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-1 Tree establishment and growth using forage ground covers in an alley-cropped system in Midwestern USA Kathleen Delate1,*, Eric Holzmueller2, DeAnn Davis Frederick1, Carl Mize3and Charles Brummer1 1Department of Agronomy, University of Florida, Gainesville, FL 32611, USA;2School of Forest Resources and Conservation, University of Florida, Gainesville, FL 32611, USA; 3Department of Natural Resource Ecology and Management, Iowa State University, Ames, IA 50011, USA; *Author for correspondence (e-mail: ; phone: 515 294-7069; Fax: 515 294-0730) Received 27 May 2003; accepted in revised form 21 October 2004 Key words: Black walnut, Fescue, Oak, Poplar, Red clover, Silver maple Abstract The integration of forage crops in an alley-cropped system was examined as a method of encouraging tree planting to increase farm income, improve soil quality, and enhance biodiversity on Midwestern U.S. farms. Crop and tree performance were evaluated in an alley-cropped system using four forage intercrops grown in tree alleyways to simulate a potential hay crop oat (Avena sativa L.) and red clover (Trifolium pratense L.); oat, red clover, and red fescue (Festuca rubra L.); oat, red clover, and orchardgrass (Dactylis glomerata L.); and oat and hairy vetch (Vicia villosa Roth) compared to a herbicide, mowing and control (no management) treatment. Five tree species, divided into fast-growing hardwoods of two poplar (Populus spp.) clones Crandon (P. alba L. P. grandidentata Michx.) and Eugenei (P. deltoids Bartr. P. nigra L.), and silver maple (Acer saccharinum L.) were compared with two high-value, slow-growing hardwoods planted from seed and as seedlings: red oak (Quercus rubra L.) and black walnut (Juglans nigra L.). Tree survival in the fi rst year was greater for the fast-growing species, and by the fourth year, tree height among the four forage treatments was equivalent for all upland locations. The oat/hairy vetch ground cover was associated with the shortest trees in the bottomland site. Herbicide-treated trees were taller than the average of the four forage treatments for all four planting conditions. Tree height in the mowing and the control treatments was not signifi cantly diff erent for all four planting conditions. The nutritive value of the forage crop was excellent in the second year of tree establishment, with crude protein content and digestibility at 17 and 71%, respectively, in the oat/red clover/red fescue treatment, suggesting the viability of forage crops as alternatives to herbicides in alley-cropped systems. Introduction The demand for continuous production of com- modity crops has led to depauperate native and introduced tree populations on Midwestern U.S. farms (Van Der Linden and Farrar 1993). The integration of trees with agronomic crops has been actively promotedthroughouttheworldto increase biodiversity, to optimize production and resource conservation, and to improve wildlife habitat (USDA 2004). Alley-cropping of trees within crop fi elds or pasture systems has been shown to be an economical method of re-integrating trees into thefarmlandscape(Nairetal.1995).Soil Agroforestry Systems (2005) 65:4352? Springer 2005 DOI 10.1007/s10457-004-5228-x improvement, enhanced crop yields, and increased farm income have been reported from alley-crop- ped systems (Gordon and Williams 1991). Less competitive, forage-based agroforestry systems havebeenshown tobemorecompatible thangrain- crop-based alley-cropping systems, due to the shading from mature tree species having a greater impactongraincropscomparedtoforages(Garrett and Kurtz 1983; Gillespie 1996). Greater system productivity (increased timber production, higher crop yields and/or livestock growth) has been ob- tained in various silvopastoral systems, including cattle grazing under loblolly pine (Pinus taeda L.) (Harwell and Dangerfi eld 1991), subterranean clo- ver (Trifolium subterraneum L.) intercropped with Douglas-fi r (Pseudotsuga menziesii L.) (Sharrow et al. 1996), rhizoma-peanut (Arachis glabrata L.) under pine (Pinus spp.) (Johnson et al. 1994), tall fescue (Festuca arundinacea L.) and orchardgrass (Dactylis glomerata L.) under mature (35-yr-old) black walnut (Garrett and Kurtz 1983), and red clover with black walnut and honeylocust trees (Gleditsia triacanthos L.) (Alley et al. 1999). During the critical period of tree establishment, competition from weeds or living ground covers for nutrients, light, and water can severely hinder tree survival and growth. Traditional methods, such as herbicide applications, have proven eff ec- tive in controlling weed populations (Cogliastro et al. 1990), but herbicides are not universally aff ordable, may cause phytotoxicity in cultivated plantings, and have limited eff ects on maintaining soil quality (Pimentel et al. 1995). Forage yields in silvopastoral systems also depend on suffi cient forage establishment and management, including managing shading eff ects from trees. Frequency of cutting or grazing under trees must be based on the shade tolerance and concomitantgrowthrateofpasturespecies. Genotypic responses of forages to shading have ranged from a negative to equal growth rate compared to non-shaded plants (Thompson 1993). Nitrogen content was shown to increase in le- gumes under shade, while dry matter, leaf area and stem length increased in shade-grown grasses (Kephart and Buxton 1993). Overall benefi ts from the integration of forage species in agroforestry systems include facilitation of mutual growth of tree and hay crops, potential revenue from dual crops, and increased carbon storage from grassand legume species. The objectives of this experiment were fourfold: (1) to evaluate the eff ect of four forage mixtures on survival and growth of trees, compared to mow- ing, herbicides, or no weed management in an alley-cropped system in Iowa; (2) to determine the performance of each forage mixture in tree alleys between seven tree types; (3) to evaluate growth and survival of propagative material (tree seeds vs. seedlings) in this system; and (4) to determine the nutritive value of the forage crop harvested from the tree alleys. Materials and methods Study site and design The fi eld research was conducted at the Iowa State University Rhodes Research Farm, near Rhodes, IA. Moderately uniform upland and bottomland sites (0.8 ha each) were selected for planting in the spring of 1998. The predominant soils on the up- land site were Downs soil (fi ne-silty, mixed, mesic Mollic Hapludalf) and Gara (fi ne-loamy, mixed, superactive, mesic Mollic Hapludalf) on the bot- tomland site. Sites were selected on former pasture ground of mediumhigh fertility that was plowed in the spring of 1998 to kill existing vegetation and create a suitable seedbed for the experiment. Seven ground management treatments, made up of four forage mixtures: (i) oat and red clover; (ii) oat, red clover, and red fescue; (iii) oat, red clover, and orchardgrass; and (iv) oat and hairy vetch, and (v) herbicide, (vi) mowing and (vii) a control (no management) were compared. Five tree spe- cies, divided into two groups fast-growing and slow-growing were evaluated in this system. The fi rst group (fast-growing hardwoods) contained silver maple and two poplar clones, Crandon and Eugenei. The second group consisted of two high- value, slow-growing hardwoods planted from seed and as seedlings: red oak and black walnut. The upland and bottomland sites were each divided into six blocks. On each site, three blocks were planted with fast-growing trees, and three blocks were planted with slow-growing trees. The overall design was four randomized complete block experiments with three blocks per experi- ment. Fast-growing tree blocks were established in a split-plot design, with ground management as the whole plot treatments and tree species as the 44 sub-plot treatments. Slow-growing tree blocks were established in a split-split plot design, with ground management as the whole plot treatments, tree species as the sub-plot treatments, and prop- agative material (seed vs. seedling) as the sub- sub-plot treatments. The whole plots for fast-growing trees were 7.3 m 33.5 m, and the tree species were ran- domly assigned to subplots within each whole plot. Tree spacing within plots was 0.9 m 3.9 m. The wholeplotsforslow-growingtreeswere 7.3 m 30.5 m. Tree spacing within plots was ei- ther 0.6 m 3.9 m (seedlings) or 15 cm 3.9 m (seeds). The plots for slow-growing trees were divided into two subplots, and the subplots were randomly assigned to be planted with either seed or seedlings. The subplots were divided into two sub-subplots, which were randomly assigned to be planted to either red oak or black walnut. Whole plots were seeded on 5 May 1998 with a grain drill (oat seed) and a BrillionTMdrill (forage seed). Oat (Ogle), red fescue (Flyer), red clover (Marathon), and orchardgrass (Duke) seeds were planted at 72, 4, 11, and 7 kg ha?1, respec- tively. After seeding the forage treatments, trees were planted in two rows, 3.9 m apart, in all whole plots. Each row of trees was planted 1.7 m from the edge of the whole plot, and the 1.7-m of edge functioned as the unsown buff er area between trees in whole plots or between trees and areas outside the blocks. For each fast-growing species, eight seedlings per row were planted 0.9 m apart. The slow-growing species had nine seedlings per row planted 0.6 m apart and 35 seeds planted 15 cm apart per row. In all, 3528 seedlings and 5900 seeds were planted on 79 May 1998. Slow-growing species were planted with narrow spacing between seedlings and seeds in order to secure adequate numbers of seedlings in each plot to estimate fi rst- year mortality with reasonable precision. The herbicide treatment consisted of a 1.5-m- wide weed-free strip centered on each row of seedlings,createdbyapplying oxyfl uorofen (1.1 kg ha?1) and pendimethalin (2.25 kg ha?1) immediately after planting in the fi rst year, and simazine(2.25 kg ha?1)andpendimethalin (2.25 kg ha?1) applied each spring in the second to fourthyears.Glyphosatewasapplied,when needed, to any weeds in the strip that emerged during the season. The remaining two ground managementtreatmentsconsistedofmowing when weeds were 20 cm in height and the control (no management). In order to reduce weed com- petition and promote forage growth, plots were mowed in July, August, and November 1998, and at approximately the same time during the second through fourth years. Forage samples were taken in the second year of establishment, at which time plots were mowed after sampling events. Cut for- age material was not removed from the plots (ex- cept for samples) due to low yields. On 16 May 1998, an electric fence (three-strand) was erected around the perimeter of each site to prevent browsing by deer and cattle in the area. Tree seedlings were planted at twice the normal density for initial establishment in 1998 and thin- ned to the target population of 50% of initial planting rate in 1999. Seedling trees were thinned on 14 and 15 May 1999 by manually clipping every other tree and applying glyphosate herbicide to tree bases to prevent re-growth. Seedlings from seed were not thinned. Measurements: trees Tree survival was determined, and diameter and height were measured, at the end of the growing seasons in SeptemberOctober of 19982001. Tree height was measured to the closest 5 cm, using a marked PVC pole, with the terminal bud as the primary reference point. Diameter of the trees, except red oak trees from seed, which were gen- erally too short to be measured, was measured at 20 cm above ground level. Measurements: forage yield and nutritive quality Forage biomass accumulation was measured in three randomly selected areas in each of the forage treatments on 23 June, 14 July, and 17 September 1999. Forage samples were taken from tree alleys, representative of areas where hay crops would be mowed. An estimate of forage height was deter- mined by measuring forage plants in the 0.1-m2 quadrat with a hand-held meter stick with the highest terminal bud as the primary reference point. Yield was estimated by hand-clipping for- ages under the 0.1-m2quadrat. Clipped forage samples were placed in paper bags for transport andfurtherlaboratoryanalysis.Foreach 45 treatment within tree type, fi ve sub-samples were taken on 23 June, six on 14 July, and twelve on 17 September 1999. After forage samples were dried at 60 ?C for 5 days, dry matter yield was deter- mined. Samples were then separated into legume orgrasscomponents.Eachcomponentwas ground through a Wiley Mill (Thomas Manufac- turing, Philadelphia, PA) to pass through an 8-mm screen and a Cyclone Mill (UDY Manufacturing, Fort Collins, CO) to pass through a 1-mm screen. The ground samples were stored in plastic jars until analyzed. Crude protein and in vitro digest- ible dry matter (IVDDM) were determined by the micro-Kjeldahl method (Bremner and Breitenbeck 1983) and the two-stage IVDDM method (Marten and Barnes 1980), respectively. Rumen fl uid was collected on the day of inoculation from a fi stu- lated steer that was fed a 100% hay diet. Statistical analysis Tree data were analyzed separately for the four experimental groups: upland slow-growing, up- land fast-growing, bottomland slow-growing, and bottomland fast-growing. Analysis of variance was used to test for interactions between the species and ground treatments and main eff ects (SAS Institute 1989). The arcsin of the percentage sur- vival was analyzed to increase homogeneity of variances (Snedecor and Cochran 1980). Duncans test was used to compare treatment means among species and treatments for the fi rst and fourth yearsresults(SnedecorandCochran1980). Planned contrasts (Snedecor and Cochran 1980) were performed to compare tree height after four growing seasons in the following groups: control vs. mowing; forage treatments vs. control and mowing; forage treatments vs. herbicide treatment; and the four forage treatments (SAS Institute 1992). Results Tree survival At the end of 1998 (Year 1) survival rates for fast- growing trees in the upland site were greater for Eugenei than Crandon and silver maple (Table 1). In the bottomland location, survival rates were also greater for Eugenei than silver maple, and silver maple was greater than Crandon (Table 1). First year survival rates were not aff ected by ground management treatments in the bottomland location (p = 0.57), but on the upland site there was a signifi cant management treatment eff ect (p = 0.002) in which the herbicide treatment was lowest with 69% survival while the other treatment means were not diff erent (averaging 93%). There was no management treatment by species interac- tion for the bottomland site (p = 0.57), but there was an indication of a management treatment by species interaction on the upland site (p = 0.049), which seemed to result from silver maple seedlings having similar survival for all management treat- ments while survival of the other species was more variable among the management treatments. After Table 1. Tree survival and growth of fast-growing tree species in a Midwestern USA alley-cropped system. First yearFourth year EugeneiCrandonSilver mapleSEEugeneiCrandonSilver mapleSE Upland site Survival (%)95aA85b89b1BBBB Height (m)0.27b0.32a0.20c0.013.5b5.1aC0.1 Diameter (cm)BBBB1.0b1.7aC0.05 Bottomland site Survival (%)97a76c90b2BBBB Height (m)0.53a0.39b0.26c0.026.1a5.7b2.6c0.1 Diameter (cm)BBBB2.2a1.9bD0.05 A Treatment means followed by the same letter in the same row in the same year are not statistically diff erent at p 0.05. BData not taken in this year. CSilver maple on upland site was heavily browsed by deer; height and diameter not measured in Year 4. DSilver maple developed many small stems; diameter not measured. 46 four years of growth (Year 4), survival for the fast- growing species was quite high (8097%) on both sites, with no diff erences evident among ground treatments or species (p 0.28). On upland and bottomland sites, combined rates of survival for red oak and black walnut seedlings in Year 1 were similar and greater than establishment of seedlings from seeds of those species (Table 2). On the upland site, black walnut from seed had higher establishment rates than red oak, but on the bottomland site their establish- ment rates were similar (Table 2). Slow-growing tree survival rates did not diff er among manage- ment treatments on the bottomland site (p = 0.38) but there was an indication of a diff erence (p = 0.06) on the upland site, due to variability in the response of the trees to the management treatment. There were no management treatment by species interactions for the upland (p = 0.98) nor the bottomland (p = 0.57) sites. Because the number of surviving red oak seed- lings from seed was so low (1.7% of sown seed in the bottomland and 1.2% on the upland), with many of the plots having no surviving seedlings, this treatment was dropped from the experiment. Thirty-one percent of the black walnut seed that produced a seedling in the fi rst year survived through the fourth growing season, and there were no diff erences in black walnut survival rates among ground treatments on either site (p 0.75). Tree size At the end of Year 1, Crandon height exceeded Eugenei, which in turn was greater than silver maple in the upland location (Table 1). On the bottomland site, Eugenei height was greatest, fol- lowed by Crandon and silver maple (Table 1). On both sites, the ground treatments resulted in modest diff erences among average heights, with no treatment clearly better than the others. On the bottomland site, there was no ground treatment by species interaction (p = 0.97), but the upland site had an interaction (p = 0.049) that seemed to re- sult from the silver maple height being relatively unaff ected by the ground treatments, possibly due