TEMPORAL VARIATION IN PHOTOSYNTHETTCALLY ACTIVE RADIATION (PAR)' IN MESIC SOUTHERN APPALACHIAN HARDWOOD FORESTS

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1 i^-tslii.>- TEMPORAL VARIATION IN PHOTOSYNTHETTCALLY ACTIVE RADIATION (PAR)' IN MESIC SOUTHERN APPALACHIAN HARDWOOD FORESTS WITH AND WITHOUT RHODODENDRON UNDERSTORIES TV '-.'..' Bartn D. Clintn 1 Abstract: Understanding spatial and tempral variatin in the understry light regime f suthern Appalachian frests is central t understanding regeneratin patterns f verstry species. One f the imprtant cntributrs t this variability is the distributin f evergreen shrub.species, primarily Rhddendrn maximum L. We measured phtsyntheticaily active radiatin (PAR) in the understry f clsed frest canpies in areas wuh-and withut R. maximum. Measurements were taken frm March thrugh Nvember 1993 with a frequency f apprximately 3 weeks; In 1994 measurements resumed frm March thrugh May. PAR under sub-canpies f &. maximum averaged 77% (e.g., 14 vs. 65 ^ml m'v) lwer than in areas withut R. maximum during the grwing seasn, and 70% (e.g., 179 vs. 641 ^ral m"y L ) lwer in the drmant seasn. Variatin in PAR during the grwing seasn ranged frm 73% t 86% lwer in R. maximum versus nn-& maximum understries. During the grwing the seasn, light levels beneath Rhddendrn were bserved t be < 2% f full sun. Lw-light envirnments assciated, with R. maximum understries are extremely limiting with respect t regeneratin f imprtant hardwd species.. INTRODUCTION The smicture f frest canpies is extremely hetergeneus and the spatial distributin f light at the frest flr is a cnsequence f this hetergeneity (Pukkala and thers, 1991). Althugh sil nitrgen availability cmmnly limits prductivity in mst temperate frests (Vitusck and thers, 1982), light is typically the mst limiting resurce under clsed canpies in suthern Appalachian frests (Wayne and Bazzaz, 1993). The quantity f light in the understry is a functin f the rate f light extinctin thrugh the canpy. Utilizatin efficiency f the residual radiant energy in the understry depends upn the understry tlerance levels f wdy plant species and determines their ability t survive and develp under light-limited cnditins.. It is this difference in understry tlerance amng species which determines the cmpsitinal and structural characteristics f frests in the suthern Appalachians. The evergreen shrub Rhddendrn maximum L. is an imprtant cppnent f mesic frest sites in the central and suthern Appalachians. This species has been recgnized fr its influence n micrrjjmaric cnditins (Rmancer, 1971) and understry cmpsitin (Clintn and thers, 1994; Mnk and thers, 1985; Phillips and Murdy, 1985). Even thugh the mechanisms (eg., cmpetitin fr water and nutrients, allelpathy, litter depth and quality, light extinctin) f its influence n abve and beiwgrund cnditins are nt cmpletely understd, there is little dubt that this species impacts the light envirnment beneath it..the purpse f this paper is t present data n the tempral variatin in the light envirnments f pen versus Rhddendrn ccupied understries in mesic suthern Appalachian frests. 'Research Eclgist. USD A Frest Service, Sutheastern Frest Experiment Statin; Cweeta Hydrlgic Labratry, 999 Cweeta Lab Rad. Ott, NC 2S763 10th Cantral Hardwd Frest Cnference 534

2 METHODS 5 cudy Design Sampling was cnducted in mature stands f mixed-hardwds at the Cweeta Hydrlgic Labratry lcated in the Suthern Appalachian physigraphic prvince f western Nrth Carlina. Basal area f these stands is appprximatcly 25 tn' : ha' 1 mst f which is accunted fr by Quercus prinus, Q. cccinea, and Acer rubrum. The basal area f Rhddendrn is apprximately 2 m" 2 ha" 1. All plts are mid-elevadnal ( m) and generally have a nrtherly aspecc. Phtsyntheticaily active radiatin (PAR) was measuredn sunny days between che hurs f 1030 and 1330 in pen understries and in understries cntaining cntinuus Rhddendrn sub-canpies. Measurements were made n each f 8 plts (6 nn-rhddendrn and 2 Rhddendrn plts). Plts were apprximately 300 m : each cntaining a permanently marked transect. Alng each transect were permanendy marked measurement pints apprximately 1 m apart. Using a prtable light meter (Sunfleck Ceptmeter, Decagn Devices, Pullman, WA, USA), fur measurements f PAR ( nm) taken apprximately 1 m abve the frest flr were averaged and recrded ac each pint. Measures f full-sun were taken befre and after each set f measurements and iveraged. Measurements were made at apprximate 3-week intervals beginning early in the grwing seasn f 1993 and cntinuing int the spring f Data Analysis Cmparisns were made fr PAR between the tw understry cnditins using PROC ANOVA (SAS, 1987). Significant differences were evaluated a: the 0.05 level.. RESULTS Variability fr a given measurement perid widiin each understry cnditin was lw. particularly wichin Rhddendrn understries (Fig. 1). Hwever, cnsiderable variabilicy exists between the tw understry cnditins (Table I). Fr each measurement date, ttal PAR was signiftcandy higher in the nn-rhddendrn underscry (Table 1; Fig. 1). Maximum PAR was bserved during the spring befre full leafut when nn-rhddendrn areas had PAR fluxes frm ^m m"v (Table I). Rhddendrn areas shwed maxima during the same perid, but PAR was cnsiderably less ( t. m'v) (Table 1). Lighc levels were mst similar, bth in terms f PAR and percent full sun during the mn&s f September and Nvember (Table 1: Fig. 1 and 2), and the highest degree f variability ccurred during the mndi f July as evidenced by the high CV. fr thse dates (Table 1). Over the entire sampling perid, PAR in Rhddendrn areas ranged frm 12% - 33% f bserved PAR in nn-rhddendrn areas. Thrughut the grwing seasn, light levels in nn-rhddendrn areas were bserved t be <10% full sun. (Fig. 2). F'irtbenare, levels under Rhddendrn sub-canpies were less than 2% f full sun (Fig. 2). PAR levels early in che grwing seasn (Hg. 1) may, be sufficient fr a limited number f species t germinate during that perid th Central Hardwd Frest Cnference

3 Table 1. Means and assciated test statistics fr PAR in rhddendrn versus na-rhddendra understries by measurement date. Values in parentheses represent the fracua f fuu-sun. PAR(amls/iaVs) Date nn-rhde Rhd ' F " P>F C.V. Apr 16 Apr (C393) (0.440) (0.131) (0.148) Jun (0.052) 12.2 (O.CS07) J6 Jun (0.043) 14.0 (0.011) O.C M (0.060) 13.9 (0.011) Jul (0.076) 23.4 (0.016) Aug 16 Sepl 39.8 (0.031) 37.7 (0.027) 10.8 (0.007) i.i (0.007) Nv (0324) 22.2 (0.028) Mar (0392) (0.103) Apr (0.496) (0.133) Apr (O39) (0.103) May (0.220) 5J.2 (0.043) > lotfa Central Hardwd Frest Cnference 536

4 1000 s - c nn rhddendrn ' rhddendrn a a a; a. CM Q_ CM CS CM 0_. Ul > </) v a; 0.. < >- >- < Measurement Data Figure 1. Measured phtsynthedcaily active radiatin (PAR) in Rhddendrn versus nn-rhddendrn understries. Errr bars represenc +/- 1 standard errr th Central Hard~wd Frest Cnference

5 nn rhddendrn rhddendrn c 3 1/ Oi OS Z3 0. 0_ -» < < O 0. ui > O a_ >- 2 CM 2 Maasuramanf ' Figure 2. Percent full sun in Rhddendrn versus nn-rhddendrn underscries. Errr bars represent +/- 1 scandard errr. DISCUSSION The light envirnmenc beneath Rhddendrn sub-canpies was bserved t- be significantly different frm; an- Rhddendrn underscries thrughut the sampling perid. These differences may accunt fr the general lack f hardwd regeneratin in areas ccupied by this species. The light cmpensatin pke fr mst wdy species is usually abut 2% full-sun (Salisbury and Rss. 1978) which was the level bserved fr Rhddendrn areas during mst f the grwing seasn in this study. This result suggests chat mst wdy species wuld simply be unable c becme established in such light-limited envirnments. A few exremely shade tlerant, species (e.g, Acer sacchanm Marsh, Tsuga caisadensis [L-] Carr.) have limited success in regenerating under Rhddendrn (Clintn and chers, 1994); hwever, they are nt imprtant cmpnents f mid-devatin suthern Appalachian verstdes (Day and thers. 1988)., t Phillips and Murdy (198.5) fund chat cree regeneratin a permanent plts at Cweeca was lwer in areas cntaining heavy Rhddendrn and chat che difference in regeneratin between Rhddendrn and nn-rhddendrn areas 10th Central Hardwd Frest Cnference 538

6 increased with time. Clintn et ai. (1994) bserved hardwd seedling density t be significantly less in Rhddendrn versus nn-rhddendrn areas (OJ vs. 2.1 stems ra'! ). In a germinatin experiment. Clintn and Vse (unpublished data) fund that nly 3% f planted Acer rubrum L. seed germinated under Rhddendrn cmpared with 15% in nn-rhddendrn areas. Althugh the lw lighc levels bserved under Rhddendrn in this study may accunt fr these differences, ther surces f inhibitin may als exist. Fr example, during the brief perid in early spring preceding verstry fliar develpment, PAR may be adequate, but temperature requirements fraerminatin germinatin f imrtanc imprtanc aksnecies species may mav nt nnf h«be sarisfiwl satisfied. " -. " -- - SUMMARY Rhddendrn maximum is an imprtant structural and functinal cmpnent f suthern Appalachian frests. The eclgical implicatins assciated with lw light envirnments beneath Rhddendrn are substantial. Its influence n tree replacement culd impact verstry cmpsitin (Clintn and thers. 1994; Phillips and Murdy, 1985), in additin t ptential impacts n nutrient cyclinafprcesses. Mst verstry species in the suthern Appalachians are intlerant t imtermediately tlerant t shade. Where lss f the verstry thrugh natural mrtality ccurs in areas ccupied by Rhddendrn, what is the prbability that the re-mrtality cmpsitin f the verstry will be replaced? Given the apparent influence f this species n tree regeneratin either due t lw light r ther phenmena, and the recent bserved increase in verstry mrtality in the suthern Appalachians (Clintn and diers, 1993: Starkey and thers, 1989; Tainter and thers, 1984), it is nt likely that verstry cmpsitin will be maintained in areas ccupied by Rhddendrn. LITERATURE CITED "" ' Clintn, B JX Bring, L.R,, and W.T. Swank Canpy gap characteristics and drught influences in ak frest f the Cweeta basin. Eclgy,.74(5):i55M558. Clintn, 3D., Bring, LJL, and W.T. Swank Regeneradn patterns in canpy gaps f mixed-ak frests f the suthern Appalachians: Influences f tpgraphic psitin and evergreen understry. Am. Mid. Nat. 132(2):308-32Q.. Day, F.P., Phillips, DJL, and CD. Mnk Frest cmmunities and patterns, pp In W.T. Swank and D.A. Crssley, eds. Fresc hydrlgy and eclgy at Cweeta. Springer-Veriag; New Yrk. Mnk, CD., McGinty, D.T., and FJ?. Day The eclgical imprtance f Rhddendrn maximum and Kalmia tatifliain the deciduus frest f the suthern Appalachians. Bull. Trrey Bt, Cub, 112:187*193. Phillips, DJL. and QJ. Murdy Effece f rhddendrn (Rhddendrn maximum) n regeneratin f suthern Appalachian1iardwds»Fr. Sci. 31: $ '. Pukkala, "H, Kuuluvainen, T., and P. Steaberg Belw-canpy distributin: f phtsynthetically active radiatin and its relatin t seedling grwth in a brealpinussytveszris stand. Scand.J. Fr. Res, 8: I f i Rmancier, R, Eclgy f seedling establishment f Rhddendrn maximum la the suthern Appalachians. PhD. Dissertatin, Duke University, Durham, NC 122 p. Salisbury, FJB. and CW. Rss; Phcsynchesisr envirnmetuai and agricultural aspects, pp In Plant Physilgy. Wadswnh Pub. C, Belmnt, CA, USA. 422 pp. SAS Institute, Inc SAS user's guide: statistics editin. SAS Institute, Inc-Cary, NC USA th Central Hard-wd Frest Cnference

7 - -, -.--»y-~*' 'V-. :,_i _;-r- * K. Starkey, D.A., Oak, S.W.. Ryan, G.W., Tainter. F L, Redmnd, C, and HD. Brwn Evaluatin f ak decline areas in the suth. USDA Fr. Serv.. Frest Prtectin Reprt R8-TR 17. Taiater, F-EL, Fraedrich, S.W., and D J!vd. Bensn The effect: f climate n grwth, dech'ne, and death f nrthern red aks in die western Nrth Carlina Nantahala muntains. Castanea, 49: ;-,.--( " if Vitusek. P.M.. Gsz. J.R., Grier, C.C., Melill, J.M., and W A. Reiners. A cmparative analysis f tendal nitrificatin and nitrate mbility in frest ecsystems. Ecl. Mn. 52: Wayne, P.M., and FA. Bazzaz Birch seedling respnses t daily dme curses f light in experimental frest gaps and shadehuses. Eclgy 74(5): !*S Clintn, Bartn D Teitpral variatin in:phtsynthetically active radiatin (PM) in mesic Suthern Appalachian hardwd frests with and withut Rhddendrn xmderstries. In: Gttschalk, Kurt W.; Fsbrke, Sandra L.C., eds. Prceedings, 10 Central Hardwd Frest Cnference; 1995 March 5-8; Mrgantwn, W. Gen. Tech. Rep. NE-197. Randr, PA: U.S. Department f Agriculture, Frest Service, Nrtheastern Frest Experiment Statin m- i Sir *! " ' ^»" - " " 10th Central Hardwxxl Frest Cnference 540