Restoration of Coast Redwood (Sequoia sempervirens) Forests through Natural Recovery

Will Russell, Jeff Sinclair, Kristin Hageseth Michels

Department of Environmental Studies, San José State University, San Jose, USA.

DOI: 10.4236/ojf.2014.42016                

Abstract

The management of second-growth Sequoia sempervirens (coast redwood) forests for the purpose of restoration and ecological conservation is a growing trend. However, little is known about the long-term regenerative potential of this forest type in the absence of post-harvest management techniques such as thinning and planting. Data on forest composition and structure were collected on a chronosequence (80 - 160 years) of mature recovering stands in the southern coast redwood range using a replicated, randomized, plot design. Results indicated that many stand characteristics including tree density, canopy cover, redwood dominance, species richness, herbaceous cover, and shrub cover reached levels statistically equivalent with old-growth reference sites in recovering stands within the time frame of this chronosequence. The recovery of individual herbaceous understory species was inconsistent however. While the cover of redwood-associated species (Oxalis oregana, Trientalis latifolia, and Disporum hookeri) reached levels statistically equivalent to old-growth reference sites, others (Trillium ovatum and Viola sempervirens) did not. Total basal area and species evenness also trended toward, but did not reach, old-growth conditions. The arboreal aspects of coast redwood forests appear to be remarkably resilient following a single logging event, and recover rapidly in the absence of active restoration techniques. The protracted recovery of certain redwood associated herbaceous understory species will require further study.

 

Keywords

Coast Redwood; Sequoia sempervirens; Restoration; Timber Harvest; Natural Recovery

Table 1.

Received December 27^th, 2013; revised January 26^th, 2014; accepted February 9^th, 2014

Figure 2.

Relative dominance of the three most common tree species―Sequoia sempervirens (Coast Redwood), Pseudotsuga menziesii (Douglas-fir), and Notholithocarpus densiflorus (tanoak)―on ten sites in the Santa Cruz mountains.

Figure 3.

Linear Regression of percent canopy cover on a chronosequence of second-growth stands with old-growth percent canopy cover mean re- ference line and standard error confidence band.

Conclusion

The results of this study indicate that natural recovery is an effective technique for the restoration of coast redwood forests. The overall density of trees declined over time in recovering stands reaching statistical equivalence with old-growth refer- ence sites for most tree species. The dominance of S. sempervi- rens also reached statistical equivalence with old-growth, as did canopy cover, understory cover, and species richness. Asso- ciated herbaceous species also trended toward recovery, though the cover of some species (Trillium ovatum and Viola semper- virens) did not reach statistical equivalence with old-growth reference sites in the time frame of this chronosequence. Active management tools such as stand thinning are counter indicated as they are generally employed as a restoration technique to

Figure 4.

Mean averaged herb cover in coast redwood second-growth and old- growth stands with 95% confidence intervals.

reduce tree density in overstocked stands and to promote the dominance of desired species. Additionally, the added distur- bance of mechanical thinning is likely to impact sensitive un- derstory herbaceous species that are dependent on the moist shady environment provided by a mature forest canopy.

Discussion

Forest structure and composition on the chronsequence ana- lyzed exhibited significant trends toward old-growth conditions for stand density, the relative dominance of Sequoia sempervi- rens, tree canopy cover, shrub cover, species richness, and total herbaceous cover. However, results were mixed for the shade- adapted understory species that are common in healthy coast redwood stands, with some recovering fully while others did not. This finding was concerning, though not unexpected. Pre- vious research on a chronosequence in the central part of the

Table 2.

coast redwood range offered similar results (Russell & Michels, 2010). Overall, the arboreal features of the forest appeared to be quite resilient to human disturbance, while shade loving herba- ceous species were not. With this in mind, implementation of mechanical restoration techniques, such as stand thinning, are counter indicated.

Mechanical disturbance in the form of stand thinning has been presented as an effective restoration technique for many forest types as a means of quickly returning stands to old- growth density levels (Boe, 1965; Bosch, 1971; Oliver et al., 1994; Brown et al., 2004; Lindquist, 2004; Smith et al., 2005; North et al., 2007). Such treatments can also increase the growth rate of selected trees through the removal of competing individuals thereby promoting the growth of large individual trees (Cole, 1983; Oliver et al., 1994; Lindquist, 2004; O’Hara et al., 2007).

In coast redwood forests, however, this model is not well ap- plied, as regenerative clonal sprouts do not compete in the tra- ditional sense. In addition, mechanical thinning can have unin- tended consequences. Removal of trees exposes the forest floor to increased levels of solar radiation (Kjeldsen-Ederer & Rivas, 1998; Russell & Jones, 2001) allowing opportunistic non-native species to colonize disturbed areas, and can alter soil conditions through compaction and reduction of nitrogen levels (Corns, 1988; Ebrect & Schmidt, 2003; Jussy et al., 2004). Thinning has been shown to decrease epicormic sprouting response of coast redwood (Powers & Wiant, 1970; Cole, 1983) and allows for non-native shade intolerant species to persist in the commu- nity. In addition, stand density often increases in coast redwood stands a few years following thinning as a response to increased solar radiation (Russell & Jones, 2001), thus the effects of me- chanical thinning are temporary.

The purpose of stand thinning as a restoration tool is to speed forest development in order to produce certain old-growth cha- racteristics more quickly. However, the development of a forest with a dominant species, such as Sequoia sempervirens, that commonly persists for 1500 to 2000 years is a lengthy process. And though the temptation to manipulate a regenerating forest so that management goals can be reached in a human time scale is compelling, the ecological costs of continued disturbance, particularly when using the same tools that caused the original damage, must be considered. The reason why some associated understory species reached old-growth equivalence, and others did not, is not clear. However, the answer likely relates to soil conditions and the relationships between plant roots and com- munities of soil microorganisms. Therefore, the protection of the soil from further human disturbance is essential so that nat- ural soil development processes can occur. In addition, allow- ing natural processes to restore a system allows for stochastic factors to create stand complexity, while eliminating the sim- plifying influence of subjective human management.

Busing and Fujimori (2002; 2005) determined that small- scale natural disturbances and unmanaged processes of com- munity development were sufficient for the regeneration of coast redwood forests. The results of this study support Busing and Fujimori’s conclusions by providing quantitative data on the development of coast redwood stands over a significant period of time. For maximum effectiveness, the focus of resto- ration should shift from arboreal canopy species to the recovery of old-growth associated understory species, such as Trillium ovatum, that have been shown to be severely impacted through logging (Jules & Rathcke, 1999).

Acknowledgements

This research was supported by grants from the Save-the- Redwoods League and the Mendocino Institute. Logistical support was provided by the California Department of Parks and Recreation.

References

Conflicts of Interest

The authors declare no conflicts of interest.

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