I Am Presently Analyzing My Root Tip Samples from Last Summer S Research at Harvard Forest

I am presently analyzing my root tip samples from last summer’s research at Harvard Forest. As the longest running continuous nitrogen fertilization experiment in the US, Harvard Forest seemed like a good place to investigate the effects of N deposition on ectomycorrhizal communities. My interests lie largely in how base cation depletion brought on by nitrogen deposition may alter the factors shaping mycorrhizal symbiosis ie. a shift from N limitation to limitation by base cations. I thought this study would be a good first chapter.

I extensively sampled the ectomycorrhizal community of a mixed hardwood forest Harvard Forest last summer and these were my hypothesis:

• Long term N fertilization altered mycorrhizal species composition and decreased mycorrhizal diversity in the high N plots.

• Nitrogen fertilization significantly decreased percent root length colonized in the organic layer, but did not affect or increased percent root length colonized in the mineral horizons.

• Not all tree species have the same mycorrhizal species, nor do the mycorrhizal assemblages of all tree species react the same way to nitrogen fertilization.

SITE DESRIPTION:

The Harvard Forest is located in Petersham, Massachusetts. My study site is in a mixed hardwood forest dominated by red/black oak, red maple, black birch, and American beech (in that order of dominance). The Harvard Forest chronic N enrichment study is an NSF funded LTER study begun in 1988. Three 30m X 30m plots are each broken up into 36 5m X 5m subplots. Each plot has either 0, 50, or 150 kg N/ha/yr added as NH4NO3 six times per year by backpack sprayer.

Note:

Only 1 plot per treatment. There are subplots but as many people are fond of pointing out to me this is pseudoreplication.

BASIC METHODS:

I chose 4 out of the inner 16 sub plots from each plot. I extensively sampled the mycorrhizal community in these four subplots in each of the three treatment plots for a total of 12 subplots. I took 5-8 cores from each subplot and 10-15 tips from the mineral soil and another 10-15 tips from the organic layer from each core. From 15 cores from each plot I separated an aliquot of mycorrhizal roots and assessed the % root length colonized using a gridline intersect method.

Note:

I performed my study in late June, July, and early August. It was difficult to find live roots by August so I wonder how much I may have hampered the effectiveness of my sampling by not beginning in early June or late May.

RESULTS:

Very little so far.

Insufficient sequences to make a rank abundance curve.

Colonization data seems to show no significant effect of nitrogen or soil horizon on percent root length colonized.

Note:

My results for % root length colonized are so hyper variable that I wonder how large an effect would be needed to see any difference.

FUTURE RESEARCH

I want to study the ability of mycorrhizae to relieve calcium deficiency in their host trees brought on by acid deposition induced leaching. Calcium has been repeatedly suggested to be the base cation most likely to come to limit the productivity of temperate forests under sustained anthropogenic N deposition. Mycorrhizae have been shown to have significant calcium uptake and translocation abilities, and to forage for Ca rich mineral sources like apatite granules in the soil. However plants seem to have a limited ability to respond to calcium deficiency because, though essential for many plant functions, it is not a metabolic nutrient and seems to have no active root uptake. Thus, though mycorrhizae are very important to forest calcium nutrition it is very unclear what the effects N deposition induced changes in mycorrhizal communities may have on the calcium status of calcium poor forests.

I want to use semihydroponic sand microcosms with calcium poor seedlings and separate hyphal and rooting chambers to study the role of mycorrhizae in calcium nutrition and how this varies between mycorrhizal species. I want to build these microcosms this summer, but am unclear as to whether I will have the time to do so given that I want to take my orals in August.

I would like to examine the effects that wolastonite (a very Ca-rich silicate rock) fertilization has had on ectomycorrhizal communities of high elevation red spruce at Hubbard Brook experimental forest in New Hampshire. An interesting study recently found that %AM colonization was significantly increased as was tree health in the lower elevation sugar maple communities in the same plots. I would like to do this, provided someone doesn’t do it first and the powers that be allow it, next summer.