not vertical, the flow path tends to be on the underside and obstructions can divert stemflow to drip. The width of the stemflow collector, the size and position of the obstacle, and the angle of lean all influence the effect.

5. Bark. There is great variation in thickness and bark type within and between species for trees of similar size (DBH). Wettability and thickness have substantial effects on stemflow yield. Smooth, easily wet bark gives high stemflow yields whereas thick absorptive bark (e.g. Eucalyptus rnacrorhyncha) results in small yields because the bark has to be saturated before stemflow commences.

For other species, the bark surface area can also be important, particularly for smaller rainfall events; the larger the surface area of bark/unit area of trunk, the more water used to wet the bark before stemflow commences. Smooth bark eucalypts debark to some degree each year and pieces of shedding bark can remain attached for many months forming drip points. The following rainfall event characteristics influence stemflow yield:

(a) The continuity of rainfall events may be such that there are no, or very few, small dry gaps (rainless periods); or there may be multiperiod events during which the dry gaps vary in number and size (from a few minutes to many hours). The length and frequency of dry gaps coupled with the air temperature, wind run, and relative humidity influence stem flow yield, i.e. the greater the evaporation, the lower the stemflow yield. Hence dry periods in winter events do not reduce stemflow yield as much as similar periods in a summer event.

(b) High rainfall intensity may produce branch flow that exceeds the capacity of the flow paths (Herwitz, 1987), and drip occurs. This causes stemflow yield to be lower than for an event of similar total volume but with lower intensity.

(c) Rain angle is particularly important in a forest with a fairly open canopy. Lower angled rainfall (to the horizontal) allows stemflow to commence when only half the trunk bark (one side) is wet. This is particularly noticeable for thick absorptive bark species (e.g. Eucalyptus macrorhyncha).

The purpose of this paper is to state and discuss the stemflow volume outputs of Pinus radiuta and the major species in the eucalypt forest, as well as the major effects of event type.

Other relevant aspects of this project already reported are; a general study reporting fully individual event volumes (Crockford and Richardson, 1983) and implications of measurement errors for rainfall, throughfall, and stemflow on the estimation of interception (Crockford and Johnson, 1983).

Descriptions of study area and general instrumentation are contained in Part I.

INSTRUMENTATION AND METHODS

Stemjlow (SF) For SF collection, split plastic hose (internal diameter = 14 mm) was wrapped around the tree, attached

with galvanized iron staples then sealed with neutral silicone sealant. Refitting these SF collectors was necessary after debarking of smooth barked eucalypts and E. meliodora each summer. The use of staples for attachment made removal and refitting quick and easy.

The Rose and Farbrother (1960) device was used for measurement of rain angle and direction. The two major species E . rossii and E . rnannifera are smooth bark trees which debark each year. The extent

of debarking varies from almost nothing in a drought year to almost complete after an unusually wet year (Crockford and Richardson, Part I). E. meliodora has fibrous bark which debarks except for the lower section of the trunk. E. macrorhyncha has thick stringy bark and does not debark except for small pieces from the outer layer. Its periderrp thickens with time.

With all species the amount of branching and branching patterns are very variable.

PARTITIONING OF RAINFALL: I1 147

Attachments were placed on seven trees per plot. In the eucalypt sites attempts were made to select the sample trees according to species, size, and type. After event 16, the number was increased to 1 1 per plot in order to extend the range of tree type. For combined plots the sample tree species and numbers were: E. rossii, 11 ; E, mannifera, 12; E . macrorhyncha, 7; and E . meliodora, 2.

In the pine areas, SF collectors were fitted to seven trees per plot, according to tree size and type. This narrow type of stemflow collector collects very little throughfall and stem and branch drip. Most stem

drip for the smooth bark eucalypts, E. rossii and E . mannifera occurs when retained pieces of detaching bark (from annual debarking) drain off some stemflow. This is more likely if the bark pieces are on the major stemflow paths. (Greater detail is contained in Part I.)

Voigt (1960) used stemflow collars from 2.5 cm to 48 cm wide in a dry weather stemflow test on one red pine (Pinus resinosa). He found that the 2.5 cm collar collected only 60 per cent of the water applied to a 3 m section of trunk while 95 per cent was collected by the 48 cm wide device, i.e. the wider the device the more stem drip collected. He described this species as a type prone to stem drip.

For eucalypts the spatial variation of drip points is very large for the following reasons: the trunks of most trees are not vertical (only six of the 33 sample trees were vertical, the angle of the others varied from 3" to 25"); the bark drip points occur anywhere on the branches and trunk; these drip points change as the bark pieces are finally shed, a process which can take many months.

As most trees in the Pinus radiata plantation were vertical or near vertical, there were very few trunk drip points. Branch drip however was obvious, and its extent was influenced mainly by branch angle, second order branches and rainfall intensity.

In this study therefore, stemflow included water from trunk flow plus as much throughfall and trunk drip as was caught in the 14 mm wide collector, i.e. the stemflow collected was almost all trunk flow. Trunk and branch drip was therefore collected as throughfall. This was collected in nine randomly placed 5 x 0.23 m troughs (3 per plot) in both forests.

For the eucalypt area the total collection period, March 1978 to December 1981 can be broken into debarking periods (DP) and interdebarking periods (IDP). For reference purposes, the events are numbered. Table I contains details of this subdivision. Stemflow was not collected from the debarking species during debarking periods except for DP3 (5/1/81-28/3/81).

RESULTS AND DISCUSSION

The six minute pluviograph data encompassing the whole period of this project are presented in Crockford and Richardson (1983), as are rainfall, throughfall, stemflow, and interception data for each of the 130 events for both the eucalypt and pine process study areas.

Table I. Collection period data

Period * Event numbers

Period dates

~ ~ -

IDP 1 1- 16 23-3-78 to 4- 2-79 DP 1 17- 22 5-2-19t026- 5-19 IDP 2 23- 39 27-5-19 to 7- 1-80 DP 2 40- 48 8-1-80 to 7- 3-80 IDP 3 49- 83 8-3-80to 4- 1-81 DP 3 84- 91 5-1-81 to 28- 3-81 IDP 4 92- 129 29-3-81 to 4-11-81

* DP: Debarking period IDP: Interdebarking period

148 R. H. CROCKFORD AND D. P. RICHARDSON

Calculation of stemflow on an annual basis When comparing the stemflow yields of eucalypt species and pines, events from the IDP2, 3, and 4 and

DP3 have been used. As the pine PSA is only 600 m from the eucalypt PSA, event rainfall at each site was similar: those few events for which the sites had quite different rainfall were not used in the comparisons.

To present SF on an areal basis, it was necessary to relate the volumes to some easily measured tree characteristic. Characteristics commonly used have been crown area (e.g. Peterson and Rolfe, 1982) and DBH (e.g. Feller, 1981). Because of the open crown structure of dry sclerophyll eucalypts, DBH was the more appropriate choice. Correlation coefficients for SF volume versus DBH and basal area (BA) (calculated from DBH) are shown in Table 11. The values are the means of those calculated for 37 events; these were chosen because there were very few missing data and they covered a wide range of event sizes (5-50 mm rainfall). Values for E. rnacrorhyncha are for events over 15 mm, because for smaller events SF from this very thick barked species was small and very variable.

The SF versus BA correlation was marginally better than for DBH, particularly for E. mannifera. For this reason BA was used. The correlation coefficient for E. mannifera (066) was substantially smaller than that of the other species; this is almost certainly due to its debarking behaviour. The detaching bark of E. mannifera expands and contracts laterally with wetting and drying. This plus the random grain pattern, cause detaching pieces to be smaller and for many to remain attached to the tree, generally on the underside of branches. Many of these can remain attached for months and act as drip points.

In contrast, the detaching bark of E. rossii expands and contracts longitudinally with wetting and drying. The detaching pieces are large (relative to E. mannifera) and come away smoothly. Virtually no bark pieces remain attached to the trees.

After debarking, the SF yield of some E. mannifera changed by a factor of 4 or 5 ; with E. rossii there was little change.

The conversion of SF volume to an areal basis was made by extrapolating the sample tree SF volumes to all trees, using the basal areas of sample trees and all trees. The result was then expressed as millimetres of SF.

Comparison of stemflow yields of each eucalypt species and Pinus radiata Table 111 shows the SF yields of the eucalypt and pine PSAs for the same events for each period and for the

periods combined. These yields are also expressed as percentages of rainfall. A substantial proportion of the events cited in Table I was used in this comparison. The proportion of each year’s total rainfall represented by Table I events is shown in Table IV.

The SF yield of the pines (as percentage of rainfall) is 2.3 times that of the eucalypts. Stand density or tree size (DBH) do not account for this difference; the densities are 1525 and 1700 trees ha- and the mean DBHs are 15.4 and 15.6 cm respectively for the eucalypt and pine areas (Table I, Part I).

The difference is due to the performance of the different species as SF yielders. A convenient means of comparison of species and trees within a species is SF volume/unit of basal area/unit of rainfall, called stemflow factor (SFF).

Also shown in Table I11 are the SFFs of each species for each period and for periods combined. The unit used is L m-’ basal area per mm rainfall.

The combined period values show the relative SF yielding capacity of the pines and eucalypt species. The observation that E. macrorhyncha is the smallest yielder is not surprising because of its thick absorptive bark.

Table 11. SF us. DBH and BA: correlation coefficients

Relationship Correlation coefficient E. ros. E. man. E. mac. Pines

SF us. DBH 0.79 0.6 1 0.96 0.82 SF us. BA 0.82 0.66 0.96 0.83

PARTITIONING OF RAINFALL: I1 149

Table 111. Stemflow volume data: all species: all periods

Period Species No. of Stemflow Rainfall Stemflow Species Stemflow events volume ha- (mm) as % basal area factor

(e x 10-4) rainfall (mZ ha- l ) (G m-* BA

per mm rain

IDP2

IDP3

DP3

IDP4

ALL

E. ros. E . man. E . Mac. E . mel. TOTAL Pine E. ros. E . man. E . mac. E. mel. TOTAL? Pine E. ros. E. man. E. rnac. E . mel. TOTAL* Pine E . ros. E . man. E. rnac. E . mel. TOTAL E. ros. E . man. E . Mac. E . mel. TOTAL? Pine

9 3.1 1 9 1.34 9 0.289 9 0.351

5.09 9 11.43

24 8.51 24 3.90 24 0.90 1 10 0.351

14.10 24 35.62 6 3.79 6 0.878 6 0.386 0

5.35 6 14.9

17 9.25 17 2.93 17 0.774 15 0.616

-

13.60 56 24.6 56 9.0 56 2.66 33 1.32

37.82 55 91.1

127.0 127.0 127.0 127.0 127-0 125.4 299.4 299-4 299.4 128.4 299.4 283.7 120.7 120.7 120-7 -

120.7 118.6 230-5 230.5 230.5 2 17.6 230.5 777 777 777 473 777 798

4.0 9.1

4.7 12.5

4.4 12.5

5.9

4.8 11.2

14.6 10.8 7.3 1.4

34.1 35.1 14.6 10-8 7.3 1 -4

34- 1 35-1 14.6 10.8 7.3

34.1 35.1 14.6 10.8 7.3 1.4

34.1 14.6 10.8 7.3 1.4

34.1 35.1

-

16.7 9.8 3.1

19.7 12.3 25.9 19.5 12.0 4.1

19.5 13.8 35.8 21.5 6.7 4-4

13.0 35.8 27.5 11.8 4.6

20.2 17.3 21.7 10.7 4.7

19.9 14.3 32.6

-

t Total includes E. me[. volume adjusted to the same events total rainfall as for other eucalypt species * Total includes E . me[ volume calculated from the relationship between E. mel. and the other eucalypt species in the

other periods

Table IV. Rainfall and events measured each year

Year Events Events Rain Rain total measured total measured

(mm) as events (mm)

1978 53 16 922 232 1979 37 23 519 404 1980 51 46 498 486 1981* 62 48 774 660

* Debarking commenced during November, and measurements ceased, i.e. the last 12 events (81 mm) were not measured

150 R. H. CROCKFORD AND D. P. RICHARDSON

Its SF yield is particularly small for events less than 15 mm, but for large events (P 2 40 mm) the SFF approaches that of E. mannifera; E. macrorhyncha is a good collector but the bark must be near saturated before most of the collected water becomes SF.

Feller (1981) and Westman (1978) both attribute the low SF yield of thick fibrous bark species (E. obliqua and E. umbra respectively) to the absorptive nature of the bark. Crockford and Richardson (Part 111) established the mean bark water storage capacity of E. macrorhyncha and E . rossii to be 92 and 12.8 mg cm-') respectively. If the difference (79 mg cm-') is applied as SF to the 2220 m2 of E. macrorhynchu wood area per hectare in this forest (Part 111) for the 56 events noted in Table 111, the SFF for E. macrorhyncha becomes 22.4 L' m-' BA per mm of rain, very similar to that of E. rossii (21.7). This suggests that absorption of rainfall into its bark is the major reason for its lower SF yield, i.e. differences in tree structure and drip points are of little importance.

The stemflow yields of individual trees within each species is reported in detail in Crockford and Richardson (1987). There was considerable variation in the stemflow factors of individual trees within each species. For example in IDP3, the mean SFF of individual trees of E. rossii ranged from 6.5 to 28-1 L' m-' BA per mm P; E. macrorhyncha trees varied from 2.0 to 6-0 and E. mannifera ranged from 2.8 to 23-5. For a similar series of events the mean SFF of individual pines ranged from 5.8 to 50.0 r?' m-'BA per mm P.

There was little change in the SFF order of individual trees of E. rossii, E . macrorhyncha, and pines over time and between IDPs, i.e. if the species mean SFF increased or decreased between IDPs (principally due to event type differences) almost all individual trees responded similarly.

The SFF of a tree is determined by the physical and structural characteristics mentioned in the introduction. Observation of these characteristics enabled reasonable assessment of a tree's stemflow potential. Although this applied to E. rossii, E. rnacrorhyncha, and the pines, for E. mannifera prediction was much more difficult, due mainly to its particular debarking behaviour (mentioned earlier).

The SFF of E. mannifera (10.6) is about half that of E. rossii, although both species are similar in structure and appearance. Drip points carried by detaching pieces of E. munnifera bark reduce the stemflow yield, disturb the stemflow volume versus basal area correlation, and cause erratic changes of stemflow factors between IDPs.

The yield effect can be seen in the changes for each species' SFF value between IDP3 and DP3 (Table 111). There is a small increase in the SFFs of E. rossii and E. macrorhyncha but a 44 per cent decrease for E. mannifera. How quickly the SFF recovers depends on the extent of debarking and the frequency of rainfall; the expansion and contraction of the detaching pieces caused by wetting and drying finally cause most pieces to detach. The more frequent the wetting and drying cycles, the shorter the residence time.

The SFF total for the eucalypt PSA for the four periods ranges from 12-3-17.3 em- ' BA per m a P, increasing through IDP2, 3, and 4. The drop in DP3 is due to the reduced yield of E. mannifera (discussed earlier). The low SFF values for both forests for IDP2 is probably due to the larger number of multiperiod events andfor higher rain angles.

In areas of dry sclerophyll eucalypt forest, species composition varies from almost 100 per cent of any one of E. rossii, E. mannifera, E. macrorhyncha, and several other species to mixed areas such as that under study. The stemflow factor values in Table 111 can be used to assess the likely stemflow yields of forests of different composition, e.g. if all trees were E. rossii then SF as a percentage of rainfall would be 8.2 per cent instead of 4.8 per cent (see Table 111); and if trees all were E. rnacrorhyncha the SF yield would be only 1.8 per cent of rainfall.

Stemflow and event size All events from numbers 1-130 were split into nine size classes as shown in Table V. Also shown are the

mean P, the mean SF, and the SF as % P. The SF (as % P) values are plotted against P (Figure 1) for both areas. The number of events for eucalypts and pines in each class varies somewhat due to:

1. Events 101-130 were not used for the pine PSA because collection of SF had ceased except for five trees of

2. SF was not collected from the eucalypts during debarking period 1 and 2. particular interest.

PARTITIONING OF RAINFALL: I1 151

Table V. Event size classes: rainfall, stemflow and stemflow as per cent of rainfall ~~~ ~ ~~

Size PSA Event No. of Rainfall Mean Stemflow Mean Stemflow class rainfall events total rainfall total stemflow as per cent

range (mm) (mm) (mm) (mm) of rainfall (mm)

Euc. Pine Euc. Pine Euc. Pine Euc. Pine Euc. Pine Euc. Pine Euc. Pine Euc. Pine Euc. Pine

1- 3.0 1- 3.0

3.1- 6.0 3.1- 6.0 6.1- 8.0 6.1- 8.0 8.1-10.0 8.1-10.0

10.1 - 15.0 10.1 - 15.0 15.1 -20.0 15.1-20.0 20.1-25.0 20.1-25.0 25.1-30.0 25.1-30.0 > 30 > 30

13 14 28 18 10 13 15 7

20 16 10 7 9

10 3 4 7 9

27.7 31.0

114.6 72-8 69.4 91.7

139.6 59.9

248.3 205.1 171.3 120.0 204.3 225.5 85.9

114.1 288.7 380.8

2.1 2.2 4.1 4.0 6.9 7.1 9.3 8.6

12.4 12.8 17.1 17.1 22.7 22-6 28.6 28.5 41,2 42.3

0.1 1 0.06 2,62 2.58 2.42 4.89 6.40 4.52

11.6 18.7 10.8 9.7

11.1 22.3 4.65

15.0 17.6 42.0

0.009 0.004 0-094 0-143 0-240 0-376 0-427 0.646 0.581 1.17 1 -08 1.39 1.23 2.23 1.55 3.74 2.5 1 4.66

0-40 0-19 2-28 3.54 3-49 5.33 4.58 7.50 4-70 9.10 6.32 8.09 5.4 1 9.88 5.40

13.10 6.09

11.0

0

12-

Stemflow

as yo of 8 -

Pine 0

0 10 20 30 40 50 Rainfall (mm)

Figure 1. Stemflow (as % of rainfall) versus rainfall for discontinuous events

3. For four events small differences in the rainfall at the eucalypt and pine PSAs caused the events to be in adjoining size classes, not the same class.

For all these combined events, stemflows (as % P) are 5.0 and 9.3 for the eucalypts and pines respectively. These are similar to the values in Table I11 for combined events (4.8 and 11.2 respectively for eucalypts and pines). As Table 111 presented species comparisons, events used were those with complete data for each species. As it happened fewer small events were used, which is consistent with the Table I11 values for SF and % P being slightly higher than in Table V.

152 R. H. CROCKFORD AND D. P. RICHARDSON

Figure 1 shows curvilinear relationships between SF (as a per cent of rainfall) and rainfall, gently sloping through 11.0 per cent for pines and asymptotic to 5.5 per cent for the eucalypts; again this is consistent with those mentioned above.

Stemflow and event type From size classes 2-9 the lowest and highest SF yielding events were selected for examination of event type

effect. Class 1 (1.0-3.0 mm) events could not be used for this purpose as most gave zero SF. The 14 highest SF yielding events from size classes 2-9 were examined for effect of event type. The most

obvious common feature of these events is rain angle; 13 had rain angles less than 50" (to the horizontal). Most also had few dry gaps and the rain intensity was never above 10 mm hr-' (mostly < 5 mm hr-'). Such rain intensity is very common in winter events.

By contrast seven of the eight low SF yielding events for which rain angle data were available, had rain angles from 65"-75", averaging 70". Although most of these events had longer dry gaps than the higher yielders, intensity patterns were similar.

The relative importance of event dry gaps and rain angle is to some extent resolved by examination of the SF yields from continuous events, i.e. those with no dry gaps longer than 6 minutes (the resolution time of the pluviograph). Actually two of the events had a dry gap, but they were of less than 30 minutes duration and occurred during the night in winter events. Figure 2 shows SF yield (% of P) versus P(mm) for these events for both the eucalypt and pine areas. Events 35, 38, and 56 (marked on Figure 2) are events where the SF yields are very different from the mean curve. These events had similar rainfall at both sites and the SF yield behaviour was similar at both sites.

Event 38 had a low SF yield and had a rain angle value of 88 '. Events 35 and 56 had rain angles of <45" and the SF yields were high.

Table VI shows the rain angles and stemflow factors for pines and the individual eucalypt species for some other continuous or near continuous events (including event 38, for comparison). Data show that the rain angle effect applies to each species. Events 14 and 76 data do not include stemflow factors for E. macrorhyncha because this thick barked species yields little or no SF for small events.

Events with pine data only occurred after the pine plantation was thinned (removal of 50 per cent of the trees) in September/October 1982. The SF trees were retained and SF measurement recommenced. Due to the 1982 drought there was negligible tree growth during the 1982-83 summer. The drought broke in March 1983 and a very wet year ensued. Measurement ceased in November 1983, when there was a noticeable

Stemflow

as % of

rainfall

Eucalyptus

56 0

0 0 '

30

I I I I

0 10 20 30 40 Rainfall (mm)

Figure 2. Stemflow (as % of rainfall) versus rainfall for continuous events

PARTITIONING OF RAINFALL: I1 153

Table VI. Stemflow factors, individual species and rain angle

Event Rainfall Stemflow factor (L' m-2 BA per mm rain) Rain (mm) E. ros. E. man. E. mac. Pine angleo

14 76 57 32 38

194 195 193 180 200 183

7.5 8-3

23-8 23.0 22.6 9.8 9.2

10.4 14.8 14.9 14.9

7-7 8-6 - 22.0 50 6.5 4.8 - 15.5 69

26.2 20.5 11.4 33.2 52 18.5 8.5 4.1 24-2 68 15.3 5.6 2.6 22.3 87

45.2 42 47-2 48 28.3 73 48-8 46 40-7 54 28.9 75

increase in DBH. Many of the events tested over this period were continuous or near continuous and again show the effect of rain angle on SF yield. The correlation coefficient for SFF versus rain angle for the two size groups combined is -0.97.

Over all event sizes combined, thinning almost doubled the stemflow factor. This is discussed in Part IV as is the effect of thinning on interception.

One event, No. 84 (29.0 mm, RA = 62", 6/1/81) shows an SF response to rainfall intensity shown mainly by the eucalypts. As this was a continuous event of only four hours duration, the yield could be expected to be moderate. It was in fact the lowest in its size class (class 9) for the eucalypts. In the second twelve-minute period 1 1 mm rain fell. The bark of E. rossii and E. mannifera is somewhat water repellent; gentle wetting allows development of the flow paths on the underside of the branches. If however a high intensity period occurs close to the start of an event (e.g. event 84) many drip points form on the sides of the trunks and branches and the flow paths do not form properly. Even when properly formed, some loss can occur during high intensity periods due to the capacity of the flow paths being exceeded; drip then occurs.

As pine bark is not hydrophobic, flow paths are more numerous thus causing less loss particularly during high intensity periods. The pine SF yield for event 84 was equal to the mean yield (12.7 per cent of P) of its size group whereas for the eucalypts the yield (4.4 per cent of P) was 25 per cent less than the mean class value.

The fact that the pines can handle larger and higher intensity events better than the eucalypts is shown in Figure 2 (stemflow yield as % P for continuous events). For the eucalypts the plot is curvilinear and asymptotic to SF = 5-0 per cent of P. For pines however the linear part of the plot is still increasing at the largest rainfall event (P = 40 mm, SF = 15.4 per cent of P). The eucalypt curve is very similar to that in Figure 1 (for all events) whereas for pines the Figure 1 curve sloped gently to SF = 11.0 per cent of P at 40 mm, i.e. for the eucalypts the potential gains due to event continuity are negated by loss of stemflow for the larger, higher rainfall intensity events.

The overall mean of 11-2 per cent for Pinus radiata is very similar to the 11.0 per cent found by Langford and O'Shaughnessy (1978) but very different to the 1 per cent reported by Feller (1981) and the 3 per cent by Smith (1974); all studies were made in southeastern Australia. The plantation studied by Langford and O'Shaughnessy was 17 years old with a stand density of 1500 trees ha-' (58 mz basal area ha-'); similar to our study area (1708 ha-'). Feller's study area contained 670 trees ha-' (51 m2 basal area ha-'), Smith's 39 year old mature plantation contained 98 trees ha-' (19.3 mz basal area ha-').

Assuming crown cover of the mature and unthinned stands to be similar, the big difference in SF yields must be related to tree structure and event type. The branches of mature pines are fewer in number, much thicker, longer and flatter (often curving downward towards the outer end). The longer flatter branch characteristics particularly, will reduce the channelling of SF to the trunks. This may be the major reason for

154 R. H. CROCKFORD AND D. P. RICHARDSON

the difference in SF yields between mature and unthinned plantations. Rain angle of course may also be involved.

Another possible reason is that older trees have thicker fissured bark. The fissuring increases the surface area to be wetted before SF commences. However this possible reduction of SF could be offset by the fact that the fissures form secure flow paths for SF, because they mostly run vertically; there is very little trunk drip with larger trees.

The increased SF yield with lower angled rain suggests that tree trunks are greater collectors of rainfall than the branches and crowns. If a tree is considered to be a vertical pole, then lower angle rain will see a greater pole surface area than higher angle rain, and this produces greater SF volume. Several SF pine trees were called ‘pole trees’ because they had very few branches and small crowns. The stemflow factors of these trees were on average among the highest, particularly for very angled rain.

One of the felled pines, after removal of all branches and the crown, was stood vertically in an open grassland area and SF measured. It responded predictably to rain angle, its stemflow factor being similar to the mean of the SF trees in the thinned plantation for angled rain, e.g. event 184, rain angle 44 O ; the stemflow factor of the pole was 32 and that of the trees was 36.

Trunks being vertical or near vertical will retain more of the incident rain than the branches; and only one side of the trunk needs to be saturated for SF to commence. In the majority of events a large proportion of the lee side of the trunks was observed to be dry.

CONCLUSIONS

The difference between the overall SF yield of eucalypts and pines is due to a number of factors, the most important being the low yields of E. rnannifera (because of drip points caused by detaching bark) and E . macrorhyncha (due to its thick absorptive bark).

To focus on effects of other differences it is useful to compare Pinus radiata with E . rossii which does not have the ‘debarking’ drip points. Table 111 shows the stemflow factor of the pines to be 50 per cent higher than E. rossii. The likely reasons for this, as suggested in the previous section, are that Pinus radiata has fewer trunk drip points due to the more vertical trunks, coupled with less hydrophobic bark. In addition a proportion of pine needles are angled above the horizontal and collected water can be channelled to the branches. For E . rossii (and the other eucalypt species) most leaves hang vertically or near vertically and the only chance of transfer from leaf to branch is if leaf drip happens to land on a branch.

When discussing SF yield and event type for the eucalypt and pine PSAs in the previous section the influence of rain angle and intensity were stressed. The influence of rain angle on stemflow yield has not previously been noted in the literature. This study suggests that it may be of importance in forests with relatively small canopy cover (or LAI). Its significance will also depend on tree structure, the exposed area of wood, and the surface area of the trunk relative to the branches.

Although in stemflow and interception studies it is common for stemflow to be collected from trees that are representative of the tree size (DBH) or tree crown areas of all trees in the site, the relationship between the size (Basal area) and stemflow yield has not previously been published. This relationship could be established for some forest types, but in forests where the trees exhibit substantial variation in trunk lean, branch angle, and trunk and branch drip points, low correlation coefficients would result, limiting the value of such an exercise.

In selecting trees for assessment of stemflow yield attention should be paid to characteristics such as leaf shape and orientation, branch angle, bark type, and flow path obstructions (potential drip points) as well as tree size, crown size, and exposure.

Stemflow will also be affected by other factors such as event evaporative dry gaps, antecedent weather conditions, and season (which will affect leaf area).

PARTITIONING OF RAINFALL: I1 155

REFERENCES

Crockford, R. H. and Johnson, M.E. 1983. ‘Some errors in the measurement of precipitation, throughfall and stemflow and the implications for the estimation of interception’, in Hydrology and Water Resources Symposium, Hobart, The Institution of Engineers, Australia, Conference Publication No. 83/13.

Crockford, R. H. and Richardson, D. P. 1983 ‘Some hydrological influences on vegetation’, CSIRO Div. Water and Lurid Resources Technical Memorandum $31 18.

Crockford, R. H. and Richardson, D. P. 1987. ‘Factors affecting the stemflow yield of a dry sclerophyll eucalypt forest, a Pinus radiafa plantation and individual trees within the forests’, CSIRO Div. Water and Land Resources Technical Memorandum 8711 I .

Crockford, R. H. and Richardson, D. P. in press. ‘Partitioning of rainfall in a eucalypt forest and pine plantation in southeastern Australia. 111. Determination of canopy storage capacity of a dry sclerophyll eucalypt forest’.

Crockford, R. H. and Richardson, D. P. in press. ‘Partitioning of rainfall in a eucalypt forest and pine plantation in southeastern Australia. I. The effect of throughfall measurement in a eucalypt forest: effect of method and species composition’.

Feller, M. C. 1981. ‘Water balances in Eucalyptus regnans, E. obliqua and Pinus radiara forests in Victoria’, A w t . For., 44, 153-161. Herwitz, S. R. 1987. ‘Raindrop impact and water flow on the vegetative surfaces of trees and the effect on stemflow and throughfall

Langford, K. J. and O’Shaughnessy, P. J. 1978. ‘A study of canopy interception in native forests and conifer plantations’, Melbourne and

Peterson, D. L. and Rolfe, G. L. 1982. ‘Precipitation components as nutrient pathways in floodplain and upland forests of Central

Rose, C. W. and Farbrother, H. C. 1960. ‘A method of obtaining average bearing and incidence of rainfall’, Q. J . R. Meteorol. Soc., 86,

Smith, M . K. 1974. ‘Throughfall, stemflow and interception in pine and eucalypt forests’, Awr. For., 36, 190-197. Voigt, G. K. 1960. ‘Distribution of rainfall under forest stands’, Forest Sci., 6, 2-10. Westman, W. E. 1978. ‘Inputs and cycling of mineral nutrients in a coastal subtropical eucalypt forest’, J. Ecol., 66, 513-531.

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