Lagascalia 19(1-2), 1997
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Edaphic specialization
Several genera and species c o m p l e x e s show a clear adaptation to either base-rich
o r b a s e - p o o r s o i l s . W i t h i n t h e g e n u s Helictotrichon
t h e H. parlatorei-,
t h e H.
sarracenorumand the H. bromoides g r o u p are strongly basiphilic whereas the H.
marginatum
group is acidophilic. Also the species of the genus
Pseudarrhenatherum
differ in this respect from each other since P. longifolium is acidophilic and P. pallens
b a s i p h i l i c ( R O M E R O Z A R C O , 1 9 8 5 a ) . All s p e c i e s of Arrhenatherum
and also the
p e r e n n i a l Avena macrostachya
are b a s i p h i l i c . S o m e of the a n n u a l g e n e r a (Aira,
Corynephorus,
the annuals of Trisetum, etc.) are specialised to sandy places with poor
or completely absent soil development (cf. Table 1). Sands are usually poor in nutrients
and bases (leaching out of nutrients), only in littoral zones and dry climates sands may
be sometimes comparatively base-rich. It seems that the annual Avenaeae are much less
selective than the perennials with respect to the chemical properties of the substrate.
S e e m i n g l y they only take a d v a n t a g e from either m e c h a n i c a l l y unstable or shallow
grounds with irregular or hazardous availability of water, which are important factors
that eliminate perennial competitors including phanerophytic plants.
T h e specialization to either base-rich or base-poor soils of the perennial Aveneae
has an e n o r m o u s impact on distribution and the representation of the particular taxa
in various types of Mediterranean vegetation. In several species or species groups the
ranges are thus defined rather by edaphic than climatical reasons. T h e most impressive
e x a m p l e is the differential distribution of the H. marginatumand the H.
bromoides
group on the Iberian Peninsula which largely accords to the distribution of base-poor
and base-rich soils produced over the different bedrocks of this region (Fig. 4).
Altitudinal zonation
Most annual Aveneae are colonisers of the Mediterranean lowlands. Lowlands fulfill
their ecological requirements (open places) and enable them to complete their life cycle
w i t h i n a g i v e n t i m e ( a v a i l a b l e w a t e r in t h e e s s e n t i a l p e r i o d s of g e r m i n a t i o n and
vegetative growth coupled with suitably high temperatures and insolation).
In perennial Aveneae the altitudinal patterns of distribution are m u c h m o r e varied.
A n u m b e r of g e n e r a or s p e c i e s g r o u p s of p e r e n n i a l Aveneae c o n c e n t r a t e s on l o w
altitudes. In the Helictotrichon
sarracenorum-,
H. bromoidesand H.
marginatum
g r o u p , in Arrhenatherum,
Pseudarrhenatherum,
p a r t s of Koeleria
a n d in t h e H.
pratense- and H. desertorum group, the latter ones being situated largely outside of the
Mediterranean Region (nos. 6, 11 in Fig. 1), most species occur in lowlands to lower
m o n t a n e zones.
Others are strictly orophytic (Fig. 1), e.g., Avena
subgen. Tricholemma,
the H. sedenense-, H. parlatorei-,
perennial species of the genus Trisetum.
macrostachya,
H. versicolor-
Helictotrichon
group and most
F u r t h e r o r o p h y t i c t a x a with partly n a r r o w g e o g r a p h i c a l d i s t r i b u t i o n b e l o n g to
o t h e r w i s e clearly l o w l a n d - c e n t r e d species c o m p l e x e s . E x a m p l e s w o u l d be
Helictotrichon
leve (central Sierra N e v a d a , endemic) in the H. marginatum
group, H.