Heterogeneity of Kentucky Bluegrass (Poa pratensis L.) Seed Germination After Controlled Burning

Prescribed burning is sometimes advocated as a means for controlling Kentucky bluegrass (POPR) in invaded grazing lands. However, little is known about the effects of fire on POPR seed survival. We exposed seeds of POPR (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 above ground with thermocouples and assessed subsequent seed germinability. Maximum surface temperatures during the test burns averaged 271 degrees C +/- 23 degrees C but varied widely (range 41 degrees C-509 degrees C) while maximum temperature at 10 cm above ground was slightly higher (301 degrees C +/- 25 degrees C). Burning inhibited seed survival or the ability to germinate (Kolmogorov-Smirnov P <.0001). Germination of the POPR seeds in the control dishes averaged 93% +/- 1%. Average germination for burned locations was 37% +/- 7% and was distributed bimodally; it was absent or strongly inhibited in 59% of the samples but much less affected (>= 60% normal germination) in the remaining 41% of locations. Germination success was similar in burned plots previously managed with both grazing and fire (35% +/- 10%) or previously managed by fire alone (38% +/- 11%), but it was significantly and inversely correlated to maximum surface temperature during the burn (Spearman r = -0.49, P < 0.005). However, we observed a binary pattern of high and low seed germination response across the entire gradient of recorded surface temperatures, including instances of highly disparate values for seed survival in samples located within 60 cm of each other. Such extreme variability may result from unburned or superficially affected safe sites that originate from heterogeneity of fire impacts. This study suggests prescribed burning can kill POPR seeds near the soil surface, especially those located in standing litter and dry thatch. However, some seeds under these layers and closer to the mineral soil surface may be less impacted. Published by Elsevier Inc. on behalf of The Society for Range Management.