In conjunction with aerial photography, remote sensing, and bathymetry, on-the-ground plant surveys provide information on biological responses to habitat restoration. Vegetation sampling is conducted during summer when vegetative cover is at its maximum; data collection began in 2009 and is on-going. Permanent, 50-m point-intercept transects (1 m intervals) have been established to determine the composition, height, and percent cover of plant species and to detect changes in vegetation through time. A 0.25 m² quadrat is also examined at the beginning, middle, and end of each transect (3 quadrats per transect) to estimate mean stem density, height, and ocular estimates of percent cover of each species. The location of target plants or invasive species (such as reed canary grass Phalaris arundinaceae) is tracked with Real-time Kinematic GPS (RTK) ground surveys or high precision color infrared aerial photographs. Data are used to describe vegetation colonization and establishment (species composition, % cover, height, and density) compared to reference and older restorations in the Nisqually Delta.
Vegetation monitoring sheds light on available habitat for birds, insects and other animal species. From this data, scientists are able to understand the potential for increased insect numbers in relation to plant density. It also provides information on the amount of bare ground compared to the amount of live vegetation. Additionally, the plant species present at a certain site are determined by soil salinity levels. For example, the invasive reed canary grass is highly salt intolerant. After the restoration, this species quickly disappeared from locations inundated by tidal waters.
A recent study found that soil salinity and elevation were the two driving factors behind vegetation colonization (Belleveau, L et al. 2015). For instance, 12 of the 27 species surveyed preferred soil salinities of less than 30 PSU; above that level, those plants were either absent or had comprised growth. The effects of salinity on plant growth were weaker than the impacts of elevation on plant growth. The study found that no plant species were documented below 2 m elevation, thus setting a lower elevation limit for plant growth. Based on the data, the scientists modeled elevation across the delta to determine the extent of suitable habitat. Based on that analysis, 49% of the 2009 restoration site was subtidal habitat or channels, 45% supported low marsh species, and 6% was classified as high marsh.
Belleveau, L.J., Takekawa, J.Y., Woo, I., Turner, K.L., Barham, J.B., Takekawa, J.E., Ellings, C.S., Chin-Leo, G. 2015. Vegetation Community Response to Tidal Marsh Restoration of a Large River Estuary. Northwest Science, Vol. 89, No. 2, pg. 136-147.
Davis, M.J., Ellings, C.S., Grossman, E.E., De La Cruz, S., Nakai, G. Modeling the long-term intersection of sediment deposition and vegetative colonization in a restored tidal estuary. [DRAFT].