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-1Tree establishment and growth using forage ground covers in an alley-croppedsystem in Midwestern USAKathleen Delate1,*, Eric Holzmueller2, DeAnn Davis Frederick1, Carl Mize3and CharlesBrummer11Department of Agronomy, University of Florida, Gainesville, FL 32611, USA;2School of Forest Resourcesand Conservation, University of Florida, Gainesville, FL 32611, USA;3Department of Natural ResourceEcology 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 2004Key words: Black walnut, Fescue, Oak, Poplar, Red clover, Silver mapleAbstractThe integration of forage crops in an alley-cropped system was examined as a method of encouraging treeplanting 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 intercropsgrown in tree alleyways to simulate a potential hay crop oat (Avena sativa L.) and red clover (Trifoliumpratense L.); oat, red clover, and red fescue (Festuca rubra L.); oat, red clover, and orchardgrass (Dactylisglomerata 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 (Populusspp.) clones Crandon (P. alba L. P. grandidentata Michx.) and Eugenei (P. deltoids Bartr. P. nigraL.), and silver maple (Acer saccharinum L.) were compared with two high-value, slow-growing hardwoodsplanted from seed and as seedlings: red oak (Quercus rubra L.) and black walnut (Juglans nigra L.). Treesurvival in the first year was greater for the fast-growing species, and by the fourth year, tree height amongthe four forage treatments was equivalent for all upland locations. The oat/hairy vetch ground cover wasassociated with the shortest trees in the bottomland site. Herbicide-treated trees were taller than the averageof the four forage treatments for all four planting conditions. Tree height in the mowing and the controltreatments was not significantly dierent for all four planting conditions. The nutritive value of the foragecrop was excellent in the second year of tree establishment, with crude protein content and digestibility at17 and 71%, respectively, in the oat/red clover/red fescue treatment, suggesting the viability of forage cropsas alternatives to herbicides in alley-cropped systems.IntroductionThe demand for continuous production of com-modity crops has led to depauperate native andintroduced tree populations on Midwestern U.S.farms (Van Der Linden and Farrar 1993). Theintegration of trees with agronomic crops has beenactivelypromotedthroughouttheworldtoincreasebiodiversity, to optimize production and resourceconservation, and to improve wildlife habitat(USDA 2004). Alley-cropping of trees within cropfields or pasture systems has been shown to be aneconomical method of re-integrating trees intothe farm landscape (Nair et al. 1995). SoilAgroforestry Systems (2005) 65:4352 C211 Springer 2005DOI 10.1007/s10457-004-5228-ximprovement, enhanced crop yields, and increasedfarm income have been reported from alley-crop-ped systems (Gordon and Williams 1991). Lesscompetitive, forage-based agroforestry systemshavebeenshowntobemorecompatiblethangrain-crop-based alley-cropping systems, due to theshading from mature tree species having a greaterimpactongraincropscomparedtoforages(Garrettand Kurtz 1983; Gillespie 1996). Greater systemproductivity (increased timber production, highercrop yields and/or livestock growth) has been ob-tained in various silvopastoral systems, includingcattle grazing under loblolly pine (Pinus taeda L.)(Harwell and Dangerfield 1991), subterranean clo-ver (Trifolium subterraneum L.) intercropped withDouglas-fir (Pseudotsuga menziesii L.) (Sharrowet al. 1996), rhizoma-peanut (Arachis glabrata L.)under pine (Pinus spp.) (Johnson et al. 1994), tallfescue (Festuca arundinacea L.) and orchardgrass(Dactylis glomerata L.) under mature (35-yr-old)black walnut (Garrett and Kurtz 1983), and redclover 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 coversfor nutrients, light, and water can severely hindertree survival and growth. Traditional methods,such as herbicide applications, have proven eec-tive in controlling weed populations (Cogliastroet al. 1990), but herbicides are not universallyaordable, may cause phytotoxicity in cultivatedplantings, and have limited eects on maintainingsoil quality (Pimentel et al. 1995).Forage yields in silvopastoral systems alsodepend on sucient forage establishment andmanagement, including managing shading eectsfrom trees. Frequency of cutting or grazing undertrees must be based on the shade tolerance andconcomitant growth rate of pasture species.Genotypic responses of forages to shading haveranged from a negative to equal growth ratecompared to non-shaded plants (Thompson 1993).Nitrogen content was shown to increase in le-gumes under shade, while dry matter, leaf area andstem length increased in shade-grown grasses(Kephart and Buxton 1993).Overall benefits from the integration of foragespecies in agroforestry systems include facilitationof mutual growth of tree and hay crops, potentialrevenue from dual crops, and increased carbonstorage from grass and legume species. Theobjectives of this experiment were fourfold: (1) toevaluate the eect of four forage mixtures onsurvival and growth of trees, compared to mow-ing, herbicides, or no weed management in analley-cropped system in Iowa; (2) to determine theperformance of each forage mixture in tree alleysbetween seven tree types; (3) to evaluate growthand survival of propagative material (tree seeds vs.seedlings) in this system; and (4) to determine thenutritive value of the forage crop harvested fromthe tree alleys.Materials and methodsStudy site and designThe field research was conducted at the Iowa StateUniversity Rhodes Research Farm, near Rhodes,IA. Moderately uniform upland and bottomlandsites (0.8 ha each) were selected for planting in thespring of 1998. The predominant soils on the up-land site were Downs soil (fine-silty, mixed, mesicMollic Hapludalf) and Gara (fine-loamy, mixed,superactive, mesic Mollic Hapludalf) on the bot-tomland site. Sites were selected on former pastureground of mediumhigh fertility that was plowedin the spring of 1998 to kill existing vegetation andcreate a suitable seedbed for the experiment.Seven ground management treatments, made upof 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 andslow-growing were evaluated in this system. Thefirst group (fast-growing hardwoods) containedsilver maple and two poplar clones, Crandon andEugenei. The second group consisted of two high-value, slow-growing hardwoods planted from seedand as seedlings: red oak and black walnut.The upland and bottomland sites were eachdivided into six blocks. On each site, three blockswere planted with fast-growing trees, and threeblocks were planted with slow-growing trees. Theoverall design was four randomized completeblock experiments with three blocks per experi-ment. Fast-growing tree blocks were established ina split-plot design, with ground management asthe whole plot treatments and tree species as the44sub-plot treatments. Slow-growing tree blockswere established in a split-split plot design, withground 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 were7.3 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. Thewhole plots for slow-growing trees were7.3 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 weredivided into two subplots, and the subplots wererandomly assigned to be planted with either seedor seedlings. The subplots were divided into twosub-subplots, which were randomly assigned to beplanted to either red oak or black walnut.Whole plots were seeded on 5 May 1998 with agrain drill (oat seed) and a BrillionTMdrill (forageseed). Oat (Ogle), red fescue (Flyer), red clover(Marathon), and orchardgrass (Duke) seedswere planted at 72, 4, 11, and 7 kg haC01, respec-tively. After seeding the forage treatments, treeswere planted in two rows, 3.9 m apart, in all wholeplots. Each row of trees was planted 1.7 m fromthe edge of the whole plot, and the 1.7-m of edgefunctioned as the unsown buer area between treesin whole plots or between trees and areas outsidethe blocks. For each fast-growing species, eightseedlings per row were planted 0.9 m apart. Theslow-growing species had nine seedlings per rowplanted 0.6 m apart and 35 seeds planted 15 cmapart per row. In all, 3528 seedlings and 5900 seedswere planted on 79 May 1998. Slow-growingspecies were planted with narrow spacing betweenseedlings and seeds in order to secure adequatenumbers of seedlings in each plot to estimate first-year mortality with reasonable precision.The herbicide treatment consisted of a 1.5-m-wide weed-free strip centered on each row ofseedlings, created by applying oxyfluorofen(1.1 kg haC01) and pendimethalin (2.25 kg haC01)immediately after planting in the first year, andsimazine (2.25 kg haC01) and pendimethalin(2.25 kg haC01) applied each spring in the second tofourth years. Glyphosate was applied, whenneeded, to any weeds in the strip that emergedduring the season. The remaining two groundmanagement treatments consisted of mowingwhen weeds were 20 cm in height and the control(no management). In order to reduce weed com-petition and promote forage growth, plots weremowed in July, August, and November 1998, andat approximately the same time during the secondthrough fourth years. Forage samples were takenin the second year of establishment, at which timeplots 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 May1998, an electric fence (three-strand) was erectedaround the perimeter of each site to preventbrowsing by deer and cattle in the area.Tree seedlings were planted at twice the normaldensity for initial establishment in 1998 and thin-ned to the target population of 50% of initialplanting rate in 1999. Seedling trees were thinnedon 14 and 15 May 1999 by manually clipping everyother tree and applying glyphosate herbicide totree bases to prevent re-growth. Seedlings fromseed were not thinned.Measurements: treesTree survival was determined, and diameter andheight were measured, at the end of the growingseasons in SeptemberOctober of 19982001. Treeheight was measured to the closest 5 cm, using amarked PVC pole, with the terminal bud as theprimary reference point. Diameter of the trees,except red oak trees from seed, which were gen-erally too short to be measured, was measured at20 cm above ground level.Measurements: forage yield and nutritive qualityForage biomass accumulation was measured inthree randomly selected areas in each of the foragetreatments on 23 June, 14 July, and 17 September1999. Forage samples were taken from tree alleys,representative of areas where hay crops would bemowed. An estimate of forage height was deter-mined by measuring forage plants in the 0.1-m2quadrat with a hand-held meter stick with thehighest terminal bud as the primary referencepoint. Yield was estimated by hand-clipping for-ages under the 0.1-m2quadrat. Clipped foragesamples were placed in paper bags for transportand further laboratory analysis. For each45treatment within tree type, five sub-samples weretaken on 23 June, six on 14 July, and twelve on 17September 1999. After forage samples were driedat 60 C176C for 5 days, dry matter yield was deter-mined. Samples were then separated into legumeor grass components. Each component wasground through a Wiley Mill (Thomas Manufac-turing, Philadelphia, PA) to pass through an 8-mmscreen and a Cyclone Mill (UDY Manufacturing,Fort Collins, CO) to pass through a 1-mm screen.The ground samples were stored in plastic jarsuntil analyzed. Crude protein and in vitro digest-ible dry matter (IVDDM) were determined by themicro-Kjeldahl method (Bremner and Breitenbeck1983) and the two-stage IVDDM method (Martenand Barnes 1980), respectively. Rumen fluid wascollected on the day of inoculation from a fistu-lated steer that was fed a 100% hay diet.Statistical analysisTree data were analyzed separately for the fourexperimental groups: upland slow-growing, up-land fast-growing, bottomland slow-growing, andbottomland fast-growing. Analysis of variance wasused to test for interactions between the speciesand ground treatments and main eects (SASInstitute 1989). The arcsin of the percentage sur-vival was analyzed to increase homogeneity ofvariances (Snedecor and Cochran 1980). Duncanstest was used to compare treatment means amongspecies and treatments for the first and fourthyears results (Snedecor and Cochran 1980).Planned contrasts (Snedecor and Cochran 1980)were performed to compare tree height after fourgrowing seasons in the following groups: controlvs. mowing; forage treatments vs. control andmowing; forage treatments vs. herbicide treatment;and the four forage treatments (SAS Institute1992).ResultsTree survivalAt the end of 1998 (Year 1) survival rates for fast-growing trees in the upland site were greater forEugenei than Crandon and silver maple (Table 1).In the bottomland location, survival rates werealso greater for Eugenei than silver maple, andsilver maple was greater than Crandon (Table 1).First year survival rates were not aected byground management treatments in the bottomlandlocation (p = 0.57), but on the upland site therewas a significant management treatment eect(p = 0.002) in which the herbicide treatment waslowest with 69% survival while the other treatmentmeans were not dierent (averaging 93%). Therewas no management treatment by species interac-tion for the bottomland site (p = 0.57), but therewas an indication of a management treatment byspecies interaction on the upland site (p = 0.049),which seemed to result from silver maple seedlingshaving similar survival for all management treat-ments while survival of the other species was morevariable among the management treatments. AfterTable 1. Tree survival and growth of fast-growing tree species in a Midwestern USA alley-cropped system.First year Fourth yearEugenei Crandon Silver maple SE Eugenei Crandon Silver maple SEUpland siteSurvival (%) 95aA85b 89b 1 BBBBHeight (m) 0.27b 0.32a 0.20c 0.01 3.5b 5.1a C0.1Diameter (cm) BBBB1.0b 1.7a C0.05Bottomland siteSurvival (%) 97a 76c 90b 2 BBBBHeight (m) 0.53a 0.39b 0.26c 0.02 6.1a 5.7b 2.6c 0.1Diameter (cm) BBBB2.2a 1.9b D0.05ATreatment means followed by the same letter in the same row in the same year are not statistically dierent 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.46four years of growth (Year 4), survival for the fast-growing species was quite high (8097%) on bothsites, with no dierences evident among groundtreatments or species (p 0.28).On upland and bottomland sites, combinedrates of survival for red oak and black walnutseedlings in Year 1 were similar and greater thanestablishment of seedlings from seeds of thosespecies (Table 2). On the upland site, black walnutfrom seed had higher establishment rates than redoak, but on the bottomland site their establish-ment rates were similar (Table 2). Slow-growingtree survival rates did not dier among manage-ment treatments on the bottomland site (p = 0.38)but there was an indication of a dierence(p = 0.06) on the upland site, due to variability inthe response of the trees to the managementtreatment. There were no management treatmentby 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 inthe bottomland and 1.2% on the upland), withmany of the plots having no surviving seedlings,this treatment was dropped from the experiment.Thirty-one percent of the black walnut seed thatproduced a seedling in the first year survivedthrough the fourth growing season, and there wereno dierences in black walnut survival ratesamong ground treatments on either site (p 0.75).Tree sizeAt the end of Year 1, Crandon height exceededEugenei, which in turn was greater than silvermaple in the upland location (Table 1). On thebottomland site, Eugenei height was greatest, fol-lowed by Crandon and silver maple (Table 1). Onboth sites, the ground treatments resulted inmodest dierences among average heights, with notreatment clearly better than the others. On thebottomland site, there was no ground treatment byspecies interaction (p = 0.97), but the upland sitehad an interaction (p = 0.049) that seemed to re-sult from the silver maple height being relativelyunaected by the ground treatments, possibly dueto browsing by deer, while the height of the otherspecies varied somewhat among treatments. Be-cause of heavy browsing by deer on the uplandsite, and the resulting minimal increases in heightin the silver maple trees after 4 years of growth,this treatment was dropped from the fourth yearanalyses. Of the other fast-growing species, Cran