Hydrothermal Germination Models: Assessment of the Wet‐Thermal Approximation of Potential Field Response

Abstract

Hydrothermal (HT) models can be used to characterize seed germination response to field‐variable conditions of temperature (T) and water potential (Ψ). Hydrothermal response data are relatively difficult to generate, which limits their utility for large‐scale comparisons of inter‐ and intraspecies germination response. Previous studies have hypothesized that HT germination response can be estimated using a simple model for thermal‐time accumulation above a fixed threshold of environmental Ψ (wet‐thermal [WT] model). The purpose of this study was to test this hypothesis by explicitly comparing HT and WT germination rates and estimated cumulative germination response of 13 rangeland grass seedlots under simulated conditions of field‐variable T and Ψ. We used a 44‐yr weather record to parameterize a seedbed‐microclimate model for estimation of hourly T and Ψ at seeding depth for a sandy loam soil type at the Orchard field test site in southwestern Ada County, Idaho. Hydrothermal and WT germination responses for each hour of the simulation were estimated for 13 range grass seedlots. We found that overestimation of germination rate by thermal accumulation at low Ψ contributed relatively little to WT model error rates, and that >95% of the variability in predicted germination response could be explained by WT germination response above a Ψ 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.

Improving dryland seedling recruitment using fungicide seed coatings

“…One way that weather could influence seed and seedling disease severity is by affecting germination timing and growth of both plants and pathogens (Allen et al., 2018; Lamichhane et al., 2018; Hardegree et al., 2020). Germination and growth rates of plants and fungal pathogens are proportional to the amount that temperature and water potential exceed a threshold value (Allen et al., 2018; Barth et al., 2015; Bradford, 2002; Hardegree et al., 2018). Because threshold values and response rates are diverse and unique to individual species, it is likely that seeds and pathogens respond differently to microsite temperature and water potential (Allen et al., 2018; Lamichhane et al., 2018; Richardson et al., 2018; Hardegree et al., 2020).…”

“…Because seed dormancy is prevalent in over 80% of dryland species, fall dormant seedings are typical of dryland restoration projects (Baskin & Baskin, 2014). Seeding in the fall allows seeds to reach the hydrothermal accumulation thresholds required to overcome their dormancy, thereby priming them for emergence when conditions are favourable for plant growth in the spring (Beyers, 2004; Hardegree et al., 2018; James et al., 2019). In temperate drylands, the winter incubation period is conducive to fungal activity and growth due to the wet, cool conditions associated with snow cover (Aanderud et al., 2013; Gornish et al., 2015; Kuhnert et al., 2012).…”

Post-Fire Management Impact on Natural Forest Regeneration through Altered Microsite Conditions

“…Seedling establishment and survival on dry south-facing slopes greatly depend on soil water content, especially in a harsh post-fire environments. Indeed, even pioneer and drought-resistant species (i.e., Pinus sylvestris), at the establishment stage, need moisture to germinate [4,14].…”

“…Successful seedling establishment is based on microclimatic requirements that are usually more restrictive than those required for adult plant survival [14]. During ontogenetic stages, trees develop structures allowing limiting factors to be overcome (i.e., a deep or large root system able to reach water deeper in the soil).…”

Modelling seedling emergence in Paspalum species using environmental data from field experiments

“…Simplified approaches based on HTT models were developed to predict field emergence timing for annual weeds using data from field experiments and weather stations (Leguizamón et al., 2009; Martinson et al., 2007; Royo‐Esnal et al., 2010). One of these is the wet‐thermal time (WT) index (Hardegree et al., 2018; Roundy et al., 2007), where air or soil temperature above a given temperature threshold is accumulated over days when soil water potential is higher than a given threshold. Logistic functions have been used to fit the increase in emergence relative to the observed maximum as a function of wet‐thermal time accumulation.…”