Using historical data to guide forest restoration in the central Sierra Nevada

Background/Question/Methods

Managers and the public are increasingly interested in restoring forests to a condition that is not only less likely to burn in an uncharacteristically severe wildfire, but is also ecologically diverse. Unfortunately, lack of adequate reference information hampers our ability to define restoration targets in many areas. In order to better understand how the structure and composition of forests has changed since logging and fire exclusion, we repeated data collection in three approximately 4 hectare plots that were completely stem-mapped in the old-growth condition in 1929. Plots were then logged using three different treatments and have not been disturbed since, making them the oldest continuously monitored forest plots known to exist in California.  In 2008, all trees larger than 10 cm in diameter were mapped using a laser rangefinder and compass module linked to a GPS, and status, species, and diameter recorded.  In addition, cover of shrubs and number of stems of herbaceous understory vegetation were quantified in plots located along a belt transect traversing each unit, retaking the same data collected in 1929.

Results/Conclusions

Density of trees increased from 286 per hectare in 1929 to 682 per hectare in 2008. The greatest discrepancy between 1929 and 2008 conditions existed for trees 30 to 60 cm in diameter. Trees in 1929 were arranged in groups interspersed with small gaps ranging between 0.03 and 0.20 ha in size, while many of these gaps had filled by 2008.  Shrubs covered 29% of the forest floor in 1929 and 2% of the forest floor in 2008. Loss of herbaceous species was less pronounced. Correlations with tree density variables suggest that lack of light may be an important cause of the decline. Using information about variability in density, canopy conditions and gap sizes from the historical data, we developed a new thinning tool for managers to restore structural complexity. Whether more variable thinning in these second growth forests better maintains canopy structure thought to be important for wildlife habitat and generates greater understory diversity is being tested in a new “Variable Density Thinning” study.