Olympic Habitat Development Study


Station 4 - GAPS

Although gaps serve multiple purposes such as increasing development of understory vegetation (learn how this worked on the Understory Tour), one of the goals of the gaps was to increase growth rates of the trees in and around the gaps. Trees left inside the perimeter of the gaps were those too small to be harvested (<20 cm)<8 inches or were species that we wanted to encourage or favor in the treatment (that is, they were not cut and were given more growing space). There were not very many trees left in the gaps but those that were left are growing well. Red alder is one of the species that did not regenerate in the control stand or in the general thinned area but was present in the gaps; there were 165 red alder seedlings and 105 red alder saplings per ha70 red alder seedlings and 42 red alder saplings per acre in the thinned area but all of these were located in the gaps.

Trees whose stems were within 10 m33 feet of the edges of the gaps (in the thinned matrix) had additional growing space with the gap on one side of them and grew well (graph).Diameter growth of trees in the thinned matrix near to gap edges or away from gap edges. These gap-edge trees grew 6.4 cm2.5 inches in diameter in 10 years (compared to an average of 4.3 cm1.7 inches in the general thinned area). In addition, the additional light from the gaps will help retain existing branches, and especially for Sitka spruce, can also trigger development of epicormic branches (new branches which form from buds under the bark when the tree is damaged or there is a major increase in light to the tree bole) on trees without many lower branches. Thus, the gaps are a very important part of the overall VDT, (variable-density thinning) even though much of their effect is outside the actual line on the ground for the gap.

The gaps also had a "halo" effect on conifer regeneration with many more seedlings around the edges of gaps than in the center of the gaps or further into the thinned matrix.


A young western hemlock (Tsuga heterophylla) grows faster in a gap.
Gap seen from the forest interior at Fresca. Light abundance is greater in the gaps. A young western hemlock (Tsuga heterophylla) grows faster in a gap. Conifer regeneration increased in the gaps.
A young western hemlock (Tsuga heterophylla) grows faster in a gap.