Environmental Engineering Reference
In-Depth Information
successional habitat as a future condition drives the silvicultural prescriptions. The
maximum allowable annual harvest is half of the annual growth increment on the
“working landscape” portion, excluding the reserves that comprise 35 percent of the
land base. The main threats to the ecological integrity of the Arcata Community For-
est are urbanization on the forest edge, invasive plants, and potential severing of eco-
logical corridors that link the community forest to other intact forest areas to the south
and east.
The Reference Condition
A reference ecosystem is an actual or historically known ecosystem that is used in set-
ting goals and planning a restoration project, and later in the evaluation of project suc-
cess (Egan and Howell 2001; Gann and Lamb 2006). Fortunately, there are reference
stands of old-growth redwood in the region that can serve as blueprints for the Arcata
Community Forest. In fact, many of the missing ecological qualities can be found in
nearby Redwood National Park and local state parks where recent research has docu-
mented redwood's ability to increase wood production through old age (Sillet et al.
2010). Using tree spacing as a reference indicator, forests with tree densities of
120-200 trees per acre would need to be slowly thinned to achieve a relative tree den-
sity of 20-35 trees per acre as found on nearby old-growth stands. This could happen
naturally through competition and mortality. It can also be stimulated through me-
chanical thinning because second-growth redwood has the ability to dramatically in-
crease basal growth following thinning (Jameson, Reuter, and Robards 2005). Re-
cruiting the structural elements commonly found in older forests is recognized as an
important management objective in younger forest stands to address issues of biologi-
cal diversity and forest integrity (Spies et al. 2002).
In an old-growth forest, natural disturbances in the formof landslides, fire, and wind
create andmaintain gaps in the canopy. The gaps, allowing light to hit the ground, give
young seedlings and saplings the chance to grow and, thus, provide variety in the age
and physical structure of a forest's trees. Thinning and group or “gap” cuts in a second-
growth forest are attempts to mimic natural disturbance. They relieve the forest's un-
natural, uniform growth created by the initial clear-cut operation. Single-tree selection
with a focus of thinning from below and group selection with green tree retention are
the main disturbance regimes used in Arcata. City staff has employed a blend of adap-
tive restorative treatments: variable retention (Franklin et al. 1997; Mitchell and Beese
2002), group selection, group selection with green tree retention, and single-tree selec-
tion. This has allowed them to learn from new information and key ecological indica-
tors when designing new interventions. The forest is a shifting mosaic of patches,
thinned areas, and gaps with the goal of allowing for tree ages in excess of one hundred
to three hundred years. The development of a multilayered forest canopy with a shade-
tolerant, shrub- and tree-dominated understory provides an indicator of the shift to an
uneven-age condition (fig. 8.2). Efforts to increase species diversity have also included
underplanting of shade-tolerant conifer species using transplanted stock from stands
nearby. Other restoration projects include road decommissioning, erosion control, and
improving fish passage opportunities at stream crossings.
