Barrier-Induced Microclimate and its Influence on Growth and Yield of Winter Wheat'

E. L. Skidmore:/

Abs t rac t . Wind b a r r i e r s reduce windspeed, modify t h e microcl imate , and a f f e c t p l a n t growth. Vegetat ive growth and d ry matter product ion of winter wheat are usua l ly h ighe r i n t h e area s h e l t e r e d by t h e wind b a r r i e r than i n t h e open f i e l d . Sometimes g r a i n y i e l d is a l s o increased. However, because growth and y i e l d next t o t h e b a r r i e r are reduced and t h e land occupied by t h e b a r r i e r is unavai lable f o r crop production, t h e n e t e f f e c t on g ra in y i e ld is o f t e n negl ig ib le .

INTRODUCTION

She l t e r research i n t h e Great P l a i n s attempts t o p red ic t q u a n t i t a t i v e e f f e c t s of b a r r i e r s on crop y i e l d s , wind eros ion , evapo- r a t ion , and assoc ia ted f a c t o r s . This r e q u i r e s an understanding of s e v e r a l r e l a t i o n s h i p s : 1) the r e l a t i o n s h i p between b a r r i e r and a i r f low must be e s t ab l i shed so t h a t t h e na tu re of t h e leeward a i r f low may be a s soc ia t ed wi th b a r r i e r c h a r a c t e r i s t i c s and c h a r a c t e r i s t i c s of t h e inc i - dent wind; 2) t he r e l a t i o n s h i p between leeward airf low and microclimate a s soc ia t ed wi th b a r r i e r - modified a i r f low must be e luc ida ted ; and 3) the e f f e c t of t h e barr ier- induced microcl imate on p l an t processes (photosynthesis , r e s p i r a t i o n , t r ansp i r a t ion , ce l l d i v i s i o n , growth, etc.) t h a t a f f e c t crop y i e l d s must be determined and re- l a t e d t o c h a r a c t e r i s t i c s of t h e b a r r i e r and wind climatology.

In t h i s paper, I d i scuss a i r f l o w as a f f ec t ed by ba r r i e r and inc ident wind, microcl imate a s influenced by barrier-modified a i r f low, and growth and y i e l d of win ter wheat as inf luenced by barrier-induced microclimate.

Many review a r t i c l e s (Caborn 1957; Fores t ry Committee Great P la ins Agr i cu l tu ra l Council 1966; Guyot 1963; Jensen 1954; Marshal l 1967; Read 1964; Rosenberg 1967; S toeckeler 1962; van der

'1 Paper presented a t t h e symposium: "ShelTerbelts on t h e Great Plains" , Denver, Colorado, A p r i l 20-22, 1976.

21 soil S c i e n t i s t , USDA, ARS, NCR, Manhattan, Kansas 66506.

Linde 1962; van Eimern, Karschon, Razumwa and Robertson 1964), i nc lud ing two (Kreutz 1952; Lupe 1952) c i t e d by Marshal l (1967), have appeared r ecen t ly on wind b a r r i e r s , s h e l t e r b e l t s , and t h e i r in f luence on microclimate and crop y ie lds . Severa l (Fores t ry Corni t tee Great P la ins Agr icu l tura l Council 1966; Read 1964; Rosenberg 1967; Stoeckeler 1962), as w e l l as an ear ly sum- mary by Bates (1911), were wr i t t en f o r d i r e c t appl ica t ion t o a g r i c u l t u r a l problems of the Great P la ins .

BARRIER-MODIFIED AIRFLOW

Permeabi l i ty

Barrier c h a r a c t e r i s t i c s t h a t a f f e c t leeward a i r f low include permeabi l i ty , he ight , shape, width, and r e s i l i e n c e . Of those, permeability (porosi ty o r dens i ty) is most important. Resul ts of many experiments have been presented i n tenns of permeability (Jensen 1954; van Eimern e t a l . 1964).

Windspeed reduct ion pa t t e rns are determined pr imari ly by the po ros i ty and d i s t r ibu t ion of pores i n t h e b a r r i e r . Woodruff e t a l . (1963) measured windspeed-reduction pa t te rns of many s h e l t e r b e l t s and found t h a t they may be e i t h e r too dense o r too porous t o be e f f ec t ive b a r r i e r s . For windbreaks with low p o r o s i t i e s , leeward wind- speed i s minimum near t h e windbreak and, a f t e r reaching minimum, tends t o increase more quickly than do windspeeds leeward of more porous wind- breaks (Harshal l 1967; Skidmore and Hagen 1970a; van Eimern e t a l . 1964; Woodruff, Fryrear and Lyles 1963). Very dense windbreaks s t imula te

57

turbulence (Baltaxe 1967; Marshall 1967; Skid- more and Hagen 1970a; van Eimern e t a l . 1964). A t low permeabi l i t i es t h e a rea of she l t e red ground decreases and a t high permeabi l i t i es t he degree of s h e l t e r provided becomes negl ig ib le .

Optimum permeabi l i ty depends somewhat on Windbreaks de- t h e purpose of t he windbreak.

signed t o d i s t r i b u t e snow may be more porous than those designed t o con t ro l wind eros ion . Windbreaks wi th optimum permeabi l i ty w i l l mark- edly reduce windspeed without inducing s t rong turbulence. Marshall (1967) c i t e d numerous papers before he concluded t h a t optimum protec- t i on f o r vege ta t ion is provided by a b a r r i e r with a geometric permeabi l i ty of 40 t o 50 per- cent.

Height

The d i s t ance a f f ec t ed o r she l t e red by a wind b a r r i e r is increased propor t iona te ly by increas ing t h e b a r r i e r ' s height; thus , he ight of b a r r i e r is important i n consider ing ex ten t of she l t e red area. She l te red d i s t ances are genera l ly expressed as mul t ip les of t h e b a r r i e r height H.

Wind Character is t ics

Wind c h a r a c t e r i s t i c s t h a t a f f e c t a i r f low leeward of a windbreak include: speed, thermal s t a b i l i t y , d i r e c t i o n (angle of inc ident wind), and turbulence l e v e l . To compare t h e wind- reducing e f f e c t s of b a r r i e r s , r e l a t i v e va lues a r e gene ra l ly used, which au tomat ica l ly assumes t h a t windspeed reduct ion is independent of t h e absolu te va lue of t h e open windspeed (van Eimern e t al . 1964).

According t o s e v e r a l pub l i ca t ions (Chepil, Siddoway and Armbrust 1964; Johnson 1965; Skidmore and Woodruff 1968; Zingg 1950), f r e - quency-intensity and d i r e c t i o n of winds vary widely i n the Great P la ins . d i r e c t i o n o r low preponderance i n p reva i l i ng d i r e c t i o n means t h a t a b a r r i e r w i l l no t always be or ien ted noma1 t o t h e wind d i r ec t ion . With wind blowing a t an angle of less than 90 degrees, a b a r r i e r p ro t ec t s a s h o r t e r d i s t ance than when wind blows a t a wider angle. However, even wi th wind blowing p a r a l l e l t o t h e b a r r i e r , wind is reduced up t o 5H behind i t ; van E i m e r n et al . (1964) c i t e d o ther work a s evidence t h a t t h e p ro tec t ive e f f e c t wi th a wind p a r a l l e l with t h e b e l t is about one-fourth of t h a t which is per- pendicular. l e l wind r e s u l t s from t h e i n e v i t a b l e v a r i a t i o n i n wind d i r e c t i o n and t h e f r i c t i o n a t and above the b e l t .

V a r i a b i l i t y of wind

The p r o t e c t i v e e f f e c t wi th a paral-

MICROCLIMATE AS INFLUENCED

BY BARRIER-MODIFIED A I R n O W

Many important microclimate f a c t o r s i n soil-water-plant r e l a t ionsh ips are influenced by a b a r r i e r and t h e assoc ia ted reduced wind- speed.

Radiat ion

Radiation, one of t h e most important f a c t o r s i n crop environment, is only s l i g h t l y a f f ec t ed by a b a r r i e r and is a f fec t ed only i n t h e imme- d i a t e v i c i n i t y of t h e b a r r i e r (Marshall 1967; Rosenberg 1966; Rosenberg 1967; van Eimern e t al. 1964). The b a r r i e r may i n t e r c e p t , r e f l e c t , and r e r a d i a t e some s o l a r o r terrestrial radia- t ion . Depending on t h e b a r r i e r ' s o r i en ta t ion , i t may r e f l e c t s o l a r r a d i a t i o n from one s i d e and shade an area on t h e o ther . However, as Rosen- berg (1967) pointed ou t , long shadows a r e c a s t only when t h e sun is l o w and s o l a r r ad ia t ion is low, so t he e f f e c t may be unimportant.

A i r Temperature

Reduced vertical d i f f u s i o n and mixing of t h e air usua l ly means h igher daytime a i r temperature and lower night t ime a i r temperature (Marshall 1967; Rosenberg 1966a; Rosenberg 1966b; Skidmore, Jacobs and Hagen 1972; Skidmore and Hagen 1970b). However, Woodruff, Read and Chepil (1959) found both h o t t e r and cooler daytime a i r leeward of a b a r r i e r . Leeward a i r temperature pa t t e rns were c lose ly r e l a t ed t o the eddy zone produced by t h e b a r r i e r ; warm zones were loca ted near t h e ground and near t h e b a r r i e r where eddy cu r ren t s were r i s i n g ; and during the day the warm zone extended 5 t o 10H leeward, t h e daytime a i r temperature . being lower than the open a i r beyond 5 t o 10H leeward. Hagen and Skidmore (1971) a l s o ob- served t h a t when mean v e r t i c a l flow was d i r ec t ed up, t h e temperature was h igher , and when mean v e r t i c a l flow was down, t h e daytime a i r tempera- t u r e leeward of t h e b a r r i e r w a s lower than cor- responding open-field temperatures.

Micrometeorological observat ions of Skidmore and Hagen (1970a) showed ambient a i r temperatures over evaporat ing sudangrass a t 2H leeward was higher than a t 6H windward by 0.9, 1.2, and 1.5 degrees C. f o r bo-, 40-, and 0-percent porous b a r r i e r s , respec t ive ly . The temperature tended t o match open-field temperatures a t g rea t e r d i s - tances from the b a r r i e r .

Rosenberg (1967) c i t e d Guyot (1963) s t a t i n g t h a t t h e e f f e c t s of s h e l t e r on air temperature may be predic ted based on whether evapotrane- p i r a t i o n is increased o r decreased. t r a n s p i r a t i o n uses more ava i l ab le energy, less is

When evapo-

58

. -

ava i l ab le t o hea t t h e a i r . Cer ta in ly , i f t h e evaporation rate is decreased with a l a r g e bu t unchanged r ad ia t ion load , temperature of evapo- r a t ing sur face would rise.

A i r Humidity

The humidity regime leeward of a wind b a r r i e r is no t always s t r a i g h t forward. Severa l f ac to r s , l i k e soil moisture , evaporat ion and t r ansp i r a t ion , d i f fus ion and a i r mixing, as w e l l as temperature and r a d i a t i o n inf luence t h e air humidity and complicate t h e condi t ions (van E imern e t al. 1964). Many s t u d i e s showed only s l i g h t va r i a t ion of r e l a t i v e humidity i n she l - tered a reas as compared wi th t h a t of unshe l te red (Marshall 1967; van Eimern et a l . 1964). Rosenberg (1966a) found absolu te humidity con- t e n t of t h e air above sugarbeets no t in f luenced by snow fence and two rows of corn. (1966b) found t h a t absolu te humidity remained cons is ten t ly higher (2 t o 3 mb) i n s h e l t e r e d areas of an i r r i g a t e d bean f i e l d .

But he

Skidmore and Hagen (1970a) found t h a t absolute humidity was s l i g h t l y h igher 2H leeward of a b a r r i e r than i n t h e open. were 1.5, 3.1, and 2.6 mb, r e spec t ive ly , f o r 60-, 40-, and O-percent poros i ty b a r r i e r s . A t 12H leeward the vapor pressure was less than windward by 0.7, 2 . 0 , and 2.5 mb, r e spec t ive ly , f o r 60-, 40-, and O-percent po ros i ty b a r r i e r s .

The d i f f e rences

Soil Temperature

Soil temperature, l i k e s o i l moisture , can be a f f ec t ed by b a r r i e r s i n two ways. F i r s t , increased s o i l moisture from snowmelt leeward from a b a r r i e r lowers soil temperature. The higher water content of t h e s o i l r a i s e s t h e hea t capacity of the soil -- more energy is requi red t o warm it. r a t ion , energy is used i n evaporat ing water t h a t otherwise.would con t r ibu te t o soil heat s torage . Second, as the b a r r i e r modif ies leeward a i r f low,

. heat t r a n s f e r t o and from t h e s o i l is a l t e r e d . Rosenberg (1966b) observed t h a t s o i l temperature i n she l te red a reas was usua l ly elevated dur ing the day and s l i g h t l y depressed a t n ight . Ac- cording t o reviews by Marshall (1967) and van E i m e r n e t al . (1964). most researchers who ob- served s o i l temperature found it was s l i g h t l y higher i n s ide t h e s h e l t e r . Increases were g rea t e s t when t h e s o i l was bare and dry , l e a s t when the s o i l su r f ace was moist o r t h e sky was cloudy.

I f more water causes more evapo-

Carbon Dioxide

The p l an t canopy provides both a source ( r e s p i r a t i o n ) and a s ink ( a s s imi l a t ion ) f o r C02. Respirat ion, a s s imi l a t ion , and d i f fus ion a l l a f f e c t CO2 concentrations. matter, and s o i l r e s p i r e cont inuously, whereas they a s s i m i l a t e only during day l igh t ; then a s s imi l a t ion consumes C02 much f a s t e r than r e s p i r a t i o n produces it (van Eimern et al . 1964). Therefore , a t low windspeeds and under conditions f o r low d i f fus ion r a t e s , C02 concent ra t ion i n t h e crop canopy tends t o inc rease above atmos- phe r i c concentrat ion during t h e n igh t and de- crease below i t during t h e day. Rusch (1955) found t h e unshel tered atmosphere l m above the ground was about 4-percent r i c h e r i n C02 between 10 a.m. and 3 p.m. than a t o the r t i m e s . Any decrease i n C02 content , induced by a b a r r i e r , has not been r e f l e c t e d i n y i e l d and, as Rosen- berg (1966b) observed, C02 quan t i ty unaccompa- n ied by a simultaneous measurement of C02 f l u x can be mis in te rpre ted .

P l a n t s , organic

Evaporation

Evaporation rate i n t e g r a t e s many of t he climatic va r i ab le s modified by a wind b a r r i e r . The high temperature and humidity i n t h e shel- t e r ed reg ion tend t o o f f s e t each o the r i n changing evaporat ive demand (Skidmore and Hagen 1970a; Skidmore and Hagen 1973). Because neg lec t ing barrier-induced changes i n tempera- t u r e and humidity is of small consequence i n r egu la t ing evaporation, t h e inf luence of a wind b a r r i e r on p o t e n t i a l evaporat ion can be approxi- mated wi th an appropriate model by accounting f o r e f f e c t s of reduced windspeed on p o t e n t i a l evapo- r a t i o n .

A cha rac t e r i za t ion of t h e con t r ibu t ion of wind t o p o t e n t i a l evapot ranspi ra t ion f o r a c l imate t y p i c a l of t he Great P l a ins demonstrated t h a t f o r high temperature/low humidity environ- ments a decrease i n windspeed a l s o profoundly decreases evaporation from f r e e l y evaporating su r faces (Skidmore and Hagen 1970a; Skidmore and Hagen 1973; Skidmore, Jacobs and Powers 1969). On representa t ive and consecutive "nonwindy" and "windy" days a t Manhattan, Kansas (average d a i l y windspeeds a t 45 cm were 0.88 and 2.26 m/sec) , a wind-dominant term cont r i - buted 33 and 113 percent, r e spec t ive ly -- a8 much a s a radiation-dominant term -- t o t o t a l ca l cu la t ed p o t e n t i a l evapot ranspi ra t ion (Skidmore e t al . 1969).

Calcu la t ions (Skidmore and Hagen 1970b; Skidmore and Hagen 1973) using c l imato logica l d a t a (May through September, 1960-1969) from two sample loca t ions i n t h e Great Plains showed a 31- and 26-percent average-potent ia l -

59

evaporat ion reduct ion from 0 t o 1OH no r th of east-west o r i en ted b a r r i e r s near Dodge C i ty , Kansas, and Bismark, North Dakota, r e spec t ive ly . When t h e a r e a was extended t o 30 b a r r i e r he igh t s , t h e average evaporation.decreaee was 14 and 7 percent f o r Dodge Ci ty and Bismark, r eepec t ive ly .

GROWTH AND YIELD OF WINTER WHEAT

AS INFZUENCED BY

BARRIER-INDUCED MICROCLIMATE

Expected Benefit

The lowering of evaporat ion demand wi th reduced windspeed i n t h e a rea she l t e red by t h e wind b a r r i e r provides an environment f o r im- proved water r e l a t i o n s . Although wind may no t rank wi th l i g h t i n t e n s i t y , l ea f temperature, l eaf water, and COP concentrat ion i n t h e hierachy of environmental parameters t h a t a f f e c t ne t photosynthes is ( Idso 1968), it is important i n water-stress r e l a t i o n s h i p s (Hagen and Skid- more 1974; Waggoner 1969). p o t e n t i a l evaporat ion with b a r r i e r s , y i e l d s have been increased and water used more e f f i c i e n t l y (Bouchet 1963; Bouchet, D e Parcevaue and Arnox 1963).

By decreasing

The p o s s i b l e y i e l d bene f i t from decreas ing p o t e n t i a l evapot ranspi ra t ion with wind b a r r i e r s was demonstrated by Skidmore (1969), using a hypo the t i ca l example. Using c l imato logica l da ta a t Dodge C i ty , Kansas, h e ca l cu la t ed windspeed-reduction pa t t e rns ; assuming t h a t t u rgo r l o s s and assoc ia ted y i e l d decrease, when p o t e n t i a l evaporat ion w a s above a speci- f i e d l e v e l . similar t o t h a t leeward of b a r r i e r s repor ted by Marshall (1967) and Stoeckeler (1962).

H e generated a r e l a t i v e y i e l d curve

For a n e t y i e l d increase , y i e l d inc rease i n s h e l t e r e d area must more than o f f s e t t h e ab- sence of y i e l d i n t h e area occupied by t h e bar- rier and t h e small a rea of reduced y i e l d t h a t u sua l ly occurs n e a r t h e b a r r i e r . I n a d e t a i l e d experiment wi th sp r ing wheat, McMartin et al. (1974) found a s l i g h t decrease i n t o t a l wheat product ion f o r an ent i re f i e l d . s tudy S t a p l e and Lehane (1955), a l s o tak ing i n t o account t h e area occupied by t h e b a r r i e r , found a modest net increaee i n y i e l d of 47 kglha (0.7 bu/ac) i n one group of she l t e red f i e l d s .

In a 5-year

Vegetat ive Growth

Aaee and Siddoway (1974) inves t iga t ed t h e development of win te r wheat as inf luenced by double rows of t a l l wheatgrass b a r r i e r s epaced 15.2 m (1OH) apa r t . The f i r s t two r m of wheat next t o t h e b a r r i e r s were poor; production

peaked a t about 1 t o 3H from t h e b a r r i e r s . A l l t h e b a r r i e r wheat, except t h e f i r s t two rows next t o each b a r r i e r , grew more vigorously than d id the check wheat. Dry-matter y i e lds were 6,293 and 5,723 kg/ha f o r b a r r i e r and check, respec t ive ly , i n 1971. i n 1972 were 8,545 and 8,243 kg/ha f o r b a r r i e r and check, respec t ive ly . The higher y i e lds i n t h e she l t e red areas were s i g n i f i c a n t a t the 5 percent l e v e l i n 1971 and nons igni f icant i n 1972. The s h e l t e r a l s o enhanced leaf-area development, he ight , and number of head8 per meter row length (Table 1 ) .

The dry-matter y i e l d s

Table 1. Winter wheat grown between t a l l wheatgrass b a r r i e r s and on check near Culbert- eon, Montana compared (after Aase and Siddway, 1974).

1971 1972 Growth f a c t o r Bar r i e r Check Barr ie r Check

Height (cm) 102* 94 109* 106 Heads/rn 99.4* 87.2 129* 109 Leaf a rea index, O . s l / 0.1 - - Leaf a rea index, 2.5- '' 1.5 - - Dry Matter 6,293* 5,723 8,545 8,243

Grain (kslha) 2,770* 2,519 3,545 3,482

28 April

31 May

(kg/ha)

* - 11 S t a t i s t i c s no t given.

Comparisons wi th in yea r s d i f f e r e n t a t 5-percent l e v e l of s ign i f i cance .

In a 3-year (1970-1972) s tudy of winter- wheat response t o barr ier- induced microclimate at subhumid Manhattan, Kansas, Skidmore et al. (1974) found t h a t p l a n t s i n t h e she l te red a rea generally grew t a l l e r , had l a r g e r leaves, and improved water-s t ress r e l a t i o n s h i p s as compared t o those i n open f i e l d . The d i f f e rence i n vege- t a t i v e growth between p l a n t s i n s h e l t e r and open f i e l d was e spec ia l ly pronounced in 1970, with Pawnee v a r i e t y wheat, dur ing a 3-wk period i n May, when warm souther ly winds prevailed. 25 the height t o heads was 11 cm more i n shel- t e red than i n unshe l te red areas. The l ea f area index of t h e f l a g leaves of p l an t s i n s h e l t e r was 43 percent g rea t e r than those i n t h e open f i e l d . I n eubsequent years of t h e s tudy , most wheat v a t i e t i e s responded vege ta t ive ly t o ba r r i e r - induced microclimate, but because the genera l climate was more favorable f o r wheat production, response was less, and hence a l s o benef i t of t he ba r r i e r .

On May

In another s tudy (Skidmore, Hagen and Gwin 1975) i n semiarid western Kansas, winter wheat

.

60

produced more vegeta t ive growth i n t h e she l t e red a rea than i n the unshel tered areas (Table 2) . The b a r r i e r s i n t h e Kansas s tud ie s were slat- fence 1.4 m (8 f t ) t a l l and were i n s t a l l e d a f t e r winter snows had melted.' Therefore, the Kansas loca t ions d id not b e n e f i t from snow catch and d i s t r i b u t i o n across t h e f i e l d as d i d t h e Mon- tana loca t ion (Aase and Siddmay 1974).

Table 2. Growth and y i e l d of winter wheat i n area she l t e red by s la t - fence b a r r i e r compared with t h a t i n open f i e l d , Tribune, Kansas, 8 June 1973 ( a f t e r Skidmore e t al . 1975)

Height t o top of head, cm 95.2 78.6

Height t o base of f l a g 69.4 61.3

19.7 11.4 Area of f l a g l e a f , cm

Functional area of f l a g 15.4 5.6 leaf (green), cm2

D r y matter, 100.9 80.2 g/3O-cm row length

Mean g ra in y i e l d (kglha) 2,381 1,924

l e a f , cm 2

Table 3. Average wheat y i e l d a t ind ica ted pos i t i on from s la t - fence wind b a r r i e r , Manhattan, Kansas. are averages of f i v e varieties and f i v e r e p l i c a t i o n s each (after Skidmore, Hagen, Naylor, and Teare, 1974).

The 1971 and 1972 da ta

Year Ave r age P o s i t i o d l 1970 1971 1972 (1971-1972)

-12.5

-8.0

-4.5

-2.0

2.0

4.5

8.0 12.5

4 g I h - 4150

1440 4180

4300

930 3830 630 4070

1300 4330

4400

1450 3940

- -

-

- 3330

3320

3400

3280

3150

3390

3410

3560

3740

3750

3890

3590

3610

3860

3910

3750

- 11 Distance from b a r r i e r i n b a r r i e r heights . Pos i t i ve and negat ive ind ica t e north and south s i d e s of an east-west b a r r i e r , respec t ive ly . P reva i l i ng wind d i r ec t ion was souther ly .

LITERATURE CITED Grain Yield

Though winter wheat p l an t s in t h e i r vege- t a t i v e growth s t age responded favorably t o bar- rier-induced microclimate, grain-yield response w a s less predictable . A t Manhattan, Kansas, i n 1970 t h e gra in y i e ld was lowest (Table 3) i n a reas where the p l an t s had appeared most vigorous during vegeta t ive development. Af te r ke rne l s were about one-fourth f i l l e d , a w e t per iod s t a r t e d , which impaired head f i l l i n g and w a s conducive t o incidence of d i sease ,

With favorable p rec ip i t a t ion and no t so much hot wind i n 1971, t he b a r r i e r in f luence was minimal, as might be expected (Skidmore, Hagen and Teare 1975). Comparing t reatment means of gra in y i e l d shows t h a t r e l a t i v e posi- t i o n of wheat t o b a r r i e r only s l i g h t l y inf luenced the d i f f e rence i n y i e ld between p l a n t s i n she l - ter and those i n open f i e l d (Table 3). Yield da ta of 1971 i l l u e t r a t e a poesible t rend: lowest-yield pos i t ions were near and f a r from the fence; y i e ld a t intermediate pos i t i ons apparent ly was favorably influenced by t h e b a r r i e r . These r e s u l t s agreed with those of Aase and Siddoway (1974). i n Table 2 f o r Tribune, a l s o show y i e l d response was favorable in t he she l te red area. But if we consider da t a from o the r pos i t i ons i n t h e f i e l d , favorable y i e l d response might be quest ioned (Skidmore et a l . 1975).

The da ta , presented

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