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EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 15,5541 (1990) STEMFLOW AND THROUGHFALL IN A TROPICAL DRY FOREST MARTIN KELLMAN AND NIGEL ROULET Department of Geography, York University, North York. Ontario. M3J IP3, Canada Received 7 December I988 Revised 7 May 1989 ABSTRACT The rainfall received by a small plot of tropical deciduous forest on sand dunes in Veracruz, Mexico, was partitioned into stemflow and throughfall components to determine whether funnelling by stemflow could reduce soil leaching by transmitting large volumes of water through vertical soil pathways beneath each stem. Although soil infiltration capacities were high, only a very small proportion of incoming rainfall was funnelled by canopy stems. This is attributed to the widely-branched morphology of mature trees. Smaller trees and shrubs were more effective funnellers of rainfall, and a crude estimate of the magnitude of stemflow in the understorey stratum in one rain event suggested a contribution approximately ten times that of canopy stemflow. However, even if augmented by the understorey stratum in this way, total stemflow is unlikely to have exceeded 10 per cent of gross precipitation, implying that it does not represent an important leaching-avoidance mechanism in this forest. KEY WORDS Sand dunes Stemflow Throughfall Tropical forest INTRODUCTION The rate and magnitude of water delivery to a soil surface is an important controller on the soil water flux, and hence is important in element transport. The presence of a forest canopy can significantly alter the pattern of delivery of water to the soil surface. This paper examines the delivery of water to a dry tropical forest floor to assess its impact on soil water flow. The primary emphasis of interception work has been on the partitioning of gross rainfall between canopy storage, throughfall, and stemflow (e.g. Jackson, 1975) and modelling canopy drainage and interception evaporation (e.g. Rutter et al., 1975). While values vary, interception, throughfall, and stemflow in wet and dry tropical forests range between 10-35 per cent, 50-95 per cent, and <3-18 per cent respectively (Hopkins, 1960; Dabral and Rao, 1968, 1969; Odum et al., 1970; Ray, 1970; Smith, 1974). Differential rates and magnitudes of stemflow and throughfall can influence the spatial and temporal variability of infiltration. In a wet tropical forest Herwitz (1986) found that stemflow produced infiltration excess at the base of trees, which resulted in localized overland flow. The infiltration rate in the ponded water zones would be at capacity, while the infiltration rate for the remaining forest floor should be at subcapacity rates. Consequently, leaching and soil element flux may be accelerated along the stemflow-induced pathways. However, if the magnitude of stemflow is insignificant or the rate of stemflow is not significantly different than the throughfall rate, delivery- induced preferred pathways would not be produced. In this study we have measured the proportion of gross precipitation that arrives at the soil surface via stemflow and throughfall beneath a tropical dry forest occupying recently formed dunes in Veracruz, Mexico, and compared flux rates with the capacity of the soil to absorb water. Infiltration capacity of the sand appeared to be extremely high and the vertical profiles of soil solution chemistry suggested a high mobility of nutrient ions (Kellman, unpublished data). Despite the severe leaching regime that these observations imply, 01 97-93 37/90/0 1 OO55507$05.OO 0 1990 by John Wiley & Sons, Ltd.

Stemflow and throughfall in a tropical dry forest

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Page 1: Stemflow and throughfall in a tropical dry forest

EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 1 5 , 5 5 4 1 (1990)

STEMFLOW AND THROUGHFALL IN A TROPICAL DRY FOREST

MARTIN KELLMAN AND NIGEL ROULET

Department of Geography, York University, North York. Ontario. M3J IP3, Canada

Received 7 December I988 Revised 7 May 1989

ABSTRACT

The rainfall received by a small plot of tropical deciduous forest on sand dunes in Veracruz, Mexico, was partitioned into stemflow and throughfall components to determine whether funnelling by stemflow could reduce soil leaching by transmitting large volumes of water through vertical soil pathways beneath each stem. Although soil infiltration capacities were high, only a very small proportion of incoming rainfall was funnelled by canopy stems. This is attributed to the widely-branched morphology of mature trees. Smaller trees and shrubs were more effective funnellers of rainfall, and a crude estimate of the magnitude of stemflow in the understorey stratum in one rain event suggested a contribution approximately ten times that of canopy stemflow. However, even if augmented by the understorey stratum in this way, total stemflow is unlikely to have exceeded 10 per cent of gross precipitation, implying that it does not represent an important leaching-avoidance mechanism in this forest.

KEY WORDS Sand dunes Stemflow Throughfall Tropical forest

INTRODUCTION

The rate and magnitude of water delivery to a soil surface is an important controller on the soil water flux, and hence is important in element transport. The presence of a forest canopy can significantly alter the pattern of delivery of water to the soil surface. This paper examines the delivery of water to a dry tropical forest floor to assess its impact on soil water flow.

The primary emphasis of interception work has been on the partitioning of gross rainfall between canopy storage, throughfall, and stemflow (e.g. Jackson, 1975) and modelling canopy drainage and interception evaporation (e.g. Rutter et al., 1975). While values vary, interception, throughfall, and stemflow in wet and dry tropical forests range between 10-35 per cent, 50-95 per cent, and <3-18 per cent respectively (Hopkins, 1960; Dabral and Rao, 1968, 1969; Odum et al., 1970; Ray, 1970; Smith, 1974). Differential rates and magnitudes of stemflow and throughfall can influence the spatial and temporal variability of infiltration. In a wet tropical forest Herwitz (1986) found that stemflow produced infiltration excess at the base of trees, which resulted in localized overland flow. The infiltration rate in the ponded water zones would be at capacity, while the infiltration rate for the remaining forest floor should be at subcapacity rates. Consequently, leaching and soil element flux may be accelerated along the stemflow-induced pathways. However, if the magnitude of stemflow is insignificant or the rate of stemflow is not significantly different than the throughfall rate, delivery- induced preferred pathways would not be produced.

In this study we have measured the proportion of gross precipitation that arrives at the soil surface via stemflow and throughfall beneath a tropical dry forest occupying recently formed dunes in Veracruz, Mexico, and compared flux rates with the capacity of the soil to absorb water. Infiltration capacity of the sand appeared to be extremely high and the vertical profiles of soil solution chemistry suggested a high mobility of nutrient ions (Kellman, unpublished data). Despite the severe leaching regime that these observations imply,

01 97-93 37/90/0 1 OO55507$05.OO 0 1990 by John Wiley & Sons, Ltd.

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56 M. KELLMAN AND N. ROULET

the sands support well-developed tropical deciduous forest whose existence must require either very large external inputs of nutrients to offset losses, specialized mechanisms of biological retention of nutrients, or avoidance of leaching. Here, we test the hypothesis that funnelling of rainfall by stemflow in this forest significantly reduces the potential for intense leaching of surface soils.

The study was carried out at the La Mancha Biological Research Station located at 19" 30N, 96" 3 0 W on the Gulf coast of Veracruz, Mexico. The area receives approximately 1300 mm of rainfall annually, almost all of which falls in the June-September period (Moreno-Casasola, 1982). During the rainy season, most rainfall is received as relatively high-intensity nocturnal thunderstorms. Seventy-two per cent of all daily rainfall totals recorded in a ten month observation period covering three wet seasons exceeded 10mm, with the largest daily rainfall recorded being 119 mm, which took place during the stemflow observations reported here. Hurricanes are infrequent in this section of the Gulf of Mexico, although the passage of those tracking farther north may induce large rainfalls, such as the one mentioned above. Short-term rainfall intensities were not measured directly, but those for total daily rainfall can be estimated from the rainfall duration provided by stemflow measurements (Table 111). Intensities estimated in this way ranged from 0.1 1 to 1.05 mm min- '.

The forest studied occurs in an area of recent sand movement. The age of this dune system is unknown, but the absence of any clay formation or slope erosion suggest that it is no more than several centuries old. The sand shows no textural differentiation within the upper 200 cm that have been examined (Kellman, unpublished data). The native forest vegetation of the area is classified as low semideciduous tropical forest by Rzedowski (1978). On the recent sands this forest consists of a main canopy at 8-10 m, with a discontinuous overstorey of larger trees 15-20 m tall, and an understorey layer of shrubs and small trees approximately 3-5 m tall. Species prominent in the main canopy include Brosimum alicastrum, Bursera simaruba, Tabernaem- ontana alba, Nectandra sp., and Ficus spp. The overstorey is comprised mainly of Cedrela odorata, while the understorey is dominated by Coccoloba barbadensis and Nectandra sp. In the research plot, 2.7 per cent of the surface area was estimated to be beneath canopy gaps. A prominent component of the forest community is the extralittoral crab Gecarcinus lateralis Frem., which acts as a litter detritivore and rapidly removes most falling litter during the wet season. As a consequence, the forest floor consists of bare sand throughout most of this period, which precludes the development of a litter layer nutrient storage compartment and further exacerbates the potential loss of nutrients by leaching.

METHODS

Field observations were designed to allow partitioning of net precipitation received at the soil surface ( N P ) , into canopy-induced stemflow (Sc) and net throughfall (7%) (including canopy drip) components. This was accomplished by measuring the stemflow at all trees and large lianas in a small representative plot of forest, and sampling net throughfall in this plot with a dense network of collectors. Gross precipitation (GP) was sampled with a recording rain gauge located approximately 800 m from the forest plot. In the previous year, rainfall collected from this gauge was compared with that measured by several gauges located adjacent to the forest plot studied. Gross precipitation received at the recording gauge deviated less than 6 per cent from that measured at the forest.

The forest plot was 16 x 10 m in size with the long axis traversing a topographic section from swale to ridge crest of one dune arm with an average slope of 13" 2 0 . The location of all tree and liana stems > 15 cm girth (17.9 cmz basal area) were mapped, and each stem enclosed with a watertight plastic collar for stemflow collection. The 23 collars installed consisted of 0 9 mm thick ployethylene strips, 15 cm wide, whose lower edges were stapled to each stem through a bead of silicone sealant to form acutely-angled cones, with the ends of the strips overlapped, taped and sealed with silicone. Outflow from each collar was by way of a rigid 6 mm internal diameter polypropylene tube inserted through the lowest point on the collar and run into 10 1 and 20 1 collecting tanks. In a preliminary trial with this equipment, stemflow was observed during rainstorms and collars showed no tendency to overflow. On the largest tree, five outflow tubes were fed into an 80 1 plastic container upon which was mounted a water-level recorder that had been calibrated for water volumes in the collector. The location of tree canopy edges was recorded on a map, as was the location of all major clusters of shrub stems.

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STEMFLOW AND THROUGHFALL IN A TROPICAL DRY FOREST 57

Net throughfall was sampled using 40 evenly-spaced plastic containers located at the soil surface. Thirty-six of these consisted of rectangular troughs 13.5 x 35.5 cm in surface area and 14 cm deep, and the remaining four were circular buckets of 21.2 cm diameter. The rectangular surface of the throughfall samplers should not have created a problem, since turbulence in the forest is minimal. Together these collectors sampled a total area of 1.87 m2, or 1.17 per cent of the total plot area. A large number of samplers reduces systematic errors, and Kimmins (1973) recommends a minimum of 30 samplers for a fixed location network. Ideally, linear troughs or plastic-sheet collectors of net precipitation should be utilized in this kind of study (Institute of Hydrology, 1977), but logistical problems prohibited their use at La Mancha. Sampling began on August 29, 1988 and was terminated on September 21, 1988. During this period seven dialy rain events were experienced, that varied from 2-1 19 mm. Net throughfall during the largest rain event was probably underestimated slightly due to splash-out from the rectangular troughs.

Net throughfall and stemflow at the plot have been expressed as water depths and percentages of gross precipitation depths that these represent. Stemflow depths were calculated by summing all stemflow volumes recorded in the plot and expressing these as a water depth equivalent for the entire 16 x 10 m plot. Funnelling ratios have been calculated for individual stems and rain events following the procedure used by Herwitz (1986). Each ratio represents the total stemflow volume collected at a stem, divided by the volume of rainfall that would have been collected by a rain gauge possessing the same aperture as the stem’s basal area.

Soil water infiltration capacity was measured adjacent to the study plot using a 10cm diameter ring infiltrometer sunk 20 cm into the soil surface. Soil around the perimeter of each ring was presaturated and saturation was maintained throughout each run. Infiltration rate was determined at 15-20 s intervals during the first 3-5 min of each run, when rates were high and changing rapidly, and thereafter at 1 min intervals. Runs continued for 30-45 min, by which time a constant infiltration rate had been obtained. A total of 15 runs was conducted, comprising three sets of five randomly-located rings in swale, mid-slope and ridge-crest locations.

RESULTS

Initial soil water infiltration rates were exceptionally high, varying from 26.5 to 238 mm min-’. Mean constant infiltration capacity was also high relative to most soils (Dunne, 1978): %f lSD=9.0 f2-7 mm min-’. Mean time to constant infiltration capacity was 3.5 min, with a range of 1.5-8.0 min.

Gross precipitation, net throughfall, and stemflow measured for the seven rain events observed is summarized in Table I . With the exception of the two small rainfalls of 2 and 3 mm, net throughfall spatial variability was moderate (c.v. 20-30 per cent). The proportion of gross precipitation delivered as net throughfall in the five larger rain events was in the range of 57-82 per cent, with the larger percentages occurring in the heavier rainstorms.

Table I. Net throughfall and canopy-generated stemflow recorded at the study plot on seven rain days. Data are expressed as total water depths and the percentages of gross precipitation that these depths represent. Net throughfall water depth as

X i 1 SD, n=40

Gross precipitation Net throughfall Stemflow Period (mm) (mm) (Yo) (mm) ( Y O )

8/29-30 3 0.14 f 0 3 1 4.7 Tr T r 8131-9/1 33 20.69 f 6.01 62.7 0.31 0.9 9/1-2 2 0.57 0.42 28.5 0-002 0.1

9 / 3 4 32 18.35f4.17 57.3 0.39 1.2 912-3 119 85.16522.83 71.6 Overflow

9j4-5 99 81.32f 17.34 82.1 Overflow 9/12-13 16 9.48k2.26 59.3 0.09 0.6

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58 M. KELLMAN AND N. ROULET

Stemflow could not be accurately measured for the total stem population during the two heaviest rainfalls, because some tanks overflowed. However, the rate of water input by stemflow at the largest tree was measured during these events and is discussed further below. The total quantity of stemflow measured during the remaining five rainfalls was surprisingly small, representing only ca. 1 per cent of the precipitation (Table I). Data on the funnelling ratios of individual stems is presented in Table 11. Ratios for the smallest rain event of 2 mm were, with only two exceptions, less than 1, indicating canopy retention. For the three larger rain events, measured funnelling ratios varied from < 1 to 45, with considerable variability (X 1 SD = 6.8 f 9.4). Individual liana stems showed little consistency in funnelling ratios between events, but those of tree stems were more consistent. In each rain event there was a significant negative correlation between tree stem girth and the logarithm of the funnelling ratio ( I = -0.45, -0.56, -052; P<O.O5), indicating that the capacity to concentrate rainfall decreases with increasing tree size.

Data for the rate of stemflow input to the soil surface at the largest tree on the plot are provided in Table 111. Data are presented separately for the total period of rainfall and for the period of maximum input as indicated by the water level recorder. Infiltration area represents the soil surface area necessary to absorb these inputs, assuming a constant infiltration capacity of 9.0 mm min- '. As initial infiltration rates may greatly exceed this value, these infiltration area estimates represent probable maxima. A peak stemflow input of 2 1 min-' was recorded during the second rainfall on September 2-3. However, the high infiltration capacity of these sands resulted in a surface area of only 0.22 mz being necessary to absorb this influx, and all other infiltration area estimates were far smaller than this figure. Not surprisingly, there was little evidence of overland flow at the base of even the largest trees in this forest.

Table 11. Funnelling ratios for individual tree and liana stems on the study plot during four rain days. Gross precipitation received on each day is indicated in parentheses

Period Stem Girth 8/31-9/1 9/1-2 9 / 3 4 9/12-13 no. Species (cm) (33mm) (2mm) (32mm) (16mm)

T1 T2 T5 T6 T7 T8 T9

TlO T12 T16 T4

T14 T15 T3

T11 T13

L1 L2 L3 L4 L5 L6 L7

Karwinskia humboldtiana Karwinskia humboldtiana Coccoloba barbadensis Coccoloba barbadensis Coccoloba barbadensis Coccoloba barbadensis Coccoloba barbadensis Coccoloba barbadensis Cedrela odorata Tabernaemontana alba Eugenia capuli Casearia sp. Casearia sp. Crataeva sp. Indeterminate No. 5 Indeterminate No. 5 Liana Liana Liana Liana Liana Liana Liana

21 3.4 100 1.8 22 1.9 59 3.8 23 6.0 57 3.5 45 7.4 23 0.6

200 1.4 50 17.3 79 2.2 25 6.2 22 40.5 15 30.6

100 0.7 62 1.2 17 7.1 16 15.2 18 10.7 17 2.1 20 3.9 16 10.4 17 0.5

0.3 4.1 0.04 2.2 0 2.5 0 5 2.7 0.2 13.4 0 4 5 0.7 3 5 0.2 1.9 0 1.6 0.1 4.3 0.1 4.0 1.3 8.7

29.9 44.6 0.6 31.3 0.1 1.8 0.1 2.3 0.4 6.3 0.7 25.5 0.4 0.7 0 8.0 0.5 6.5 0 25.5 0 1 .ti

3-6 1.7 0.3 1 *6 3.3 3.3 6.5 1 .o 0.4 1.3 2.9 4.4

13.0 20.3 0.6 1.2 0 1-8 1 *7 4.4 4.7 6.1 0

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STEMFLOW AND THROUGHFALL IN A TROPICAL DRY FOREST 59

Table 111. Stemflow fluxes at the largest tree on the study plot (Cedrela odorata, 200 cm girth). Two rainfall events occurred during the September 2-3 period. Infiltration area is calculated assuming a soil infiltration rate of

9.0 mm min- ’ Total period of rainfall Peak rainfall period

Duration Volume Rate Infil. area Duration Volume Rate Infil. area Period (min) (ml) (ml min-’) (cm2) (min) (mI) (ml min-’) (cm’)

8/31-911 38 14160 373 414 13 10000 769 855 9/2-3 (a) 46 14500 315 3 50 13 7000 538 598

9 / 3 4 29 1 14 000 48 53 24 6000 250 278 914-5 237 72000 304 34 9 8000 889 988

(b) 67 61500 918 101 1 7 14000 2000 2222

DISCUSSION AND CONCLUSIONS

Although the number of rain events occuring within the observation period were limited, these encompassed the full range of conditions that are normally experienced during the wet season in this area. Consequently, we believe that the data are representative of the throughfall and stemflow magnitudes that prevail in this forest.

The infiltration results indicate that stemflow arriving at the soil surface in this forest would easily penetrate the soil surface, and that little overland flow would be generated, in contrast to the situation described by Herwitz (1986). Given the high infiltration capacity and the textural homogeneity of the soil, it is unlikely that there would be much lateral divergence in percolating water passing through the soil. However, while the soil has the capacity to absorb and transmit large volumes of water from point sources, the total volume of water delivered in this pattern by stemflow is small. Comparably small contributions by stemflow in other tropical forests have been reported by Jackson (1975) and Manokaran (1979).

The funnelling ratios recorded in Table I1 lie within the lower range of those reported by Herwitz (1986). It is unlikely that funnelling ratios during the two heaviest rainfalls would have exceeded these values greatly, as the throughfalls measured during these events were the highest recorded (Table I) and funnelling ratios for the largest tree were only 2.0 and 2.3 at these times. The low efficiency of funnelling by these trees appears to be primarily a function of their specialized morphologies. In contrast to the tall slender trees characteristic of moist tropical forest, canopy trees at La Mancha tend to be short and have broadly spreading branch systems that are almost horizontal for much of their length. Raindrop detention and branchflow has been shown to be closely correlated with branch inclination, and to approximate zero at low inclination angles (Herwitz, 1987). The negative correlation between tree girth and funnelling ratio recorded here suggests that a local tree’s effectiveness at concentrating rainfall deteriorates with maturation, and that understorey saplings and shrubs may be more effective in this capacity.

The net throughfall and stemflow measured on the plot may be combined to provide an estimate of net precipitation arriving at the soil surface during each rain event (Table IV). Assuming that:

G P = N P + N C (1) where G P is gross precipitation, N P is net precipitation, and N C is net canopy storage, the water volumes listed as ‘unaccounted for’ in Table IV should be attributed to net canopy storage. However, the magnitude of these volumes for the larger rain events appear to be unrealistically high. For example, Jackson (1975) estimates net canopy storage by rain forest in Tanzania to be approximately 2.5 mm for 30 mm rainfalls, while our data (Table IV) imply storages of 12-13 mm for rainfalls of similar magnitude. It would therefore appear that net precipitation has been underestimated in our observations.

In Table IV, net precipitation is assumed to be comprised only of net throughfall and canopy stemflow, but o.ther pathways may exist. A significant contribution by stemflow down small liana stems is discounted because of the low contribution by those larger stems that were collared. However, the tendency for smaller tree stems to be more effective funnellers of precipitation (Table 11) suggests that the understorey canopy and

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60 M. KELLMAN AND N. ROULET

Table IV. Net precipitation received at the study plot during five rain days

Gross precipitation Net precipitation Unaccounted for Period (mm) (mm) (mm) O h

8/29-30 3 0.1 2.9 97 813 1-911 33 21.0 12.0 36 911-2 2 0.6 1.4 70 9 / 3 4 32 18.7 13.3 42 9/12-13 16 9.6 6.4 40

stems may have been important in intercepting and concentrating precipitation that would otherwise have been measured as net throughfall, i.e.:

N P = T n + S c + S u (2) where Tn is net throughfall, Sc is canopy stemflow, and Su is understorey stemflow.

The logistics of instrumenting large numbers of multistemmed shrub clusters made it impractical to attempt a thorough assessment of this possibility, but some evaluation of the contribution of this stratum was achieved by instrumenting two clusters. Small saucer-shaped collectors of 0.9 mm polyethylene sheet were constructed and installed at two multistemmed clusters of the understorey shrub Nectandra sp. and connected to collecting tanks as in the stemflow instrumentation. Total stemflow for each shrub was calculated by deducting the net throughfall that would have been expected on a collector of this surface area had it acted as a ‘normal’ net throughfall collector. Only two rainfall events occurred after these shrubs were instrumented and the water volumes collected and funnelling ratios calculated for these events is provided in Table V. In one instance overflow occurred, but the three measured ratios were all far in excess of any measured for tree or liana stems. While these results suggest that an important water concentrating role may be played by the understorey, the extrapolation of the results to a plot-wide contribution by this stratum is very tenuous. A total of 36 shrub clusters, comprising 235 individual stems, were enumerated in the study plot. If shrub stemflow volume is assumed to be proportional to stem number, then the water volumes collected by the instrumented shrubs suggest that on September 12-13 the total shrub stratum would have funnelled approximately 147 1 of stemflow, representing 0.9 mm of precipitation. While this represents ten times the canopy stemflow measured during this rain event (Table I), it augments total stemflow to only ca. 6 per cent of gross precipitation. Moreover, it reduces the magnitude of water flux unaccounted for in Table IV only marginally, to 5.5 mm. Unless a very large canopy storage is assumed, one must conclude that some other unmeasured water flux pathway also existed.

Based upon field observations during rainstorms, and the existence of drip lines in the sand beneath shallowly-angled branches, we believe that concentrated water flow from inflexion points of branches is the most likely pathway to have remained unmeasured. Such a pathway probably accounted for the high inputs recorded in a few throughfall collectors. Shuttleworth (1988) has recorded high spatial variability in throughfall in an Amazonian rain forest, which he attributes to concentrated water flow from canopy drip points. A throughfall sampling protocol involving many gauges, moved randomly over long time periods, is recommended by this author as the only procedure likely to provide accurate estimates of throughfall in tropical forests. Without more data it is impossible to assess how concentrated, or spatially stable, such drip points are within the forest. Field observations made during the course of rainstorms suggested that the points of concentrated flow varied considerably in space and time, which would make it unlikely that this pathway of water flux was a significant contributor to ‘leaching avoidance’ in the forest.

Taken together, these data indicate that concentration of incoming rainfall by plant structures is not a major contributor to ‘leaching avoidance’ in this forest. Although the sandy soils are capable of absorbing large quantities of water delivered to them at point sources, the actual delivery volume is small. Even if an understorey stemflow contribution ten times that of the canopy is assumed, the total stemflow contribution is

Page 7: Stemflow and throughfall in a tropical dry forest

STEMFLOW AND THROUGHFALL IN A TROPICAL DRY FOREST

Table V. Stemflow received at two Nectandra sp. shrub clusters during two rain events

Sept. 4-5 Sept. 12-13 Net Net

Total basal area stemflow Funnelling stemflow Funnelling Shrub No. of stems (cm2) (ml) ratio (mu ratio

61

N1 3 6.19 7 504 122.6 loo1 101.1 N2 6 21.19 Overflow 4 588 135.3

unlikely to exceed 10 per cent of gross precipitation. Movements of water, and presumably also of nutrients, in this forest soil are dominated by spatially relatively uniform vertical fluxes. It would be unwise to generalize these results to other tropical dry forests, as the absence of canopy stemflow generation seems closely related to tree morphology, and at La Mancha an extreme morphological form may predominate. However, it would be equally unwise to generalize the large stemflows recorded in some tropical moist forests to tropical forests generally.

ACKNOWLEDGEMENTS

We thank Kathryn Outerbridge and Tim Hodkinson for assistance in the field, and are grateful to the Instituto Nacional de Investigaciones sobre Recurcos Bioticos for permission to work at the La Mancha Biological Research Station. The research was supported by an operating grant from the Natural Sciences and Engineering Research Council of Canada to M.K. and a grant from the York University President’s Ad Hoc Fund to N.R.

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