In my previous post on Acute Oak Decline I posed the question of whether ecological factors are predisposing oaks to this “new” disease. My concern is that research groups led by the plant pathologists will remain focused on the disease model and not consider the ecology of the problem. I encourage scientists studying Acute Oak Decline to pause for a moment, take a breath, and consider things like rain pH, soil fertility, and ecosystem structure before launching into a costly vortex of biological studies on whatever bacterial species is determined to be the “cause”.
In this post I would like to present some preliminary findings of soil fertility in a stand of diseased and non-diseased English oaks (Quercus robur) in Bushy Park, London, UK. Several oaks showed bleeding symptoms characteristic of Acute Oak Decline (see photos below), although the bacterial species was not positively ID’d.
Upon inspecting the soils I noticed that near the diseased trees there were large mats of mosses, whereas few mosses were found around the non-diseased oaks. Having a strong interest in the influence that mosses may have on soil fertility, and thus oak health, I, along with Neville Fay of Treework Environmental Practice and Vinodh Krishnamurthy of Soil Foodweb Lab Services and Research, devised a simple test.
Joel Williams assisted us in the field and Agnes Michalik assisted in the laboratory.
Two surface soil samples from the upper A horizon were collected, one from underneath a moss mat, and another about 2 meters away from an area with no mosses. The samples were taken back to the laboratory, dried and sieved, and their nutrient composition analyzed using industry standard techniques.
There were three types of nutrient analyses conducted: soluble, exchangeable+extractable, and acid extractable. From a plant/tree perspective, the ‘soluble’ nutrient values are the most relevant. These are the more readily available nutrients to the roots and other soil organisms. The ‘exchangeable+extractable’ values, which tend to be higher than the soluble values, indicate the levels of nutrients that may be available to plants under certain conditions such as a high moisture influx, or if the roots can produce enough exudates to mobilize these nutrients. In general, however, these nutrients are more tightly bound to the soil particles than the soluble nutrients. The ‘acid extractable’ nutrients are mostly those that are very tightly bound to the soil particles and are generally not available to the plants. These values are quite high and tend to reflect the composition of the bulk soil material.
Summary of Preliminary Results
Keep in mind the following results are preliminary. They are based on only a handful of samples with no replications of laboratory analyses.
The pH of both moss and non-moss soils was measured at 6.6. These data indicate that the soils are not very acidic. Based solely on the pH readings one does not get an indication of a pH problem in the soils. The similarity in pH for both sites I interpret as reflecting the strong buffering capacity of mineral soils at this site.
However, when one looks closer at the nutrients, the base cations seem to be depleted. Also, aluminum seems to be a bit on the high side in both samples. The cation exchange capacity, a common measure of soil fertility, is lower in the moss site vs. no moss site (22.5 vs. 28.9 M.E./100g, respectively).
Below is a figure which summarize the variables that I like to focus on when examining soil chemistry data:
Let me first draw your attention to the bottom two graphs that show the ‘acid extractable’ nutrients. Here the differences between the moss and no moss sites are assumed to reflect the background variability in the bulk soil chemistry. Acids produced by the mosses are too weak to be able to alter the soils to this extent. So a fair null hypothesis would be that any differences in the ‘soluble’ nutrient levels and ‘extractable+exchangeable’ nutrient levels between the sites would be mainly the result of variance in parent material composition. Thus, the differences in moss and no moss sites for ‘soluble’ and ‘extractable+exchangeable’ nutrient levels should be similar to the trends in the ‘acid extractable’ nutrient levels, assuming no effect of mosses.
As you can see, however, this is not the case. For both the ‘soluble’ and ‘extractable+exchangeable’ nutrients, the Ca/Al ratios of moss vs. no moss sites are opposite of those found in the ‘acid extractable’ nutrients. In fact, for the ‘soluble’ nutrients the Ca/Al ratio in the moss sites is nearly half that of the no moss sites. For the base/acid cation ratio in the ‘extractable+exchangeable’ nutrients, we see a similar pattern of lower values under the moss sites compared to the no moss sites. Unfortunately ‘soluble’ Na was not reported, so total base cation concentration cannot be determined for the soluble nutrients. For both the ‘soluble’ and the ‘extractable+exchangeable’ nutrients, total acid cations (Fe+Al) are higher in the moss vs. no moss sites. This, again, is opposite the pattern found in the ‘acid extractable’ nutrients. Total base cations in the ‘extractable+exchangeable’ nutrients is lower under the moss vs. no moss sites.
The reason I focus on calcium and aluminum is that calcium deficient soils can cause serious problems for trees, especially in bark and wood health; and high levels of aluminum can cause toxicity and root mortality. Similarly, an abundance of base cations is generally characteristic of fertile soils, whereas high levels of acid cations are an indication of infertility.
I find this all quite interesting because despite the fact that soil pH was the same for both sites, we clearly see large differences in the base vs. acid cation concentrations and ratios between moss and no moss sites. This is one of the reasons I focus so much on mosses. Relying only on pH we would have concluded that mosses had no effect on soils, when in fact it appears that mosses are having a significant effect (in the non-statistical sense) on the soils here.
These findings raise a number of relevant questions regarding predisposing factors in Acute Oak Decline. What is the role of mosses in soil fertility, root health, and mycorrhizae abundance? What is the role of calcium deficiency bark splitting and deterioration? What is the role of high levels of aluminum in fine root and mycorrhizae dynamics of diseased trees?
For more information, readers are encouraged to look over some of my previous work on the role of mosses in tree decline:
Klinger, L.F., 1990. Global patterns in community succession. 1. Bryophytes and forest decline. Memoirs of the Torrey Botanical Club, 24, p.1-50.