PLANT SPECIES COMPOSITION AND GROUNDWATER LEVELS IN A PLATTE RIVER WET MEADOW
21



winter and peaks in spring and early summer. Total precipitation
was above normal in all years except 1986. Although cyclical, the
timing of peak ground water levels differed between years. For
instance, in 1983 and 1984, peaks occurred during the May through
August growing season, while in 1985 and 1986 the peaks occurred
earlier in the spring (February) and probably had little direct in-
fluence on plant growth (Figure 1). Groundwater levels in 1983
and 1984 reflected extremely high sustained flows in the Platte in
mid-1983 (maximum = 666 mt/sec) and in mid-1984 (maximum
= 425 m3/sec). Environmental conditions during the May through
August growing season generally paralleled annual patterns (Table
1). Ground water and discharge were high in 1983 and 1984, while
precipitation was high in 1982 and low in 1986. Temperature was
relatively uniform throughout the growing season during the six
years of the study.

Species Distribution
  During the study, 91 species were recorded in the permanent
plots. Individual species were, for the most part, distributed un-
imodally along a moisture gradient from swale to ridge top (Figure
2). It should be emphasized that these were general distribution
patterns, since species abundance and distribution shifted along
the gradient in response to environmental conditions. Water smar-
tweed (Polygonum amphibium L.) was narrowly distributed in
wetland sites, but other wetland species such as water sedge (Carex
aquatils Wahl.) and American bulrush were widely distributed.
The major prairie grasses, switchgrass, indiangrass, and big blues-
tem had overlapping distributions in the middle of the moisture
gradient, however, switchgrass was associated with wetter sites,
indiangrass with intermediate sites, and big bluestem with drier
sites. Redtop (Agrostis stolonifera L.) and smooth brome (Bromus
inermis Leyss.), are introduced grasses that appeared along the
gradient in direct competition with switchgrass and big bluestem,
respectively. Many-flowered aster and common ragweed (Ambro-
sia artemisizfolia L.) were ubiquitous along the gradient. Aster
was skewed towards higher elevations, while ragweed had a bi-
modal distribution with peaks at both high and low elevations,
where disturbed sites allowed colonization.



Temporal Changes in Species Cover
  Of the 91 species sampled in the study, six were chosen for
detailed analysis because they showed statistically significant (P
<0.05, one-way ANOVA) responses from year to year (Table 2).
These species included water sedge, spikerush, and American bul-
rush, which are characteristic of lowland sites, and indiangrass,
big bluestem, and switchgrass, which dominate intermediate slopes.
Species responses were similar for each of the four moisture classes
(Figure 3). Sedge ( +37%) and bulrush ( + 14%) increased sub-
stantially between 1982 and 1984, when discharge and ground-
water levels also increased. Spikerush (+36%) and indiangrass
(+ 22%) also responded positively to this wet period but they
peaked early in 1983. At medium wet sites, however, sedge and
spikerush continued to increase until 1985, even though ground-
water and discharge levels had already declined substantially (Ta-
ble 1). Following the peak cover values in 1983, 1984, and 1985;



tYZ


0
C-



FIG. 2. Distribution of major species along the topographic (mois-
ture) gradient. Wet represents the bottom of the swales while dry
represents ridge tops. (CAAQ = water sedge, SCAM = American
bulrush, POAM = water smartweed, PAVI - switchgrass, SOAV
= indiangrass, ANGE - big bluestem, AGAL = redtop, BRIN
= smooth brome, ASER = many-flowered aster).



Table 2. Statistically significant (P < 0.05, one-way ANOVA) responses
of major species during the 1982-1987 period (+ = significant increase, -
ignificant decline). The year of peak increase or decline is provided for
the overall population and for each moisture class. NS = not significant.



Species



Overall



Water sedge
Spikerush
American bulrush
Indiangrass
Big bluestem
Switchgrass



(+) 1984
(+) 1983
(+) 1984
(+) 1983
(-) 1984
(+) 1985



Wet



(+) 1984
(+) 1983
(+) 1984
  NS
  NS
  NS



Moisture Class
   Medium
     wet



(+) 1985
(+) 1985
(+) 1984
(+) 1983
(-) 1984
(+) 1985



Medium
  dry



  NS
(+) 1983
  NS
(+) 1983
  NS
(+) 1984



Dry



NS
NS
NS
NS
NS
NS



Tdble 3. Stepwise multiple regression equations associated with mean cover
P'laleters were measured during the growing season (May-August).



of major species during



the 1982-1987 period'. Environmental



Water sedge          V = (27-R8)(MFANrW)-(in MluINOD)-18R Q



Spikerush
Alerican bulrush
lanpgrass



y = (13.15)(MEANGW)-10.41
Y = (5.57)(MAXGW)-(0.09)(MINQD)-6.39
y = (7.l8)(MAXGW)-R.25



Dtg bluestem         y = (14.68)(BURN)-(7.25)(MAXGW) +46.65             
                                           Rt = 0.11

                     9b y = (-0. 17)(TOTPc) + (8.70)(BURN) + 84.33      
                                           R' = 0.20
      i,- mEgngruund98ge elevation, MINQD = minimum river discharge, MAXGW
= maximum groundwater elevation, BURN = percentage of plots burned, TOTPC
= total precipita-



Rt = 0.18
Rt = 0.28
R2 = 0.24
R2 = 0.29