Educational Module 3
Microclimate and Plant Response (Germination and Emergence Syndromes)
- Understand how different plant materials might respond to weather and microclimate variability.
- Evaluate alternative germination/emergence syndromes and how different plant materials might respond at different sites and in different years.
- Compare potential microclimate response of seeded grasses to potential response of cheatgrass.
- Assess the relative effect of planting date on potential germination and pre-emergence seedling mortality.
- Discuss how knowledge of differences in weed and seeded species response might affect or inform: choice of planting date; use of alternative seedbed preparation and planting treatments; selection of alternative plant materials; weed control strategies; our understanding of ecological resilience and resistance to weed invasion; bet-hedging strategies for increasing the likelihood of establishment success.
The general importance of climate is acknowledged in most seeding guides in the form of tables that list species and cultivar suitability as a function of mean annual precipitation (Jensen et al., 2001; Lambert 2005; Ogle et al., 2008a, 2008b). Unfortunately, the microclimatic requirements for germination, emergence and seedling establishment are much more restrictive than the longer-term climatic requirements for maintenance of mature plant communities (Call and Roundy, 1991; Peters, 2000; Hardegree et al., 2003). Current state-and-transition models acknowledge that there are a limited number of potential trajectories for moving between undesirable and desirable vegetation states (Westoby et al., 1989; Batabyal and Godfrey, 2002; Bestelmeyer et al., 2003; Briske et al., 2003, 2005, 2006, 2008; Bashari et al., 2008) and many transition pathways require a specific and infrequent series of climatic events (Westoby et al., 1989).
Planting date recommendations are usually based upon getting the seeds into the ground in time to take advantage of the most favorable season for plant establishment (Plummer et al., 1968; Vallentine 1979; Roundy and Call 1988; Monsen and Stevens 2004). In the Great Basin, the majority of rangeland seeding takes place in the fall, well in advance of the optimal growing season. Relatively recent research indicates that germination of fall-planted seeds is probably not a bottleneck for perennial grass establishment, and that germination in the field is typically much higher than seedling emergence (Hardegree and Van Vactor, 2000; Roundy et al., 2007; James et al., 2011; Boyd and Lemos, 2013). Newly germinated plants, however, are particularly vulnerable to abiotic soil conditions and post-germination/pre-emergent mortality from relatively short-term thermal and drought stress (James et al., 2011; Boyd and Lemos, 2013; Hardegree et al., 2016). Planting-date effects on germination timing, therefore, may have a large impact on vulnerability of seedlings to mortality events during seedling emergence. Hardegree et al. (2003, 2008, 2010, 2013) has documented the relatively high germination rate of cheatgrass but also significant variability in germination rate among perennial grass seedlots. Both time of planting, and germination syndrome (fast vs slow; uniform vs spread out) may have significant impacts on the probability of post-germination and pre-emergent mortality of seeded species (Hardegree et al., 2016).
The purpose of this exercise is to provide you with the tools to interpret microclimatic effects by their impact on the germination and emergence response of both perennial grasses and their weedy competitors. In this exercise, we will introduce potential germination response as a bio-assay for the favorability of seedbed microclimate; how different species and seedlots may respond to spatial and temporal variability in seedbed microclimate; and how this variability may interact with respect to decisions about planting date.
Suggested Reading/Viewing Assignments
- Boyd, C.S. and J.A. Lemos. 2013. Freezing stress influences emergence of germinated perennial grass seeds. Rangeland Ecology and Management 66:136-142.
- Boyd, C.S. and J.J. James. 2013. Variation in timing of planting influences bluebunch wheatgrass demography in an arid system. Rangeland Ecology and Management 66:117-126.
- Hardegree, S.P., C.A. Moffet, G.N. Flerchinger, J. Cho, B.A. Roundy, T.A. Jones, J.J. James, P.E. Clark and F.B. Pierson. 2013. Hydrothermal assessment of temporal variability in seedbed microclimate. Rangeland Ecology and Management 66:127-135.
- Hardegree, S.P., R.L. Sheley, S.E. Duke, J.J. James, A.R. Boehm and G.N. Flerchinger. 2016. Temporal variability in microclimatic conditions for grass germination and emergence in the sagebrush steppe. Rangeland Ecology and Management 69:123-128.
- Hardegree, S.P., T.A. Jones, B.A. Roundy, N.L. Shaw and T.A. Monaco. 2016. Assessment of range planting as a conservation practice. Rangeland Ecology and Management 69:237-247.
- James, J.J., T.J. Svejcar and M.J. Rinella. 2011. Demographic processes limiting seedling recruitment in arid grassland restoration. Journal of Applied Ecology 48:961-969.
- Historical daily weather information from the 4-km Gridmet database for the period January 1, 1979 through the current year (Abatzoglou, 2013).
- Microclimatic simulations from the Simultaneous Heat and Water (SHAW) model for characterizing seedbed microclimate (Flerchinger and Hardegree, 2004; Flerchinger et al., 2012).
- Hydrothermal germination response models for cheatgrass and perennial grass species (Hardegree et al., 2013; 2015).
- Restoration Climatology Report
Step 1 - Use the Microclimate and Plant Response Form (below) and pick a location, soil type, slope and aspect characteristics and seedlots that you want to include in your analysis and download weather, microclimate, and seedlot response data.
Depending on what comparisons you want to make, download one or several location/microclimate/plant-response scenarios for comparison. The focus of this exercise is to use seedlot response as a bioassay for assessing site variability; or to evaluate relative seedlot response to alternative planting locations, or planting dates.
Fill out the Microclimate and Plant Response form.
Step 2 - Spreadsheet analysis
Open your downloaded files in a spreadsheet application and manipulate the data to assess the impact of planting location or planting date on the timing of germination response. Identify at least three elements or patterns that you believe would have a significant impact on the relative success of rangeland restoration at your site; that may indicate major seedlot differences in germination syndrome; or that may show fundamental differences between perennial grass and annual weed species.
Step 3 - Evaluate the Restoration Climatology Report
From the Climatology Report, pick 5 random years for analysis and evaluate how predicted germination response may be interacting with seasonal patterns of both favorable weather, and potential seedbed mortality factors of temperature and water stress.
Step 4 - Synthesis.
Synthesize your new awareness of topography, soils and weather on seedbed microclimate relative to seed population response. Select from one of the synthesis/discussion questions below, or generate your own discussion topics. This synthesis is not a simple discussion of the results of your hypothesis testing but should be broader in scope and address how your results inform broader management questions related to restoration planning, policy issues, or adaptive management.
- How does seedlot germination response vary as a function of soils, topography, location and/or year of seeding?
- Can you identify alternative germination syndromes/strategies that may be affecting restoration success at a given location?
- How might you take advantage of your new knowledge of germination response to microclimate to inform decisions about planting date?
- How does cheatgrass compare to the perennial grass seedlots relative to potential germination response?
- Seed germination is not usually limiting for perennial grass establishment. What about the potential impacts of soil freezing and near-surface drought on post-germination/pre-emergence mortality in any given year?