|The mid-Holocene Ulmus decline: a new way to evaluate the pathogen hypothesis|
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In modern elm trees infected with Dutch Elm Disease the wood tissue forms resistance mechanisms limiting the spread of the pathogen. If the mid-Holocene elm decline was caused by a similar disease, identical mechanisms must be present in contemporary elm wood. Can this be shown and thus indirectly prove the presence of the pathogen 6000 years ago?
The debate about the cause(s) of the mid-Holocene elm decline has been running for decades and in recent years several researchers have argued the case strongly for the involvement of an elm-specific disease similar to, or identical with, the present day Dutch Elm Disease.
The problem with this hypothesis is, however, that it is particularly difficult to prove or disprove.
The disease hypothesis has been given greater credence since the detection of the mid-Holocene presence of the elm bark beetle (Scolytus), which is the major carrier of Ophiostoma ulmi, the fungus which today causes Dutch Elm Disease. Mid-Holocene presence of elm bark beetles has so far been detected at three sites in Europe, in England, Switzerland and Denmark:
At Hampstead Heath, London, fossil remains (two wingcases) of Scolytus scolytus were found in peat deposits just predating the elm decline.
On the Neolithic site Weier, Switzerland an elm trunk was found which exhibited beetle galleries of Scolytus scolytus.
On the late Mesolithic (Ertebølle) site Præstelyngen, Zealand, Denmark a piece of elm wood was found with galleries of Scolytus laevis. The Præstelyngen find is AMS-dated to 3870 cal. BC (1 stdv. 3980-3780 BC). This is exactly contemporary with the elm decline in Denmark which is radiocarbon wiggle-dated to 3870 cal. BC.
The beetle finds from around the time of the elm decline thus show that the carrier of the major elm disease was available, but it still remains to be established if the disease itself, i.e. the pathogenic fungus, also was present.
Modern elm trees infected by the fungus Ophiostoma ulmi initiate various resistance mechanisms. Among these are two important anatomical mechanisms: barrier zone formation which limits lateral spread of pathogens, and tylose formation which impedes vertical spread of pathogens. A barrier zone is a protective non-conducting tissue consisting largely of axial parenchyma and swollen ray parenchyma which forms in response to infection as well as to mechanical wounding. Barrier zones serve to isolate or separate the infected wood on the inside from the healthy wood that continues to form on the outside of the cambium after the zone is completed.
Fig. 2. Transverse section of a modern elm trunk with Dutch Elm Disease.
A. Barrier zone at the end of a growth ring. B. Swollen ray parenchyma.
In an on-going study, subfossil elm wood from the Præstelyngen site is being histologically examined for the presence of resistance mechanisms to pathogen infection: So far 15 elm branches and twigs have been investigated.
In one of these, a branch fragment 12 cm long with a diameter of 4 cm, a distinct zone of axial parenchyma and swollen ray parenchyma was recorded.
The detected anatomical features correspond exactly to a modern barrier zone.
At the moment, samples of the barrier zone are under preparation for a detailed SEM investigation.The branch in question was recently AMS-dated to 3990 cal. BC (1 stdv. 4040-3970 BC), i.e. late Atlantic time just before, or contemporary with, the elm decline.
The preliminary evidence from the late Mesolithic site of Præstelyngen is notable in relation to the possible existence of an elm-specific disease in the mid-Holocene.
The presence of the elm bark beetle Scolytus laevis contemporary with the elm decline indicate that the carrier of the fungus pathogen, a prerequisite for a successful spread of the disease, was indeed present.
The presence of a distinct barrier zone in a piece of elm wood contemporary with the elm decline is intriguing, as such a zone is a characteristic defence mechanism in modern elm trees infected with Dutch Elm Disease.
One barrier zone does not prove that the mid-Holocene elm decline was caused by Dutch Elm Disease but it is a new and strong indication that this might have been the case. Further studies of elm wood from the Præstelyngen site and from other sites in Europe are needed in order to substantiate the pathogen hypothesis.