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Title: Hydrothermal germination models: assessment of the wet-thermal approximation of potential field response

item Hardegree, Stuart
item ROUNDY, BRUCE - Brigham Young University
item Walters, Christina
item Reeves, Patrick
item Richards, Christopher
item Moffet, Corey
item Sheley, Roger
item Flerchinger, Gerald

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2018
Publication Date: 8/23/2018
Citation: Hardegree, S.P., Roundy, B., Walters, C.T., Reeves, P.A., Richards, C.M., Moffet, C., Sheley, R.L., Flerchinger, G.N. 2018. Hydrothermal germination models: assessment of the wet-thermal approximation of potential field response. Crop Science. 58(5):2042-2049.

Interpretive Summary: Hydrothermal germination models are a way of predicting the time-course of seed germination in the field. Prediction of field germination is essential to develop an understanding of why field seedings succeed or fail, which species are most appropriate to plant in a given location, and the optimal timing of planting to ensure success. Hydrothermal models, however, are very expensive to develop and previous studies have suggested that simpler wet-thermal germination models may give the same degree of accuracy in field predictions. We developed detailed hydrothermal germination models for 13 rangeland grass seedlots to specifically test whether the less expensive wet-thermal procedure would yield the same predictive accuracy. We simulated field conditions over 44 years and determined that wet-thermal models were at least 95% as accurate as hydrothermal models during typical planting scenarios in the field. Use of wet-thermal models could greatly increase our understanding of rangeland plant establishment in the Great Basin as the underlying thermal germination models are already available for many principal plant materials collections in the western US.

Technical Abstract: Hydrothermal models can be used to characterize seed germination response to field-variable conditions of temperature and water potential. Hydrothermal response data are relatively difficult to generate which limits their utility for large-scale comparisons of inter and intra-species germination response. Previous studies have hypothesized that hydrothermal germination response can be estimated using a simple model for thermal-time accumulation above a fixed threshold of environmental water potential. The purpose of this study was to test this hypothesis by explicitly comparing hydrothermal and wet-thermal germination rates and estimated cumulative germination response of 13 rangeland grass seedlots under simulated conditions of field-variable temperature and water potential. We used a 44-year weather record to parameterize a seedbed-microclimate model for estimation of hourly temperature and water potential at seeding depth for a sandy loam soil type at the Orchard field test site in southwestern Ada County, Idaho. Hydrothermal and wet-thermal germination responses were estimated for 2 seedlots of cheatgrass (Bromus tectorum L.), 4 seedlots of bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Löve], 3 seedlots of bottlebrush squirreltail [Elymus elymoides (Raf) Swezey], and one seedlot each of Sandberg bluegrass (Poa secunda J. Presl.), big squirreltail [Elymus multisetus (J.G. Smith) M.E. Jones], thickspike wheatgrass [Elymus lanceolatus (Scribn. And J.G. Smith) Gould] and Idaho fescue (Festuca idahoensis Elmer). We found that overestimation of germination rate by thermal accumulation at low water potential contributed relatively little to wet-thermal model error rates, and that over 95% of the variability in predicted germination response could be explained by wet-thermal germination response above a water potential threshold of -0.3 to -0.5 MPa. Given the pulse-like nature of favorable germination conditions in the field, this modeling approach may have immediate and wide potential application as there are a relatively large number of thermal-germination datasets currently available for rangeland plant species.