Olympic Habitat Development Study
US FOREST SERVICE

 

Station 3 - GAPS

The biggest changes in understory vegetation at Fresca were in the gaps. Bracken fern increased (graph) Changes in percent cover of bracken fern in gaps over 10 years. from less than 1% prior to thinning to almost 60% at year 7. Other species with double digit increases (graph) Changes in percent cover of 4 understory species in gaps. in cover in the gaps were wood sorrel, deer fern, false lily-of-the-valley and graminoids. (grasses, sedges, and rushes) Species in the gaps responded to increased light and below-ground resources (soil water and nutrients), as well as exposure of mineral seed beds due to movement of logs and equipment through the gaps.

Many species which initially increased in cover, had started to decrease again by year 10. Why would this happen? There could be several explanations.

    •  As individual species expanded greatly (such as bracken fern and western hemlock (see the Tree Tour for more information on response of hemlock), the species in the gaps began competing with each other.

    •  Some species can respond rapidly after disturbance (like the groundsel mentioned at station 6) but are not very competitive once other species become established.

    •  Animals could be playing a larger role. The expansion of vegetation resulted in more animal activity in the gaps and browsing or clippingThis tree seedling has lost most of its stem and branches to mountain beaver browsing. was observed on many plant species. Animal sign seen in the gaps indicated both Roosevelt elk and mountain beaver were using these areas frequently and we suspect other browsing animals also used the increased forage that was created within the gap.


 

Graminoids increased their cover in gaps.
The most evident changes in understory vegetation happens in gaps, because of the greater amount of disturbance and increased sunlight. Graminoids increased their cover in gaps. More animal activities were observed in gaps, such as this elk herd.
Graminoids increased their cover in gaps.