Lakhani, K.H.

English

NERC Open Research Archive

7215. AN IMPROVED METHOD OF ESTIMA-TING THE CONTRIBUTION OF CROWNLEACHATES TO THE CHEMICAL COMPO-SITION OF RAIN COLLECTED BENEATHTREESK.H. LAKHANIAmounts of solutes and suspended matter inrainwater collected as stemflow and throughfallbeneath trees are generally greater than those inrainwater collected in the open. This enrichment,as rainwater passes over foliage and branches, ispartly attributable to the removal of substancespreviously deposited on foliage and branches byimpaction or adsorption, and partly to substancesleached from within plants (ie crown leaching).The former provides a new input to the system,whereas crown leaching facilitates recycling. Buthow can the contributions made by these 2 pro-cesses be separately identified?Atmospheric inputs can be estimated directly andindirectly. They can be estimated directly from(i) catches of airborne particles (White & Turner,1970), but problems arise when ascribing amountsto unit areas of forest, and (H) with methodsbased on nutrient budgets (Likens  et al.,  1977)which tend to lack sensitivity because the fluxesof different substances are usually small relativeto the sizes of the 'sinks', particularly soil. Indirectmethods have relied on comparisons made of thechemical composition of stemflow and through-fall with those of rainfall collected in the openduring the same period of time. However, thecompositions of stemflow and throughfall reflectthe extent of crown leaching in addition to drydeposition and wet deposition, whereas those ofrainfall collected in gauges in the open reflect thelatter, plus a small fraction of dry deposition(accumulated since the gauge was cleaned). Mayerand Ulrich (1974) argued that crown leaching ofdeciduous trees could be assumed to be negligibleduring leafless winter months, and then proceededto obtain estimates of dry deposition during winter.Their method, however, assumes that significantleaching occurs only through leaves, and thatrates of dry deposition to leafless trees in winterare similar to those to leafy trees during summermonths. Miller  et al. (1976),  working with Corsicanpine,  Pinus nigra var. maritima  (Ait.) Melv., observedthat amounts of solutes and suspended matter instemflow and throughfall (kg ha-1 wk-1) werelinearly and directly related to their amounts inrainfall collected in the open. They used theintercept of this regression as a measure of crownleaching, assuming that inputs in rain and from theremoval of surface deposits could be combined as asingle variable. However, the validity of thisassumption has recently been questioned byLakhani and Miller (1980) who were concernedwith the partitioning of rain in a plantation ofCorsican pine. Dr H.G. Miller of the MacaulayInstitute analysed the chemical composition of:(i) stemflow and throughfalllip rain collected in the open in Nipher-shielded rain gauges (open gauges)and (iii) rain collected in the open in funnelssurmounted by an inert wind-filter ofpolyethylene coated wire mesh (filtergauges).Samples of (i), (ii) and (iii) were taken simulta-neously and at equally spaced intervals during thecourse of 2 years, the inclusion of filter gaugesbeing essential if amounts of crown leaching wereto be separately identified from those of wet anddry deposition.1. MethodSuppose that rainwater is collected during 'n'equal time periods. Let the weight (kg ha -1) of agiven substance in stemflow and throughfall duringthe 'i'th time period be X1i; and, let the corres-ponding weights of that substance in open andfilter gauges be X2 and X3i respectively. Thus,the field data will consist of n triplets of observa-tions X1i, X , X3 i (i = 1, 2,  , n). If the wetdeposition, dry deposition and leaching of diffe-rent substances (kg ha-1) during the 'i'th timeperiod are denoted Wi, Di and Li, then the xi;values (j = 1, 2, 3; i = 1, 2,..., n) can be expressedin terms of these components and other effects.X1, the concentration of different substances incombined stemflow and throughfall, is essentiallythe total of wet deposition, dry deposition andleaching during the 'i'th time period. But somepart of some substances will be 'lost' through (i)foliar absorption, fi, and (ii) the incompleteremoval of surface deposits whatever their origin,On the other hand, there will be some gain, gi,because rain of the 'i'th time interval will succeedin removing some of the surface deposits remainingfrom earlier periods. Additionally, X1, the compo-sition of stemflow and throughfall, will be subjectto chance variations represented by the error term,ei. Thus,X 1 = Wi + Di + Li - fi - + gi + ei (1)The loss and gain terms, I; and gi, are unknownfunctions of a range of variables which tend tocancel each other. By absorbing them and theunknown foliar absorption term, fi, a new errorterm el i is evolved:eii = - fi - + gi.Thus X1 = + Di + Li + (2)Because the chemical composition of rain collectedin the open gauge, X2 j, will be mainly attributedto wet deposition, with some contamination fromdry deposition which is assumed to be proportionalto dry deposition on forests,X2 j = W i aDi + e2i (3)where 'a' is a positive constant of proportionalityand 'e2r is a random error term.Finally, the chemical constituents in the filtergauge,X3 j, will also be attributable to wet deposi-tion and dry deposition. Because their wind-filterswill intercept some of the non-vertical rainfallwhich would otherwise not be captured, the filtergauges will tend to collect greater volumes of rainthan open gauges. If the rain collection efficiencyof filter gauges, relative to that of open gauges, isassumed to be constant, and if most wet depositionis attributable to rain in contrast to fog and mist,then amounts of wet deposition in X3 j can beequated to 'kWi' where 'k' is an unknown positiveconstant. If it is assumed that filter gauges, asregards dry deposition, have a catching efficiencyof 'b' relative to the catching efficiency of forests,then:X3 j = kWi + bDi + e3 (4)where e3i is a random error term. As k is likely tobe greater than 1, and b greater than a, X3 j will beexpected to be greater than X2 j.If Vi and Ui are the volumes of liquid collected inthe open, and in filter gauges, during the 'i'th timeperiod, then the estimate (k) of the constantk is given by:k = /Dividing (4) by k gives the adjusted observation:X'3 j = X3 i/k = + b'D + e'3 (5)where b' = b/k, e'3i = e3i/k and k/k = 1.To eliminate the wet deposition term Wi fromequations (2), (3) and (5), X2 j is subtracted fromXii and X'3 j respectively to obtain the derivedvariables:Yi = X1 —X2 j = (1 - a) Di + Li + ei - e2 (6)and Xi = X'3 j - X2 j = (b' - a) Di + e'3i - e2 i (7)Thus, if Li is independent of Di, with the mean ofLi equal to ML (this condition is less strict than thespecial case that Li be constant), then Yi is linearly2. Discussion73related to Xi with the slope equal to (1 - a)/(b' - a)and the intercept equal to ML. Problems of esti-mating the parameters defining the structural rela-tionship between 2 variables (in this instance Xiand Yi), both of which are subject to randomerrors, are intrinsically difficult (Kendall & Stuart,1961). In the present instance, the problems areexacerbated because the error term, e2i, presentin both equations, reduces the degree of independ-ence. However, unlike the error terms e2; and e3i,the error term el; is a conglomeration in which,because of the large spatial variation beneath aforest canopy, even the single term ei is likelyto be large relative to e2i or e3i. On the otherhand, because k is likely to be greater than 1,e'3; will tend to be less than e3i. Thus, if it isassumed that el i is likely to be relatively largecompared with e2i or e'3i, then the parametersdefining the relationship between Y; and X; can bereadily estimated using standard regression tech-niques. Theoretically, the magnitude of the differenterrors can be controlled by varying the intensityof sampling.Previously, Miller  et al.  (1976) used the interceptof the regression of Xi; on X2 j as an estimate ofcrown leaching. But, because Xi; Wi + Di + Liand X2 j Wi aDi, it was necessary to assumethat Di was proportional to Wi, so that theycould be treated as a single variable; and alsothat Li was independent of W. In practice, Di andWi are likely to be correlated, but not propor-tionately. In contrast, Li and Wi are unlikely to beindependent (Abrahamsen et al.,  1976).By including the additional variable, X3 j, it hasbeen possible to eliminate Wi from equations(2), (3) and (5) so enabling the estimation ofleaching (Li) on the assumption that they areindependent of dry deposition (Di). In the event,the filter gauge is likely to oversample rainfall, sorequiring the adjustment of the X31 values viathe estimate of k (see equations (4) and (5)).Alternatively, the gauge may be modified to havea large funnel with inset filter.In addition to facilitating estimates of crownleaching, the method described in this chaptercan be used to estimate dry deposition (D). Fromequation (7), X — (b' - a) D and hence D X/(b'- a)which may be written as:(1 - a)D X. (1 - a)/(b' - a)with (1- a)/(b' - a) being estimated from the slopeof the regression of Yi on Xi. The term (X. slope)underestimates mean dry deposition, the extent ofthe underestimation being equal to a.D, which74tends to zero as 'a' tends to 0, as happens when theopen gauge is designed to collect negligible drydeposition.ReferencesAbrahamsen,  G., Bjor, K.,  Horntvedt, R.  &  Tveite, B.1976. Effects of acid precipitation on coniferous forest.In:  Impact of acid precipitation on forest and freshwatersystems in Norway,  edited by F.H. Braekke, 37-64. Oslo-As:SNSF. (Research report SNSF project no. 6).Kendall, M.G. & Stuart, A. 1961.  The advanced theory ofstatistics, Vol. 2,  London: Griffin.Lakhani, K.H. & Miller, H.G. 1980. Assessing the contribu-tion of crown leaching to the element content of rainwaterbeneath trees. In:  Effects of acid precipitation on terrestrialecosystems,  edited by T.C. Hutchinson and M. Havas,161-172. New York: Plenum.Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S.& Johnson, N.M. 1977.  Biogeochemistry of a forestedecosystem.  New York: Springer.Mayer, R.  &  Ulrich, B.  1974. Conclusions on the filteringaction of forests from ecosystems analysis.  Oecol. Plant.,9, 157-168.Miller, H.G., Cooper, J.M. & Miller, J.D.,  1976. Effect ofnitrogen supply on nutrients in litter fall and crown leachingin a stand of Corsican pine.  J. appl. EcoL,  13, 233-248.White, E.J.  &  Turner, F.  1970. A method of estimatingincome of nutrients in a catch of airborne particles by awoodland canopy.  J. appl. Ecol.,  7, 441-461.

An improved method of estimating the contribution of crown leachates to the chemical composition of rain collected beneath trees

http://nora.nerc.ac.uk/7045/1/N007045CP.pdf

An improved method of estimating the contribution of crown leachates to the chemical composition of rain collected beneath trees

Abstract

Similar works

Full text

Available Versions

NERC Open Research Archive