Heterogeneity of Kentucky Bluegrass (Poa pratensis L.) seed germination after controlled burning

Authors: Halvorson, Jonathan; Toledo, David; Hendrickson, John

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2022
Publication Date: 7/1/2022
Citation: Halvorson, J.J., Toledo, D.N., Hendrickson, J.R. 2022. Heterogeneity of Kentucky Bluegrass (Poa pratensis L.) seed germination after controlled burning. Rangeland Ecology and Management. 83:112-116. https://doi.org/10.1016/j.rama.2022.04.001.
DOI: https://doi.org/10.1016/j.rama.2022.04.001

Interpretive Summary: Fire is sometimes advocated as a means for controlling Kentucky bluegrass (KBG) in invaded grazing lands. However, little is known about the effects of fire on KBG seed survival. We placed seeds of KBG in shallow metal dishes at ground level, and exposed them to fire while monitoring temperature at the soil surface and at 10 cm. We then measured subsequent seed germination. Seed germination from burned plots, 37±7%, was much lower than for unburned control dishes,93 ±1%. Germination success was similar in plots previously managed with both grazing and fire (35 ± 10%) and plots previously managed by fire alone (38±11%). Maximum surface temperatures during the test burns averaged 271±23 oC but varied widely while maximum temperature at 10 cm above the soil surface was slightly higher, (301±25 oC). Seed germination decreased with increasing soil temperature measured during the burn. However, seed survival often was quite different between sample dishes that were close (= 60 cm) to each other. Such extreme variability may result from unburned areas remaining after the fire that acted as safe sites for seeds. This study indicates that fires can kill KBG seeds at the soil surface. However, use of fire to reduce viable seeds must be evaluated carefully since while fire may kill existing seeds stocks, it may also induce subsequent seed production.

Technical Abstract: Prescribed burning is sometimes advocated as a means for controlling Kentucky bluegrass (KBG) in invaded grazing lands. However, little is known about the effects of fire on KBG seed survival. We exposed seeds of KBG (c.v. Kenblue), placed in shallow metal dishes, at ground level, to prescribed burns while monitoring temperature at the soil surface and at 10 cm. and assessed subsequent seed germinability. Successful germination for the control dishes averaged 93 ±1%, much higher than from burned plots, 37±7%. Germination success was similar in burned plots previously managed with both grazing and fire (35 ± 10%) and plots previously managed by fire alone (38±11%). Maximum surface temperatures during the test burns averaged 271±23 oC but varied widely (range 41 – 509 oC) while maximum temperature at 10 cm above the soil surface was slightly higher, (301±25 oC). Germinability was significantly and inversely correlated to maximum soil temperature during the burn and Pearson and Spearman correlation coefficients, -0.47 and -0.55, respectively, were significant (P<0.005). However, there were many instances when we observed disparate values for seed survival in sample dishes = 60 cm of each other. Such extreme variability may result from unburned or superficially affected areas that acted as seed refugia and originate from heterogeneity of fire impacts. This study indicates that prescribed burning can kill KBG seeds at the soil surface. However, use of fire to reduce viable seeds must be evaluated carefully. While fire may kill existing seeds stocks, it may also induce subsequent seed production. Fire will most dependably kill the relatively high proportions of seeds that congregate in standing KBG litter and dry thatch but seeds under these layers and closer to the mineral soil surface likely will be less impacted.