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BEN # 260

Adolf Ceska aceska at victoria.tc.ca
Wed Nov 8 04:00:32 EST 2000


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No. 260                              November 8, 2000

aceska at victoria.tc.ca                Victoria, B.C.
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 Dr. A. Ceska, P.O.Box 8546, Victoria, B.C. Canada V8W 3S2
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ANEMO-OROGRAPHIC SYSTEMS AND THEIR IMPACT ON PLANT LIFE
From: Jan Jenik [jenik at natur.cuni.cz]

More  than  three decades ago, a preliminary paper followed by a
monograph (Jenik 1959, 1960) attempted to describe  and  explain
the  origin  and  unequal  distribution  of  biodiversity in the
Sudetes and other Hercynian/Variscan mountains  in  Europe.  The
essence  of  these  publications, the theory of anemo-orographic
systems  (A-O  systems  for  short),  has  become  a  reasonably
feasible model for describing and explaining large-scale ecosys-
tem  patterns  encountered in the summit areas of these "middle-
mountains". Some geographers positively responded to  this  con-
cept, which aims at integration of biogeographical knowledge and
topoclimatic  and  geomorphologic  evidence. Many ecologists and
conservationists have utilized and even further  developed  this
theory  (see  a  review  in Srutek 1990) and applied it to other
European mountains (Jenik, 1990).

Essential features of the Hercynian middle-mountains

In Central Europe, the Hercynian mountains  create  an  arch  of
separate  massifs  situated  to  the NW and N of the Alps. Their
major representatives are the  Vosges,  Black  Forest,  Bohemian
Forest,  the Ore Mountains and the High Sudetes, the latter with
three dominant elevations called Giant Mts. (1602 m), Snow  Mts.
(1423 m) and High Jesenik Mts. (1491 m). A product of Palaeozoic
mountain-building, these massifs - despite various specificities
-  are  similar  in  age,  lithology, georelief, climate and, in
their plant and animal life.  They  consist  mostly  of  gneiss,
crystalline  schists  and  granite,  and the base-rich carbonate
rocks and volcanic rocks are scarce.

Due to  advanced  denudation  in  the  Tertiary,  their  rounded
georelief lacks rugged rock-faces, precipitous cliffs and deeply
cut  ravines.  Seldom  surpassing the altitude of 1500 m a.s.l.,
they are  transformed  by  periglacial  cryogenic  processes  or
glacial  erosion  only  in  their  topmost areas. In contrast to
rugged landforms and glaciated valleys met with in the  Alps  or
Carpathians,   these   middle-mountains,  called  also  "forest-
mountains" ("Waldgebirge" in  German),  offer  only  constrained
areas  for treeless ecosystems and fine-grained alpine biodiver-
sity.

Contrasting distribution of biodiversity

The surface of Hercynian mountains is largely  covered  by  con-
iferous  and mixed forests. A product of Postglacial succession,
the flora and  fauna  consist  of  widely  distributed  "common"
European  species. In the German and Czech botanical literature,
the expression "Hercynian flora" is a pejorative term indicating
species-poor and monotonous floristic composition. Zonal  forest
communities  consist  of  only a few dozens of vascular species,
and similar uniformity refers  to  other  life  forms,  such  as
mosses,  lichens,  fungi,  and  to many groups of vertebrate and
invertebrate animals.

There exists, however,  a  small  number  of  local  exceptions,
species-rich  sites, which are recurrently reported in documents
of European natural history.

These species-rich sites

 1. are colonised by outstanding numbers  of  plant  and  animal
    populations,
 2. excel  in  a remarkable coexistence of biogeographically and
    ecologically contrasting species, e.g., glacial relics  near
    thermophilous populations,
 3. display  remarkable  diversity  of  contrasting  biotic com-
    munities and ecosystems.

For example, about 500 vascular species, coexisting in  30  dif-
ferent  plant  communities,  have  been  recorded  from  a small
valley-head called Velka Kotlina (Jenik et al.  1980).  Species-
rich  fauna usually accompanies the botanical wealth. Occurrence
of endemic and/or biogeographically and  taxonomically  isolated
populations  suggests that these localities serve as (i) refuges
of retreating biota, (ii) forerunner sites of  spreading  biota,
and  even (iii) centres of active microevolution for new species
(Jenik 1983).

Earlier explanations of outstanding biodiversity in  these  par-
ticular localities were rather inconsistent and confusing (Jenik
1961:  54-74). Many explanations linked the biotic wealth with a
"sheltered site",  a  general  qualification  which  was  seldom
described  in  terms of physical parameters. By many hypotheses,
mainly locally  outcropping  base-rich  rocks  were  taken  into
account.  Detailed  geological and geobotanical research did not
confirm the primary  role  of  this  factor:  some  species-rich
communities  evidently  occur  within the acid parent rocks and,
vice versa, the existence of mineral-rich rocks in  the  subsoil
does not always support species-rich ecosystems.

In  order to understand the identity of species-rich localities,
broader environs of these  sites  must  be  taken  into  account
(Soukupova  et  al.,  1995).  Though  numerous arctic and alpine
organisms participate, their refuges do not occupy  the  highest
altitudes   reached  in  particular  ranges.  In  contrast,  the
majority of diversified ecosystems occurs  at  lower  altitudes,
often on treeless avalanche tracks below the present-day timber-
line.  Remarkably,  many  arctic/alpine/nordic/boreal species of
the Hercynian mountains coexist  in  association  with  numerous
southern/lowland plants and animals.

A  detailed  analysis  of  the  biodiversity pattern in the High
Sudetes disclosed repeated geomorphological  and  meteorological
components  in  these  species-rich  localities:  (i) linkage to
east-facing slopes, and situation on the eastern margin of large
summit plateaux or saddles, (ii) situation on back  side  of  an
upland  plateau or saddle, with regard to a funnel-shaped valley
ascending the summit area and accelerating air currents from  W,
SW  or NW, (iii) coincidence with recurrent snowdrifts, cornices
and active avalanche action  in  winter,  and  (iv)  coincidence
within  various  concave  landforms, such as nivation niches and
glacial corries.

In terms of  georelief,  the  high-biodiversity  sites  coincide
mostly with E-facing (NE, E, SE) concave landforms, particularly
with  shallow  hollows  or  deeply  cut  cirques, which are con-
tinually subjected to nivation and, in the past,  were  sculpted
by  glaciers.  Remarkably,  the  topmost  Hercynian  peaks  lack
prominent glacial cirques  even  on  their  eastern  flanks.  If
altitude  lacks  importance, what is then the general setting of
the remarkable biodiversity centres?  The  Giant  Mts.,  in  the
Czech  and  Polish  territory  offer  a  particularly meaningful
picture: 15 cirques with botanical  and  zoological  sanctuaries
situated  to  the  east  of  large  upland plateaux. In the High
Jesenik Mts. and in Black Forest two large funnel-shaped valleys
join in the topmost area, and the famous  cirques  and  species-
rich  habitats  are  situated at the eastern side of these junc-
tions. In a range stretched from north to  south,  such  as  the
Vosges,  understandably,  a  variety  of  nivation  hollows  and
cirques is developed along the long E-facing flank of the ridge.

A generalized model called anemo-orographic system

Existing interplay of  landforms  and  meteorological/ecological
effects  in  culminating  areas  of the Hercynian mountains have
been  summarized  in  a  generalized  model  called  the  Anemo-
Orographic System.

According to the above quoted monograph (Jenik 1961), this model
consists of a

  1. funnel-shaped windward section,
  2. wind-accelerating summit section, and
  3. leeward turbulent section.

Summary of the individual sections of A-O systems:

Funnel-shaped windward section

   Landforms: large valley, gradually ascending
   Wind action: laminar air currents, prevailing local wind
   Precipitation: enhanced cloudiness, rainfall, snowfall
   Snowpack:  moderately  deep,  stratified according to weather
   sequence
   Temperature: according to altitudinal gradient
   Soils: deeply weathered zonal soils, podzols, cambisols
   Ecosystems: forests, controlled  by  altitude,  species-poor,
   coarse- grained pattern

Wind-accelerating summit section

   Landforms: high elevation summit plateau, flat saddle
   Wind action: high-speed, laminar air currents
   Precipitation: maximum rainfall, snowfall, rime
   Snowpack:  shallow  and  short  durations,  reduced by eolian
   ablation
   Temperature: equable over flat relief due to wind and  advec-
   tion
   Soils:  azonal soils in cryo-eolian zone, partly relic soils,
   organic soils
   Ecosystems: treeless, controlled by  wind  action,  upwelling
   springs and mires

Leeward turbulent section

   Landforms: concave hollow, niche, cirque, corrie
   Wind  action:  turbulent currents, reduced velocity, calms in
   shelter
   Precipitation: fog, drizzle, dew, plenty of melt water
   Snowpack, avalanches: cornices, snowdrifts, avalanche tracks,
   long duration
   Temperature: variable in rugged relief, due to differences in
   insolation and reduced convection
   Soils: variety of lithosols and  rankers,  locally  nutrient-
   rich outcropping rocks
   Ecosystems:  treeless,  controlled  by avalanche action, high
   biodiversity, fine-grained pattern

A great variety of real A-O systems is encountered in  different
ranges  and parts of these ranges. In the above quoted monograph
(Jenik 1961: 195-196) various  models  have  been  proposed  (1)
according  to  their  distance  or  proximity to the generalised
model as "imperfect" or "perfect", respectively, and (2) accord-
ing to occasional combination of two  or  several  funnel-shaped
windward valleys, as "simple" and "composed" A-O systems. Obser-
vations  in  the  past  four  decades disclosed a number of less
perfect A-O systems in numerous mountains  of  Europe.  Wherever
unilateral  winds  blow  over  suitably  sculpted landforms, the
respective windward, topmost and leeward sites develop  in  con-
trasting  ecosystems  which are clearly indicated by plant life.
However, only long-term  interaction  of  broader  relief,  wind
action  and  plant  succession results in the scenery of a "per-
fect" A-O system.

Applicability of the model

The late Prof. Askell Love,  University  of  Colorado,  was  the
first  to  indicate  the  potential applicability of A-O systems
outside Europe, i.e., in the area  of  Mount  Washington,  White
Mountains.  This  massif  is  marked  by terminal situation in a
funnel-shaped configuration of the Presidential  Range,  and  by
extremely  unilateral  western  winds. Elevation, georelief with
distinctive climate, position of the timberline  and  vegetation
are  similar to the European middle-mountains. Though covered by
an ice-sheet in the  Glacial  period,  the  famous  species-rich
Alpine  Garden,  Tuckerman's Ravine, Huntington Ravine and Great
Gulf were very likely  developed  and  preserved  due  to  their
stabilised  leeward position and avalanche action throughout the
Postglacial period. The  A-O  system  model  seems  to  fit  the
spatio-temporal   scale   and   evolutionary  history  of  Mount
Washington more satisfactorily than  the"alpine  mesotopographic
gradient"  described  by  Billings (1973) and illustrated in the
new  edition  of  "_North  American   Terrestrial   Vegetation_"
(reviewed in BEN No. 252).

References

Billings,    W.D.   1973.   Arctic   and   alpine   vegetations:
   similarities, differences, and susceptibility to disturbance.
   BioScience 23: 697-704.
Jenik J. 1959. Kurzgefasste Uebersicht der  Theorie  der  anemo-
   orographischen Systeme. Preslia 31: 337-357. (In German.)
Jenik  J.  1961.  Alpine vegetation of the Giant Mts., Snow Mts.
   and High Jesenik Mts.  Theory  of  anemo-orographic  systems.
   Naklad. CSAV, Praha, 409 p. (In Czech with German summary.)
Jenik  J.  1983.  The evolutionary stage of the Sudetic cirques.
   Biol. Listy 48: 241-248. (In Czech.)
Jenik J. 1990. Large-scale pattern of biodiversity in  Hercynian
   massifs. Pp. 251-259 in: F. Krahulec, A.D.Q. Agnew, S. Agnew,
   J.H.  Willems [eds.]: Spatial processes in plant communities.
   Academia, Praha.
Jenik J. 1997. Anemo-orographic systems in  the  Hercynian  Mts.
   and  their  effects  on  biodiversity. Acta Univ. Wratislav.,
   Prace Inst. Geogr., ser. C, Meteorol. i Klimatol. 4: 9-21.
Jenik J., L. Bures, & Z. Buresova. 1980. Syntaxonomic  study  of
   vegetation  in  Velka  Kotlina  cirque,  the  Sudetes.  Folia
   Geobot. Phytotax. 14: 337-448.
Soukupova L., M. Kocianova, J. Jenik, & J. Sekyra  [eds.]  1995.
   Arctic-alpine  tundra  in  the  Krkonose,  the Sudetes. Opera
   Corcontica 32: 5-88.
Srutek M. 1990. Application of the  theory  of  anemo-orographic
   systems  in  natural history. Opera Corcontica 27: 47-58. (In
   Czech.)

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