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Germination, Survival, and Growth of Grass and Forb Seedlings: Effects of Soil Moisture Variability

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Germination, Survival, and Growth of Grass and Forb Seedlings: Effects of Soil Moisture Variability Acta Oecologica 35 (2009) 679–684 Contents lists available at ScienceDirect Acta Oecologica journal homepage: www.elsevier.com/locate/actoec Original article Germination, survival, and growth of grass and forb seedlings: Effects of soil moisture variability Philip A. Fay a,*, Megan J. Schultz b,1 a USDA-ARS, Grassland Soil and Water Research Laboratory, 808 E. Blackland Road, Temple, TX 76502, USA b Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811, USA article info abstract Article history: Seed germination and seedling growth, survivorship, and final biomass and their responses to watering Received 31 January 2009 interval were studied in two grass and six forb species to assess germination and seedling growth Accepted 13 June 2009 responses to increased soil moisture variability as might occur with future increases in precipitation Published online 14 July 2009 variability. Seeds were planted in prairie soil and watered at 1, 2, 4, or 7 d intervals (I). Seed germination peaked at I ¼ 4 d whereas leaf growth in grasses and forbs, and final biomass in grasses peaked at I ¼ 7d, Keywords: suggesting that growth and biomass were favored at greater soil moisture variability than seed germi- Allocation nation. Biomass responses to I were stronger than the germination responses, suggesting that soil Biomass Climate change moisture variability more strongly influenced post germination growth. Individual species responses to I Community fell into three groups; those with responses to I for: (1) seed germination and seedling survival, (2) Establishment biomass, or (3) both germination and biomass production. These species groups may be more useful than Extreme events life form (i.e., grass/forb) for understanding seed germination and seedling dynamics in grasslands Functional group during periods of soil moisture variability. Seed germination and early growth may assume more Grassland importance in grassland plant community dynamics under more variable precipitation patterns. Plant life-form Published by Elsevier Masson SAS. Rainfall Seed germination and seedling growth are critical stages of associated with growth form or life history reflect adaptations plant development leading to the establishment of new individuals to environmental variability (Matthews, 1976; Qi and Redmann, in plant communities. These two aspects are highly sensitive to 1993; Dubrovsky, 1998; Perez-Fernandez et al., 2000; Flores and environmental variability, and require favorable conditions of soil Briones, 2001). moisture, which is rarely static. As a result, seeds in the soil in natural Understanding how seeds and seedlings fare under varying soil conditions routinely experience periods of alternating wet and dry moisture is important for determining how soil moisture variation conditions (Baskin and Baskin, 1982) caused by the episodic nature may affect recruitment into plant communities. For example, several of precipitation. Tolerance of varying soil moisture also depends on studies report that pulses of seed germination and seedling estab- the stage of plant development (Wilson et al., 1974). For example, in lishment in field populations follow pulses of precipitation (Wilson, the C4 grass Bouteloua gracilis, levels of water stress which promoted 1973; Vincent and Cavers,1978; Roberts and Potter,1980). The timing high germination were unsuitable for seedling survival (Qi and of such events may have a marked effect on competitive success Redmann, 1993). In shortgrass steppe, grass species that germinated (Larsen et al., 2004; Weltzin and McPherson, 2000). The duration well under a wide range of temperature and moisture conditions between precipitation inputs is a primary determinant of soil mois- were intolerant of drought as established plants, whereas other grass ture variability. Most climate change scenarios forecast increasingly species with narrow germination and establishment requirements extreme precipitation patterns (Christensen and Hewitson, 2007). survived drought when established (McGinnies, 1960). Thus, These will likely be manifest as longer droughts broken by large conditions that may favor germination may not favor continued precipitation events. Increases in the frequency of extreme precipi- growth and survival of seedlings or later stages (Lloret et al., 2004). tation events have been documented (Groisman et al., 2005; IPCC, Several studies have shown that differences in germination patterns 2007). Knapp et al. (2002) reported that experimental rainfall applications that increased the occurrence of extreme events resul- ted in increased plant diversity and species turnover in grassland. Changes in plant community composition under more extreme * Corresponding author. Tel.: þ1 254 770 6533; fax: þ1 254 770 6561. E-mail address: [email protected] (P.A. Fay). precipitation patterns, particularly during spring, could arise in part 1 Present address: Mayo Clinic, Rochester, MN, USA. from changes in seed and seedling success. 1146-609X/$ – see front matter Published by Elsevier Masson SAS. doi:10.1016/j.actao.2009.06.007 680 P.A. Fay, M.J. Schultz / Acta Oecologica 35 (2009) 679–684 To evaluate the association between seed/seedling success and surface. When no further germination occurred (day 21), seedlings soil moisture variability we conducted a watering interval experi- were thinned to one per tube. Growth of these post germination ment addressing two questions: (1) How does soil moisture varia- seedlings was recorded at 10–14 d intervals by measuring plant tion arising from longer periods (I) between water inputs affect seed height from the soil surface to the distal leaf tip in the grasses, and by germination and seedling survivorship and initial growth? (2) How counting expanded leaves for the forbs. Survivorship of the seedlings do species and growth forms differ in germination and growth was estimated from the number of tubes containing plants at the last responses to soil moisture variation? The approach was a green- growth measurement (day 56). The plants were then harvested and house study using seeds planted in soil, to approximate the substrate the roots were carefully washed free of soil. The roots and tops and drying rate encountered by seeds in natural populations. The were separated, dried to constant mass, and weighed to determine primary hypothesis was that soil moisture variability would affect biomass and allocation above vs. belowground. germination and seedling growth differently both among species and growth forms. 1.3. Data analysis 1. Methods The tube was the experimental unit for statistical analysis. Percent germination in each tube was computed for each obser- The study focused on two grasses, Andropogon gerardii Vitman vation day. Germination and growth curves were analyzed for and Sorghastrum nutans (L.) Nash, and six forbs, Amorpha canescens I, day, and I  day effects by life form (grass/forb) and by species Pursh, Asclepias tuberosa L., Echinacea purpurea (L.) Moench, using a repeated measures ANOVA. Logistic regression was used to Lespedeza capitata Michx., Liatris punctata Hook., and Psoralea analyze the number of seeds germinated per tube on day 21 and the tenuiflora (Pursh) Rydb. A. gerardii and S. nutans are C4 tall grasses number of surviving plants at day 56. Biomass and allocation and dominant in large portions of their ranges, accounting for over variables were assessed for I, life form, and I  life form effects 50% of net primary production in some cases (Freeman, 1998; using ANOVA. Analyses were conducted using Proc Mixed and Proc Silletti and Knapp, 2001). The forbs are also widespread and locally Logistic in SAS 9.1 (SAS Institute, Cary, NC, U.S.A.). abundant in the North American grasslands. These species were of further interest because of their inclusion in other precipitation 2. Results variability studies (Fay et al., 2008) 2.1. Soil moisture 1.1. Culture method and treatments The mean and variability in soil moisture was strongly affected The experiment was conducted in a temperature-controlled by watering interval. The soil dried between waterings at about the greenhouse in Duluth, MN U.S.A. during July and August, 2004. The same rate in all four treatments, with tubes losing approximately À1 mean afternoon air temperature was 25.1 Æ 0.5C and the mean 2% of their mass, approximately 3 g, of H20d (Fig.1). Thus, the soil nighttime minimum was 17.6 Æ 0.3 C, which represented spring in tubes watered at longer intervals varied more and became temperatures commonly encountered by these species during significantly drier between successive waterings than those with germination and establishment. Illumination was from natural shorter watering intervals (F 3,59 ¼ 226.1, p < 0.0001; Fig. 1 inset). daylight, with midday peaks varying between 500 and 1800 mmol À2 À1 m s photosynthetic photon flux density. 2.2. Germination We prepared 640 plastic tubes (3.8 cm diameter  21.0 cm deep, 3 164 cm , ConetainerÔ, Stuewe and Sons, Corvallis, OR, U.S.A.) con- Germination in the grasses averaged 30–45%, compared to 20– taining 80% sieved native prairie soil, 15% peat and 5% vermiculite 25% for the forbs (p < 0.0001, Table 1, Fig. 2). Germination for both (v:v:v). Eighty tubes per species were planted with 3 seeds per tube forbs and grasses were affected by the watering treatments, with at 1 cm depth in the soil mix. Seeds were obtained from commercial significant I and days since planting effects for grasses and forbs, sources, and were not pre-conditioned, but the resulting germina- tion rates were adequate for the objective of comparing watering interval treatments. The tubes were top-dressed with 2 g of slow 100 release fertilizer (OsmocoteÒ, The Scotts Company, Marysville, OH, Watering interval (d) U.S.A.). Twenty tubes per species were then randomly assigned to be 1 watered at 1, 2, 4 or 7 d intervals (I). Watering was performed with 98 2 a hand nozzle with a gentle flow pattern. Water was applied to the 4 7 tubes until the soil appeared fully infiltrated. The small diameter of the tubes made watering individual tubes impractical, so all tubes 96 assigned to an I were grouped on the greenhouse bench and watered together.
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