Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills

RAMA-00338; No of Pages 9 Rangeland Ecology & Management xxx (xxxx) xxx–xxx

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Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills Mitchell B. Stephenson a,⁎, Jerry D. Volesky b, Walter H. Schacht c, Nevin C. Lawrence d, Jon Soper e, Jessica Milby e a

Range Management Specialist, University of Nebraska−Lincoln, Panhandle Research and Extension Center, Scottsbluff, NE 69361, USA Range and Forage Specialist, University of Nebraska−Lincoln, West Central Research and Extension Center, North Platte, NE 69101, USA Professor, Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE 68583, USA d Weed Management Specialist, University of Nebraska−Lincoln, Panhandle Research and Extension Center, Scottsbluff, NE 69361, USA e Research Technologist, Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE 68583, USA b c

a r t i c l e

i n f o

Article history: Received 23 March 2018 Received in revised form 31 August 2018 Accepted 4 September 2018 Available online xxxx

a b s t r a c t Several studies have evaluated the spatial distribution of cool- and warm-season grasses across different topographic positions in the Nebraska Sandhills, but limited research has explored topographic differences in total plant production or production of plant functional groups in relation to variable amounts of precipitation. This study evaluated how spring and growing season precipitation influenced plant production at four topographic positions common in the eastern Nebraska Sandhills. Plant production data were collected from annually moved grazing exclosures in mid-June (peak cool-season grass production) and mid-August (peak warm-season grass production) during a 17-yr period from 2001 to 2017. Total plant production and precipitation use efficiency were 35−58% greater on interdune positions, and precipitation marginal response for total plant production was more sensitive to increases in spring and growing season precipitation on interdune compared with dune positions in both mid-June and mid-August. The greater precipitation marginal response of total plant production on interdune positions was driven primarily by greater increases in cool-season grass production with increasing spring or growing season precipitation. Warm-season grass precipitation marginal response was not different among the topographic positions, but production was 23−70% greater on interdune compared with dune topographic positions in mid-August. When differences in the amount of each topographic position at the study location were accounted for, growing season precipitation explained 49% of the variation for total plant production in midAugust, but spring precipitation only explained 23% of the variation for total plant production in mid-June. Because of the differential response of plant production at dune and interdune positions, incorporating the relative amount of each topographic position into estimates of plant production at the pasture or ranch scale will provide better information for adjusting stocking rates to more accurately match animal demand with forage availability. © 2018 The Society for Range Management. Published by Elsevier Inc. All rights reserved.

Introduction The Nebraska Sandhills is a unique, mixed-grass ecoregion in the northern Great Plains characterized by stabilized, grass-covered sand dunes. The geomorphic terrain of the dunes creates a heterogeneous landscape with a variety of different topographic positions in close proximity to one another (i.e., slopes, dunetops, and interdune swales). Topographic position on a landscape can influence soil moisture (Briggs and Knapp, 1995), soil organic matter and resource abundance (Schimel et al., 1985; Burke et al., 1999; Schacht et al., 2000), aboveground net primary productivity (Briggs and Knapp, 1995; Nippert et al., 2011), and selective preference by grazing livestock (Senft et al., 1985; Milchunas et al., 1989; Bailey, 2005). In the Nebraska Sandhills, several descriptive studies ⁎ Correspondence: Mitchell B. Stephenson, University of Nebraska−Lincoln, Panhandle Research and Extension Center, Scottsbluff, NE 69361, USA. E-mail address: [email protected] (M.B. Stephenson).

have documented differences in plant species composition across dune (i.e., slopes and dunetops) and interdune topographic positions (Pool, 1914; Tolstead, 1942; Burzlaff, 1962; Barnes and Harrison, 1982; Barnes et al., 1984; Schacht et al., 2000), but limited research has evaluated the influence of topographic position on total plant production or plant production of different functional groups (Stephenson et al., 2013). Failure to account for differences at the topographic position level can lead to large overestimates or underestimates in plant production when averaging across a landscape (Nippert et al., 2011). Interannual variability in precipitation strongly influences plant production in the Great Plains, and several studies have examined relationships between annual or seasonal precipitation and plant production at regional and local scales (Smoliak, 1986; Sala et al., 1988; Milchunas et al., 1994; Epstein et al., 1997; Lauenroth et al., 2000; Wilcox et al., 2015; Petrie et al., 2018). Because of uncertainty in future precipitation amounts, planning for and predicting the amount of available forage for grazing livestock is challenging and often requires a high degree of

https://doi.org/10.1016/j.rama.2018.09.001 1550-7424/© 2018 The Society for Range Management. Published by Elsevier Inc. All rights reserved.

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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flexibility by livestock producers to maintain appropriate stocking rates without overgrazing or undergrazing the forage resource in dry and wet years, respectively (Derner and Augustine, 2016). In addition to the influence of temporal precipitation variability on plant production, spatial variability in available resources and the dominance of different plant functional groups can influence relationships between precipitation and plant production (Smart et al., 2007; Derner et al., 2008; Irisarri et al., 2016). Understanding how both spatial (e.g., topographic position) and temporal (e.g., precipitation) variability influence plant production is important for livestock producers to better match grazing livestock forage demand with fluctuating forage availability across and within years (Andales et al., 2006). The potential interacting influence of seasonal precipitation and topographic position on plant production has not been extensively researched at different times during the growing season. Greater information on the sensitivity of plant production to variable precipitation amounts across topographically diverse landscapes could improve predictions for ranchlevel forage inventory estimates, as well as enhance forage availability to animal demand relationships. In this study, we examined the relationship between plant production and seasonal precipitation at different topographic positions in the eastern Nebraska Sandhills over 17 yr (i.e., 2001 through 2017). The main objectives of the study were to 1) evaluate differences in plant production among the four main topographic positions in the Sandhills (i.e., dunetops, interdunes, south-facing slopes, and north-facing slopes, see Fig. 1) in mid-June (peak cool-season grass biomass) and mid-August (peak warm-season grass biomass), 2) determine if relationships between precipitation and plant production were differentially expressed among the four topographic positions, and 3) evaluate the relationship between precipitation and plant production at a pasture scale with representative proportions of the different topographic positions. On the basis of the differences in species composition between dune and interdune topographic positions, we hypothesized that plant production and the response to precipitation would differ among the topographic positions.

Sandhills near the town of Rose, Nebraska (42°13′32′′N, 99°38′09′′W; elevation = 765 m). Study pastures are dominated by sands ecological sites (R065X6055NE) with scattered sandy (R065XY054NE) and choppy sands (R065XY056NE) ecological sites throughout the area. Topography of the uplands consists of grass-covered dunes typically oriented west by northwest to east by southeast with dune heights up to 40 m from interdune valleys. Soils are classified in the Valentine series (mixed, mesic Typic Ustipsamments) with loamy sand texture and 84−89% sand. No differences in soil texture were observed at the different topographic positions, but soil organic matter in the top 0−15 cm was greater on interdune (1.5%) compared with dune positions (0.7 − 0.8%) (Schacht et al., 2000). The proportion of different topographic positions calculated in a subsample of the study pastures with a handheld Global Positioning System unit was 15% dunetops, 15% interdunes, 34% north-facing slopes, and 36% south-facing slopes (M. Stephenson unpublished data 2010). The eastern region of the Sandhills is on the transitional border between semiarid and subhumid climate types. As a result, vegetation in the study pastures consists of a mixture of shortgrass, midgrass, and tallgrass species common to the shortgrass and tallgrass prairies. Kentucky bluegrass (Poa pratensis L.) and switchgrass (Panicum virgatum L.) dominated the cool- and warm-season grass species composition on interdune topographic positions, respectively. Common warm-season grass species on the dune positions were little bluestem (Schizachyrium scoparium [Michx.]), switchgrass, prairie sandreed (Calamovilfa longifolia [Hook] Scribn.), and sand bluestem (Andropogon hallii Hack). The most common cool-season grasses on the dune positions were needlegrass species (needleandthread [Hesperostipa comata Trin. & Rupr.] and porcupine grass [Hespersotipa spartea Trin.]). Several sedge species (Carex spp.) were common at all topographic positions throughout the study site. Western ragweed (Ambrosia psilostachya DC.) was the most common forb observed across all topographic positions, and the shrub leadplant (Amorpha canescens Nutt.) was found more commonly on dune than interdune positions (see Schacht et al., 2000).

Material and Methods Plant Production Sampling Study Site Research was conducted at the University of Nebraska − Lincoln Barta Brothers Ranch located in the eastern region of the Nebraska

25 m

Figure 1. Representation of the four common topographic positions in the Nebraska Sandhills.

Plant production data were collected from 1.2 × 1.2 × 1.2 m wire livestock exclosures placed at four topographic positions in eight pastures from 2001 to 2008 and three pastures from 2009 to 2017. Topographic positions included dune tops, interdunes, north-facing slopes, and south-facing slopes. Before the beginning of the study in 2000, cattle had grazed continuously at moderate stocking rates during the growing season from about mid-May to mid-October. From 2001 to 2008, cattle were managed within a four-pasture deferred rotation or an eight-pasture short-duration grazing rotation during the growing season for a study evaluating differences in plant production and species composition between these grazing methods (Stephenson et al., 2013). Study pastures were grazed at moderate stocking rates (i.e., 1.8 − 1.9 animal unit months (AUM) ∙ ha −1). At the conclusion of the grazing methods study, three representative pastures were selected in 2009 to continue collection of plant production data. Representative pastures were chosen on the basis of their similarity in variance to the overall mean. All pastures were grazed within four-pasture deferred rotations from 2009 to 2017 at moderate stocking rates. The timing of grazing on the pastures was rotated annually so that each pasture was deferred from grazing until early September in 1 of 4 yr. At each topographic position within the study pastures, five exclosures were placed toward the middle of 50- to 100-m long transects at intervals of 5 − 10 m. Exclosures were systematically moved 2−3 m from the previous location in a yearly defined direction at the beginning of each growing season to avoid sampling in the same location and to capture the previous year’s grazing treatment effect. Aboveground plant production data were collected from a randomly selected side of the exclosure in a mid-June harvest and the other side of the

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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exclosure in a mid-August harvest from 0.25-m2 (0.25 × 1.0 m) quadrats. A mid-June harvest was selected to capture the estimated peak standing crop for cool-season grasses and a mid-August harvest to capture peak standing crop for warm-season grasses. All current-year, aboveground herbaceous biomass of plants rooted within the quadrat was hand clipped at ground level and separated into different plant functional groups (i.e., warm-season grasses, cool-season grasses, sedges, and forbs). Current-year growth of shrubs (i.e., leaves and new stem tissue) rooted within the quadrat were collected and included in the estimations for total plant production but were not analyzed individually for this study. All standing dead plant material was separated from current-year plant production, and only current-year production was used in the analysis.

with the driest spring and growing season (i.e., 2002) and a year with a relatively wet spring (i.e., 2005, Fig. 2) during the 17-yr study period. Soil moisture content at different depths was compared among the topographic positions in 2002 and 2005, representing years with wet and dry spring precipitation. Time-domain reflectometry (TDR) was used to determine the percent volume of soil moisture from clear plastic tubes that were installed at central locations at each of the topographic positions (i.e., midslope for dune slope positions and the center of interdune and dunetop positions). Soil moisture measurements were collected at depths up to 3 m in the early to midpart of each month.

Precipitation and Soil Moisture Data

Plant production data at the different topographic positions were analyzed using a linear mixed model, repeated measure analysis of variance conducted in JMP (SAS Institute Inc., Cary, North Carolina). Topographic position and spring precipitation were used as fixed effects for the mid-June harvest, and topographic position and growing season precipitation were fixed effects for the mid-August harvest. Pasture was treated as a random variable. Compound symmetry covariance structure was used to account for the repeated measurements in the same pastures. Data were square root transformed to achieve normal distribution in the analyses, but actual, untransformed means were reported. To analyze differences in soil moisture between topographic positions at different depths of the soil profile, data were analyzed using a similar mixed model analysis. Data from a year with a dry spring (i.e., 2002) and a wet spring (i.e., 2005) were analyzed separately during the months data were collected. Topographic position and measurement depth were treated as fixed effects in the models, and pasture was treated as a random variable.

Precipitation data were collected from an on-site weather station from 2001 through 2017 and from a longer-term weather station in the nearby town of Ainsworth, Nebraska (40 km northwest of the Barta Brothers Ranch, HPRCC, 2018). Measurements of spring (1 April to 15 June) and growing season (1 April to 15 August) precipitation were selected to correspond with the harvests taking place in midJune and mid-August. Mean annual precipitation in Ainsworth, Nebraska was 56.4 cm ± 13.8 standard deviation (SD) (1940 − 2017) with 38.4 cm ± 10.8 SD, or 68%, falling during the growing season. Two of the lowest annual precipitation yr since 1940 occurred during the study period in 2002 and 2012 with only 51% and 48% of the longterm, mean annual precipitation falling in these years, respectively. Soil moisture data were collected at monthly intervals from April through July from 2002 through 2007 in four of the study pastures. This 5-yr period when soil moisture data were collected had the year

Statistical Analysis

Figure 2. Spring (1 April−15 June), summer (16 June−15 August), growing season (1 April to 15 August) and total annual precipitation at the University of Nebraska Barta Brothers Ranch in the eastern Nebraska Sandhills from 2001 to 2017.

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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Precipitation use efficiency (PUE) was estimated as the ratio of midJune plant production and spring precipitation (1 April–15 June) and the ratio of mid-August plant production and growing season precipitation (1 April–15 August). Precipitation marginal response (PMR) was determined by estimating the slope of the linear regression between plant production and spring or growing season precipitation for the mid-June or mid-August harvest, respectively. For PMR, we developed slopes from data at each of the topographic positions in each pasture from 2001 to 2017 and compared the slope coefficients among the different topographic positions. Precipitation use efficiency and PMR are both valuable measures in determining relationships between plant production and weather factors (Verón et al., 2005; Irisarri et al., 2016). The PUE expresses the amount of output (i.e., plant production) by a unit amount of input (e.g., spring or growing season precipitation), whereas PMR, or the slope of the regression, provides an indicator of the sensitivity of production to changes in input (Verón et al., 2005). Differences among topographic positions in PUE and PMR were analyzed using a mixed model analysis of variance in JMP with topographic position as the fixed effect and pasture as a random variable. Data for PUE were square root transformed for normality, but PMR data were normally distributed and untransformed means were used in the analysis. Lastly, we determined the relationship between precipitation and total plant production on the basis of the amount of each topographic position in the study pastures. Linear regression models were developed using total plant production values that were weighted by the estimated proportion of each topographic position (i.e., 15% dunetop, 15% interdune 34% north-facing slope, and 36% south-facing slope) in the

study pastures. Based on greater R2 values, total growing season precipitation was used for the mid-August harvest and spring precipitation for the mid-June harvest. Results Precipitation and Soil Moisture Precipitation data collected at the Barta Brothers Ranch from 2001 through 2017 indicated that 39% ± 11 SD of the annual precipitation occurred in spring (1 April to 15 June) and 28% ± 9 SD occurred in summer (16 June to 15 August) (see Fig. 2). The 3 driest yr for spring precipitation, in order of their severity, were 2002, 2006, and 2012 and the 3 driest yr for growing season precipitation were 2002, 2012, and 2006 (see Fig. 2). A 4-yr period from 2008 to 2011 exhibited above-average growing season precipitation. Mean high temperatures during the growing season ranged from a minimum of 22.9○C in 2009 to a high of 27.9○C in 2012. Soil moisture in 2002 and 2005 exhibited a significant (P b 0.01) topographic position-by-measurement depth interaction in each month of the years analyzed except in April 2005, which had significant topographic position and measurement depth main effects (P b 0.05). During the dry conditions in 2002, minimal differences were detected between dune and interdune positions at depths of ≤ 1 m in each month data were collected, but interdune positions had greater soil moisture at deeper depths compared with dune positions (Fig. 3A−D). Soil moisture during the wet spring in 2005 was greater on interdune positions

Figure 3. Mean soil moisture (volume %, ± 1 SE) collected at five depths in early to mid-April (A), May (B), June (C), and July (D) at the Barta Brothers Ranch in a year with a dry (i.e., 2002) and a wet (i.e., 2005) spring at dunetop (DT), interdune (ID), north-facing slope (NS), and south-facing slope (SS) topographic positions.

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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Table 1 Mean values and comparisons of total plant production, cool-season grass production, and warm-season grass production and mean relative abundance of cool- and warm-season grass production to total plant production collected at dunetop (DT), interdune (ID), north-facing slope (NS), and south-facing slope (SS) topographic positions in mid-June and mid-August from 2001 to 2017. DT

Relative abundance June Cool-season grass Warm-season grass August Cool-season grass Warm-season grass 1

ID

NS

SS

Topo

Precipitation

Topo X Precipitation

Prob N F

1111 B1 451 B 335 A

1752 A 896 A 408 A

1190 B 520 B 261 B

1159 B 469 B 387 A

69 44 26

b 0.01 b 0.01 b 0.01

b 0.01 b 0.01 0.89

b 0.01 b 0.01 0.31

1769 B 495 B 722 B

2683 A 940 A 1039 A

1804 B 537 B 610 C

1870 B 532 B 843 B

109 42 49

b 0.01 b 0.01 b 0.01

b 0.01 b 0.01 b 0.01

b 0.01 b 0.01 0.83 Prob N F

% of total plant production 38% B 31% A

47% A 25% B

41% B 22% B

38% B 34% A

2% 2%

b 0.01 b 0.01

b 0.01 b 0.01

0.75 0.94

28% B 42% AB

32% A 39% BC

29% AB 35% C

29% AB 45% A

2% 2%

b 0.01 b 0.01

b 0.01 0.90

b 0.01 0.22

Different letters represent differences (P b 0.05) between topographic positions.

compared with dune positions at all depths in April (see Fig. 3A) and at depths of ≥1 m in May, June, and July (see Fig. 3B−D). Soil moisture on interdunes was not different than other topographic positions at depths b 0.5 m in May, June, and July in 2005.

Aboveground Plant Production and Precipitation Use Efficiency Mid-June Harvest Averaged over all years, total plant production in mid-June was 62−66% of the production collected in mid-August at the different topographic positions. Total plant production was 47− 58% greater and precipitation use efficiency (PUE) was 35 − 46% greater on interdune compared with dune topographic positions (Tables 1 and 2). No differences (P N 0.30) were detected in plant production across the dune topographic positions. Total plant production increased significantly (P b 0.01) with increasing spring precipitation at all topographic positions, but increases were greater on interdune compared with dune topographic positions (see Table 1 and Fig. 4A). Table 2 Mean precipitation use efficiency and precipitation marginal response for total plant production, cool-season grass production, and warm-season grass production at dunetop (DT), interdune (ID), north-facing slope (NS), and south-facing slope (SS) topographic positions collected in mid-June and mid-August from 2001 to 2017. DT

ID

NS

SS

SE

Prob N F

kg · cm−1 · ha−1 Precipitation use efficiency (PUE) June Total 57.6 B1 Cool-season grass 27.8 B Warm-season grass 18.2 A August Total 51.2 B Cool-season grass 13.9 B Warm-season grass 21.3 B

83.9 A 39.8 A 20.4 A

62.1 B 25.0 B 13.8 B

59.3 B 22.2 B 20.5 A

2.9 b 0.01 2.4 b 0.01 1.6 b 0.01

75.0 A 23.7 A 28.7 A

50.6 B 14.4 B 17.2 C

53.9 B 14.8 B 24.4 B

2.1 b 0.01 0.2 b 0.01 1.4 b 0.01

Precipitation marginal response (PMR) June Total 11.8 B 38.5 Cool-season grass 9.4 B 27.9 Warm-season grass −2.2 B 2.3 August Total 36.2 B 70.9 Cool-season grass 10.3 B 35.5 Warm-season grass 17.0 20.9 1

SE

kg · ha−1

Plant production June Total Cool-season grass Warm-season grass August Total Cool-season grass Warm-season grass

A 16.3 B 15.1 B 4.3 b 0.01 A 12.4 B 8.7 B 3.9 b 0.01 A −1.3 AB 0.1 AB 1.1 0.05 A A

30.9 B 12.8 B 14.6

39.0 B 12.3 B 14.5

6.1 b 0.01 3.5 b 0.01 3.1 0.29

Different letters represent differences (P b 0.05) between topographic positions.

Cool-season grass production in mid-June was greater (P b 0.01) on interdune compared with dune positions, and a stronger relationship between spring precipitation and production was observed on interdune compared with dune positions (see Table 1, Fig. 4E). Precipitation use efficiency for cool-season grasses was 1.4- and 1.8-fold greater on interdune compared with dune topographic positions (Table 2). Relative abundance of cool-season grass production to total plant production was greater (P b 0.01) on interdune compared with dune positions, and relative abundance was positively influenced by increasing growing season precipitation at all topographic positions (see Table 1). Warm-season grass production and PUE were greater on interdune, dunetop, and south-facing slopes compared with north-facing slopes (see Tables 1 and 2). No significant relationships were observed between spring precipitation and warm-season grass production at all of the topographic positions (see Table 1, Fig. 4C). However, the relative abundance of warm-season grass production to total plant production when averaged across topographic positions decreased with increasing spring precipitation, in contrast to what was observed for cool-season grass production (see Table 1). Warm-season grass production comprised a greater proportion of the total plant production on south-facing slopes and dunetops compared with north-facing slopes and interdunes (see Table 1). Mid-August Harvest Similar to the mid-June harvest, total plant production was 43−52% greater on interdune compared with dune positions, but no differences were observed between the dune positions (see Table 1). Precipitation use efficiency for total plant production was 39 − 48% greater on interdune compared with dune positions (see Table 2). Total plant production increased at all topographic positions with increasing growing season precipitation, but the relationship between growing season precipitation and total plant production was greater on interdune compared with dune positions (Fig. 4B). The relationship between growing season precipitation and total plant production was not different between the dune topographic positions. Cool-season grass production was 1.7- to 1.9-fold greater on interdune compared with dune positions (see Table 1). Growing season precipitation positively influenced cool-season grass production at all topographic position, but the increase was significantly greater on interdune compared with dune positions (see Table 1 and Fig. 4F). Cool-season grass PUE was 1.6- to 1.7-fold greater on interdune compared with dune positions (see Table 2). The relative abundance of cool-season grass production to total plant production increased

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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June Harvest

August Harvest

A.

B.

C.

D.

E.

F.

Figure 4. Relationship between total plant production (A and B), warm-season grass production (C and D), and cool-season grass production (E and F) and spring precipitation (1 April−15 June) for a mid-June harvest (A, C, and E) and growing season precipitation (1 April−15 August) for a mid-August harvest (B, D, and F). Points are mean values for each year of the study (kg · ha−1, ± 1 SE) at dunetop (DT), interdune (ID), north-facing slope (NS), and south-facing slope (SS) topographic positions, and lines show the linear regression between plant production and precipitation.

significantly more (P b 0.01) on interdune than dune positions with increasing growing season precipitation (see Table 1, Fig. 4F). Warm-season grass production was 23−70% greater on interdune compared with dune topographic positions, and production was 38% and 18% greater on south-facing slopes and dunetops compared with north-facing slopes, respectively (see Table 1). Precipitation use efficiency followed a similar pattern to production for the interdune and dune positions (see Table 2). Warm-season grass production at all the topographic positions increased similarly (P = 0.83) with increasing amounts of growing season precipitation (Fig. 4D). Relative abundance

of warm-season grass production to total plant production remained constant (P = 0.90) across the topographic positions regardless of growing season precipitation, but the relative abundance of warm-season grass production was greater (P b 0.01) on south-facing slopes compared with interdune and north-facing slope positions (see Table 1). Precipitation Marginal Response Precipitation marginal response for total plant production was 2.4to 3.3-fold greater in mid-June and 1.8- to 2.3-fold greater in mid-

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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growing season precipitation = 48.1 ± 3.9 cm) plant production was 17% (2291 ± kg · ha−1) greater than average. The relationship between growing season precipitation and total plant production explained a moderate proportion of the variation in total plant production (R 2 = 0.49, P b 0.01). In mid-June, less variation (R2 = 0.23, P b 0.01) in total plant production was explained by spring precipitation (Fig. 5A). Average total plant production in mid-June was 1251.8 ± 71.9 kg · ha −1 with 22% less production (970.7 ± 198.9 kg · ha −1) in the 3 driest springs (i.e., 2002, 2006, and 2012, mean spring precipitation = 11.5 ± 1.3 cm) and 28% greater production (1599.2 ± 161.4 kg · ha −1) in the 3 wettest springs (i.e., 2010, 2016, 2005, mean spring precipitation = 35.7 ± 4.0 cm). Discussion

Figure 5. Relationship between total plant production (kg · ha−1, ± 1 SE) and spring precipitation in mid-June (A.) and growing season precipitation in mid-August (B.). Values represent estimates of total plant production weighted by the proportion of different topographic positions on pastures at the Barta Brothers Ranch (i.e., 15% interdune, 15% dunetop, 34% north-facing slope, and 36% south-facing slope) from 2001 to 2017.

August on interdunes compared with dune positions (see Table 2). Precipitation marginal response for cool-season grass production on interdunes was between 2.3- and 3.4-fold greater on interdunes than dune positions in mid-June and mid-August. No differences (P N 0.05) were detected in PMR between the dune positions for both total production and cool-season grass production. Warm-season grass PMR in mid-June exhibited a negative or neutral response on the dunes and a slight positive response on the interdunes with the only difference observed between dunetop and interdune positions (see Table 2). In mid-August, no differences were detected in the PMR of warm-season grasses at the different topographic positions (see Table 2). Total Plant Production at Pasture Scale Average total plant production in mid-August when weighted by the proportions of the different topographic positions observed on the study pastures was 1955 ± 124.2 kg · ha −1 (Fig. 5B). In the 3 driest yr (i.e., 2002, 2012, and 2006, mean growing season precipitation = 20.8 ± 1.0 cm), total plant production was 39% (1193 ± 113.4 kg · ha − 1) below average and in the 3 wettest yr (i.e., 2010, 2008, 2016,

Plant production data collected at the different topographic positions in the eastern Nebraska Sandhills indicated that total plant production, PUE, and PMR were greater on interdune compared with dune topographic positions in both the mid-June and mid-August harvests. An important driver in the response of total plant production to spring and growing season precipitation was a greater response of cool-season grass production to increasing precipitation at the interdune compared with dune topographic positions. Warm-season grass production was greater on interdune compared with dune positions, but the marginal response of warm-season grass production to precipitation was similar across the topographic positions at peak production in mid-August. Although other research in the Great Plains has reported greater plant production on lowland positions (i.e., interdunes) compared with associated uplands (Briggs and Knapp, 1995; Nippert et al., 2011), our results indicate that cool-season grass and total plant production are more sensitive to differences in spring and growing season precipitation on interdune compared with dune topographic positions in the Sandhills, which provides important insight into the spatial variability of warm- and cool-season grass production across the landscape in wet and dry years. The greater PUE and PMR of cool-season grasses on interdunes compared with dunes was likely the result of greater early-season soil moisture availability in wet years and differences in cool-season grass species composition across the topographic positions. Kentucky bluegrass dominated the cool-season grass species composition on interdunes within the study pastures. Schacht et al. (2000) indicated that frequency of occurrence of Kentucky bluegrass at the Barta Brothers Ranch was 72.5% on interdunes compared with an average of only 6.6% on the dune positions. Kentucky bluegrass is a shallow-rooted, introduced grass that has invaded and altered native plant communities in many regions of the northern Great Plains (Toledo et al., 2014). Kentucky bluegrass is productive during wet and cool conditions in early spring, but it becomes dormant during hot, dry periods (Wedin and Huff, 1996). Within our study pastures in years with dry springs, coolseason grass production on the interdunes was closer to production amounts on dune positions. For example, in 2002 and 2006, the 2 driest yr for spring precipitation during the study, cool-season grass production was only 8% greater on interdune compared with dune positions, but during 2010 and 2005, the 2 yr with the wettest springs of the study, cool-season grass production was 103% greater on interdune compared with dune positions. The reduced cool-season grass production in the years with dry springs influenced total plant production in mid-August with only 23% greater production on interdune compared with dune positions in the years with the driest springs, but 70% greater production on interdunes during the years with the wettest springs. While considered invasive in the Sandhills, greater amounts of Kentucky bluegrass may have benefits for livestock production in the northern Great Plains. Reeves et al. (2014) indicated that weight gains of yearlings on Kentucky bluegrass − invaded rangelands were greater post invasion compared with preinvasion and that cattle production on rangelands invaded by Kentucky bluegrass was positively influenced

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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by wet, cool springs and wet conditions during the previous year. Research in the shortgrass steppe and northern mixed grass prairie also has indicated that increases in cool-season grasses during years with above-average precipitation strongly influences greater total plant production (Derner et al., 2008). Increased total plant production with increasing precipitation on interdune positions in our study was attributed mostly to increases in cool-season grass production, which exhibited larger increases in both production and relative abundance on interdune positions compared with dune positions as precipitation increased. Because of this differential response of total plant production on interdune compared with dune positions in both mid-June and midAugust, differences in forage available for grazing livestock during wet and dry years will vary by the relative amount of the different topographic positions within a pasture. Warm-season grass production and PUE were greater on interdune compared with dune positions in mid-August, but the response of production to increasing growing season precipitation (i.e., PMR) did not differ among the topographic positions. Warm-season grass production and PUE also were greater on dunetops and south-facing slopes compared with north-facing slopes. In addition, relative abundance of warm-season grass production to total plant production varied by topographic position but remained relatively constant regardless of growing season precipitation. In contrast, cool-season grass relative abundance increased more on interdune compared to dune positions with increasing growing season precipitation. The warm-season tallgrass, switchgrass, was the most frequently observed warm-season grass on interdune positions at our study site but was also an important warmseason grass species on the dune positions. Warm-season species more associated with dune positions (i.e., prairie sandreed, little bluestem, and sand bluestem) were typically infrequent on interdune positions. Aspect influences species distribution in the Sandhills with rhizomatous warm-season tallgrasses, sand bluestem and prairie sandreed, typically being more prevalent on south-facing slopes and dune topes compared with north-facing slopes in the Sandhills (Tolstead, 1942; Bragg, 1978; Schacht et al., 2000). Greater light intensity and the subsequent increase in soil surface temperatures on south-facing slopes in grassland systems provide better plant habitat for warm-season grass species with higher water use efficiency compared with associated cool-season grass species (Barnes et al., 1983). However, the PMR of warm-season grass production at our study site suggests that warm-season grasses exhibit a similar sensitivity to growing season precipitation across the topographic positions and differences in plant species or other variables affected by topographic position did not influence the response. The greater amount of cool- and warm-season grass production and the greater PMR of cool-season grass and total plant production to increasing precipitation on interdune compared with dune positions were likely influenced by differences in soil moisture availability early in the growing season during wet and dry years. Soil moisture was greater on interdune than dune positions at shallow depths (i.e., b 1 m) early in a year with a wet spring, but soil moisture was not different among the topographic positions at shallow depths early in a year with a dry spring (see Fig. 2A). Soil moisture throughout the growing season was greater on interdunes than dunes at depths N 1 m in a wet year and 2 m in a dry year, but research in the tallgrass prairie has indicated that soil moisture at these greater depths may not strongly influence plant production (Nippert et al., 2012). The coarse-textured soils found in the Sandhills have a high rate of rainfall infiltration that is presumably used differently by deep-rooted and shallow-rooted grass species at the dune and interdune topographic positions, respectively. In the western part of the Sandhills, Barnes et al. (1984) reported that deeperrooted warm-season tallgrass species were more common on the coarser-textured soils found on the dunes because of the high water infiltration and the greater use of this deeper soil moisture throughout the growing season. The authors also indicated that shallower-rooted species found on the finer-textured soils of the interdunes were better

able to use soil moisture in the upper soil profiles of interdunes. Schacht et al. (2000) reported no differences in soil texture among the dune and interdune positions at our site in the eastern Sandhills but found that soil organic matter, phosphorus, and potassium in the upper 15 cm were greater on interdune compared with dune positions, which further explains the potential for greater plant production on interdunes. With greater soil moisture availability, soil nutrient abundance, and plant production, interdunes provide an important component of the landscape in the Sandhills and should be considered separately from the dune positions when managing these grasslands for grazing, resiliency, and diversity. The PMR of plant production at the different topographic positions and the linear regression models at the pasture scale provide valuable information for estimating total plant production outputs based on additional spring and growing season precipitation inputs. Growing season precipitation explained 49% of the variation in total plant production in mid-August, but less variation in plant production was explained by spring precipitation for the mid-June harvest. Including fall and winter precipitation may have increased the amount of plant production variability that was explained during the mid-June harvest. Patton et al. (2007) found that the amount of precipitation from the end of the previous year growing season to the middle of the current year growing season strongly influenced plant production on Kentucky bluegrass and smooth bromegrass (Bromus inermis Leyss.)−dominated rangelands in the northern Great Plains. Other weather variables including freeze date and accumulated growing degree days (Smart et al., 2007), timing of precipitation and temperature (Craine et al. 2012), and the frequency and size of precipitation events (Wilcox et al., 2015) also may provide greater insight into variability in plant production. Additional research is needed to better understand how these variables influence plant production at the topographic positions in the Sandhills. Overall, our results highlight key differences in plant production and response of warm- and cool-season grasses to precipitation among different topographic positions within grazed pastures in the Nebraska Sandhills. Including topographic position as a variable in estimating plant production will provide more accurate measurements and predictions for total plant production and production of plant functional groups, especially in years with wet springs. Climate change projections for the northern Great Plains indicate that the Sandhills region is expected to have greater annual temperatures, greater winter precipitation, and increased variability in growing season precipitation by 2050 (Polley et al., 2013). Earlier spring warm-up and greater winter precipitation will likely be more favorable for cool-season grass production in the northern Great Plains (Derner et al., 2018). Increased early spring precipitation in our study and the resulting increased soil moisture during a wet year were important drivers in the greater cool-season grass production on interdune compared with dune positions. The greater response of cool-season grass production to spring and growing season precipitation suggests that plant production on interdunes may be more responsive to future climate scenarios with wetter winters and earlier growing seasons. Management Implications The Nebraska Sandhills region is a topographically diverse mixedgrass prairie. The variable topography influenced production of cooland warm-season grasses, which are the primary sources of forage for livestock grazing on these landscapes. Interdune positions at our study site typically had greater production of cool- and warm-season grasses and total plant production compared with dune positions. Including the greater precipitation marginal response of cool-season grass and total plant production on interdunes in models will provide better estimates of aboveground plant production. In addition, incorporating the relative spatial area of each topographic position (e.g., 15% interdune) into estimates of total plant production at the pasture or ranch scale

Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001

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Please cite this article as: Stephenson, M.B., et al., Influence of Precipitation on Plant Production at Different Topographic Positions in the Nebraska Sandhills, Rangeland Ecology & Management (2018), https://doi.org/10.1016/j.rama.2018.09.001