Stemflow nutrient inputs to soil in a successional hardwood forest

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1 Plant and Soil 4: , 992. (E) 992 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 963 Stemflow nutrient inputs to soil in a successional hardwood forest CHRISTOPHER S. POTTER Biology Department, Emory University, Atlanta, GA 3322, USA. Present address: NASA-Ames Research Center, Mail Stop 239-2, Moffett Field, CA 9435, USA Received 3 April 99. Revised October 99 Key words: acid deposition, hardwood forests, nutrient leaching, soil inputs, stemflow, succession Abstract Stemflow and throughfall from a regenerating (8-year-old) southern Appalachian hardwood forest were collected to examine the relative importance of tree bole nutrient leaching in response to acid deposition. Samples from nine (2m 2 ) stemflow collection plots were analyzed for four dormant season and growing season rainstorm events. Results showed that, relative to throughfall fluxes, stemflow accounted, on average, for approximately 8.5% of total water reaching the forest floor during both dormant and growing season storms. Relative to foliar leaching, K-, SO 4 -, and PO 4 ions appear to be the most easily leached ions from young tree stems. Proportional nitrate and base cation stemflow fluxes increased significantly (p<.5) with growing-season storm-event duration, suggesting that the stemsurface nutrient pool is depleted by precipitation more slowly than the foliar pool. On average, proportional stemflow fluxes of SO 4 (2%) and K (4%) were consistently higher than reported maximum values for more mature forest stands, which indicates that small-scale stemflow inputs of ions such as these to the forest floor may be important in early successional ecosystems. Introduction Although forest stemflow contributions to total below-canopy fluxes of water and dissolved nutrients average only about 2% (depending on the element and stand type), stemflow represents a major mineral input to certain areas of the forest floor (Parker, 983). From an ecosystem perspective, stemflow can be a particularly important transfer route for nutrients such as SO 4 - and Ca (Mayer and Ulrich, 972), whereas fluxes of N and P over the tree bole surface may be relatively minor (Parker, 983). Little information exists regarding the importance of stemflow as a nutrient transfer pathway to the soil at different stages in ecosystem succession. Potter et al. (99), however, conducted storm event analysis of total below-canopy nutrient transfers which suggested that stemflow is a significant mineral input to soil in a regenerating hardwood forest. Inclusion of stemflow fluxes increased canopy exchange rates of SO 4 -, PO 4 -, C-, K, and Mg ions by more than 2% in a regression model of total (throughfall + stemflow) below-canopy element fluxes. On an annual basis, atmospheric dry deposition supplied roughly one-third, whereas canopy exchange supplied two-thirds of total element enrichment of throughfall. Furthermore, addition of stemflow to the model decreased area-based net canopy absorption rates of NH 4 - and H ions, indicating that (relative to leaf surfaces) little ion exchange occurs on bole surfaces. These results raise several important questions concerning stemflow in the forest nutrient cycle. Does the stemflow-to-throughfall nutrient flux ratio change on a storm-by-storm basis? Is leaching from tree boles affected by storm event characteristics such as rainfall amount, duration, and intensity? Are proportional stemflow contri-

2 25 Potter butions to total below-canopy nutrient fluxes in early successional forests markedly different from those in more mature aggrading stands? The objectives of this study, therefore, were to ) quantify the importance of stemflow (relative to throughfall) fluxes in a regenerating southern Appalachian forest canopy, 2) analyze the relationship of storm event characteristics to stemflow nutrient fluxes, and 3) compare proportional stemflow fluxes in an early successional forest to previously studied aggrading forest stands. Methods The study area was watershed 7 (WS7) as the USDA Forest Service Coweeta Hydrologic Laboratory, near Franklin, North Carolina. This watershed was clearcut and harvested in 977 by cablelogging, a technique which minimized disturbance to the forest floor and promoted rapid stump sprouting and forest regeneration. Details on site history and sample plots are provided, respectively, by Boring et al. (98) and Potter et al. (99). In brief, leaf area index and net primary productivity on WS7 had nearly returned to pre-clearcut levels by 985 (Boring and Swank, 986). Study plots were located near the middle of WS7 in an area characterized as a xeric chestnut oak (Quercus prinus; see Radford et al. (968) for authorities on species names) forest community (Boring and Swank, 986). Stemflow (along with throughfall and incident precipitation) collection and analysis methods were explained by Potter et al. (99). All stems (representing various tree species and size classes) were collared in each of nine ( m x 2 m) stemflow areas located near the middle of WS7. Stemflow was collected as an areal composite sample from all living stems >lcm diameter (at 4cm from stem base) during individual rain events. Plastic troughs (.x.m) were used to collect throughfall. Troughs were randomly placed, ten per plot on each of three 2 x 2m plots, for a total of 3 collectors. Sulfate, P, NO 3 -N, and NH 4 -N in aequeous samples were determined photometrically (McSwain, 973; Technicon Industrial Systems, 97). Sample ph was measured using a Corning model 2 ph meter and a calomel combination electrode. Ca, Mg and K were analyzed on a Perkin-Elmer 36 Atomic Absorption Spectrophotometer. As a quality-control measure, all elemental concentrations were determined for Environmental Protection Agency (EPA) aqueous reference samples (EPA, 984). Standard reference material concentrations were within the % range of accepted values. Storm characteristics and stemflow nutrient flux data were tested for significant regression relationships using SYSTAT Multiple General Linear Hypothesis programs (Wilkinson, 986). Results and discussion Characteristics of the various dormant-season (November to April 3) and growing-season (May to October 3) storms include event amount (mm), event duration (h), event intensity (mmhf ), antecedent dry period (h), and stemflow amount (mm) (Table ). Stemflow amounts ranged from 5.4 to 9.4% of incident precipitation for dormant-season storms and from 3.3 to 7.5% of incident precipitation for growingseason storms. There was a positive correlation (R 2 =.98; /?<.) between growing-season stemflow amount and storm-event amount (as shown in Equation ). Neither event duration nor intensity accounted for a significant (p <.5) portion of the variability in growing-season storm stemflow amounts. Stemflow amount (mm) =. x Event amount (mm) -.8 () The general pattern of stemflow nutrient fluxes (yu,eqm~ 2 ) was the same for both dormant-and growing-season storms: SO 4 and K highest; Ca, H and NO 3 intermediate; Mg, NH 4 and PO 4 lowest (Table 2). Proportional stemflow nutrient fluxes (:TH), calculated as Stemflow/(Stemflow + Throughfall), provides a relative index of the importance of stemflow as an input to soil in the forest nutrient cycle. During the dormant season, : TH values were highest for base cations (K-, Ca- and Mg- ions). Potassium, SO 4 and PO 4 showed the highest : TH values for

3 Stemflow nutrient inputs to soil 25 Table. Storm event characteristics Date Dormant Amount (mm) Duration (h) Intensity (mm IT ) Dry period (h) Stemflow 2 (mm) (.5) (.2) (.2) (.2) Growing Values are means (N = 9) and standard errors (.9) (.33) (.3) (.42) (.36) (.4) (.6) (.5) (.4) (.4) (.5) growing-season storms (Table 2). Since average : TH values for these ions exceeded the average hydrologic : TH value of 8.5%, it appears that, relative to foliar leaching, K-, SO 4 - and H 2 PO 4 ions are the most easily leached ions from young tree boles. Conversely, :TH values for NO 3 - and NH 4 ions were consistently lower than storm hydrologic : TH values, suggesting that nitrogen leaching from stem surfaces was relatively unimportant. Regression analysis revealed no significant (p<.5) relationships between (:TH) hydrologic or nutrient flux values and growingseason storm-event amount, intensity, or antecedent dry period. There were, however, significant (p<.5) correlations between growingseason storm duration and : TH values for NO 3 -, K-, Ca- and Mg ions (Fig. ). These trends suggest that precipitation depletes the stem surface nutrient pools more slowly than foliar nutrient pools, i.e. the longer the storm, the relatively more that stemflow contributes to total below-canopy element transfers to the forest floor. Parker (983) calculated the percentage stemflow contribution to below-canopy nutrient deposition (: TH) for a variety of forests (including coniferous stands). Results showed proportional stemflow hydrologic contributions of up to 5%, and maximum : TH nutrient contributions of 3% S, 5% N, % P, 9% K, 7% Ca, and 9% Mg. Results from the present study of an early successional forest suggest that, on average, proportional stemflow fluxes of SO 4 (2%) and K (4%) were higher than Parker's (983) maximum reported values for more mature forest stands. Similarly, Ewel et al. (975) reported a relatively high (28%) :TH value for K stemflow transfer in certain hardwood forests. Swank and Reynolds (987) found elevated : TH values of 25% for SO 4 and 26% for K ions in one out of five storms in a white pine stand at Coweeta (Watershed ), although it should be noted that coniferous and deciduous stands may display distinctly different canopy nutrient processing mechanisms (Cronan and Reiners, 983). Stemflow is frequently the largest pathway for nutrient transfer to the area immediately surrounding tree boles and the associated root zones, so that its effects on soil chemistry and plant nutrition are potentially significant (Falkengren-Grerup, 989; Gersper and Hollowaychuk, 97; Wittig and Neite, 985; Zinke, 962). Compared to throughfall, stemflow nutrient concentrations (particularly S, K, Ca, and Mg) are generally high (Mahendrappa, 974), whereas stemflow ph is characteristically low (Carlisle et al., 967; Nicholson et al., 98).

4 252 Potter Table 2. Sternflow nutrient fluxes (; units are ^,eq m~ 2 (ISE)) and proportional contributions to total below-canopy nutrient fluxes (:TH) a Storm Date Dormant S 4 85 (9) 6(4) 7 (22) 4(3) 47 :TH P 4 6() 2 :TH NO,-N 5(4) 35(7) 25(5) 2 :TH NH 4 -N 2(3) (3) 4 :TH Growing (3) 468(8) 5(7) 68 (44) 54 (39) 32(6) 5(8) 36(8) 257(57) 24(7) 22(5) (3) (5) 4(3) () 7(3) (8) 56(3) 24(6) 7(4) 6(2) 5(2) 5(2) (2) 3 7(2) 7(4) () () Dormant K 97 (46) 5(2) 4(6) (4) Ca 7(5) 3(3) 28(6) 53() Mg 5(6) 3() 9(3) 7(4) H 2(7) 9(4) 29(7) 4(2) Growing (28) 62 (58) 6 (25) 7(3) 8(3) 2(6) 3(3) 7 (23) 9(7) 22(6) "Calculated as Stemflow/(Stemflow + Throughfall). 46 (5) 67(7) 44(6) 27(8) 7(2) 3() 2(3) 22(5) (3) (3) () 39(6) 6(4) 8(2) 3() 3(4) 5() (6) 99 (2) 36(2) 32(9) 22(4) 7(3) 5(4) 9(3) The total area of stemflow zones on a forest floor is probably not larger than the stand basal area (Abee and Lavender, 972). Forest regeneration following clearcutting in the southern Appalachians results in a larger number of stems per ha and lower overall basal area, compared to pre-disturbance (Boring and Swank, 986; Boring et al., 988) and control watershed (Day et al., 988) stand measurements. Nevertheless, because proportional stemflow contributions of SO 4 and K on WS7 were found to exceed most previously reported values for more mature forests, it appears that small-scale stemflow fluxes of certain ions represent relatively important inputs to the forest floor in early successional ecosystems.

5 Stemflow nutrient inputs to soil Storm Duration (h) 4 6 Nitrate * Potassium Calcium ' Magnesium Fig.. Regression analysis of proportional stemflow fluxes (: TH = Stemflow/(Stemflow + Throughfall)) versus duration of growing-season storms on Coweeta WS7. R" >.5 (N = storms) for all nutrients. Acknowledgements The author thanks H Ragsdale, W Swank, and S Lindberg for valuable discussion and thoughtful reviews of an earlier version of the manuscript. J Buchannan, J Deal, K Reynolds, and S Waidroop assisted in various aspects of the field and analytical work. L Mammel assisted in field sampling. This research was supported by USD A Forest Service Grant No , National Science Foundation-LTER Grant No. BSR 8293, and Sigma Xi, the Scientific Research Society. References Abee A and Lavender D 972 Nutrient cycling in throughfall and litterfall in 45-year-old Douglas Fir stands. In Research on Coniferous Forest Ecosystems First Year Progress in the Coniferous Forest Biome. Eds. J F Franklin, L J Dempster and R H Waring, pp US/IBP Proc. of a Symposium of the Northwest Scientific Association, Pacific Northwest Forest Experiment Station, Forest Service, USDA, Portland, OR. Boring L R, Monk C D and Swank W T 98 Early regeneration of a clearcut southern Appalachian forest. Ecology 62, Boring LR and Swank W T 986 Hardwood biomass and net primary production following clearcutting in the Coweeta basin. Southern Forest Biomas Workshop, 6-9 June, 986, Knoxville, TN. Boring L R, Swank W T and Monk C D 988 Dynamics of early successional forest structure and processes in the Coweeta basin. In Forest Hydrology and Ecology at Coweeta. Eds. W T Swank and D A Crossley Jr. pp 6-8. Springer-Verlag, New York. Carlisle A, Brown A H F and White E F 967 The nutrient content of tree stemflow and ground flora litter and leachates in a Sessile Oak (Quercus petraea) woodland. J. Ecol. 55, Cronan C S and Reiners W A 983 Canopy processing of acid precipitation by coniferous and hardwood forests in New England. Oecologia 59, Day F P, Phillips D L and Monk C D 988 Forest communities and patterns. In Forest Hydrology and Ecology at Coweeta. Eds. W T Swank and D A Crossley Jr. pp 4-5. Springer-Verlag, New York. Environmental Protection Agency (EPA) 984 Quality control samples for mineral analysis. Government Printing Office, no , Washington, DC. Ewel K C, Gamble J F and Lugo A E 975 Aspects of mineral nutrient cycling in a southern mixed hardwood forest in north central Florida. In Mineral Cycling in Southeastern Ecosystems. Eds. F G Howell, J B Gentry, and M H Smith, pp Proc. of U S Energy Research and Development Administration Symp. Augusta, GA, -3 May, 974. Falkengren-Grerup U 989 Effect of stemflow on beech forest soils and vegetation in southern Sweden. J. Appl. Ecol. 26, Gersper D L and Hollowaychuk N 97 Effects of stemflow water on a Miami soil under a beech tree. I. Morphological

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