Wednesday, July 13, 2011

Quaking bogs and other Shaky Ground: some thoughts on the history of phytosociology

In the early 1980s I volunteered to work in Killarney National Park in Ireland on a project to rid the oak woodlands of Rhododendron ponticum, an invasive shrub that was encroaching in the understory of this habitat.  The concern was that this invasive species prevented the regeneration of oak.  Since oak woodland, or in fact any woodland type other than spruce plantation, is rare in Ireland, Rhododondron, that most beautiful of vandals, could wreck a national treasure.  Many of the volunteers were, like me, students from one of the colleges in the National University of Ireland system.  We stayed in a deconsecrated church at the edge of the park and would travel by boat across the delightful Lakes of Killarney to our worksite for the day.  At the foot of the mountainous slopes upon which these woodlands are found, the ground is boggy.  These small patches of bog are themselves of some interest botanically – often dominated by heather and gorse, with an occasional butterwort (a carnivorous species). Some of these bogs were so spongy underfoot that if one hopped up and down a little (ah youth!) the entire landscape would respond – slowly at first, and then like a slow green sea the ground would sinuate out towards the woody margins where the trees themselves would seem to sway.
Each of these adjacent communities, bog and woodland, are classifiable into distinct vegetation types that are quite obvious to the observer, even if he is leaping on the bog.  The rather arcane European school of phytosociology favored by Irish botanists, employing the Braun-Blanquet phytosociological method, is an elaborate quantification of both the obvious and the more subtle patterns of vegetation.  The method can make distinctions between habitats that would not be obvious otherwise.  The technique works by taking extensive vegetation information collected in the field and then grouping plant species into communities of associates.  This requires ferocious amounts of field work, and frustratingly complex statistical analyses.  The approach has been regarded witheringly by those outside a select priesthood of practitioners, but passionately defended from within.  I recall that the chair of botany in my early undergraduate years at University College Dublin, the tall, bespectacled and inspirational Professor J. J. Moore S J, reported that initially he had not been convinced of the correctness of this version of vegetation classification.  He was “converted” after accepting an invitation to meet with his European counterparts.  He records his change of heart in the acknowledgment section of one of his papers in which he defended the techniques.  There he thanks “Professor Tüxen for the patience, tact and skill with which he expounded the system to me, [and] for the almost overwhelming hospitality with which he welcomed me at his institute”.[i] Reinhold Tüxen, a founder of phytosociology, had been director of the Federal Institute for Vegetation Mapping in Germany at that time.  Another Botany Department faculty, a seasoned and highly regarded practitioner had, however, confided in a lecture, that vegetation classification was a “Sunday afternoon activity”.
Thus, the well-studied Killarney woodlands are assigned to a variety of distinct vegetation associations.  Employing the baroque language of the Braun-Blanquet school the oak woodlands on Devonian aged Old Red Sandstone are placed in the Blechno-Quercetum Association; some of the mossiest sections of the woods, ones that might not to the unassisted observer seem all that distinct, are placed in their own “subassociation”, called Blechno-Quercetum scapanietosum.  Elsewhere in the National Park, in woodland growing over Carboniferous Limestone, the prevalence of Yew trees, a relative rarity, lends an exotic element to the Corylo-Fraxinetum[ii]  Just as is true of the woodlands, the adjacent bog can be classified quite minutely based upon its characteristic vegetation into its various associations of mountain blanket bog. Association.
Now my interest here in this post and tomorrow’s post concern only certain aspects of the vegetation problematic posed by the scene sketched above. 
1.The status of European phytosociology: The appropriateness of the techniques, described above, and initially developed in Europe in the early decades of the 20th Century, for describing, classifying and understanding natural patterns have been questioned from both within and beyond the phytosociological tradition. What is the usefulness, if any, of this approach?  What is the critique; what is the defense?  Is there any need for those outside the phytosociological brethren to concern themselves with this business?

2. Static versus dynamic patterns: A central issue in discussions about phytosociology is the degree to which this structure are compatible with the conceptual framework of Anglo-American ecology developed by Frederic Clements, Alfred Tansley, and other; described in previous posts.  Are both of these traditions addressing the same questions in ecology but in different ways? 
The description of vegetation units (whether we call these “associations” or “climax communities”) found in a landscape depends upon the reliable re-occurrence of plant species, patterns classified into associations.  When we call a woodland Blechno-Quercetum, it should mean that other almost identical woodlands can be found.  It is to verify this that vegetation methods utilize multiple samples – being a vegetation scientist in Ireland requires miserably bending over quadrat (a standard sampling unit) after quadrat in the lashing rain, being drained by blood lusty midges, until one finally has enough date for statistical analysis.  The identification of these reoccurring units suggests a rather static concept of vegetation patterns rather than a dynamic one.  Are associations – oak woodlands, bogs – “real” things in nature – things like you and me?  Or should these terms be regarding as convenient abstractions, like, say, Harry Potter fans at a movie, temporarily aggregating before dissipating? 

3. Directional change in vegetation: In addition to describing and classifying vegetation, early 20th Century ecologists wanted to explain how vegetation dynamics change.  As we have already seen Frederic Clements, the American ecologist, described such change largely as progressive, as developmental in the way that individual organisms develops.  This development leads to the concept of the “climax state”.  In fact, an early account of succession, one that Clements refers to in his work, comes from an examination of Irish bogs.  Starting with this I will describe a brief history of succession – stressing the degree to which the concept incorporates both stability and change.
4. Finally, the invasion of woodlands with invasive species like Rhododendron in the Killarney poses challenges for the ways in which we regard vegetation patterns.  If a community are dynamic and constantly in flux, never in equilibrium or balance, never developing to a stable end point (as some contemporary successional theories would have it), then in what sense should we be concerned about returning the Irish woodlands to their pre-invaded conditions.

[i] Moore, J. J. (1962) The Braun-Blanquet System: A Reassessment  Source: Journal of Ecology, Vol. 50, No. 3 pp. 761-769
[ii] Kelly, Daniel L. (July 1981). "The Native Forest Vegetation of Killarney, South-West Ireland: An Ecological Account". The Journal of Ecology 69 (2): 437–472.

No comments:

Post a Comment