Population trends, demography, and the effects of environment and disturbance on Cypripedium fasciculatum in Southern Oregon

Cypripedium fasciculatum Kellogg ex S. Watson (clustered lady’s slipper, Orchidaceae) is a rare woodland orchid that can be found in scattered populations throughout northwestern North America. Although several studies have explored its relationship with mycorrhizal fungi and habitat associates, there has been little information on the species’ demography, including determining the likelihood that this species has a dormant life stage and population growth rates. Information is also limited on the effects of disturbance on this species. This report summarizes a ten-year demographic study of 28 Cypripedium fasciculatum populations in southwest Oregon that was established to address some of these information gaps.

The number of emergent stems and proportion of flowering plants varied significantly between sites and years. In general, 13% – 45% of emergent plants will become dormant the following year. The majority of dormant plants re-emerged after one year. Demographic analyses found that the growth rate for these populations was less than one, suggesting that they will eventually decline to zero.

Analyses of habitat variables found that the population size and stability in flowering and plant size were generally associated with higher basal area of trees and cover of low- to mid- level vegetation. However, it is important to note that our studies were conducted in relatively large populations and thus habitat characteristics are expected to be within the preferred range for this species. It is likely that there would be a negative response by C. fasciculatum if cover of any of these habitat layers were to significantly increase or decrease.

Poor replication and a lack of records prohibited making strong conclusions regarding the effects of tree thinning and fire on C. fasciculatum. We recommend future studies using replicated treatments (including thinning of the mid-story vegetation and low severity ecological burns) to determine if these management techniques could maintain C. fasciculatum habitat while having a neutral to positive effects on the species.

Although there was significant variability in population size and reproduction, we did not find any habitat nor environmental factors to explain this variation. As these populations have a declining growth rate, we recommend that future studies focus on determining the best methods for reintroduction of this species and methods to manage habitat that do not harm existing populations.