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Genetic diversity of yellow birch seedlings in Michigan1
KENNETH A . WEARSTLER, JR. Sclzool of Forest Resources, North Carolirirr Stote University, Raleigh, NC, U.S.A. 27607
AND
BURTON V. BARNES Scl~ool of Nat~rralReso~rrces, University of Michigrrn, Ann Arbor, MI, U . S . A . 48104
Received April 27, 1976
WEARSI-LER, K . A,, JR., and B. V. BARNES. 1977. Genetic diversity of yellow birch seedlings in Michigan. Can. J. Bot. 55: 2778-2788.
The objective of the study was to determine genetic differentiation of populations of yellow birch (Bet~rla alleghaniensis Britton) along ecological gradients in Michigan and the Appalachian Mountains. Seeds were collected from populations in each of six physiographic regions of Michigan and in the northern and southern Appalachian Mountains. Seedlings were grown in a common garden in southeastern Michigan. First-year height of progeny, seed weight, initial germination. and germination percentage for 141 yellow birch trees in 30 populations were determined and compared.
Significant differences were found among the six Michigan regions for all characters. Strong south-north clinal trends of decreasing height, increasing seed weight, earlier initial germination, and increasing total germination were revealed. Comparisons between physiographic regions showed that tall seedlings were produced by relatively light, late-germinating seeds. Within each region, however, seedlings from earlier germinating seeds tended to exhibit greater height growth. Significant differences were found among populations within a given region for specific characters. In the Appalachian Mountains, southern populations had taller progeny and lighter and poorer germinating seeds than northern populations.
Regressions relating each character to 13 site variables were computed. Variation in total height and initial germination were interpreted as the result of differences in the length of the growing season associated with changes in elevation, terrain type, and aspect. Variation in seed weight was associated with differences in the length ofgrowing season and soil moisture regimes.
WEARSTLER, K . A,, JR., et B. V. BARNES. 1977. Genetic diversity of yellow birch seedlings in Michigan. Can. J. Bot. 55: 2778-2788.
Le but de cette etude Ctait de determiner la diffkrenciation gCnCtique des populations de bouleau jaune (Betula allegllaniensis Britton) le long de gradients Ccologiques au Michigan et dans les Appalaches. Des graines furent recoltkes dans des populations de chacune des six rkgions physiographiques du Michigan, ainsi qu'au nord et au sud des Appalaches. Les plantules furent cultivCes dans un m&me jardin dans le sud-est du Michigan. La taille de premiere annCe des progCnitures, le poids des graines, le dtbut de la germination et le pourcentage de germination furent dCterminCs et comparCs pour 141 arbres provenant de 30 populations.
Des diffkrences significatives ont CtC trouvies entre les six rCgions du Michigan pour tous les caracteres. De fortes tendances clinales de direction sud-nord ont CtC dCmontrCes pour la diminution de la taille, I'augmentation du poids de la graine, la prCcocitC du dCbut de la germina- tion et I'augmentation de la germination totale. Des comparaisons entre regions physiog- raphiques ont rCvCIC que les plantules de taille ClevCe sont produites par des graines relativement ICgkres qui germent tard. Cependant, a I'intCrieur d'une m&me region, les plantules provenant des graines germant tat ont tendance a prCsenter une meilleure croissance en hauteur. A I'intCrieur d'une rCgion donnke, des diffkrences significatives ont CtC trouvCes entre populations pour certains caracteres. Dans les Appalaches, les populations mCridionales ont une progCniture de taille plus ClevCe, ainsi que des graines plus ICgeres et germant moins bien que les populations nordiques.
Les auteursont calcule des rkgressions reliant chaque caractere a 13 variables des sitesetudiis. La variation dans la taille et dans le dCbut de lagermination est interprCtCe comme Ctant le resultat de differences dans la longueur de la saison de croissance, ces diffirences Ctant associCes a des
'Based on a thesis submitted in partial fuIfillment of the requirements f i r the degree of Master of Science, Horace H. Rackham School of Graduate Studies, University of Michigan, Ann Arbor, MI.
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WEARSTLEK A N D B A R N E S
changements d'altitude, de type de terrain et d'exposition. La variation dans le poids des graines est associte a des differences dans la longueur de la saison de croissance et dans les regimes d'humidite du sol.
[Traduit par le journal]
Introduction
Yellow birch (Betula alleghaniensis Britton) is an ecologically and commercially important species of the Eastern North American Deci- duous Forest. It occurs throughout much of the western Great Lakes Region and ranges east through northeastern United States and south- eastern Canada, and from Newfoundland south in the Appalachian Mountains to northern Georgia. Yellow birch is found throughout Michigan as a member of the northern hard- wood; forest type, on upland sites in the Upper Peninsula, and in southern Michigan as a com- ponent of the swamp hardwoods community.
Range-wide studies by Clausen (1968) indicated that variation in height of 2-year-old individuals was random while that of growth cessation was clinal. Genecological studies along latitudinal and elevational gradients in the Appalachian Mountains by Sharik and Barnes (1976) in- dicated genetic difference~ in a strong clinal pattern for growth cessation and leaf flushing. Dancik (1974) and Dancik and Barnes (1974), using multivariate analysis of 13 leaf character- istics from yellow birch populations throughout Michigan, found major phenotypic differences ainong northern and southern populations and among populations growing in different habitats. The variation pattern suggested genetic differ- ences attributed to site conditions.
The purpose of this study was to determine genetic differentiation of yellow birch along ecological gradients in Michigan based on the sampling scheme and the phenotypic studies of Dancik and Barnes (1974). Clinal patterns of phenotypic and genetic variation strongly cor- related with latitude have been repeatedly demonstrated for tree species when conducted over a wide geographic range (Spurr and Barnes 1973). A inajor feature of this experiment was to examine population differentiatibn intensively throughout a limited portion of the species range spanning only 5" of latitude in Michigan.
Methods Six major physiographic regions in Michigan were
identified and seeds were collected in 1970 from three to six populations of five trees in each region (Table 1 , Fig.
1). In addition, seeds from selected yellow birch popula- tions in the Appalachian Mountains, two each from the northern (latitude 44.5" N) and southern (latitude 33.3" N) Appalachians (Sharik and Barnes 1976) were included.
Average seed weight was determined for each of the 147 trees. In February 1973, 10 seeds from each of the trees were placed on 'Jiffy-7' peat pellets on a greenhouse bench at the University of Michigan Matthaei Botanical Gardens. A completely randomized design with 20 replications was used. Continuous light was used for 8 weeks until most of the seeds had germinated. The seedlings were thinned to leave one vigorous seedling and potted in 3-in. (1 in. E 2.54 cm) plastic pots. In June the seedlings were transplanted to nursery beds of the southein Michigan state Forest ~ u r s e r y near Brighton, Michigan (latitude 42.6" N, longitude 83.7" W, elevation 280 m). At transplanting, 4 replications and the progeny of 6 trees were eliminated, leaving 16 replications of 141 trees in the experiment. Snow fencing was used initially to shade the seedlings for 3 weeks, and the plants were watered periodically by the overhead sprinkler system.
Beginning 1 week after sowing, germination data were recorded on a weekly basis for 5 weeks. Total germination (percentage) was computed from these data. Initial germination is the number of days after sowing that germination was first observed. Total height of each seedling was measured June 1, July 9, and at the end of the growing season, September 15. Height measurements were used in correlations with seed weight. The September measurement was used in the analysis of genetic differ- ences in height among regions and among populations within regions.
Analysis of variance was used to determine differences among and within regions. Four dependent variables were used: total height, seed weight, initial germination, and total germination. Pairwise comparisons between populations within a region were based on /-tests. Three stepwise, multiple regression analyses incorporating various site factors were performed for three of the four seedling traits (total height growth, seed weight, and initial germination) to determine which site variables were most closely associated with differences among the traits. Populations were evaluated for the following 13 site variables: (1) latitude; (2) longitude; (3) elevation; (4) slope in percentage from the horizontal; (5) aspect (0 = level; 1 = north; 3 = east; 5 = south; 7 = west); (6) soil type (1 = muck; 3 = clay; 5 = loam; 7 = sandy loam; 9 = sand); (7) general site drainage (1 = un- drained, standing water; 5 = fresh, well drained; 9 = dry, excessively well drained); (8) pH of the A soil horizon; (9) pH of the B soil horizon; (10) general terrain type (1 = swamp; 2 = riverbank; 5 = hollow; 7 = ridge or dry lake bed; 9 = dune); (11) mean annual pre- cipitation; (12) mean annual temperature; and (13) mean annual days below freezing.
The first regression analysis was performed on seedling and seed data from all the Michigan populations for which site information was available. (Site information was not available for populations 2, 3, 4, 5, and 18 (Table
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2780 CAN. J . BOT. VOL. 55, 1977
TABLE 1. Populations by region, number of seed trees representing each population, and population means for height growth and seed characteristics of Michigan and Appalachian Mountain yellow birch
Total height growth, cm* Seed Initial Total Population No. seed weight, germina- germina-
Region No. Population trees June1 July9 Sept.15 mg tion,days tion,%
Southern 5 Lower 4 Michigan 1 (SLM) 3
2 Regional mean
Northern 7 Lower 9 Michigan 6 (NLM) 8
Regional mean Eastern 15
Upper 14 Peninsula -- 13 (EUP) 12
11 10
Regional mean Huron 16
Mountains 17 (HM) 18
Regional mean Superior 23
Upland 19 (SU) 22
20 21
Regional mean Keweenaw 26
Peninsula 24 (Kp) 25
Regional mean Northern 28
Appalachian 27 Mountains
Regional mean Southern 30
Appalachian 29 Mountains
Regional mean
Fox Hills (FH) Liberty Swamp (LS) Walsh Lake (WL) North Lake (NL) Mud Lake (ML)
Bear Lake (BL) Flotte Lake (FL) Platte River (PR) Jordan River (JR)
Little Bay de Noc (LB) St. Ignace (SI) Creighton Lake (CL) Deadman's Lake (DM) McNearney Lake (MN) East Lake (EL)
McCormick Tract (MT) Yellow Dog River (YD) Dead River (DR)
Deadman Lake (DL) Sunset Lake (SL) Mallard Lake (MD) Lake 17 (17) Gibbs City (GC)
Calumet (CA) Point Abbaye (PA) Mohawk (MO)
Centennial Woods Mt. Mansfield
Devil's Courthouse Davidson River
*Populations are ranked within regions on the basis o f total height growth o n September 15. tPopulation significantly different (P < 0.05) from other populations in a region. NOTE: Populations within a region having values followed by the same letter (a , b, o r c) are not significantly different ( P > 0.05).
I).) A second regression analysis was performed on seedling and seed data for populations from southern Lower Michigan, northern Lower Michigan, and the eastern Upper Peninsula, a latitudinal gradient of 4". A third regression was performed for populations in the eastern Upper Peninsula, Huron Mountains, Superior Upland, and the Keweenaw Peninsula, a rather large area exhibiting diverse site conditions over a narrow range of latitude. The computer programs for the analyses are documented in MIDAS, the Michigan Interactive Data Analysis System, developed at the Statistical Research Laboratory, University of Michigan, Ann Arbor.
Results and Discussion
Regional Differences Striking and significant differences (P < 0.01)
were found among the six Michigan regions for total height, seed weight, initial germination, and total germination (Table 2). Similar results were found between the two Appalachian regions except for initial germination (Table 2).
Strong south-north clinal trends of decreasing
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WEARSTLER A N D B A R N E S
TABLE 2. Regional means of height growth and seed characteristics of yellow birch
Total Seed Initial Total height, weight, germination, germination,
Region cm mg days %
Southern Lower Michigan Northern Lower Michigan Eastern Upper Peninsula Keweenaw Peninsula Superior Upland Huron Mountains Southern Appalachian
Mountains Northern Appalachian
Mountains
'Total height of seedlings in September 1973; growth period was 7 months. ?Regions significantly different (P c 0.01) from all other regions. NOTE: Regions having values followed by the same letter (a, b , c, o r d ) are not significantly different (P > 0.01).
FIG. I . Map of locations of Michigan yellow birch populations (numbers refer to populations in Table I).
height, increasing seed weight, earlier initial germination, and increasing total germination were found in Michigan (Table 2). From the Huron Mountain populations westward in the Upper Peninsula, height increased and initial germination was later, whereas seed weight decreased.
Regressions of the six Michigan regional means indicated that tall seedlings tended to be developed from relatively light, late-germinating seeds that exhibited relatively poor germination (Figs. 2a, 2b, and 2c). Late-germinating seedlings
came from light seeds that germinated poorly (Figs. 2d and 2e). Light seeds germinated poorly (Fig. ?f).
0bs&vations of germinating seeds from differ- ent trees indicated a strong positive correlation between seed weight and height immediately after germination. Correlations of seed weight and total height of progeny of each tree were weak and not significant for the June (r = 0.04) and July (r = 0.05) height measurements (Table 1). The early effect of seed weight on height had been overcome. By September there was a significant (P < 0.01) negative correlation (r = - 0.20), indicating that the seedlings were ex- pressing their genetic potential rather than environmental preconditioning.
Seedlings from the colder climate and higher latitude of the Huron Mountains were over 37% shorter than populations from southern Lower Michigan. Seedlings from all populations in the Upper Peninsula were 31% shorter than popula- tions from southern Lower Michigan. The Huron Mountain seeds germinated 8 days earlier, and, on the average, all Upper Peninsula seed germinated 7 days earlier than southern Lower Michigan seeds. There is a strong genetic- ally based response of the seedlings associated with the photoperiodic regime of their origin. Seedlings of trees from the North Temperate Zone cease growth earlier than those native to
L 2
the latitude of a planting site farther south where the growing season is longer (Spurr and Barnes 1973). The northern seedlings are typically re- duced in size corresponding to the shorter growing season of their native habitat.
The performance of seedlings from the six
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C A N . J . ROT. VOL. 5 5 , 1977
a. 42
y --31.9~ + 67.4
$ 34 I
26 1 0.90 130 1.30
Seed Weight, mg
;HM I I
10 14 18
ln\t~al G e r m c ~ t ~ o n . days
FIG. 2. Regressions of total height to seed weight, initial germination, and percentage germination; initial germination to seed weight and percentage germination; and seed weight to percentage gerniina- tion for the six Michigan regions (letters refer to regions in Table l ) .
Michigan regions was examined in relation to length of growing season. Growing season data were calculated by averaging United States Weather Bureau records from all available weather stations in the regions (United States Weather Bureau 1964).
Frost-free season, Region days
Southern Lower Michigan 220 Northern Lower Michigan 209 Eastern Upper Peninsula I87 Huron Mountains 183 Superior Upland 169 Keweenaw Peninsula 193
Generally, seedling height decreased and seed germination was earlier for regions with in- creasingly shorter growing seasons (Fig. 3). The
effect of slight variations in regional climate on the responses of tree populations is indicated by the performance of populations of the Upper Peninsula. Huron Mountain seedlings were shorter and seeds germinated earlier compared with those of the three other Upper Peninsula regions. Westward from the Huron Mountains to the Keweenaw Peninsula there was a general increase in height and later initial germination. Seedlings from the eastern Upper Peninsula populations were taller and seeds germinated later. These responses match relatively well the respective regional climates. The Superior Upland and Huron Mountains have colder climates, the frost-free season increasing east- ward to the Straits of Mackinaw and west- northwest to the Keweenaw Peninsula. Lake Michigan and Lake Superior modify the regional climates, and these modifications may be
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WEARSTLER A N D B A R N E S
NLM
n
Decreasl~g Length ol Frosl Free Seauln-
SLM 18 4 -
Decreasing Length ol Frost Free Season-
k r e a s ~ n g Length of Frosl Free Season-
FIG. 3. Relationship of Michigan regional means of total height, initial germination, and seed weight to the length of frost-free season (letters refer to regions in Table 1).
reflected in the greater height growth of seedlings and later germination of seed sources near the lakes.
The Superior Upland has the coldest and most continental climate of the Upper Peninsula, yet it did not have the shortest seedlings or earliest germinating seeds; instead, the nearby Huron Mountains did. The results of populations from the Huron Mountains may be the exception to the regional trend. Two problems are apparent, limited sampling of populations and lack of sufficient data to adequately characterize re- gional and local climates of the sources. A larger sample (only three populations were sampled in the Huron Mountains compared with five for the Superior Upland) might have better defined the response of seedlings of both regions. Also, the regional climate, which was hard to define precisely because of inadequate weather data, may not be representative of the seed collection
sites in either region. Here, conditions may act to increase or decrease the severity of the local climate.
Nienstaedt and Olson (1961) studied the effects of photoperiod and seed source on seedling growth of eastern hemlock, which has a range similar to that of yellow birch. They found that total seedling height and cessation of growth were strongly related to length of the growing season. Similar to yellow birch, hemlock seed- lings from regions with longer frost-free seasons tended to be taller and grew for a longer period than those from regions with shorter frost-free seasons.
Trends in total height and seed weight similar to those of the Michigan regions were found for seedlings from the two Appalachian regions. Seedlings from the northern Appalachians were shorter and seeds were heavier than those from the southern Appalachians (Table 2).
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2784 CAN. J . BOT. \.
Total germination increased along a south- north gradient and was significantly different among the Michigan and the Appalachian regions. Poor germination was found for both southern Michigan and southern Appalachian populations. Catkins and seeds of these popula- tions were heavily damaged by weevils. The weevil damage was such that it was almost impossible to determine whether seeds were sound and viable before inclusion in the test. Thus, inviable seeds resulting from this damage may have been selected despite efforts to eliminate damaged seed.
Pop~llation DifSerences Witlzin Regions The means of the populations within each
region (Table 1) illustrate the differences among populations for the four characters. Differences in total height in September among populations were significant (P < 0.05) within all regions except the Huron Mountains, the Keweenaw Peninsula, and the southern Appalachians. Seed weight differences among populations were significant within all regions except the eastern Upper Peninsula, the Superior Upland, and the Keweenaw Peninsula. Initial germination differed significantly among populations of all regions. Populations within all regions except northern Lower Michigan and the southern Appalachians were significantly different in total germination.
Considerable variation existed among popula- tions within a given region. The kinds of within- population variation encountered within a region are illustrated for two regions, southern Lower Michigan and the eastern Upper Peninsula. In southern Lower Michigan, Mud Lake seedlings were 20% shorter than the seedlings from Fox Hills (Table 1). Walsh Lake seeds were sig- nificantly heavier than the other populations; seeds from the Fox Hills population were the lightest and differed significantly from all popula- tions except Liberty Swamp. Mud Lake and North Lake seeds germinated at similar times but significantly later than all other popula- tions.
The Walsh Lake, Liberty Swamp, and Fox Hills populations each were significantly different from the other four populations in only one of the four traits. This emphasizes the fact that there was no well-defined trend for population differences within this region for all traits. For example, Fox Hills seedlings were the tallest,
had the lightest seed, and the poorest germina- tion (Table l). However, Fox Hills seedlings did not differ greatly from those of the other populations in initial germination. In contrast, the Mud Lake population had the shortest seedlings and had the latest initial germination, a relationship opposite that of the regional trend of late germination associated with tall seedlings.
Although the Fox Hills population had significantly taller seedlings than the other populations, it had significantly poorer total germination than the others. This indicates that poor germination is not necessarily related to good growth and that total germination, although easily tested, would not be an appro- priate early test in culling slow-growing popula- tions.
The eastern Upper Peninsula contains the most ecologically diverse habitats in the study. East Lake, St. Ignace, and Little Bay de Noc are basic limestone sites; McNearny Lake, Dead- man's Lake, and Creighton Lake are acid sites. Considerable variation was exhibited among all populations for total height, initial germination, and percentage of total germination (Table 1). Seedlings from the East Lake and McNearny Lake sites were the shortest and were not significantly different; they were significantly different from the four other populations. Seed- lings from Little Bay de Noc were the tallest and differed significantly from all other populations; they were 30% taller than those of the shortest population. Seeds from East Lake and Little Bay de Noc germinated significantly later than most other populations.
Considering the association of characteristics, seeds from Little Bay de Noc were lightest, had late and the poorest germination, and produced the tallest seedlings. In contrast, East Lake did not vary significantly from Little Bay de Noc in initial germination yet it had the shortest seed- lings and relatively heavy seeds. Within this region late germination was associated both with fast and slow height growth and light and heavy seeds. These relationships may reflect adapta- tions to local selection pressures; it emphasizes the need for detailed studies of populations within regions related to their specific micro- habitat conditions.
No systematic differences among populations from acid and basic sites were revealed. A larger sample of populations and reciprocal plantings
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WEARSTLER A N D BARNES
, Southern Lower Michigan b, Northern Lower Michigan
iH B.L
14 18 22 8 12 16
Initlal Germination, Initial Germination, days days
C. Eastern Upper Peninsula d. Huron Mountains
L.B
Initial Germi~t ion , days
initial Germination, days
Superior Upland Keweenaw Paninsula
8 12 16 lnitlal Germination.
days
8 12 14 Initial Gormlnation,
days
FIG. 4. Relationship of total height and initial germination for populations in the six Michigan regions (letters refer to populations in Table 1).
on the respective substrates are needed to deter- mine a genetic response to acid and basic sites.
Two general findings emerged from the within-population analyses. First, no population in any of the Michigan regions was significantly different from the other populations within the region in all traits. There were only three populations in the six Michigan regions that differed significantly from the other populations of their region in two traits. Second, although no clearly defined relationships were found within a region, compared with those found among regions, a general trend was found for earlier germinating seedlings to exhibit greater height growth (Fig. 4). This is discernible especially in southern Michigan, eastern Upper Peninsula, Huron Mountain, and the Keweenaw Peninsula (Figs. 4a, 4c, 4c1, and 4 f ) . This within-population
trend is just the opposite of the strong trend of regional means (Table 2) of late-germinating seeds producing tall seedlings and vice versa. The within-region and among-region trends empha- size the importance of the association of initial germination with total height of populations. A detailed analysis of the adaptive significance of this relationship and the relation of site factors to these traits is clearly needed.
Association of Site Factors to Seedling Traits Regression analyses revealed that total height
is most closely associated with latitude (Tables 3 and 4). The ecological factors associated with changes in latitude eliciting genetic adaptation are probably photoperiod and temperature. However, over the narrow range in latitude (1.5") from the eastern Upper Peninsula to the
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C A N . J. BOT. VOL. 55, 1977
TABLE 3. Results of stepwise regressions for 13 site variables on height growth and two seed characteristics of yellow birch from all (21) populations in Michigan*
Step 1 Step 2 Step 3 --
Dependent Site Site Site variable factor R factor R2 factor R
-
Total height Latitude 0.32 Drainage, 0.38 Annual precipitation 0.42
Seed weight Terrain 0.13 pH soil, B 0.20 Initial horizon Mean annual days
germination Terrain 0 .17 Aspect 0.23 below freezing 0 .27
'Site information was not available for populations 2, 3, 4, 5, and I8 (Table I). NOTE: All regressions significant ( P < 0.01).
TABLE 4. Results of stepwise regressions for 13 site variables on height growth and two seed characteristics of yellow birch from 11 populations in southern Lower Michigan,
Northern Lower Michigan, and the eastern Upper Peninsula*
Step 1 Step 2 Site 3
Dependent Site Site Site variable factor RZ factor R factor R
Total height Latitude 0.42 Longitude 0.50 Annual temperature 0.57
Seed weight Terrain 0.21 Soil type 0.30 Initial
germination Terrain 0.21
'Site information was not available for populations 2, 3, 4, and 5 (Table 1). NOTE: All regressions significant ( P < 0.01).
TABLE 5. Results of stepwise regressions for 13 site vari- ables on height growth and two seed characteristics of yellow birch from 16 populations in the eastern Upper Peninsula, Huron Mountains, Superior Upland, and
Keweenaw Peninsula*
Step 1 Step 2
Dependent Site Site variable factor RZ factor RZ
Total height Terrain 0.14 p H soil, B horizon 0 .23
Seed weight Terrain 0.16 Slope 0.21 Initial
germination Elevation 0.04
'Site information was not available for population 18 (Table 1). NOTE: All regressions significant (P < 0.01).
Keweenaw Peninsula, more subtle site character- istics became important, terrain type being the dominant factor (Table 5). There are several specific microsite conditions which could vary with changes in terrain type; going from lowland areas to surrounding uplands: these are soil moisture, soil drainage, soil type, pH of the
various soil horizons, soil temperatures, and average annual air temperature.
Temperature is probably the single most important selection force upon height growth performance along a latitudinal and terrain gradient. For example, the seedlings from the warmer climatic origins (southern Michigan) grew taller because of the longer growing season, whereas seedlings from colder climatic origins ceased growth in southern Michigan much earlier and, as a consequence, were shorter.
Seed weight was shown above to increase with latitude (among the Michigan regions). In addition, terrain type, slope, soil type, and pH of the soil B horizon are related to differences in seed weight (Tables 3-5). In Michigan. soil type and terrain type are probably strongly correlated, since lowland or swampy areas would tend to have poorly drained, organic soils whereas uplands tend to support well-drained, mineral soils.
Differences in seed weight are probably most
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AND B A R N E S 2787
closely associated with an interaction between moisture availability and length of the growing season. Through genetic selection, differences in available moisture and length of the growing season may have affected the variation in seed weight, causing increases in seed weight with decreasing soil moisture availability and shorter growing seasons.,
Virtually every experiment involving the germination and establishment of yellow birch seeds and seedlings has found moisture to be the most significant environmental factor (Godman and Krefting 1960; Linteau 1948; Tubbs 1969; Wang 1965). Under field conditions Wang (1965) reported the growth of yellow birch seedlings t o be good in spite of strong conlpetition froln sedges and grasses, elderberry, raspberry, and sugar maple under favorable soil and moisture conditions. Godman and Krefting (1960) simi- larly reported that if moisture is plentiful during the critical period seedlings will become estab- lished, even with heavy con~petition.
Within-region population differences in seed weight were associated with changes in moisture availability and length of the growing season. F o r example, Flotte Lake, the most northerly of the four populations in northern Lower Michi- gan, occupies an upland type of terrain and has the coldest and shortest growing season.
Seed Mean No. days below O'C weight,
Population (U.S. Weather Birreau 1964) mg
Bear Lake 148 Platte River 152 Jordan River 162 Flotte Lake 198
Flotte Lake had significantly heavier seeds than the three other populations in the region, which tends to support the hypothesis that heavier seeds are produced on relatively dry yellow birch sites where the growing season is short.
St. Ignace and Little Bay de Noc have the lightest seeds of the populations sampled in the eastern Upper Peninsula. Their sites are the lowest in elevation and exhibit the most inoder- ate climates of the region because of the influence of Lake Michigan. Their habitats are similar in slope, aspect, terrain, and soil type (poorly
TABLE 6. Relationship of elevation to initial germination and seed weight for popi~lations in the Superior Upland
Initial Seed Elevation, germination, weight,
Population m days mg
Gibbs City 465 14 0 .92 Deadman Lake 480 1 1 1.04 Sunset Lake 495 1 1 1.20 Lake 17 520 11 1.26 Mallard Lake 520 9 1.14
drained, basic). The relatively greater moisture availability associated with the poorly drained site conditions and the somewhat longer growing seasons may be dominant among the selective forces acting t o produce relatively light seeds for both populations.
The adaptive value of heavier seeds on dry sites or sites with shorter growing seasons or both is apparent. If seeds are heavier because they contain more stored food energy, they would have an advantage over lighter seeds after germination. Baker (1972) studied seed weights of plant species in California and suggested a similar hypothesis of a moisture gradient - seed weight relationship. His data support the fact that there is a marked tendency for increases in seed weight with increasing likelihood of the seedling being exposed to drought or moisture stress.
Terrain type, aspect, elevation, and mean number of days below freezing were the iin- portant site variables associated with variation in initial germination (Tables 3-5). Temperature is a key environmental factor resulting in the genetic variation associated with these four site variables. Within a given region, higher average temperatures would be expected a t low eleva- tions, on south-facing slopes, and in upland areas. The result would be longer growing seasons or more suitable growing conditions in the former environments.
An example illustrates how elevation and aspect may affect variation in initial germination and seed weight. Fo r populations in the Superior Upland, earlier germination ( r . = - 0.81) and heavier seeds (I. = 0.85) are correlated with pro- gressively higher elevation (Table 6). Even small differences in elevation in the Superior Upland, the coldest region in Michigan, may reflect growing conditions different enough t o cause
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significant differences in initial germination and seed weight.
In the Keweenaw Peninsula, aspect was apparently the major site variable distinguishing the Mohawk and Calumet populations.
Initial Seed germination, weight,
Population Aspect days mg
Mohawk Southeast 12 1 .30 Calumet Northwest 10 1.17
The Mohawk population produced heavier seeds that germinated later, possibly because its southerly aspect provides a longer growing season and creates more moisture stress. The argument above was that longer growing seasons yielded lighter seeds. In this case, mois- ture stress could override the effects of growing season on seed weight. The relationship between initial germination and seed weight for these two populations cannot be directly explained. This emphasizes the need of a detailed study of populations and sites to determine local popula- tion differentiation in this region.
Conclusions Total height, seed weight, and initial seed
germination displayed the considerable amount of genetic variation existing among yellow birch populations from a relatively small portion of its range in Michigan. In addition the results revealed clinal rather than random differentiation of height growth of yellow birch seedlings along environmental gradients. Trends in seedling height and seed characteristics of yellow birch from populations in the northern and southern Appalachian Mountains were similar to the Michigan regions.
The amount of genetic variation and the differentiation of yellow birch populations pro- vide two practical considerations for the manage- ment of yellow birch. First, there is a potential for considerable gains to be made from genetic manipulation because of the large amount of
variation among and within populations. Second, the need to define and delineate vellow birch populations by site requirements is a must if full potential of the species is to be realized.
Acknowledgments Funds provided for this study by the McIntire-
Stennis Cooperative Forestry Research Act (P.L. 87-788) are gratefully acknowledged. The authors thank the University of Michigan Matthaei Botanical Gardens for the use of green- house facilities and the Southern Michigan State Nursery for nursery space and assistance.
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SHARIK, T. L., and B. V. BARNES. 1976. Phenology of shoot growth among diverse populations of yellow birch (Berrrla allegl~aniensis) and sweet birch (B . lenta). Can. J . Bot. 54: 2122-2129.
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