Between 1992 and 1995, two experiments totaling 27 experimental units were implemented in five blocks. Experimental units of about 7 ha comprising apparently uniform mature Douglas-fir were identified within blocks selected to distribute replicates across the different soil parent materials. Pretreatment assessment indicated moderately uniform initial conditions within blocks (See Table 1: http://andrewsforest.oregonstate.edu/data/studies/tp122/tp122_tables.pdf).
Each 7-ha experimental unit had a central 1.5-ha measurement area. Four vegetation treatments were applied in 1997: Douglas-fir and early-seral plantations and unaltered and thinned mature forest (See Table 2: http://andrewsforest.oregonstate.edu/data/studies/tp122/tp122_tables.pdf). Except for Unaltered, all treatments included units with either high or low organic matter retained on site (0 or 15% of the merchanTable harvest). The Early-seral and Douglas-fir plantation treatments had about 30-40, while the Mature thinned treatments had about 10-15 stems ha-1 left on the ground as downed wood. The low-retention treatments removed easily accessible pre-existing downed wood as well.
On July 12, 2002, the Biscuit Complex wildfire was sparked by multiple lightning strikes about 50 km east of the study site, and it burned out of control for 3 months. The wildfire, and backburns set to control it, burned two study blocks and a portion of a third. Where the wildfire entered, all of the Early-seral and Douglas-fir plantations and Thinned treatments burned at high intensity (about 700°C on average as measured by the melting of gridpoint aluminum tags), while the Unaltered plots burned at lower intensity (Bormann et al. 2008). The wildfire disrupted the initial experimental design, but eight treatment–burn combinations resulted, with replicate experimental units of each combination occurring in at least two blocks: Unaltered , 2 unburned and 2 burned units; Thinned, 5 unburned and 3 burned units; Early-seral plantation, 2 unburned and 4 burned units; Douglas-fir plantation, 4 unburned and 2 burned units. The two units with prescribed fire are not included in this analysis, reducing the comparison to 25 experimental units.
Five strategies were developed to increase the chances that this study would live up to its long-term billing: 1) tried to capture a range of management actions likely to include future prescriptions, an approach more durable over the long-run; 2) installed large measurement plots (1.5 ha) with very wide buffers to follow long-term development of trees as a stand, to protect a central measurement plot, and to allow for destructive sampling in buffers; 3) maintain interim support with associated short-term studies to address a wider range of emerging issues and develop new methods useful in monitoring long-term experiments; 4) installed permanent plot corners and 25m grid points to make them easy to find (and a local GIS helps manage activities through time); and 5) stored soil and plant samples and photos as well as the numeric data.
Between 1992 and 2011, the experimental units have been repeatedly measured. All measures are tied to a marked 25-m grid system array in a 4 by 4 or 5 by 3 matrix within the central 1.5-ha measurement area. Five tree plots (18 by 18 m) were tracked in each 1.5-ha measurement area since 1992. Standard measurements on all live trees (dbh > 3.5 cm) were: species, dbh, and treetop and base of live-crown heights. Additional measurements of sapwood thickness and rings in sapwood on all trees > 20 cm dbh. A subsample of live trees was cored for breast-height age. We determined biomass of mature Douglas-fir trees by applying locally derived equations of biomass versus dbh (Nay and Bormann in press). We also developed a biomass-increment equation that used sapwood and live-crown length as predictor variables, in addition to dbh (Bormann et al. in review). We calculated hardwood biomass using equations developed in and for the general area (Snell and Little 1983). Equations for large knobcone pine were not available, so we substituted the Nay and Bormann (in print) equation for these relatively few individual trees.
QA/QC protocol calls for trees to be randomly selected in each plot and measured by a different individual from the crew. If DBH is not within 1%, both individuals re-measure the test tree. If the primary DBH measurer was in error the plot is re-measured for DBH. If height measurements are not within 10%, the same procedure as above is implemented.
We analyzed statistics for 25 experimental unit means (Table 2). Tables and figures show 95% confidence intervals around these means to aid in comparisons among treatments at specified years. Significance with 95% confidence can be visually interpreted when intervals do not include other means.
Additional attribute information:
TREEPLOTNO_NOM: Nominal pretreatment tree plot code: these codes (A-E) were used in 1992 (Blocks 1, 2, 3) and a few times in 1995 (three tree plots within Block B (codes 2, 4, 5)) to identify initial tree plot location within experimental unit (EXPUNIT). Plot codes (TREEPLOTNO_PERM) were changed to numbers (1-5) in subsequent years but not all plots were included in future surveys.
TREEID: Unique 14-character code specific to each tree: Experimental unit (4 characters), X-coordinate (3 characters), Y-coordinate (3 characters), Tree tag number (4 characters). NOTE: tree tag numbers preceeded by an 'N' are reference numbers for trees measured during pretreatment in 1992 (none of which were physically tagged) or 1995 and do not directly correspond to trees subsequently measured.
DEADCROWNBASE: Height to base of dead crown; measured from the base of the tree to the lowest dead branch. In the case of unburned trees, measured to the lowest point at which dead branches could have carried fire to the crown. In the case of burned trees, measured to the lowest dead branch which still had fine twigs (indicating that it was probably the live crown base pre-fire).
