Variability of understory response to ecological thinning and gap creation in Douglas-fir/hemlock forests

Background/Question/Methods  

Forest managers are increasingly using thinning and other silvicultural manipulations to enhance the ecological values of managed forests.  However, most experiments in ecological thinning have been poorly replicated—either within or among sites—making it difficult to know whether results are repeatable or to generalize about responses to treatments.  In this study, we explore variation in vegetation response (changes in plant cover and richness) to three treatments replicated four to five times at each of three sites (Bear, Pine, and Pine-N) in the Cedar River Municipal Watershed, western Washington.  Treatments were (1) gap creation (all trees removed in a 20-m diameter gap centered in the 40 x 40 m experimental unit), (2) spatially structured thinning that preserved the largest trees and removed smaller trees in clumps (30% reduction in basal area), and (3) control.  All three sites lie at low elevations (640-710 m) with similar slopes (10-30%) and aspects (S-SW).  All were dominated by 65-yr-old Douglas-fir and western hemlock of similar total basal area (75-80 m²/ha) and had very low understory cover (0.8-3.5%) and diversity (0.9 -2.0 species/m²).  Cover of all vascular plant species was estimated in ~40-60 1-m² quadrats per experimental unit before and 3 yr after treatment.

Results/Conclusions

Both removal treatments increased plant cover at all sites compared to controls, but the magnitude of increase did not differ between gap creation and thinning.  However, responses varied greatly among replicates and sites.  In the gap treatment, for example, increases in mean cover averaged over the whole experimental unit ranged from 0.1% (3.8 to 3.9%) to 17.1% (4.9 to 22%).  Moreover, while gap creation and thinning had a positive effect on understory cover at all three sites, they did not have similar effects on richness.  Richness increased after treatment at Pine and Pine-N, but declined at Bear. 

 Although we anticipated stronger responses to treatment in units with greater understory development, responses relative to initial conditions actually differed among sites.  At Pine and Pine-N, increases in cover and richness were positively correlated with initial values.  At Bear, however, there was no correlation between initial cover and change in cover, and units with greater initial richness showed larger declines in richness.  We explore possible explanations for these differing responses.  Our results highlight the importance of replicating treatments within and among sites prior to implementing silvicultural manipulations at larger spatial scales.