changes in vegetation and nutrient pools during riparian succession

WETLANDS. Vol. 14, No. 2. June 1994, pp. 98-109 (¢ 1994, The Society of Wetland Scientists

C H A N G E S I N V E G E T A T I O N A N D N U T R I E N T P O O L S D U R I N G RIPARIAN SUCCESSION

Keith Boggs J and T. Weaver Biology' Department

Montana State University Bozeman, Montana 59717, USA

' Current address: Alaska Natural Heritage Program

University of Alaska Anchorage Anchorage, Alaska 99501, USA

Abstract: Changes in vegetation composition, structure, biomass, and the nutrient pools of phosphorus. nitrogen, and potassium are described for a riparian sere on the floodplain of the lower Yellowstone River, Montana. Community dominance progressed from seedlings of Great Plains cottonwood (Populus deltvides) and sandbar willow (Salix exigua), to a thicket of sandbar willow and cottonwood, to cottonwood forest, to shrubs, and finally to grassland. Sandbar willow and cottonwood were lost because they died without regeneration. Community height and structural complexity increased to a maximum in the cottonwood stages and decreased in the latter stages. Change from the cottonwood seedling to cottonwood forest and grassland stages for total above-ground biomass (1 Mg/ha to 193 Mg/ha to 2 Mg/ha, respectively), below-ground biomass (6 Mg/'ha to 94 Mg/ha to 29 Mg/ha, respectively) and soil organic matter (31 Mg/ha to 216 Mg/ha to 177 Mg/ ha, respectively) was similar, initially increasing then decreasing. Total ecosystem phosphorus mass increased from 7000 kg/ha to 9000 kg/ha as the cottonwoods matured and remained relatively constant thereafter. In contrast, nitrogen and extractable potassium mass increased as the cottonwood forests matured (3000 kg/ha to 8000 kg/ha and 500 kg/ha to 3000 kg/ha for N and K, respectively) and declined slightly as the cottonwood forests died out.

Key Words: Populus deltoides, Salix exigua, Agropyron smithii, Yellowstone River, biomass, succession, riparian, nitrogen, phosphorus, potassium

INTRODUCTION

Studies of cottonwood succession in riparian ecosystems have typically concentrated on changes in plant composition (Ware and Penfound 1949, Weaver 1960, Hosner and Minckler 1963, Wilson 1970, Keammerer et al. 1975, Shaw 1976, Hansen et al. 1991). This paper relates changes in structure, biornass, and nutrient pools to plant succession in a cottonwood-dominated riparian ecosystem (Wikum and Wall 1974, Johnson et al. 1976).

A chronosequence approach was used to measure successional changes along the floodplain of the Yel- lowstone River, the longest undammed river in the arid western USA. The formation of new land in riverine ecosystems is well documented (Leopold et al. 1964, Friedkin 1972). Along a meandering river, alluvium is typically deposited on convex curves in the river channel. The opposing concave bank is cut, providing sediment for deposition on convex curves

98

downstream and creating a series of similar bands of alluvial deposits. The channel thus meanders laterally across the floodplain. Vegetation growing on new deposits near the river may be contrasted with that on older deposits inland to recognize and measure successional processes (Linsey et al. 1961, Stevens and Walker 1970, Jenny 1980).

Vegetation in the sere described progresses from seedlings of Great Plains cottonwood (Populus deltoides Bartr. ex Marsh.) and sandbar willow (Salix exigua Null.) establishing on newly deposited alluvium, to a thicket of sandbar willow and cottonwood, to cottonwood forest, to a shrubland, and then to a grassland. The cottonwood-to-grassland sere is one of the dominant riparian seres in this region of the Northern Great Plains. Hansen et al. (1991) has described the changes in plant species dominance of the sere.

The objectives of this study were (1) to determine changes in species composition during succession in a cottonwood-dominated riparian ecosystem, (2) to re-

Boggs & Weaver, CHANGES IN RIPARIAN VEGETATION AND NUTRIENT POOLS 99

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late changes in structure, biomass, and nutrient pools to the sere, and (3) to identify the dominant processes influencing the movement of nutrients through the ecosystem. Major nutrients were measured because of their critical role in ecosystem development and their rep- resentative pathways through the ecosystem. Phos- phorus is a relatively immobile nutrient that once imported should remain within the ecosystem, and it typically enters riparian ecosystems via sediment transport from flooding (Mitsch et al. 1979). Nitrogen is often cited as the most limiting nutrient in forest ecosystems and typically enters an ecosystem through biological nitrogen-fixation, precipitation, and silt-lad- en floodwaters. Potassium is a mobile nutrient and is often imported in solution (fiver and ground water) and readily lost from the ecosystem by leaching. The results from this study will be useful for those managing river flows to affect community composition, structure (Rood and Heinze-Milne 1988), biomass, and nutrient pools on riparian sites.

STUDY AREA

The study area was a 72 km stretch of floodplain along the lower Yellowstone River in Montana; the river flows northeast from latitude 47 ° 07'N to 47 ° 45'N and longitude 104 ° 42'W to 104 ° 07'W. The elevation ranges from 577 to 625 m above sea level. The sur-

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Figure 2. Average monthly temperature and precipitation at Glendive, Montana (Ruffner 1971).

rounding upland landscape consists of gently rolling hills, wide valleys, and fiat divides with sandstone and clinker beds forming ridges and buttes (Veseth and Montagne 1980). Erosion by the Yellowstone River has formed a broad, fiat floodplain with an average width of 4.4 km. Mean streamflow discharge is highest during the months of May, June, and July and decreases dramatically between August and April (Figure 1, USGS 1910-1981).

The region has a semi-arid climate (Thornwaite 1941), which supports grass and shrub vegetation on the uplands. The mean annual precipitation is 30-35 cm (Ruffner 1971). Average monthly precipitation is highest in early summer, with 8.1 cm falling in June, arid decreases to 0.8 cm in December (Figure 2). Mean monthly temperatures range from 23.7 C ° in August to - 7 . 4 C ° in JanuaD' (Figure 2, Ruffner 1971). The number of frost free days averages 130 (Caprio 1961).

METHODS

The sere was outlined by describing seven successional stages presented in an age sequence: sandbar, cottonwood seedling, cottonwood sapling, cottonwood pole, mature cottonwood, shrub, and grassland. Fifty- nine sites representing examples of the sere were located in the study area. Nine sites were sampled in each of the sandbar, cottonwood seedling, cottonwood pole, shrub, and grassland stages, eight in the cottonwood sapling, and six in the mature cottonwood stage.

100 WETLANDS, Volume 14, No. 2, 1994

For inclusion, the sites had to have homogeneous veg- elation and show no evidence of disturbance by fire, logging, heavy grazing, or cropping. Sample plots were situated within undisturbed sites that occurred infre- quently on the floodplain, which also led to the vari- ability in sample size. Complete seres, from sandbar through grassland, perpendicular to the river were sampled at five locations; the remaining sites were located along incomplete seres at seven locations.

One transect, 60 m in length, was placed parallel to the river in each site. The following variables were recorded or sampled along the transect per site: cottonwood age, plant species, canopy cover, elevation of the land surface relative to the river surface, height of the dominant species, density of the dominant woody vegetation, standing crop of herbaceous species, and soil core samples at three depths. Sampling occurred between late June and early September in 1980 and 1981. Changes in plant canopy cover, height, density, below- and above-ground biomass, and nutrient content vary seasonally, and we were unable to adjust our sampling dates to compensate for these changes,

Cottonwood age was estimated by counting the rings of an average sized cottonwood tree from each plot. Trees with < 6 cm trunk base diameter were cored at the base, and trees with -> 6 cm trunk base diameter were cored at breast height. Four years were added to the breast height core counts on the assumption that trees required 4 yr to reach breast height (Everitt 1968, Wilson 1970).

Elevation of the land surface relative to the river surface was used to determine the rate of alluvial deposition. Stand elevation relative to the thalweg was determined with a measuring rod and hand level. The elevations were then adjusted using the lowest measured river stage at base flow (0 m elevation).

Community Composition and Structure

Canopy cover of the individual plant species was recorded to document changes in community composition during succession. Nomenclature follows Dorn (1984). Percent cover of understory plants was measured with 60 step points along the transect in each site (Evans and Love 1957). Tree canopy cover was estimated ocularly (Brown 1954). The height of the tree canopy surface relative so the ground level was measured with a measuring tape and clinometer.

Tree and shrub densities in each site were recorded and used to calculate the total biomass of each site. All densities were based on individual stems arising from the soil surface. Tree densities were measured in three rectangular plots placed at equidistant points along the transect. Plot size by stage was 1) cottonwood seedling 1 m × 0.5 m, 2) cottonwood sapling 2 m × 3 m,

3) cottonwood pole 5 m x 5 m, and 4) mature cottonwood 20 m x 20 m. Density plots for shrubs were also placed at equidistant points along the transect with a minimum plot size of 1 m x 5 m. Plot length and width were proportionately increased to include a minimum of 20 trees or shrubs. Trees were tallied by trunk diameter at breast height (dbh) classes, and shrubs were tallied into crown diameter classes.

Biomass

Above-ground live biomass was estimated by summing the mass of the tree, shrub, and herb components. The regression equations used to calculate the biomass of plant parts (leaf, wood diameters of 0-1 cm, 1-10 cm, and > 10 cm, or 0--0.5 cm and >0.5 cm) of individual woody plants per site are presented in Table 1, and all were based on harvested plants. Two regression equations were used for cottonwood trees: one based on trees with < 6 cm trunk base diameter and the other on trees with >- 6 cm trunk base diameter (the latter used dbh as the independent variable). The trunk base tree regression was based on trees ranging from 0.5 to 6 cm, and the dbh regression was based on trees ranging from 1 to 64 cm dbh; the dbh of trees in all plots ranged from 0.5 cm trunk base to 70 cm dbh. The mass of the tree layer was then determined by estimating the dry mass of each tree from its base diameter or dbh, summing across all trees in the density plots, and adjusting to a ha basis (Kira and Shidei 1967, Whittaker and Woodwell 1968).

Biomass of sandbar willow and peach-leaved willow (Salix amygdaloides Anderss.) was estimated from numbers and diameters at trunk base of shrubs measured in the density plots and the appropriate regression equation (Table 1), The biomass of above-ground parts of western snowberry (Symphoricarpos occ~den- tafis Hook.), woods rose (Rosa woodsii Lindl.), and silver sagebrush (Artemisia cana Nutt.) were similarly estimated from shrub density, crown diameters, and diameter-mass regressions (Table 1). Above-ground mass for grasses, forbs, and litter was determined by harvesting, drying, and weighing the material present in five 0.5 m 2 plots spaced equally along the transect line in each site.

Above-ground dead organic matter was estimated for the same five sites in each stage by summing the mass of litter (described above) and dead vegetation (shrubs and trees) per site. Dead tree (standing or downed) mass was first calculated as living tree mass based on its dbh and then corrected downward both for estimated loss of branches (< 1 cm and 1-10 cm diameter) and bole decomposition. Dead trees were recorded within the following branch loss categories: all branches present, half of the < 1 cm and 1-10 cm

Boggs & Weaver, CHANGES IN RIPARIAN V E G E T A T I O N AND N U T R I E N T POOLS 101

Table 1. Constants and coefficients of determination for the allometric regression equations of the form log y = a + b log x, to estimate biomass of trees and shrubs.

Regression Constants

Species Diameter Basis Plant Part a b r 2 N

Woods Rose Crown canopy Leaf 0.23 1.46 0.93 9 (Rosa woodsii) Wood -<0.5 cm 0.30 1.44 0.92

Wood >0.5 cm 0.47 1.59 0.86 Western Snowberry Crown canopy Leaf 0.18 1.48 0.94 9

(Syrnphoricarpos occidentalis) Wood <0.5 cm 0.31 1.46 0.98 Wood >0.5 cm -0.09 1.59 0.95

Silver Sagebrush Crown canopy Leaf 0.46 1.80 0.98 9 (Artemisia cana) Wood <0.5 cm 0.43 2.31 0.95

Wood >0.5 cm -0.02 3.21 0.97 Sandbar Willow Trunk base Leaf 0.98 2.14 0.99 9

(Salix exigua) Wood < 1 cm 0.89 2.43 0.97 Wood 1-10 cm 1.06 3.35 0.92

Peach-leaved Willow Trunk base Leaf 0.93 2.09 0.98 9 (Salix amygdaloides) Wood < 1 cm 1.08 2.22 0.97

Wood 1-10 cm 0.91 2.98 0.92 Great Plains Cottonwood Diameter at breast height Leaf - 1.29 1.53 0.97 5

(Populus deltoides) Wood < 1 cm - 1.78 1.93 0.99 Wood 1-10 cm -0.85 1.87 0.99 Wood > 10 cm - 1.67 2.83 0.99 Roots >-1 cm 1.14 2.45 0.99 6

Great Plains Cottonwood Trunk base Leaf 0.76 2.53 0.99 5 (Populus deltoMes) Wood < 1 cm 0.88 2.38 0.98

Wood 1-10 cm 0.87 2.85 0.99 Roots >_1 ern 0.62 2.33 0.99 6

diameter branches lost, or all the < 1 cm and 1-10 cm branches lost. Dead tree boles were recorded within the following decomposi t ion categories: no decomposition, moderate decomposit ion, or heavy decomposition. The percent decomposi t ion values were calculated by collecting five bole samples f rom each category; each was dried, weighed, vo lume determined, and wood density calculated. The weight of each dead tree was then calculated by 1) estimating live mass f rom the dbh regression equations, 2) subtracting the appropriate branch loss category, and 3) multiplying by the appropriate ratio o f wood density o f decom- posed bole to wood density of live bole.

Below-ground biomass was also est imated in the same sites used for above-ground biomass. The following categories were measured: roots with diameters > 1 cm, roots 0.1-1 cm, and roots < 0.1 cm. To estimate the mass o f roots > 1 cm, we used a root mass/s tem diameter regression equation constructed from six cot tonwood trees (Table 1) ei ther excavated or found pre-washed on sandbars. The mass o f sandbar willow roots > 1 cm in diameter was assumed to equal that of roots of similar sized cot tonwood trees.

Mass o f roots < 0. I cm in diameter (fine roots) and

0.1-1 cm in diameter were est imated from five 2-era- diameter soil cores (mineral soil only) equally spaced along each transect per site and divided into 0-10, 10- 30, and 30-150 cm soil depths. The five cores from each site were mixed, and 25 % by weight was removed and air dried for analyses o f nutrients and soil organic mat ter at the Montana State Universi ty Soil Testing Laboratory. The remaining soil was dried to constant weight at 60°C and weighed for estimating bulk density (see below). Roots < 0.1 cm and 0.1-1 cm in diameter were estimated by washing the remaining soil following the methods of Jackson (1956). Roots 0.1-1 cm in diameter were removed, dried at 60°C, weighed, and their mass calculated per site. The remaining fine roots (all < 0.1 cm in diameter), soil particles, and detritus were placed on ashless filter paper. We then visually est imated what proport ion o f this material was con- tr ibuted by roots. Finally, the material plus filter paper was weighed, ashed, and reweighed. Mass o f fine roots per site was calculated as [mass of root + soil + detritus

- ash) × (% o f roots remaining on the filter paper) (Weaver 1982).

Soil organic mat ter content o f soils was calculated as (percent organic matter) x (soil bulk density) x


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Sera] Stage Figure 3. The elevations of the ground surface above the river surface for communities of increasing age. Letters represent the seral stages: sandbar (S), cottonwood seedling (C), cottonwood sapling (CS), cottonwood pole (CP), mature cottonwood (MC), shrub (SH), and grassland (GR).

(depth of the soil layer considered). Roots > 0.1 cm in diameter were removed prior to analyses. Percent organic matter was determined colorimetrically after dichromate oxidation (Sims and Haby 1970). Bulk densities (g/cm 3) were estimated by dividing the mass of the pooled and dried cores by the volume of the coring tube segment. Because 25 % by weight of the pooled cores had been removed for nutrient and organic matter analyses, these calculations were adjusted appropriately. The mass of soil organic matter per site was then estimated as (total soil organic matter) - (mass of roots < 0.1 cm in diameter).

Nutrient Pools

Nutrient pools of phosphorus, nitrogen, and potassium in the ecosystem at each seral stage were measured to describe changing pool sizes of these nutrients through succession. The same sites used for biomass estimates were used for all nutrient mass estimates. Nutrient pools in plants were determined by multiplying the biomass of a given component by its nutrient concentration. For each woody species harvested for the biomass equations, five plants were used for nutrient analyses. Each of the five plants per species was separated into leaves, wood of diameter classes 0-1, 1-10, and > l0 cm or 0-0.5 cm and >0.5 cm, analyzed

for nutrients, and the results averaged, Plant phosphorus and potassium were determined by ashing samples and determining quantities of the nutrient elements contained by spectrophotometric methods (Olson and Dean 1965). Total nitrogen was determined by the Kjeldahl method (Bremner 1965).

Not all plant components were analyzed for nutrients, and in the interest of completing the nutrient pools, we made assumptions about the nutrient concentrations of the missing components (Rodin and Ba- zilevich 1967). Concentrations of nutrients in roots 0 - 1 cm in diameter were assumed to equal 0-1 cm twig concentrations, and concentrations in roots > 1 cm were assumed to equal those of 1-10 cm branches. Nutrient concentrations of dead boles were assumed to equal those of > 10 cm live boles.

One sub-sample of soil was removed from each of the three soil depths per site (35 sites) and analyzed for nutrient concentrations. The mass of each soil nutrient per site (kg/ha) was estimated as the product of nutrient concentrations (%) from the three soil depths (0-10, 10-30, 30-150 cm), soil bulk density (g/cm~), and soil depth (cm) in the depth concerned, Total soil phosphorus was determined spectrophotometrically after ashing and extracting with 1 M H2SO, (Olsen and Dean 1965). Soil nitrogen was determined by the Kjel- dahl method (Bremner 1965). Potassium was extracted from soils with 1 M ammonium acetate and measured by atomic absorption (Pratt 1965).

RESULTS

The cottonwood establishment and age data enabled us to estimate reasonably well the age of all the cottonwood sites sampled. Cottonwood age was a direct measure of site age because cottonwood seedlings are one of the primary colonizers on newly established sandbars (the first stage of succession), seedlings rarely establish in later stages, and all cottonwood sites were relatively even-aged (Moss 1938, Johnson et al. 1976). Ages of cottonwoods within the cottonwood seedling, cottonwood sapling, cottonwood pole and mature cottonwood stages averaged (__+ SE), respectively, 3 + 0, 7 _ 1, 34 ___ 3, and 92 + 5 yr-old. With the death of the oldest cottonwood trees, the relatively precise cottonwood chronology ceased. The shrub stage often contained decadent cottonwoods, whereas the grassland stage rarely did. Consequently, the shrub stage was considered seral to the grassland stage.

The average rate of alluvial deposition during the 0--20 yr period was 0.11 m/yr, raising the ground surface level by 2.2 m (Figure 3). The rate of deposition was fastest during the early stages of this period and had slowed by 20 yr. Thereafter, the rate of alluvial

Boggs & Weaver , C H A N G E S I N R I P A R I A N V E G E T A T I O N A N D N U T R I E N T POOLS 103

Table 2. Canopy cover of the dominant species (> 60% constancy and > 1% canopy cover) in 6 seral stages. The stages include seed (cottonwood seedling, n 9), sapling (cottonwood sapling, n = 8), pole (cottonwood pole, n = 9), mature (cottonwood mature, n = 6), shrub (shrub, n = 9), and grass (grassland, n = 9).

Canopy Cover (X _+ SE)

Plant Species Habit Seed Sapling Pole Mature Shrub Grass

Salix exigua Shrub 5 _+ 2 30 . . . . Salix amygdaloides Tree 1 _+ 1 5 < 1 < 1 -- -- Populus deltoides Tree 21 _+ 3 30 66 _+ 4 40 _+ 4 < 1 -- Etymus repens (L.) Gould Grass -- 3 _+ 1 5 _+ 2 1 __ 1 < 1 1 _+ 0 Etymus canadensis L. Grass < l 2 _+ 1 7 + 1 6 _+ 2 2 +_ 1 2 _+ 2 Muhlenbergia racemosa (Michx.) B. S.P. Grass -- 2 + 1 7 +_ 2 5 +_ 2 4 _+ 2 3 +- 2 Bromus inermis Leyss. Grass -- < l 2 +_ 1 2 _ 2 < 1 2 _+ 2 Toxicodendron rydbergii (Small ex Rydb.) Greene Shrub -- -- < 1 12 __ 3 3 _+ 1 < I Vitis riparia Michx. Liana -- -- < l 4 __ 2 -- -- Rosa woodsii Shrub - - <1 12 + 3 14 _+ 2 <! S vmphoricarpos occidentalis Shrub - -- < 1 6 _+ 2 8 _+ 1 < I Artemisia fudoviciana Nutt. Shrub - < 1 < 1 < 1 < 1 2 _+ 1 Artemisia cana Shrub . . . . . 1 _..+_ 0 Calamovilfa longifofia Grass -- < 1 < 1 - -- 11 _+ 4 Elymus smithii Grass - 1 _+ 0 1 _+ 0 1 _+ l 3 _+ 1 17 + 5

deposi t ion was considerably slower (0.01 m/yr) raising the ground surface level to 3.0 m at 110 yr.

C o m m u n i t y Compos i t ion

In te rms o f dominance, cover, and height, each seral stage can be characterized in several ways. 1) The sandbar stage was domina ted by unvegetated deposits o f alluvial silt, sand, and gravel. 2) in the co t tonwood seedling stage, sandbar willow, peach- leaved willow, cot tonwood, and herbaceous seedlings colonized the new deposits. Cot tonwood, willow, and shrub total cover averaged 27 %, and herbaceous cover averaged 8 % (Table 2). Canopy height averaged 0.3 + 0.2 m. 3) Sandbar willow and cot tonwood also domina ted the cot tonwood sapling stage. Cot tonwood and willow cover averaged 65 %, and the undergrowth was d o m - inated by a variety of herbaceous species with a com- bined average canopy cover of 28 %. Cot tonwood and willow height averaged 2.2 +_ 0.4 m. 4) Due to the loss of sandbar willow between the co t tonwood sapling and co t tonwood pole stages, co t tonwood domina ted the co t tonwood pole stage with an average canopy cover of 66 % and height o f 19 ± 1 m. The undergrowth was domina ted by a variety of herbaceous species, with an average canopy cover o f 37 %, and a trace of shrubs including woods rose and western snowberry. 5) In the mature cot tonwood stage, cot tonwood cover decreased to 40 %, and its height increased to 25 __+ 2 m. The shrub understory componen t increased dramatical ly to 19 %, and herbaceous species averaged 39 %. The site 's appearance was one o f widely spaced, dying cot ton-

woods with a dense shrub understory. 6) With the dis- appearance of cot tonwoods, a shrub stage composed of woods rose and western snowberry dominated . Shrubs had an average canopy cover o f 23 %, herbaceous cover averaged 29 %, and shrub height was 1.0 + 0.4 m. While few dead shrubs were observed in the mature co t tonwood stage, ocular es t imates of percent dead shrubs within the density plots in the shrub stage ranged f rom 20 % to 50 %. Dead shrubs were not included in the cover calculations; consequently, shrub canopy cover values do not fully characterize the pre- vailing dense shrub aspect. 7) The oldest observed stage was a grassland domina ted by western wheatgrass [Ely- mus smithii (Rydb.) Gould], prairie sandreed [Cala- movilfa longiJblia (Hook.) Scribn.], and dotted with silver sagebrush (Artemisia cana Nutt,). The herbaceous cover averaged 67 % and silver sagebrush averaged 1%.

Biomass

Mean l ea fb iomass (Mg = 106g) +_ SE rose markedly f rom the co t tonwood seedling (0.65 ± 0.25 Mg/ha) and cot tonwood sapling (1.52 + 0.33 Mg/ha) stages to the cot tonwood pole stage (6.35 +__ 0.84 Mg/ha) and decreased in the mature cot tonwood (3.03 _+ 0.30 Mg/ ha), shrub (0.53 -+ 0.07 Mg/ha), and grassland (1.41 +_ 0.09 Mg/ha) stages (Figure 4a). Average shoot mass (_+ SE) increased through the cot tonwood seedling (0.53 + 0.21 Mg/ha), co t tonwood sapling (3.07 __ 0.95 Mg/ ha), co t tonwood pole (141.14 _+ 31.82 Mg/ha), and peaked for the ma tu re cot tonwood (190.39 _+ 21.24


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Seral Stage

Figure 4. (a) Leaf and root (diameters < 1 mm) biomass, and (b) total below- and above-ground biomass (roots, shoots, and leaves), shoot mass, and mass of roots (diameters > 1 ram) for communities of increasing age. Letters represent the seral stages: sandbar (S), cottonwood seedling (C), cottonwood sapling (CS), cottonwood pole (CP), mature cottonwood (MC), shrub (SH), and grassland (GR). All lines were fit by regression analysis using polynomial functions (r 2 = 0.68-0.83); the lines are to show the trends in the data only.

Me/ha) stages (Figure 4b). Shoot mass declined through the shrub (0.86 _+ 0.09 Mg/ha) and grassland (0.28 *-- 0.09 Me/ha) stages. The maximum above-ground biomass, 193 Me/ha, is within the 100 to 300 Mg/ha range of means reported for riparian forested wetlands (Re- iners 1972, Taylor I985, Brinson 1990).

The average (_+ SE) mass of fine roots showed a similar pattern as was found for leaves, with increases from the cottonwood seedling (2.16 +_ 0.56 Me/ha) stage through the cottonwood sapling (5.52 + 1.64 Me,/ ha) and cottonwood pole (8.77 _+ 1.53 Me/ha) stages, followed by a leveling in the mature cottonwood (9.28 +_ 1.55 Me/ha) and shrub (8.32 _+ 1.20 Mg/ha) stages (Figure 4a). The grassland stage showed an increase in fine root biomass over previous stages (12.17 _+ 1.35 Mg/ha), and this stage had the highest quantity. Bio- mass of roots > 1 mm in diameter ranged from (average per stage) 3.43 Me/ha to 94.28 Me/ha, peaking in the mature cottonwood stage (Figure 4b). Root-to- shoot ratios for all stages were high, particularly the cottonwood seedling (2.9), shrub (11.4), and grassland (10.1) stages; ratios for the other three stages ranged from 0.4 to 1.9. Leaf biomass was a significant per- centage of the above-ground biomass in the cottonwood seedling, sapling, shrub, and grassland stages, ranging from 33 % to 83 %; in the cottonwood pole and mature stages, it was 2 % to 4 % only.

Dead Organic Matter

Average (+_ SE) above-ground dead organic matter increased through the cottonwood seedling (0.10 + 0.19 Me/ha) to the mature cottonwood (14.40 _+ 7.06 Mg/ha) stages and declined thereafter to the grassland (3.37 _+ 1.60 Me/ha) stage (Figure 5). Percent dead wood (> 1 cm diameter) of total above-ground dead organic matter rapidly increased through the sandbar (< 1%) to the young cottonwood (55 %) stages and declined to the mature cottonwood (30 %) and shrub and grassland (1%) stages.

Average soil organic matter increased through the cottonwood seedling (31.00 +_ 17.77 Me/ha) to the mature cottonwood (216.00 _+ 49.19 Mg/ha) stages and declined to the grassland (177.00 +__ 45.94 Me/ha) stage (Figure 5). Most of the increase in soil organic matter was caused by an increase in the average (_ SE) percent organic matter. For example, in the 0-10 cm soil layer, concentrations of soil organic matter increased from the sandbar (0.1 _+ 0.0 %) stage through the cottonwood seedling (0.9 +- 0.5 %), cottonwood sapling (1.1 _ 0.5 %), cottonwood pole (1.1 _+ 0.7 %), and mature cottonwood (3.6 +_ 1.5 %) stages, and then declined in the shrub (2.6 _+ 1.4 %) and grassland (2.9 _+ 1.0 %) stages. Similar trends were obtained for soil organic matter concentrations in the deeper soil ho- rizons, although the concentrations were lower, ranging from 0.4 % to 2 ,1% in the 10-30 cm layer and 0.2 % to 1.2 % in the 30-150 cm soil layer. The mass of soil organic matter was higher than above-ground plant biomass in all stages of succession (Figures 4b and 5).


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Seral Stage

Figure 5. Mass of soil organic matter to 150 cm depth and above-ground dead organic matter (litter and dead shoots) for communities of increasing age. Letters represent the seral stages: sandbar (S), cottonwood seedling (C), cottonwood sapling (CS), cottonwood pole (CP), mature cottonwood (MC), shrub (SH), and grassland (GR). All lines were fit by regression analysis using polynomial functions (r 2 = 0.75 and 0.78 for soil organic matter and above-ground dead organic matter, respectively); the lines are to show the trends in the data only.

Nutrient Pools

Approximately 7000 kg/ha of to ta l phosphorus was initially deposited during the sandbar stage (Figure 6a). Phosphorus rose rapidly to 9000 kg/ha after about 10 yr and showed a slight increase thereafter. Phosphorus bound in above-ground biomass rose from < 10 kg/ha in the cottonwood seedling stage to 40 kg/ha in the mature cottonwood stage and declined to <10 kg/ha in the grassland stage in parallel with total biomass.

Nitrogen mass was approximately 3000 kg/ha in the

Figure 6. Total (a) phosphorus, (b) nitrogen, and (c) potassium in above- and below-ground biomass and soil to 150 cm depth for communities of increasing age. Letters represent the serai stages: sandbar (S), cottonwood seedling (C), cottonwood sapling (CS), cottonwood pole (CP), mature cottonwood (MC), shrub (SH), and grassland (OR), All lines were fit by regression analysis using polynomial functions (r ~ = 0.52, 0.72, and 0.84 for phosphorus, nitrogen and potassium, respectively); the lines are to show the trends in the data only.

13000

110(]0 v

9000'

~ 7000 -

• • 411

• gO •

B • • 0 • • D • •

5O0O 0 20 4~ 60 80 ~QD

X

C c~ 2 Z

12000 -

10000

8000

6000

4000

2000

I e I •

7/" . : . , I I - , - - - - ,

2 0 40 6'0 8 ~ 1 00

oi,

E

r~ "G

4000 -

e

3000 - .I

2 °

n ~ • •

1000- V

o 2'o ;o 8'0 8'o 1 ;o ~' '

s c cS.-~ cP. Age (yr)

MC -~--]~SH .-]~G R Seral Stage


sandbar stage, increased to approximately 8000 kg/ha in the mature cottonwood stage, and fell slightly thereafter, in contrast to phosphorus (Figure 6b). Nitrogen bound in above-ground biomass increased to nearly 250 kg/ha in the mature cottonwood stage and decreased to 20 kg/ha in the shrub and grassland stages.

Approximately 500 kg/ha &potassium was initially deposited during the sandbar stage, followed by an increase to over 3000 kg/ha in the mature cottonwood stage and a decrease to 2500 kg/ha in the grassland stage (Figure 6c). Potassium bound in above-ground biomass rose to nearly 402 kg/ha in the mature cottonwood stage and decreased to 10 kg/ha in the shrub and grassland stages.

DISCUSSION

Changes in community structure, biomass, and nutrient pools during riparian succession on the lower Yellowstone River floodplain are directly related to species dominance. As cottonwood dominance increased through time, structure, biomass, and nutrient pools all increased. With the loss of cottonwood and the ensuing dominance of silver sagebrush and grass, structure, biomass, and nutrient pools (except phosphorus) dec:teased.

Comparisons with other Riparian Systems

Riparian seres progressing from tree (cottonwood) dominated communities to shrub or herbaceous dominated communities, in the absence of disturbance, are rare (Hansen et al. 1991) and distinct from those of more humid regions or higher elevations. The seven seral stages on the floodplain of the study area are consistent with communities described in eastern Montana (Hansen el al. 1991) and similar to vegetation described in North and South Dakota (Everitt 1968, Wilson 1970, Wikum and Wall 1974, Keammerer et al. 1975, Johnson et al. 1976). However, riparian vegetation less than 500 km distant from the study area differs significantly in composition in the late seral community. To the east, in western and central North Dakota, the late seral communities are forests dominated by green ash [Fraxinus pennsylvanica vat. lan- ceotata (Borkh.) Sarg.], American elm (Ulmus ameri- cana L.), Rocky Mountain juniper (Juniperus scopulorum Sarg.), and boxelder maple (Acer negundo L.), each of which was present in lesser amounts in our study area (Everitt 1968, Keammerer et al. 1975, John- son et al. L976). Further east in castern North Dakota and South Dakota, American basswood (Tilia amer- icana L.) occurs with the above species (Wilson 1970, Wikum and Wall 1974). To the west of our study area,

green ash, Rocky Mountain juniper, ponderosa pine (Pinus ponderosa Dougl.), silver sagebrush, or Douglas fir [Pseudotsuga menziesii (Mirbel) Franco.] dominate the late seral communities (Hansen et al. 1991). North- west of the study area in southern Alberta, quaking aspen (Populus tremuloides Michx.) and white spruce (Picea glauca Moench) dominate the late seral communities (Shaw 1976). These differences in the species composition of late seral communities suggest that other site-specific variables, such as edaphic factors or water-table height, play important roles in species establishment. For example, within our study area, cottonwood trees are able to tap the water table, but as they die, they are replaced by species, such as silver sagebrush and western wheatgrass, that have far lower water requirements and, evidently, do not tap the water table.

Accumulation of Organic Matter and Nutrient Pools

Studies of forest soil organic matter through time show rapid increases during early succession followed by stabilization in later stages (Johnson et al. 1976, Switzer and Nelson 1979), and our results show a similar trend. Due to root growth and the incorporation of above-ground litter into the soil, organic matter increases rapidly to the mature cottonwood stage but then declines. The average (+_+ SE) soil organic matter in the grassland stage is 216.0 _+ 49.0 Mg/ha, which is higher than the 130.9 + 10.3 Mg/ha reported for the solum of grasslands of the region (Weaver 1978). The range of percent organic matter from the mature cottonwood through grassland stages (2.9-3.6%) is well within the 2-5 % reported for other bottomland hardwood forests in the USA and above the 0.4-1.5 % reported for adjacent uplands (Wharton et al. 1982). Because the grassland stage continues to receive peri- odic flood waters, enhancing productivity (Hansen et al. 1991 ), soil organic matter will probably not decline to the values reported for uplands.

Changes in phosphorus, nitrogen, and potassium nutrient pools in the riparian sere mainly reflect changes in alluvial deposition, and flood- and ground-water inputs. Accumulation in biomass and dead organic matter during succession are less important because most of the tolal pool was in the soil. We review the processes influencing movement of nutrients through the ecosystem with the object of identi~4ng the dominant processes.

Phosphorus pools in our ecosystem are relatively large in comparison to other wetland systems (Heiiman 1968, Reiners and Reiners 1970). Because phosphorus inputs parallel alluvial deposition (Figure 3) and large quantities of phosphorus are transported on clays (Ry-


den et al. 1972, Mitsch et al. 1979), we attribute the influx and subsequent large accumulations of phosphorus at our site mainly to silt and clay deposited by floods. Precipitation (Likens et al. 1977, Schlesinger 1978) and river water (USGS 1975-1981) are very. dilute in phosphorus and contribute little to the total phosphorus gains in the riparian system. Phosphorus contents remain high through the older seral stages because phosphorus is tightly bound by soils and leach- es slowly (Simonson 1970, Thomas 1970, Bohn et al. 1979). The loss of phosphorus bound in above-ground and dead organic matter late in the sere has little effect on the total phosphorus content in the ecosystem because it never comprises more than 1% of total phosphorus in any seral stage.

Nitrogen in the ecosystem peaked in the mature cottonwood stage and decreased slightly as the cottonwoods died (Figure 6b). As expected, this pattern of accumulation parallels that for soil organic matter because the ratio of carbon to nitrogen in soil tends to be fairly constant. Because nitrogen fixation by non- symbiotic organisms (0.01 Mg ha-tyr -~) and forage legumes (0.15--0.30 Mg ha-lyr -~) is relatively small (Burns and Hardy 1975, Johnson ct al. 1983), it is unlikely that these sources account for much of the nitrogen enrichment in the riparian system. Like phosphorus, ground water from adjacent uplands (Peter- john and Correll 1984) and floodwaters/sediments (Brown and Peterson 1983) also may contribute significant amounts of nitrogen to riparian forests. Import of nitrogen by river water at 0.17 mg/L (USGS 1910- 1981), precipitation (Likens et al. t977, Schlesinger 1978), or birds and mammals (Verbeek and Boasson 1984) is too low to account for the majority of the increase. The loss of nitrogen bound in cottonwood shoots late in the sere has little effect on the total nitrogen content in the ecosystem because the shoots never account for more than 3 % of total nitrogen in any seral stage. In contrast to our restdts, Johnson el al. (1976) reported that percent soil nitrogen increased in late succession on the Missouri River floodplain in central North Dakota. This can be attributed to the continued high levels of above-ground biomass in late succession.

Despite potassium being a readily leached element, significant quantities accumulated in our study sites. Contents of potassium in the various seral stages increased through the mature cottonwood stage and decreased thereafter (Figure 6c) in parallel with the likely flooding regime and distance to the water source. Po- tassium is loosely bound by soils and organic matter and is delivered by river water and ground water (Brown and Peterson 1983, Peterjohn and Correll 1984, Jack- son et al. 1987). Some potassium may be weathered on site or imported by birds, captured dust, and pre-

cipitation (Likens et al. 1977), but these sources are likely to be small compared to the above processes. During the replacement of the cottonwood forest with grassland, about 100O kg/ha of potassium is lost from the ecosystem (Figure 6c). This loss may be influenced by the release of 420 kg/ha stored in above-ground biomass during the loss of cottonwoods.

Implications for Management

Management goals within the riverine systems of the western United States are primarily livestock grazing, timber harvesting, recreation, and providing water for irrigation. These water and land use practices all affect succession and community structural diversity within the spatial and temporal patchwork of the Yellowstone River floodplain. During succession, each stage is lim- ited by the degree of community development, which reaches its peak as the cottonwood forest matures. With the eventual death of the cottonwoods, community height and biomass are restricted by the lack of regeneration of any tree species in the shrub and grassland stages.

Livestock grazing may further limit the pattern and extent of the cottonwood-dominated communities and the biodiversity and density of wildlife species (Kauff- man and Krueger 1984, Schulz and Leininger 1990, Conroy and Svejcar 1991). Grazing severely reduces regeneration of cottonwood, willow species, boxelder maple, and green ash (Hansen 1991). Consequently, forested communities of the arid west are often con- verted to grass- or shrub-dominated communities. The effect of reduced community structure takes on added significance due to the importance of riparian habitat to wildlife in the arid west. These wet, structurally complex, and productive habitats support far greater wildlife density and biodiversity than the adjacent arid uplands (Pfister and Batchelor 1984). Changes in riparian community structure typically result in significant changes in wildlife density and biodiversity (Tho- mas et al. 1979, Brinson et. al. 1981, Medin and Clary 1989).

Timber harvesting of the cottonwood forests for agriculture and fuel is historically and presently extensive and has greatly reduced the extent of cottonwood-dominated communities (Damone 1987). Nutrient data from the mature cottonwood sites is of special interest to forest managers concerned with nutrient depletion due to clearcutting of forested lands (Weaver and For- cella 1977, Franklin 1989). Whole tree removal, as when cottonwoods are harvested, removes < 1% of total phosphorus, 3 % of nitrogen, and 12 % of potassium from the ecosystem stores of each nutrient. These are relatively small amounts; thus removal of biomass by tree harvesting is likely to have minimal impact on


nutrient stores of riparian systems of the Yellowstone River floodplain.

Diversion of river flow for irrigation is significant throughout the arid west and on the Yellowstone River (USGS 1910-1981). Alterations in river flow have been shown to change rates of fluvial erosion and deposition, cause plant water stress, and reduce regeneration of cottonwood and willow, thus reducing cottonwood- dominated communities on the floodplain (Wilson 1970, Johnson et al. 1976, Rood and Heinze-Milne 1988, Rood and Mahoney 1990, Smith et al. 1991). Water flow alterations may slow the delivery &alluvial and water-borne phosphorus, nitrogen, and potassium mass to the ecosystem. Nutrient pools have been shown to strongly influence community composition of many wetland ecosystems, such as bogs and fens (Van Der Valk and Bliss 1971, Sims et al. 1982). Although it has been suggested that nutrients are not limiting in forested riparian ecosystems (Brinson et al. 1981), a change in sediment loads and dissolved nutrients in river water due to alterations in river flows may change community productivity or composition over time.

ACKNOWLEDGMENTS

We thank Sandra Brown, Richard Mackie, Paul Hanson, Malvern Westcott, and Mike Scott for their valuable reviews; Gary Dusek for advice and assistance with harvesting; Jack Rumely for assistance with plant identification; and the Montana Department of Fish, Wildlife, and Parks and Montana State University for financial support.

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Manuscript received 30 January 1992; revisions received 7 April 1993, 22 September 1993, and 21 December 1993', accepted ] 1 January 1994.

changes in vegetation and nutrient pools during riparian succession

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