Tree regeneration after fire: Yukon Lodgepole Pine Surveys, seedlings analysis

Experimental Design

We used a multi-step procedure to select sample sites along the edge of the northern lodgepole pine range. We first identified all road-accessible burns between 5 and 50 years of age in southern and central Yukon Territory using federal fire inventory maps (Indian and Northern Affairs Canada, Whitehorse, Yukon). We visually inspected burns to determine whether deadwood from the recent fire was sufficiently well-preserved to identify the genus of pre-fire individuals. 

We classed the resultant set of burns into one of three categories describing their position relative to the current pine distribution, as follows: 
1) outside range: no evidence of pine being present, 
2) range interior: burns occurred in or adjacent to stands dominated (>50% cover) by mature lodgepole pine, and 
3) range edge: sites with trace amounts of mature pine but with no dominant pine stands >0.5 ha. 

We sampled all road-accessible burns along the range edge that met our sample criteria, plus two additional, isolated burns that were accessed by boat along the Pelly River. The resultant sample included 6 burns, ranging in age from 6 to 48 years. Within each burn, we sampled stands where pine was present in the pre-fire community, as evidenced by standing dead pine trees. In three of the burns, we sampled all accessible pine stands; in burns where pine was more widespread, we selected stands to represent the range of observed pine regeneration densities. We measured a total of 17 stands, with 1 to 5 stands per burn. All of the selected stands experienced 100% mortality of standing trees as a result of the most recent fire. Ages of the pre-fire stands ranged from 60-130 years. All sampling was done in 2001. 

Methods

We sampled five randomly-positioned 2x50 m transects (0.05 ha) within a 0.25 ha area in each stand and counted the total number of seedlings or saplings of each tree species. We pooled seedlings of black and white spruce (Picea mariana and P. glauca) because small seedlings of these two species could not be differentiated. We measured the basal diameters of all trees in the pre-fire community from standing or fallen deadwood that had been rooted within a sampling transect prior to the fire. Genus determinations of dead trees were based on cone, bark and branching morphology. We did not count trees that were dead at the time of the fire, judging from deep charring patterns on the bole. 

We measured organic-layer depths at 55 points at 5 m intervals along the center line of each transect. The upper 15cm of mineral soil was sampled using a 2.5 cm corer at five random points for laboratory analysis of soil pH and gravimetric moisture content at field capacity. The age of the pre-fire stand was determined from annual ring counts of 5-10 basal disks or tree cores (30 cm above the uppermost roots) obtained from a mixture of dead pine and spruce trees systematically sampled to represent the largest, and hence oldest, trees in the stand. 

Species codes: 

LP = lodgepole pine (Pinus contorta) 
WS = white spruce (Picea glauca) 
BS = black spruce (Picea mariana) 
S = unknown spruce, black or white 
PB = paper birch (Betula papyrifera) 
BP = balsam poplar (Populus balsamifera) 
TA = trembling aspen (Populus tremuloides) 
UNK = unknown tree species