Throughfall and Stemflow in the Forest Nutrient Cycle
G. G. PARKER
I. Summary . . 58 11. Introduction . . . 58
111. Definitions . . 61 A. Hydrological . . 61 B. Chemical . . . 62
IV. Magnitude and Importance of the Fluxes . . 65 A. Throughfall . . 65 B. Stemflow . . 69 C. Seasonality. . . 72 D. Recycling Rates . . . 73 E. Variability . . . 74
V. Factors Affecting Throughfall Quality. . . 75 A. Amount of Precipitation . . 76 B. Latitude . . 71 C. Proximity to Sources . . 78 D. Forest Type . . 80 E. Stand Age . . . 81 F. Site Fertility . . 82 G. Insect Consumption . . 84 H . Other Factors . . 84
VI. Processes of Throughfall Enhancement . . 85 A. Incident Precipitation . . . 86 B. Evaporative Concentration . . . 86 C . Inter-Event Deposition . . 87 D. Leaching . . . 90
VII. Partitioning Net Throughfall . . 92 A. Direct Approaches . . 92
VIII. Elements in Throughfall . . . 98 A. Carbon . . 98 B. Hydrogen . . . 98 C . Potassium . . . 99 D . Calcium . . 99 E. Magnesium . . 100
E. Foliar Uptake . . 91
B. Indirect Approaches . . 94
58 G. ti. PARKER
F. Sodium . . 100 G. Phosphorus . . 100 H. Nitrogen . . 101
J . Sulphur . . 102 1. Chloride . . . 101
K . Silicon, Iron and Aluminium . . 103 L. Heavy Metals . . 103
IX. Recommendations for Future Research . . 104 X. Conclusions . . 105
XI. Acknowledgements , . . 106 References . 101 Appendix . . 121
The quality of precipitation falling on forests is altered during a brief but significant interaction with the surfaces of plants, resulting in the transfer of additional mineral matter to the forest floor. While incident precipitation is the largest nutrient input to many forests, throughfall fluxes are substantially greater, ranging from 1.27 (for NO,-N) to 11.2 times as high (for K). These alterations in nutrient concentrations involve numerous processes and combine materials originating both within (biotic) and outside of the forest ecosystem (atmospheric). The resulting flux is a pathway of rapid mineral cycling, a route for transferring metabolic biproducts and substances of allelochemic and pedogenic importance and a method for washing the air filtering surfaces of the forest canopy.
Throughfall and stemflow are major pathways in nutrient recycling. The annual nutrient return to the forest soil for the elements K, Na and S is predominantly via throughfall and stemflow and little due to litterfall. Stemflow transfers only 5-20% of the total in precipitation-borne solutes, yet it is the major nutrient input to restricted areas of the forest floor.
In this review 1 discuss a number of factors influencing throughfall and stemflow quality and their variation. Particular attention is paid to the effect of the canopy in altering precipitation quality, and to the numerous processes involved. Foliar leaching is concluded to be the major process controlling throughfall and stemflow enhancement for nearly all elements, though foliar uptake and canopy filtration of dusts, aerosols and gases is important in particular situations.
Precipitation is an important source of nutrient input to forested ecosystems (e.g. Nye, 1961; Miller. 1963; Likens ef d., 1977; Swank and Henderson,
THROUGHFALL, STEMFLOW IN FOREST NUTRITION 59
1976), especially where rock weathering is slow (Gorham, 1953; Jordan et a/., 1980). Since nutrient transfers in throughfall and stemflow are usually substantially larger than those in incident precipitation, an increasing amount of interest has been directed at this flux (e.g. Will, 1955; Yawney et a/., 1970; Szabo, 1977; Killingbeck and Wali, 1978; Prebble and Stirk, 1980; Khanna and Ulrich, 1981). Estimation of the fluxes of elements in incident pre- cipitation, throughfall and stemflow is now a routine part of nutrient budget studies in forests (e.g. Corlin, 1971; Tsutsumi, 1971; Likens et a/., 1977; Schlesinger, 1978; Kelly, 1979; Clesceri and Vasudevan, 1980; Sollins et a/., 1980).
The alteration of the composition of water in contact with plant tissues has been recognized since de Saussure in 1804 (Tukey, 1970a). That throughfall may be important in plant nutrition and soil fertility is a comparatively recent idea, stemming from Ingham ( 1950a.b) and Tamm ( 1 950). The material added to incident precipitation due to the action of the canopy, was found to derive both from within (Tamm, 1950,1951)and from upon (Ingham, 1950a,b)plant surfaces. Will (1955) first showed the importance of forest rainwater relative to the nutrient flux in litterfall for Na, K and Mg. Madgwick and Ovington (1959) were the first to report that different forest types have unique effects in changing the concentration of precipitation.
Studies of the chemical composition of throughfall and stemflow are now commonplace and increasing in number; most of the reports on throughfall and stemflow quality reported here have been published within the past 5 years. In the past interest was directed at (1) factors affecting the hydrological partitioning of precipitation into interception, throughfall and stemflow; (2) the quantification of nutrient fluxes under various climates and canopies, and (3) the examination of factors controlling throughfall composition, its seasonality and heterogeneity. Recently, much attention is being focused on the external and internal sources of throughfall enrichment (Ulrich et a/., 1978; Lakhani and Miller, 1980; Miller and Miller, 1980; Parker et a/., 1980).
The composition of throughfall and stemflow has been studied in a number of geographical locations (Fig. l), especially populated western hemisphere regions. Some of the most extensive forests remain unstudied. High latitude forests have received little attention (Kazimirov and Morozova, 1973; Johnson, 1975; and Van Cleve, in Cole and Rapp, 1981) and there are only two descriptions of throughfall composition from the Amazon basin (Nort- cliff and Thornes, 1978; Jordan et a/., 1980).
In this review the processes of throughfall (precipitation which falls through the canopy) and stemflow (precipitation which flows down the tree stem) are considered in the context of the nutrient economy of forests. I examine the quality, magnitude and timing of these fluxes relative to other major nutrient pathways, with consideration of their biological, geographical, hydrological, and meteorological influences. Particular attention is paid to the problem of estimating the sources of throughfall enhancement (recycling and input).
Fig. 1. Studies of throughfall and stemflow chemistry around the world relative to the distribution of forests. Shaded areas are forested. Adapted from Dansereau (1957); Strahler (1973); Dassman (1976) and Neil, York Ti/tie.s (1978).
THROUGHFALL, STEMFLOW IN FOREST NUTRITION 61 This review will not concern itselfwith aspects ofthroughfall quality that d o
not pertain to the biological cycle of nutrient elements. Thus I shall avoid discussion of the information-rich allelochemic compounds whose effects are surely disproportionate to their weights (e.g. Whittaker and Feeney, 1971). leachates which can mobilize soil iron. aluminium and manganese (e.g. Malcolm and McCracken, 1968) and the exudates which may drip from the canopy in dry periods (Browne, 1932; Durant, 1932; Carrier, 1958: Wyatt- Smith, 1958).
Since throughfall and stemflow are associated with precipitation events, the transport of nutrients contained in throughfall and stemflow depends on the magnitude, timing and form of the precipitation. Thus a reliable estimate of throughfall nutrient flux demands a good forest hydrological budget. Some essential concepts in throughfall hydrology are defined and discussed in the following section.
Water falling on the forest is called incident precipitation (gross pre- cipitation (Leonard, 1966), wetfall or grossfall (Lindberg ct d., 1979)). Most commonly it impinges upon the canopy vertically (direct precipitation) but often it is laterally advected and intercepted by the canopy (indirect or occult precipitation, (Kittredge, 1948), which is important for mist, fog or cloud water. Direct precipitation is measured either above the canopy (e.g. Attiwill, 1966; Lindberg et a/., 1979) or, more commonly, in open areas adjacent to the forests. Indirect inputs are more difficult to measure (Falconer and Falconer. 1979; Falconer and Kadlecek, 1980). Precipitation which passes through the canopy and falls to the ground is called throughfall (net or effective rainfall (Helvey and Patric, 1965), canopy drip, crown runoff (Bache, 1977), drainage water (Brosset, 1976), leafwash (Cole and Rapp, 1981), rainwash (Nye, 1961; Whittaker and Feeney, 197 1 ) or pluviolessivage (Denaeyer-DeSmet, 1966; Rapp, 1969; Lemee, 1974)) and may exceed incident precipitation in quantity at some locations (Zinke, 1962; Banaszak, 1975; Prebble and Stirk, 1980). Throughfall includes incident precipitation which penetrates canopy gaps. unless the individual gaps are large and frequent (e.g. > lo",, of the area).
An additional portion of precipitation reaches the ground by running down the branches and trunk, depositing at the base of the tree. This portion is called stemflow (trunk or stem runoff). The sum ofthroughfall and stemflow is called total forest water (net precipitation or net rainfall, in Zinke. 1966).
Incident precipitation that does not appear on the forest floor by either of these routes is called the interception loss (Kittredge, 1948). This includes
62 G. G. PARKER
( I ) stemflow which never reaches the ground, (2) water evaporated from the canopy during the course of a storm (Leonard, 1966) and (3) the amount of water held in the canopy and evaporated after the storm, the canopy saturation (Zinke, 1966). The latter portion includes the canopy storage capacity (Zinke, 1966), the maximum amount of water the canopy can retain at a given time, which ranges from 1-5mm and appears to be relatively constant for a given forest and season, though affected by amount and intensity of precipitation (e.g. Leonard, 1966).
As it cascades through the canopy, a unit of water interacts with numerous plant surfaces. For example, nearly all stemflow encounters leaf surfaces first (Jordan, 1978) and throughfall may include water dripping from twigs and branches. However, the hydrological classification depends on the last surface encountered. If that surface is the tree trunk then the precipitation is termed stemflow, otherwise it is throughfall. Where measurements are made at several levels in a multi-storied canopy, interception and throughfall can be further subdivided according to the number of strata defined (Patterson, 1975); even the litter layer may be considered as a canopy (Helvey and Patric, 1965). Stemflow is not similarly partitioned. The compartments of the forest hydrological budget are summarized by the equation (after Helvey and Patric, 1965):
R = I + T + S
where R , I, T, S are incident precipitation, interception loss, throughfall and stemflow respectively, all values given in linear dimensions (mm). Only these terms will be employed in this treatment. These hydrological conventions are diagrammed in Fig. 2.
The quantity and distribution of throughfall and stemflow depends on microscale features of canopy structure, such as, crown density (Anderson et al., 1969; Paivanen, 1974; Nicholson et al., 1980), closeness of the foliar elements, distance from the nearest bole (Prebble and Stirk, 1980), or to open spaces in the canopy (Tamm, 1951). Trees with drooping branches can produce much throughfall at the canopy perimeter. Quantity of stemflow is certainly affected by bark smoothness (Voigt 1960a, Voigt and Zwolinski, 1964), stem diameter (Kittredge, 1948) and branch angle (Mina, 1965; Hutchinson and Roberts, 198 I ) . Large-crowned emergent trees with smooth bark and raised branches produce the most stemflow.
In a forest the flux of nutrients in precipitation is closely related to the amount of precipitation. However, there are important differences between nutrient concentration and water quantity: nutrient fluxes are quite variable and not directly obtainable from the water budget. Variability in nutrient fluxes is treated in Section IV. E.
THROUGHFALL, STEMFLOW IN FOREST NUTRITION 63
Fig. 2. Compartments and fluxes in throughfall hydrology.
I use the terminology proposed by Carlisle rt a/ . (1966b) and extended by Gosz el a/. ( 1 976) to describe nutrient transfer by various pathways. The flux of nutrients onto the forest by either direct or indirect precipitation is termed incident precipitation (INC), which is further called bulk precipitation if the collector is open to dry deposition between storms (Whitehead and Feth, 1964) or wet precipitation otherwise (Galloway and Parker, 1980). For a given nutrient the total flux of precipitation borne nutrients to the forest floor is the sum of the materials falling through the canopy (throughfall, designated as THF) and travelling down along the trunk (stemflow, STF), both ofwhich are conventionally expressed as elemental mass deposited per unit area per unit time. Such fluxes are termed depositions (e.g. Richter and Granat, 1978; Galloway and Parker, 1980). When the appropriate nutrient deposition in incident rainwater is subtracted from that in either throughfall or stemflow (Carlisle c>r a/., 1966b) the remainder is called net throughfall (NTF) or net stemflow (NSF), either of which can be negative. The total effect of the canopy on these depositions is obtained by subtracting the incident precipitation deposition from the sum of the throughfall and stemflow depositions. This quantity has no accepted name (Eaton rt al. (1973) call it "net stemflow and throughfall") and will be designated in this treatment as net forest water (NFW).
These conventions are summarized in equation form below (Table 1). D;, D, and D, are the amounts (depth) of water sampled in incident precipitation,
64 G. G. PARKER
throughfall and stemflow; C,, C, and C', are the respective solute con- centrations of a given nutrient (concentrations applying to depositions for more than one event should be volume-weighted). When amounts of water and concentrations are in compatible units one may calculate the deposition of nutrient elements (given by capitalized symbols) according to the following lo rm ulae .
Table I Definitions and equations relating precipitation amounts and nutrient concentrations
to nutrient depositions.
~ ~~ ~. ~ ~-
D ~ f i t 1rd I vrt? I S amount of precipitation ionic concentration
gross deposition net deposition
Precipitation pathway incident
precipitation through fa I I stenif ow . ~- ~. ~ ~~ ~~~ _ _ ~-
D , D, D , c, c-t C,
INC = D,C', THF=D,C', SF = D,C, N T F = D,(C', C,) NSF= D,(C, C,)
Several enrichment factors may be defined. The ratio of an element's concentration in throughfall (THF)...