Remember when I promised everyone results from the veg crews sometime in the near future? Well, it’s been 6 months and the future as arrived! I finally have some very exciting (preliminary) results to share.
If you need a refresher in the method behind my madness, review The Meaning of Life – Explained! It covers in depth the purpose of my PhD research and why I’ve spent the last 3 summers conducting vegetation surveys in Penn’s Woods.
Before I get to the results (note the build up to create drama and suspense), I want to update everyone on what I’ve been doing over the last 6 months.
Three of those months were spent drying and hand grinding soil samples in the greenhouse with several volunteers (thanks everyone!). These samples were collected last summer (2016) at sites that were also sampled in 2014. The end result was 730 beautifully packaged containers of soil.
After they were processed, I took a small sample from each container and had them analyzed for pH.
In addition to processing soil, I have been organizing data and beginning the analysis. I reviewed and paired 2014 and 2016 soil samples results for comparison of pH following treatment (fence, lime, and herbicide); conducted data checks for quality control; converted data to the right format; and started pairing soils data with vegetation data to view any trends. Most importantly, I received results from the NRCS lab in Lincoln, NE for my 2014 soil samples. This allows me to include important soil chemistry variables like extractable soil nutrients (Ca, Mg, and K) and extractable toxic metals (Al, and Mn) in my analysis.
As reported previously in Down in the Dirt, pH was a significant predictor of occupancy of Mountain Laurel (Kalmia latifolia) and Hay-Scented fern (Dennstaedtia punctilobula), but not of Indian cucumber-root (Medeola virigniana). Given the low pH values at our sites (3.0-4.5), it is highly likely that these plants are suffering from some level of metal toxicity. (Remember: metals like aluminum and manganese become more plant available as pH declines).
So, do the new data support this?
pH was still a significant predictor of occupancy of huckleberry and hay-scented fern. Huckleberry showed a negative trend with pH, meaning it is more likely to take up residence on a site where pH is low. Here’s the graph for huckleberry with 95% confidence intervals (dashed lines).
The results for hay-scented fern showed the opposite trend remaining unchanged from what was reported in our last post.
What about mountain laurel? The answer: it’s complicated. pH is still a significant predictor of occupancy, but so is total soil extractable manganese (Mn). As pH declines, mountain laurel occupancy increases, but as Mn increases, mountain laurel occupancy declines. [the graph below shows that occupancy declines as Mn increases, but also how this relationship differs for 3 different pH levels]
This interaction suggests that mountain laurel has a Mn sensitivity, meaning it cannot occupy sites where soil Mn is high. There was no significant influence of soil extractable aluminum on mountain laurel occupancy suggesting it is not sensitive to soil aluminum.
What about the yummy Indian cucumber-root? It has a Mn sensitivity too! Occupancy of Indian cucumber-root declines drastically once soil Mn gets above 0.2 cmol(+)/kg.
It appears that in addition to pH, soil Mn is an important factor influencing species occupancy at our sites. It is important to note that soil extractable Mn is not a direct measure of plant available Mn.
There are several complex chemical processes that occur in plants that affect their ability to take up nutrients and, inadvertently, toxic metals. Soil extractable Mn should be carefully interpreted as a surrogate for plant available Mn.
Despite these limitations, there appears to be strong relationship between soil Mn and occupancy of at least two species at our field sites.
That’s a result worth waiting for!
Department of Ecosystem Science and Management