Post 3: Ongoing Field Observations

Organism Studied: Alnus rubra (red alder)

Environmental Gradient: The environmental gradient of the study area is the rise in elevation from the shoreline to the railway and the corresponding changes in soil type, drainage and exposure to flood disturbance. Alnus rubra appears to dominate the lower elevations while coniferous species dominate the higher elevations.

I have selected 4 sites along a 100 metre stretch of shoreline. Some parts of the shoreline are very steep and rocky, with limited vegetation. I consequently selected 4 sites that had a more gradual slope, and thus had sufficient vegetation to demonstrate a response to elevation, in regards to species type, abundance and maturity.

Site 1:
Roughly one quarter of the site is a low lying flat area, within 1 metre in vertical elevation from the waterline. The soil is soft, dense and deep, with a layer of leaf and stick detritus completely covering it. It appears that only species present here is alnus rubra, with many young plants covering the area as well as 5 mature trees over approximately 8 metres tall. There are two western red cedars and one western hemlock between 6 and 8 metres tall at the top of the slope, approximately 5 metres above the waterline.

Site 2:
Young alnus rubra plants are growing densely in the area below 2 metres in vertical elevation from the shoreline. The area has deep moist soil covered in leaf and stick matter. The slope rises steeply over large stone boulders. Above 2 metres in elevation, several mature western red cedars (6-12 metres) grow in loose sandy soil on the boulders. At around 5 metres in elevation, several Douglas firs and western red cedars (all over 5m tall), and some young western red cedars are present.

Site 3:
Site 3 rises and dips in several areas, and is mostly lower than 3 metres above the water level. The southern half of the site has a rocky surface with a thin layer of course soil that rises from the shoreline for 5 metres before sloping downwards to an area of thicker moist soils. The higher rocky ground has a several mature (5-15m tall) western red cedars and Douglas fir trees. The northern side of the site is lower lying, covered in grasses, mosses or dead leaf matter, with soft deep, moist soils. There are many smaller red alder plants and 5 mature red alder trees over 6 metres tall.

Site 4:
Most of site 4 is less than 1 metre above the lake water level. These areas have deep moist soils covered either by grasses or leaf detritus. There are many young alnus rubra growing in these low lying areas and 5 mature alnus rubra trees over 5 metres in height. At the top of the slope, approximately 5 metres above the water level are some young western red cedar trees.

In all four sites the low-lying areas appear to be dominated by alnus rubra. These areas are mostly occupied by mosses, grasses and young alnus rubra, and the soils are deep, spongy and moist. The low areas are generally flat, and the land only rises where there are rock formations, which suggests to me that these areas are flood plains that have come about from erosion of softer parts of the shoreline. Walking further north along the shoreline I observed a grass and young red alder covered area beside a creek that was now submerged due to the increased creek flow from spring snow melts. This helped support my idea that these areas are likely subject to flood inundation. The vast majority of the alnus rubra in the low lying areas are young plants less than 50cm tall, which could be related to the frequency of flood disturbances, and red alder possibly being a colonizing species. The rocky, more elevated areas seemed to be dominated by mature conifers. Their age indicates that the area may not have not been subject to a significant flood disturbance for a long time, and the fact that there are no young conifer species at lower elevations might suggest that alnus rubra colonizes these areas before conifers do following floods, they out compete conifers there, or they are more resistant to flood disturbances so conifers are less likely to survive a flood.

I hence made the hypothesis that:
Alnus rubra will be the dominant tree species in flood prone areas of the Nita Lake shoreline.

Formal prediction:
Alnus rubra will be the most common tree species in areas of the riparian zone less than 2 metres in vertical elevation from the current waterline.

Predictor variable: elevation (continuous)

Response variable: abundance of tree species, age/size of trees (continuous)