Douglas Fir and Ponderosa Pine Seed Dormancy as Regulated by Grassland Seedbed Conditions

Abstract

Tree encroachment in the ecotone between grassland and forest of interior British Columbia has resulted in decreasing grazing potential of rangelands. The 2 dominant tree species in this region, Douglas fir (Pseudotsuga menziesii var. glauca) and ponderosa pine (Pinus ponderosa), require stratification for seed dormancy release. The objective of this study was to determine whether seeds of these species can be stratified and dormancy released under grassland conditions. Field stratification experiments were conducted over 4 years using 2 Douglas fir and 3 ponderosa pine seed collections. A laboratory experiment was conducted to determine the effect of seedcoat removal, light, and stratification duration on dormancy release. Dormancy in Douglas fir and ponderosa pine was released after 1 to 2 months of stratification under grassland seedbed conditions when seeds were placed in the field in late fall and early winter. Continuous stratification until the following May was correlated with higher germination rate. One week of stratification in the laboratory was sufficient to break dormancy in the 2 species and a similar effect can be achieved by exposure to light. Seed coat removal for ponderosa pine also released dormancy, indicating that this structure imposes dormancy. Therefore, the grassland seedbeds near the forest edge can provide suitable conditions to break dormancy of Douglas fir and ponderosa pine seeds, contributing to tree encroachment into adjacent grasslands. Managements aiming to control tree encroachment should take the interaction between tree seed and grassland seedbed conditions into consideration, and the control should be focused on the elimination of seeds and seedlings but not on the germination stage.