Shade, Leaf Growth and Crown Development of Quercus Ruhra, Quercus Velutina, Prunus Serotina and Acer Rubrum Seedlings
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J,.. TreePhysiology 14,735-749 @ 1994 Heron Publishing-Victoria, Canada Shade, leaf growth and crown development of Quercus ruhra, Quercus velutina, Prunus serotina and Acer rubrum seedlings KURT W. GOTTSCHALK us Department of Agriculture, Forest Service,Northeastern Forest Experiment Station,180 Canfield Street,Morgantown, WV 26505-3101, USA Received July 19, 1993 Summary The study was conducted in an open field to detennine the optimum irradiance for establishmentand growth of two oak speciesand two major associatedwoody species.Half-sib seedlings of black cherry (Prunus serotina Ehrh.), red maple (Acer rubrum L.), northernred oak (Quercusrubra L.) and black oak (Q. velutina Lam.) were grown for two years under shade-clothtents. Eight shadetreatments (94, 70, 57, 45, 37, 27, 20 and 8% of full sunlight) with three replications eachwere used. Measurementswere made on seedlingsharvested at the end of the first and secondgrowing seasons.In the second year, shading significantly decreasedthe number of leaves for all species except black cherry, but only significantly decreasedleaf area in northern red oak. Shading significantly decreasedaverage leaf size of red maple. Average leaf size of black cherry was largest in the intennediate shade treatments and decreased significantly with increased and decreasedshade. Leaf weight/leaf area (mg cm-2) increased signifi- cantly in a quadratic pattern with decreasing shade for all four species. Leaf area ratio (cm2 g-l) decreasedsignificantly with decreasingshade for all speciesexcept red maple in the first year and black oak in the secondyear. Total branch developmentincreased significantly with decreasingshade in red maple and northern red oak, whereasindetenninate branchesincreased significantly with decreasing shade only in black cherry, and short branchesincreased significantly with decreasingshade only in red maple. Keywords: black cherry, black oak, branch development,growth analysis, leaf area, leafarea ratio, leafweight/leaf area, northern red oak, red maple. Introduction Successfulregeneration of mixed oak (Quercus)forests after clear-cutting depends on the numberand size of oak seedlingsand saplingspresent in the understoryat the time of harvest(Sander 1972,1977, Sanderetal. 1976, 1984).Germination and early growth of oak seedlingsare not limited by heavyshade beneath hardwood canopies, but many seedlings fail to survive in deep shade once cotyledonary reservesare exhausted(Tryon and Carvell 1958, Carvell and Tryon 1961). Shelterwoodcutting, or thinning, increasesirradiance below the canopy,enhances the survival of hard- wood seedlings and usually increases height growth, root development and root/shootratios (Jarvis 1964,McGee 1968,Musselman and Gatherum 1969, Loach 1970,Phares 1971). On many mixed oak sites in the Allegheny Plateau region there are few oak seedlingsand saplings present beneaththe canopy.These standsare replaced after harvestby standsof red maple (Acer rubrum L.) or a mixture of red maple and black 736 GO'n'SCHALK chelTY (Prunus serotina Ehrh.). Partial removal of the canopy may promote the advance regeneration of oaks allowing them to compete more successfully with maple and cherry following complete canopy removal (Loftis 1990). The present study was undertakento determine more precisely the relationship betweenirradi- anceand the growth of oak, red maple and black cherry seedlings.The work, which analyzesgrowth as a function of leaf area,extends previous researchin which height and diameter growth, root/shoot ratio, biomass production and partitioning of the sametrees were examined(Gottschalk 1985, 1987).The results indicate the value of shelterwood cutting to promote oak regeneration. and methods A randomized block design of eight shadetreatments with three replications each was used. Twenty-four 2.4 x 2.4 m plots were covered with a 1.8-m-tall tent covered with one of eight gradesof Saranshade-cloth. The light transmittancein the different treatmentswas 94, 70, 57, 45, 37, 27, 20 and 8% of full sunlight. The study areais an abandonedagricultural site on the flood plain of the Allegheny River nearWarren, Pennsylvania.The plots were not watered or fertilized. Although there were minor differences in soil water availability among the shadetreatments, the differences were not significant becauseof the frequent and abundantrainfall during the two growing seasons.Each plot was roto-tilled beforeplanting and a border of aluminum flashing was installed to a depth of 15 cm to preventdamage by rodents.Open-pol- linated seedsfrom one tree eachof northernred oak, black oak and black cherrywere sown in November 1980. Open-pollinated seeds from one red maple tree were collected and sown in June 1981. Each specieswas planted in one row of five with aO.3-m spacingbetween and within rows (1.2 x 1.5 m). Northernred oak, black oak, and black cherry seedgerminated in May and grewunshaded until June 15-17, 1981. Shade-clothtents were then erected,and height, diameter at 2 cm aboveground and leaf number were measured.Red maple seeddid not germinate until mid- to late June, after the tents had been installed. No mortality occurred in black cherry and northern red oak during the fIrst year, but one black oak seedlingand 10 red maple seedlingsdied (no more than two seedlingsfrom anyone treatmentplot). During the secondyear, one black oak seedling and three red maple seedlings died. All four specieswere grown in the sameshade treatment plots, but were analyzedas separate experimentsby speciesbecause of the heterogeneityof variancesamong species. In mid-October 1981, the height and diameter of all seedlingswere measured.In addition, one seedling from each speciesx shade block combination was selected randomly for harvest. These seedlingswere excavatedby hand to recoveras many roots as possible. The harvestedseedlings were partitioned into stem, leaves and roots, dried at 75 °C, and weighed to the nearestmg. The leaveswere photocopied before drying and leaf areas were measured with an integrating planimeter. The shade-clothtents were dismantled in November after leaf fall was complete. The shade-clothtents were erected again in April 1982 before bud break. In late September-early October 1982, the height and diameter of all seedlings were Materials SHADE, LEAF GROWTH AND CROWN DEVELOPMENT 737 measured.By this time, some black cherry seedlingshad reachedthe tops of the tents and the study was terminated. All remainingseedlings (n = 2, 3 or 4) were harvested in the same way as in the fIrst year, except that branches were partitioned into separatecategories and leaf area was measured with a Li-Cor leaf area meter. Branches were classified by growth habit as indeterminate,determinate and short (Gottschalk 1984). Short branchesconsist of only preformed leaves in the bud with little or no internode elongation. Determinate branches consist of only preformed leaves in the bud with normal internode elongation. Indeterminate brancheshave normal internode elongationand consist of preformed leaves in the bud and leaves formed after bud break. Data analysis Leaf numbers,leaf areasand dry weights of the 2-year-old seedlingswere averaged for eachplot and the plot averagesused in the analysis of variance.Average leaf size was calculated by dividing the total leaf area of a seedling by the total number of leaves. Leaf weight per unit leaf area (W, mg cm-1 was calculated by dividing total seedling leaf dry weight by total seedling leaf area. Leaf area per unit seedling weight, or leaf area ratio (LAR, cm2 g-l), was calculated by dividing the total leaf area of a seedling by the total seedlingdry weight. Analysis of variance was based on blocks and shadingtreatments. Orthogonal contrastsbased on single degreesof freedomtested the linear, quadratic,cubic and quartic polynomials for a quantitative relationship betweenshade treatment and crown variables. Results Number of leaves In the secondyear, the shadetreatments significantly affected the number of leaves produced for all species except black cherry (Table 1). Although black cherry exhibited the largest absolutedifferences and a linear increasein leaf number with decreasingshade, none of the differenceswere statistically significant (Figure 1). In Table 1. Analysis of variance and polynomial orthogonal contrasts for leafnwnbe~ of seedlingsof four tree speciesgrown in different shadetreatments for two years. Species Source of R2 df P (F-ratio) P (polynomial orthogonal contrasts) variation Linear Quadratic Cubic Quartic Red maple Block 2 0.108 Shade treatment 0.672 7 0.026 0.006 0.207 0.236 0.990 Northern red oak Block 2 0.009 Shade treatment 0.763 7 0.008 0.010 0.003 0.617 0.020 Black oak Block 2 0.190 Shade treatment 0.637 7 0.039 0.009 0.227 0.018 0.314 Black cherry Block 2 0.045 Shade treatment 0.585 7 0.189 0.036 0.181 0.916 0.237 738 GO1TSCHALK Figure 1. Mean number of leaves (:t standard error of mean)of seedlings of four tree speciesgrown in different shade treatmentsfor two years. Significant orthogonal polynomial contrasts areplotted for the four speciesas follows: A. red maple--linear, B. northernred oak-quadratic, C. black oak-cubic, and D. black cherry-linear. 8% of full sunlight, significantly fewer leaveswere producedby red maple, northern red oak and black oak than in other shadetreatments (Figure 1). With decreasing shade,leaf production in red maple increasedlinearly, whereasin northernred oak and black oak, it increasedquadratically and cubically, respectively(Figure 1). The analysis of variance explained 58 to 76% of the variation