Tree growth from 30 years ago to 10 years ago had
been fairly constant, changing little from period to period and
both increasing and decreasing. However, from 10 years ago to
5 years ago there was a significant decrease in growth. Trees
adjacent to trails decreased growth by 20% from the previous period,
and trees not adjacent to trails decreased 22% from the previous
5 year period. Then, from 5 years ago to current both sets of
sample trees exhibited accelerated growth from the previous period,
increasing 26% and 34% for trail adjacent trees and non-adjacent
trees respectively. These changes are likely in response to climatological
influences rather than trail use trends, and in any case both
adjacent and non-adjacent trees responded similarly.
Looking only at the current growth rates, the trees sampled immediately
adjacent to the trails had a growth rate that was about 4% greater
than trees sampled that were not adjacent to trails. In looking
at the paired samples, in 40% of the pairs, trees adjacent to
trails were growing at a faster rate than trees not adjacent to
trails, in 40% of the pairs the situation was reversed, and in
20% of the pairs the growth rates were identical.
One sample tree stood out from the others in that it had a growth rate more than 400% greater than the average and almost 200% greater than the next highest growth tree. Ironically, this tree was located in the picnic area of the lower meadow, one of the highest use impact areas. However, the tree was also growing in a filled riparian area providing higher than normal available moisture. Overall, no significant differences were observed in growth rates between redwood trees growing immediately adjacent to trails and with exposed roots in the trails, and trees not adjacent to trails and not affected by trail use compaction and disturbance.
Redwood Forest Understory Vegetation
The redwood forest understory was assessed by NRM’s botanist. In particular, traffic impacts (pedestrian, equestrian, and bicycle) and the general health of the understory were noted along and adjacent to the trail system through the redwood (Sequoia sempervirens) forested portions of the park. The redwood forest supports a moist (mesic), shady, and sheltered environment for the understory vegetation, which is a unique and important environmental resource for this commonly dry (xeric) woodland to grassland and urban-developed region (East Bay hills).
The understory vascular plants encountered in the redwood forest during the field survey are presented in a species list (Table 4), and the taxonomic nomenclature used was based on The Jepson Manual (Hickman 1996). The native plants, which are in bold type in the list, comprise 71% of the total species. The greatest diversity of native plants is in the forest understory: 87% are shrubs or herbaceous species. The understory also has the greatest percentage of non-native plants (31%).
