Vegetation Changes in a Central Wisconsin Floodplain from Pre-settlement to Mid-21st Century1. James E. .... The river is free flowing from the Dexterville.
Running Head: floodplain vegetation dynamics James E. Cook College of Natural Resources, UW-SP Stevens Point WI 54481 Vegetation Changes in a Central Wisconsin Floodplain from Pre-settlement to Mid-21st Century1 James E. Cook2 University of Wisconsin Stevens Point, Stevens Point WI COOK, JAMES (College of Natural Resources, UWSP, Stevens Point WI 54481). Vegetation Changes in a Central Wisconsin Floodplain from Pre-settlement to Mid-21st Century. J. Torrey Bot. Soc. XXX:000-000. 200X. – The vegetation changes of a central Wisconsin floodplain from mid-1800’s to 2000 were documented, and the probable changes to 2050 projected. This was accomplished by use of survey witness trees in 1851, an extensive survey in 2000, and projection to ~2050 using probable gap replacement frequencies and forest development theory. Between 1851 and 2000 extensive changes in relative abundance of tree species occurred. Silver maple (Acer saccharinum L.) exhibited the largest increase, and river birch (Betula nigra L.) and eastern white pine (Pinus strobus L.) the greatest decline; bur oak (Quercus macropcara Michx.) and yellow birch (Betula alleghaniensis Britton (B. lutea Michx. f.) were noted by the surveyor but not found in 2000. Conversely, two species [hornbeam (Carpinus caroliniana Walt.)and paper birch (Betula nigra L.)]; that are present now were not found among the witness trees in 1851. The amount of open habitat remained approximately constant, but the degree of canopy closure in forests increased dramatically from 1851-2000. The probable causes of these changes were land clearing and harvesting during European settlement, supplemented by the hydrologic changes they induced. Collected data suggest that the current disturbance regime of scattered, small-scale canopy gaps will continue until the mid21st century. If true, it is likely that another major shift in relative abundance of arboreal species will occur and there will be an increase in open habitat. Green ash (Fraxinus pennsylvanica Marsh.) and red maple (Acer rubrum L.) will probably be the dominant tree species by mid-21st century due to their ability to regenerate widely, replace other species and to tolerate growing
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season flooding. In the absence of widespread, severe disturbance (e.g., clearcutting), it does not appear that the arboreal composition of the landscape will return to either 1851 or late 20th century conditions.
Keywords: disturbance regime, autecology, hydrologic regime, succession, Acer saccharinum, Fraxinus pennsylvanica, Acer rubrum 1
Funding for this work was provided by US Fish and Wildlife Service, Green Bay WI.
2
The invaluable assistance provided by Melissa Saeland is acknowledged with heart-felt
appreciation. The staff at Necedah Wildlife Refuge assisted in many ways and their efforts are much appreciated.
2
Floodplains occupy a relatively small percent of the terrestrial surface in the temperate zone, but they perform an extremely wide range of ecological functions (Malanson 1993, Costanza et al. 1997, Rood et al. 2003). Because of the need developed societies have for water (drinking water, power generation, industrial processes, etc.), and the fertility of many floodplain soils, these parts of the landscape have been heavily impacted. There has been extensive work on the effects of dams and channelization on water flows, sediment movement and vegetation immediately adjacent to the river channel, especially in arid areas (Hupp 1992, Lignon et al. 1995, Hughes 1997). Yet, despite their value to society and to the maintenance of biological diversity (Malanson 1993), we are only beginning to understand the full range of vegetation dynamics for entire floodplains. Evidence of the paucity of disturbance/succession studies in floodplain is seen in recent (last 10 yr.) reviews on [primarily] terrestrial disturbance and/or succession (Attiwell 1994, McCook 1994, Reice 1994, Cook 1996, Frelich 2002). All of these were practically devoid of references to bottomland or floodplain systems. Though the effects of harvesting have been investigated in a few areas (e.g., Timoney et al. 1997), the influence of the complete disturbance regime has not been commonly evaluated for floodplain landscapes. One notable exception is the work on primary succession in an Alaskan floodplain (Walker et al. 1986). The importance of this type of information has become greater in recent years, and will increase in the future due to recent and on-going efforts to restore riparian ecosystems (e.g., Ouchley et al. 2000, Rood et al. 2003), the shift in public land management toward landscapelevel management, efforts to prevent further losses of native biodiversity, and identify important differences from other terrestrial landscapes.
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Scattered, local, and short-term studies of vegetation dynamics have been conducted in floodplains of the eastern U.S., but the majority occurred in the South. Critical autecological characteristics of arboreal species have been identified (e.g, Hosner and Minckler 1963, Wilson 1970, Hupp 1992, Bell 1997) and the importance of facilitation to succession commonly noted (Hosner and Minckler 1963, Wilson 1970, Bell 1997, Hughes 1997). The dynamics of an oldgrowth floodplain forest in southern Illinois was driven by gap-phase replacement (Robertson et al. 1978). Following severe (sensu Frelich 2002) disturbance, both major shifts in composition and rapid convergence to pre-disturbance composition have been documented (e.g., Muzika et al. 1987, Kellum et al. 1999). Two studies in the Lake States region evaluated floodplain vegetative composition and abundance from the mid-1800’s [=pre-settlement] to the mid-1990’s (Barnes 1997, Knutson and Klaas 1998). Both studies noted large increases in some species and major declines in others, as well as some structural changes in these floodplain forests. These authors identified harvesting, agriculture and altered hydrologic regime as probable determinants of the changes. Given the biological value of floodplain forests and the limited information on floodplain dynamics in the eastern U.S., more work is clearly needed in a greater variety of floodplain landscapes. Of particular need are empirically based studies of vegetation dynamics over long periods of time, the role of various disturbance types, and the outcome of vegetation dynamics initiated by European settlement. Materials and methods. The study was conducted in the Lower Yellow River watershed (area = 1443 km2 , http://water.usgs.gov/nwis/peak), which drains a part of the Central Sand Plain Natural Division in southwestern Wood and northern Juneau Counties (Fig. 1). It is characterized by near-level topography and soils in the “alluvial land wet” and Algansee-
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Glendora associations; the texture ranges from sandy to silty (Bartline 1977, Gundlach et al. 1991). Stream gradients are very low, with a drop of one meter km-1, or less, in many stretches (Gundlach et al. 1991). The main channel meanders frequently, and oxbow lakes, cut-offs, running sloughs, and small ponds are common. The river is free flowing from the Dexterville Dam, built in 1965, downstream to the dam at the Village of Necedah, a distance of approximately 40 km (Fig. 1). We utilized an approximate 37 km stretch within this reach, avoiding the 1.6 km stretch adjacent to the dams at the north and south ends. Annual precipitation at Necedah is 841 mm, with 70% falling between April and September (http://mcc.sws.uiuc.edu/Precip/WI475786). The median annual flow is 3.34 m3 /s (118 cf3/s) and the median peak flow, 15.12 m3/s (534 cf3 /s), occurs in April (http://water.usgs.gov/nwis/monthly; station #05402000). The median monthly flow is above the annual value from March to June. PROJECT DESIGN. The overstory composition in this floodplain was determined for two points in time (1851, 2000), and then projected to 2050. Due to practical constraints, it was necessary to use three different methods to achieve this. The objective was not to document changes in the composition at any specific location, but rather to make an extensive assessment of composition and abundance throughout the landscape; therefore, though the level of precision and accuracy varies slightly among the methods, any large changes noted can be accepted with a reasonable degree of certainty. DETERMINATION of PRE-SETTLEMENT OVERSTORY COMPOSITION and STRUCTURE. To determine the relative abundances of common overstory species before extensive settlement by Europeans, the Original Public Land Survey records (PLS) were used. In this region, the survey was completed in 1851. Use of PLS data has a long history in ecology research
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(Whitney 1986, Manies and Mladenoff 2000), including the determination of overstory composition and relative abundance in floodplains of the Great Lakes region (Barnes 1997, Knutson and Klaas 1997). This information source has limitations, but is well suited to the determination of relative abundance of species in the overstory over large areas (Whitney 1986, Manies and Mladenoff 2000). Witness trees at quarter-section and section corners near the Yellow River and in the floodplains of its tributaries were located within a database of the surveyors records (obtained from Department of Natural Resources, Madison WI). The corner and quarter-section corners in the appropriate townships (T18-22N, R3E) that include the Yellow River were identified by one of the following criteria: 1. Presence of species with strong floodplain affiliation – swamp white oak (Quercus bicolor Willd.), silver maple (Acer saccharinum L.), and river birch (Betula nigra L.); 2. A suitable “ecosystem” type AND the presence of a wet-mesic species [elm (Ulmus spp.), bitternut hickory {Carya cordiformis (Wangenh.) K. Koch}]. The ‘ecosystem type’ is a general designation assigned by the surveyor for most corners. The ones used as indicators were bottom, pond/slough, swamp, low wet area and river. 3. The presence of two wet-mesic species AND the absence of any strong upland indication (a third species or the ecosystem type). Because a few of the common names of the 1840’s differ from today (Table 1), some judgment calls had to be made. Many times a tree was only identified to genus (e.g., elm or maple), and more common names were occasionally used than there are species in a genus [maple is the best example of this]. Therefore, occasionally a tree was assigned to a species even though its identity was not 100% certain. The presence of other species at that location
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and the ecosystem type were used in this process. When the identity was not reasonably clear, the tree was excluded from the analysis. By compiling the witness tree data from all corners, the overstory composition in the floodplain and the relative abundance of overstory taxa were determined. The surveyors also recorded the distance to each witness tree which makes it possible to derive an approximate estimate of density. To do so, it was assumed that the nearest tree recorded represented the nearest overstory tree. Using this distance and the density formula for the point-center quarter method (Barbour et al. 1980) provided an estimate of overstory tree density. It is acknowledged that this could under-estimate density because the surveyor might have ignored a closer tree for practical reasons (e.g, ease of marking or apparent longevity). However, two lines of evidence indicate that this was not a substantial source of error. The surveyor included witness trees as small as 10 cm, thus he probably did not by-pass many trees. The second line of evidence is the numerical estimate derived is consistent with the general characterization of these section corners by the surveyor (see Results). DETERMINATION of CURRENT COMPOSITION and STRUCTURE. Nineteen transects on 11 properties were sampled in the summer of 2000 (Table 1). The number of transects per property was roughly proportional to area and total transect length inventoried was 4020 m. The areas sampled [properties] were allocated throughout the 37-km stretch of the Yellow River using a stratified random design; we also alternated sides of the river to insure that all conditions in the floodplain were sampled. All transects were located within the same townships as used in the pre-settlement survey. This design provided a random sample that distributed the plots throughout the landscape. Each transect started at the bank of the Yellow River and ran due east or west. The overstory, intermediate and sapling layers were sampled at the following
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Table 1. Average basal areas for overstory and intermediate strata and overstory richness by property in the Yellow River floodplain, central Wisconsin, 2000.
_____________________________________________________________________ Basal Area Transect Property1
Overstory Intermed.
Length [m](#) (m^2/ha)
(m^2/ha)
Overstory Richness
____________________________________________________________________________ Wood Cty
150(1)
9.79
0.84
6
Hazelton
150(1)
10.23
1.95
5
Novacek
200(1)
15.66
2.28
9
Ironside
410(3)
22.02
1.91
13
Novotny
200(1)
14.36
1.75
9
Brandt
950(3)
15.29
1.08
11
Hamel
410(3)
8.65
0.74
10
Gotzion
550(2)
19.77
2.41
10
Rodolfo
100(1)
16.64
6.79
1
Hubka
750(2)
18.06
2.05
11
Haider
150(1)
18.75
0.76
7
Avg., landscape
15.38
2.05
Standard deviation
4.33
1.69
Coefficient variation
0.28
0.82
8.4
____________________________________________________________________ 1
Properties are listed in order north to south
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distances from the starting point: 20, 60, 100 m; and every 50 m thereafter until the edge of the floodplain. We employed a nested plot design at 100 locations with plot sizes of 400, 100 and 2 m2 for the overstory, intermediate layer and saplings, respectively. The vegetation layers were identified by height. Saplings included any woody plants from 1.5 - 3 m tall. The overstory was the upper layer in the forest, and the intermediate was any upright woody stem greater than 3 m tall, but whose crown was not part of the canopy. Our field work in 2000 and 2001 also included canopy gap surveys. We conducted a preliminary inventory in 2000 along three of the vegetation transects, but found no gaps. Ten additional transects were randomly selected and 20-m wide strip transects inventoried for gaps in 2001. When encountered, the gap widths were measured along the longest axis and along an axis perpendicular to the longest. The size was calculated from these two measurements assuming the gap shape was an ellipse (Runkle 1992). We randomly chose 25, 20-m line transects spread out across 10 of the properties to determine the degree of overstory crown closure. On each property we measured one 20-m transect and occularly estimated the other(s) The extent of cover was measured by projecting the crowns vertically and noting the beginning and ending point of each stretch of continuous canopy. By summing these values, the percent canopy cover was determined per property and placed in one of these classes: 0-20, 21-40, 41-60, 61-80 and 81-100% average cover. Climatic data from the late 1800’s to 2000 were obtained online from the National Climate Data Center (www.ncdc.noaa.nws). PROJECTION of COMPOSITION to 2050. Observations of forest structure during the study, conversations with local land managers, age structure of the current forests (approximate age
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65-85 yr, B. Peterson, Necedah National Wildlife Refuge, pers. comm.), abundance of recent gaps, longevity of the common overstory species and examination of 100’s of Wisconsin Dept. Nat. Res. landowner files were used to deduce the probable disturbance regimes since ~ 1930 and for the next 4-5 decades. These data indicated these forests have probably not experienced catastrophic disturbance since ~ 1930, and that the vast majority of harvesting since ~ 1950 was selective. Thus the disturbance regime has been predominantly small scale events. Selective harvests are prescribed for the next several decades and only a couple common species are approaching their natural longevity. At this stage of forest development, the amount of canopy lost to wind and biotic agents should increase gradually as an old-growth stage is approached (Dahir and Lorimer 1996). Thus, it is probable that the disturbance regime of the recent past (~50 yr) will continue in the near future. Furthermore, at the estimated canopy gap formation rate, turnover of the canopy could occur within 50 yr. This corroborates that projection to 2050 is a reasonable time frame for the transitions to play out. To assess the outcome of the vegetation dynamics by 2050, the relative abundance of each arboreal species in the landscape was computed as the percent of locations in which it will be part of the subsequent forest; this was done by the method outlined below. By combining these numbers with the percent of locations in each transition type, the relative importance of each species in 2050 was estimated. In upland forests following creation of a canopy gap, either the gap closes by lateral growth (e.g., Hibbs 1982) or one or more stems from a lower stratum ascend to the canopy (e.g., Runkle 1981, Cho and Boerner 1991, Dahir and Lorimer 1996). The latter mechanism was identified as dominant in a southern Illinois floodplain forest (Robertson et al. 1978). Because seedling growth is usually too slow relative to the rate at which closure occurs from the sides
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(Hibbs 1982, Poulson and Platt 1989), saplings and sub-canopy stems have a much higher likelihood of making it into the canopy (Runkle and Yetter 1987, Poulson and Platt 1989, Cho and Boerner 1991). Thus, in the absence of catastrophic disturbance, the transitions (patch scale) from current forest types in the Yellow River will be determined by the composition of the intermediate layer, where one is present. The stems in this layer average 9.7m tall (Cook and Saeland 2003), which is at least three times taller the saplings. Thus, the transition from the current to the subsequent canopy is relatively predictable based on intermediate composition. The composition of the intermediate layer at each plot location was compared to the overstory composition above it to determine the probable composition of the patch when the current overstory dies. Four general outcomes were identified. The pathways are 1) “self-replacement” when the intermediate successor(s) is/are the same species as the overstory species, 2) “partial self-replacement” when at least one of the species in the overstory is also present in the intermediate layer, 3) “turnover” if the intermediate layer and overstory have totally different species, and 4) “no clear trajectory” when neither an intermediate stem or sapling is present. Results. PRE-SETTLEMENT COMPOSITION and STRUCTURE. The overstory layer included at least 16 species, and the five most abundant taxa were bur oak (Quercus macrocarpa Michx.), river birch, northern red oak (Q rubra L.), eastern white pine (Pinus strobus L.) and northern pin oak (Q. ellipsoidalis E.J. Hill) [Table 2, Fig. 2]. This stratum contained many species – e.g., white pine, yellow birch (B. allegheniensis Britton [B. lutea Michx. F.]), and northern red oak - not commonly associated with floodplains. The forests of the floodplain in 1851 were dominated by “sparse” or “scattered” timber (88% of the locations), as indicated by notes for each corner. The first two witness trees (occasionally three were listed) had average diameters of 30.2 and 25.9 cm, respectively. The density of trees
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Table 2. Relative density (Rel. Dens.) of arboreal species and openings in 1851 as indicated by the witness trees (N=111) recorded in the Original Public Land Survey at the section corners and quarter-section corners in the floodplain of the Yellow River, Juneau and Wood County, Wisconsin.
Surveyor
Current
Names
Common Name
Rel. Dens.
Black oak
N. pin oak1
~ .15
Bur oak
Bur oak
.13
Birch
River birch
.11
Red oak
N. red oak
.10
Maple, white maple Silver maple
~ .10 [.08]2
White pine
E. white Pine
.09
Yellow birch
Yellow birch
.07
White oak
Swamp white oak
.05
Hickory
Bitternut hickory
.04
Elm
Elm [= American, red]
.04
Ash
Green ash
.03
Soft maple
Red maple
Linden
Basswood
.03
White ash
White ash (?)/green
.02
Aspen
Aspen TOTAL for FORESTED CORNERS
~ .02 [.04]2
< .01 ~ .98
12
Open habitat
< .02
_______________________________________________________________________________________________________________
1
Identification based on distribution of black oak this century, and the autecology of black vs.
northern pin oak 2
The species called ‘white maple’ by the surveyor could have been silver or red; I concluded it
was silver because red maple is known regionally as soft maple. The number in brackets indicates what the value would be if ‘white maple’ was another name used for red maple
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greater than 12.5 cm was 41/ha; thus, completely open habitat was uncommon (Table 2, Fig. 2), but few areas had a closed canopy. 2000 COMPOSITION & STRUCTURE. The overstory is currently comprised of 17 species (Table 3). The five most common species, in decreasing order of dominance based on relative density, are silver maple, red maple, green ash (Fraxinus pennsylvanica Marsh.), northern red oak, and northern pin oak (Fig. 2). Two species [hornbeam (Carpinus caroliniana Walt.) and paper birch (B. papyrifera Marsh.) not recorded by the surveyor are now present in the overstory. Sixteen species were found in the intermediate layer, 11 of which are also in the overstory (Table 3.) The four overstory species not found were white pine, paper birch and the two aspens (Populus spp.); one species (Prunus serotina Ehrh.) not recorded in the overstory was found (though rare) in the intermediate layer. The dominant species differed significantly from the overstory with green ash and silver maple switching ranks and the inclusion of bitternut hickory in the top five. Similar to the overstory, nine species contributed at least 5% of the relative density in the landscape. On a per-property basis, the average (+ 1 S.D.) overstory basal area per hectare was 15.4 m2 (+ 4.33), but ranged from 8.65 to 22.02. The average (+ 1 S.D.) basal area of the intermediate layer was 2.18 m2 / ha (+ 1.72) (Table1). These structural data are presented by property to show the variation in the landscape; each property is not necessarily a distinct ecological unit but all properties are separated by at least 2 km. The intermediate layer was three times more variable than the overstory (see coefficient of variation, Table 1), due in large part to the fact that this layer was present in only 72% of the plots surveyed. Overstory richness varied widely ranging from 1-13 per property, with a mean between 8 and 9 species.
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The average overstory diameter was 28.7 cm and the overstory density averaged 192.6 stems/ha. The intermediate layer averaged 213/ha. The floodplain is largely closed canopy forest; 85.4 % of the transects surveyed had more than 60% canopy cover, and over 70% exceeded 80% (Table 4). The pattern of domination by closed forest was relatively consistent across the landscape as the lowest average canopy cover for a property was 68.3% (Table 4). Of the 100 plots sampled, 3% had no overstory present. The canopy gap survey indicated that gaps are quite rare. We believe the sample intensity was too low to provide an accurate estimate of amount of area in recent gaps. Based on the size of gaps found and an estimate of the number per unit area from another floodplain forest of approximately the same age (Almquist et al. 1999), the canopy gap formation rate in the recent past (~ 10 yr) was estimated to be 1-2% per year (Cook and Saeland 2003). This rate is consistent with the mortality rate for the size classes of trees in this landscape (Harcombe 1987). TRANSITION from 2000 to ~ 2050. The following transitions are indicated for the 97 locations that had an overstory: 1. Twelve of 97 (12.4%) will result in self-replacement;
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Table 3. Relative density (Rel. Dens.) within the overstory and intermediate layers in a 37-km portion of the Yellow River floodplain, central Wisconsin. Species are listed in descending order of importance within each stratum. ________________________________________________________________________ Overstory Species
Intermediate Layer Rel. Dens.
Species
Silver maple
.17
Green ash
.17
Red maple
.14
Red maple
.16
Green ash
.10
Bitternut hickory
.13
N. red oak
.09
Silver maple
.10
N. pin oak
.08.
N. pin oak
.09
Populus (2)
.08
Basswood
.09
Swamp white oak
.07
American elm
.07
Basswood
.07
Bitternut hickory
.06
River birch
.06
Swamp white oak
.05
E. white pine
.02
N. red oak
.04
American elm
.02
Shagbark hickory
.02
Shagbark hickory
.02
Black cherry
.02
Jack pine
.01
Red elm
.01
Hornbeam
.01
River birch
.01
Jack pine
.01
Rare species1
< .01
Rel. Dens.__________________________
_________________________________________________________________
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1
Rare species (< .3% relative density) included paper birch, red elm and hornbeam
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2. Thirty-nine of 97 (40.2 %) will transition to a similar ‘patch type’; 3. Twenty-one of 97 (21.6%) will have complete compositional turnover, and 4. No clear pathway is indicated for 25.8% (25 out of 97 locations). The forest structure in this landscape [28% of the locations did not have an intermediate layer and 78% did not have a sapling (Cook and Saeland 2003)], is the reason why the final outcome (#4 above) was common. We interpreted this as no clear successional trend because those patches are probably “arrested” in an herbaceous-dominated stage (sensu Connell and Slatyer 1977, Abrams et al. 1985); alternatively, some arboreal species could invade later in the projection period and ascend to the overstory. The relative abundance of green ash and red maple will probably equal or exceed 13% by 2050 and three species are clustered in a second tier at approximately 8% relative abundance (Table 5). This will represent a substantial increase for green ash over this 50-yr period; in contrast, red maple, basswood and northern pin oak will maintain their current abundance and silver maple will likely experience a sharp decline. Bitternut hickory, the 6th most abundant species, will also stay more-or-less constant and American elm would show a doubling of its abundance if all the current intermediate-
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Table 4. Canopy cover class frequency and average cover by property in a 37-km stretch of the Yellow River floodplain, central Wisconsin, 2000. _____________________________________________________________________________________
Cover Class
Average
Frequency (%)
Property
Canopy Cover
____________________________________________________________________________ 0-20
3.1
Wood Cty.
87.6
21-40
2.0
Hazelton
70.4
41-60
9.4
Novacek
68.3
61-80
14.6
Ironside
90.5
81-100
70.8
Novotny
79.3
Brandt
85.9
Hamel
82.5
Gotzion
79.1
Ridolfo
82.5
Hubka
81.2
_____________________________________________________________________________
Landscape Average =
80.7%
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Table 5. Summary of probable transitions to the overstory from 2000 to approximately 2050 in the Yellow River floodplain, central Wisconsin. The probable abundance of each species is the sum of the three pathways that can lead to a species being present in the overstory (see Methods for details). ____________________________________________________________________________
Partial Sum Species Self-Repl1. Turnover _______________________________________________________________________________ Green ash 2.1 6.2 7.9 16.2 Red maple
0
5.1
7.9
13.0
Basswood
2.1
2.1
4.1
8.3
Silver maple 3.1
0
5.0
8.1
N. pin oak
4.1
3.8
0.1
8.0
Bitternut
0
1.0
5.7
6.7
0
2.1
2.8
4.9
Swamp white 0
0.2
1.4
1.6
0
1.4
1.4
hickory Amer. elm
oak N. red oak
0
TOTAL, ALL SPECIES
68.2 2
___________________________________________________________________________ 1
The pathways by which a species will become part of the future canopy are self-replacement
[Self-Repl], by replacing another species [Turnover], or by being one of a group replacing a group of which it was part [Partial].
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2
68.2 + 25.8 [“open”] = 94%; the remaining 6% of the landscape will be dominated at the patch
level by uncommon species [river birch, black cherry, shagbark hickory, Jack pine, red elm]
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size stems were to live. These projections suggest a decline in overstory richness to 14 (13 if elm is lost due to Dutch elm disease). Discussion. Large changes in arboreal abundance and landscape structure occurred between 1851 and 2000; in addition, a few species may have been locally extirpated. Smaller, but significant, changes in abundance will likely occur in this landscape by approximately 2050, and it is likely that non-forested habitat will increase. Substantial changes such as these could be the result of a change in: land use, the ‘natural’ disturbance regime, climate or hydrology. In this floodplain, land use during the Settlement period [~1870-1930] appears to be the primary determinants of current vegetative composition. These events set in motion changes in species distributions and abundances that are continuing to play out (Fig. 2). These more severe disturbances overshadowed the background disturbance effects of floods, wind and biotically induced canopy gaps. CHANGES - 1851 to 2000. Extensive settlement and development of central Wisconsin by Europeans began ~ 1870 (Curtis 1959, Johnson 1995, Knutson and Klaas 1998). Clearing for agriculture, clearing for homes, selective harvesting, and heavy partial cutting began to decline rapidly after ~ 1920, and toward the end of this period (1930’s), considerable amounts of agricultural acreage were abandoned. The dominant age class throughout the floodplain is 6585 yr, which corroborates the intense disturbance during the Settlement period. Agricultural clearing and heavy cutting would have increased flood frequency, depth, and duration. With greatly reduced amounts of vegetation, there was less precipitation interception, water uptake and transpiration. The combined effect was more run-off, and greater flood depth and/or duration. The increased volume and velocity would result in greater transport and deposition of sediment (Hornbeck and Kochenderfer 2000). These disturbances and the resulting hydrologic
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changes likely favored tree species that are tolerant of flooding, and those that could capitalize on heavily disturbed areas (Hosner and Minckler 1963). Forest succession on abandoned farm land in the Lower and Mid-Mississippi Valley can follow more than one successional pathway (Hosner and Minckler 1963, Battaglia et al. 2002). These studies found that pioneer composition was a function of distance to seed sources, dispersal mode, seed size, tolerance to sediment deposition and soil moisture conditions during seedling establishment. Barnes (1997) and Knutson and Klaas (1998) looked at changes in tree composition and abundance along the Chippewa River (Wisconsin) and the Upper Mississippi River from pre-settlement to the 1990’s. Silver maple increased in both landscapes, while river birch and willow (Mississippi) or willow and cottonwood (Populus deltoides Bartr. ex Marsh.) [Chippewa] declined. Similarly, silver maple increased dramatically and river birch declined in the Yellow River. This consistency across the region is further evidence that the dynamics playing out today were likely set in motion during the Settlement period. Silver maple is not a desirable timber species and thus was not cut extensively during Settlement. It is a typical pioneer species with rapid growth and thus can readily capitalize on heavy disturbances (Hosner and Minckler 1963, Dixon et al. 2002; Appendix A). Its late-spring seed dispersal, extended germination period (J. Cook, unpublished data) and flood tolerance enabled it to find suitable germination conditions during May, June and July, and to survive mid-growing season floods (Appendix A). Its much larger seed may explain why it increased whereas river birch, also a classic floodplain pioneer, declined. The larger seed makes a plant more competitive as a seedling by being able to become larger before uptake of site resources is necessary, and being less in need of bare mineral soil for establishment (Grime and Jeffrey 1965, Dixon et al.
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2002). Its rapid height growth and large seed would be important in abandoned fields and cutover forests because of the substantial amount of residual vegetation. A very large decrease in bur oak [we found none] and white pine between 1851 and 2000 was probably caused by the extensive harvesting around 1900 (Curtis 1959, Johnson 1995, Knutson and Klaas 1998). This greatly reduced seed sources, and thus very few seedlings became established around the turn of the 20th century. The success of white pine may have been further reduced by more growing season floods (see below), and both species were probably at a regeneration disadvantage because of fall seed maturation. The species showing large increases from 1851 to 2000, and from 2000 to ~ 2050, have partial or complete spring seed dispersal. Regeneration in this floodplain seems most affected by seed size, the timing of seed dispersal and flood tolerance in the seedling stage than by the mode of dispersal; it seems unlikely that tolerance of sediment deposition and soil moisture are important factors. Several lines of evidence indicate that the role of disturbance is paramount, but other factors should not be rejected out-of-hand. Climate is a possible cause of vegetation changes over periods of 200 yr, as has been documented in other parts of the Great Lakes region (e.g., Jacobson 1979, Parshall 2002). However, neither average annual temperature nor average annual precipitation showed any upward or downward trend during this period. Thus, climatic change is an unlikely explanation for the widespread changes in arboreal abundance noted between 1851 and 2000. Some of the more common (> 5% relative abundance) species that dominate today [e.g., northern red oak, basswood, red maple] are not ‘floodplain’ species (Loucks 1987, Knutson and Klaas 1998), and thus their success since the late 1800’s cannot be explained by flood tolerance or the hydrologic changes induced by disturbance. The success of this group is
24
probably a function of their shade tolerance (basswood, maple), ability to thrive in partially cut areas, and relatively rapid growth (Crow 1988, Abrams 1998). For basswood and red oak, small topographic differences may also be critical. Less flood-tolerant species can become established on slightly elevated area and thereby avoid regular flooding as a seedling. Trees are most susceptible to flood-induced mortality at the beginning of the growing season and as seedlings (Loucks 1987). In broad, flat and small floodplains like the Yellow River, elevational differences of less than 1 m could easily prevent inundation in most years (Menges 1986, Hall and Harcombe 1998). The prevalence of non-floodplain species in 1851 supports this explanation. In the latter half of the 20th century, two other human-induced disturbances impacted this landscape. Dutch elm disease reached Wisconsin by 1956 (Fig. 2) (www.extension.umn.edu/distribution/naturalresources/components/3756b) and probably contributed to the decline in this species by 2000. In 1965, the dam 1.6 km north of the study area was installed. Because the influence of floods on vegetation is a function of frequency, duration, timing and intensity (Hughes 1997, Cosgriff et al. 1999), flow data for the pre-and postdam periods were compared. Data (http://water.usgs.gov/nwis/peak) for 25 yr pre-dam to 34 years after dam installation indicate a roughly constant trend in average peak flow since the late 1960’s (Fig. 3). Monthly flow volumes indicate that April was the peak discharge period throughout the 56 yr period (Fig. 4), but there was a small increase in growing season flood frequency after dam installation [data not shown]. Thus, there have been minor hydrological changes since 1965, and the only impact was probably through flood-induced stress during the growing season.
25
DYNAMICS - 2000 to ~2050. As described in Methods, the outcome of the dynamics over this period is based on two critical assumptions [see Methods]. It is possible, though highly improbable, that high severity disturbances (fire, or flood) would occur that 1) eliminate the intermediate layer and 2) also allow other species to become established. Fires are very uncommon in wet areas in this region (Whitney 1986), and the flood regime (Fig. 3) and precipitation patterns indicate that this applies to the Yellow River floodplain. The peak fire season in Wisconsin is April-May, which coincides with high flow volumes and above-average precipitation. A 30-yr record of precipitation for the area indicates the average monthly rainfall from April to September exceeds 7.5 cm (http://mcc.sws.uiuc.edu/Precip/WI/4755786_psum). Thus, fires are probably very rare in this landscape. The current intermediate layer has been in place for roughly 10-30 years and thus has endured numerous floods; therefore, widespread mortality from this agent is unlikely. Even if a 1-in-50-yr event were to occur, this would probably not induce widespread mortality because these rare events only translate into a 2-5X increase in flow in low-gradient rivers (Hughes 1997). Therefore, it is probable that gap-phase type disturbances (wind, insect, disease, senescence and selective harvests) will dominate the disturbance regime over the next several decades. Because two species (river birch, aspen) are currently mature and two other common species (red maple and northern pin oak) are near their typical longevity (Appendix A), death by secondary insects, pathogens and the incidence of wind breakage may increase in the near future (Dahir and Lorimer 1995). The anticipated changes will occur gradually but regularly; the arboreal composition and relative abundance will shift from 2000 to 2050, but not in a direction that is any more similar to 1851. Of the five most abundant species in 1851, only silver maple and N. pin oak will likely remain in this group by 2050. The projections suggest both species will do so by maintaining
26
dominance were they are currently located (see Table 5). Furthermore, the number of species with a relative abundance < 1% will probably decline, producing an arboreal component with lower eveness. Green ash and red maple will probably become dominants largely by replacing other species. As noted before, small to medium-sized gaps favor some species (green ash, basswood, bitternut hickory and red maple (Hosner and Minckler 1963, Crow 1990, Kennedy 1990, Abrams 1998) over others. In addition, these two species are able to establish new seedlings over a large portion of the growing season (J. Cook, unpublished data; Appendix A) and by being able to germinate on a variety of substrates (Appendix A). Vegetative structure across the landscape will probably change in at least two ways. The average level of canopy closure in forests will decrease and the amount of truly open patches will increase; these outcomes are anticipated due to common absence of an intermediate layer and low sapling densities. These changes would have important implications for many faunal species, and for potentially for the regeneration of plant species. Similar to the changes caused during settlement, less canopy cover would result in less interception and transpiration, and more run-off, thus favoring more flood tolerant species. However, the magnitude of the hydrologic changes would be smaller and erosion/sedimentation probably would not increase substantially. Thus species like silver maple, green ash and red maple that are flood tolerant but do not need bare mineral soil to germinate, would be favored. Further work is needed to determine if changes beyond 2050 will move the landscape back closer to pre-settlement conditions or cause it to continue to diverge. However, given the conditions that gave rise to the 20th century dominants, it seems unlikely that the vegetation will move back toward that landscape structure in the absence of widespread, severe disturbance.
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ROBERTSON, P. A., G. T. WEAVER and J. A. CAVANAUGH. 1978. Vegetation and tree species patterns near the northern terminus of the southern floodplain forest. Ecol. Monogr. 48:259-267. ROGERS, R. 1990. Swamp white oak (Quercus bicolor). p. 614-617 in R. Burns and B. Honkala (tech. Coord), Silvics of North America, Vol. 2. Hardwoods. USDA For. Serv. Ag. Handbook 654. ROOD, S. B., C. R. GOURLEY, E. M. AMMON, L. G. HEIKI, J. R. KLOTZ, M. L. MORRISON, D. MOSLEY, G. SCOPPETTONE, S. SWANSON and P. L. WAGNER. 2003. Flows for floodplain forests: a successful riparian restoration. BioSci. 53:647-655. RUNKLE, J. R. 1981. Gap regeneration in some old-growth forests of the eastern United States. Ecology 62:1041-1051. RUNKLE, J. R. 1992. Guidleines and sample protocol for sampling forest gaps. USDA For. Serv. PNW, Gen. Tech. Rep. 283. RUNKLE, J. R. and T. C. YETTER. 1987. Treefalls revisited: gap dynamics in the southern Appalachians. Ecology 68:417-424. SCHOPMEYER, C. S. (tech. coord.) 1974. Seeds of woody plants in the U.S. USDA For. Serv., Agric. Hndbk 450. TIMONEY, K. P., G. PETERSON and R. WEIN. 1997. Vegetation development of boreal riparian plant communities after flooding, fire, and logging, Peace River, Canada. For. Ecol. Manage. 93:101-120. TREMBLAY, M. F., Y. MAUFFETTE and Y. BERGERON. 1996. Germination responses of northern populations of red maple (Acer rubrum) populations. For. Sci. 42:154-159.
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WALKER, L. R., J. C. ZASADA and F. S. CHAPIN, III. 1986. The role of life history processes in primary succession on an Alaskan floodplain. Ecology 67:1243-1253. WALTERS, R. S. and H. W. YAWNEY. 1990. Red maple. p. 60-69 in Silvics of North America, Vol. 1. R.M. Burns and B.H. Honkala, eds. U.S.D.A. Agricultural Bulletin 654. WHITNEY, G. G. 1986. Relation of Michigan’s presettlement pine forests to substrate and disturbance history. Ecology 67:1548-1559. WILSON, R. E. 1970. Succession in stands of Populus deltoides along the Missouri River in southeastern South Dakota. Am. Midl. Nat. 83:330-342.
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Figure legends.
Fig. 1. Yellow River study area in Central Wisconsin. Fig. 2. Timing of major disturbance events and periods for the Yellow River floodplain from time of settlement to 2050. The dominant arboreal species and amount of open habitat are indicated for each inventory period (1851, 2000) and the end of the projection period (2050).
Fig. 3. Average annual flow (cubic feet per second) from 1944-2000 at the Babcock Station, Yellow River, Wisconsin.
Fig. 4. Average monthly flow (cubic feet per second) for the period 1944-2000 at the Babcock Station, Yellow River, Wisconsin.
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APPENDIX A. Autecological characteristics1 of common arboreal species in the Yellow River floodplain, central Wisconsin. ____________________________________________________________________________________________________ Period of
Dry
Substrate for Seedling
Tolerance of
Longevity Seed Dispersal Seed Germ. Seed Weight Germination Radiation2 30-d Flood (yr) (g) ____________________________________________________________________________________________________
Species
Silver Maple
125+
May-June
June-July
0.26
Light litter, soil
High
Yes
N. red Oak
175
Oct.-Nov.
May-June
3.60
Litter
Interm.
No
Swamp White oak
300
Oct.-Nov.
Oct.-Nov.
3.80
Litter
Interm.
Yes ?
Red maple
125
May-June
June-Aug.
0.02
General
LowInterm.
Yes
Green ash
125 ?
Oct.-June
May-June
0.03
Light litter?
Interm.
Yes
River birch
80
May-June
June ?
0.01
Soil
High
Yes
_____________________________________________________________________________________________________ 1 Sources: Schopmeyer, 1974; Peterson and Bazzaz, 1984; Loucks, 1987; Crow, 1988, 1990; Gabriel, 1990; Rogers, 1990; Walters and Yawney, 1990; Harlow et al. 1996; Tremblay et al. 1996; Abrams, 1998. 2 Characteristic is a qualitative ranking of amount of radiation a seedling needs to survive
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