Colonisation by reed seedlings, Phragmites australis (Cav.) Trin. ex Steud.
is rare and usually occurs after drawdown and when shallow water prevails.
P. australia seeds have high rates of germination but successful colonisat
ion is dependent upon subsequent water depths. We investigated the capacity
of young reed plants to resist a 4 weeks submergence stress within a 5 mon
ths period, and their subsequent recovery. A pond experiment examined the i
nteractions between submergence depth and the age of the seedlings at subme
rgence. Four submergence treatments were used. In two partial submergence t
reatments, 50 and 80% of the initial leaf area was submerged. In two total
submergence treatments, plants were either submerged at 125% of their initi
al height with possible subsequent development of emerged leaves, or the wa
ter was deepened as they grew to maintain total submergence for 4 weeks. Th
e ages at submergence were 40, 60 and 80 days. Plants were harvested at 5 m
onths. Shoot elongation, biomass allocations to aerial biomass, roots and r
hizomes, and photosynthetic activity of aerial leaves were measured. Redox
potential was measured for a subsample.
Mortality (18.7%) occurred only in the permanent submergence treatment for
40-day-old seedlings. In all treatments, submerged leaves senesced, except
the terminal (youngest) leaves of permanently submerged plants. Submergence
differentially affected shoot length and biomass, depending upon the inten
sity of the treatment and the seedling age. The major differences were foun
d between the two partial and two total submergence treatments. Partial sub
mergence (50 and 80%) significantly enhanced biomass accumulation and growt
h, whereas total submergence largely decreased biomass production and growt
h in length, with less effect on shoot numbers. The 80-day-old seedlings to
lerated submergence better but growth was poorest in medium-aged plants (60
-day-old). Increased elongation of the growing internodes of up to 140% was
caused by submergence, and photosynthetic activity was enhanced by 85% in
emergent leaves of plants initially submerged but allowed to produce emerge
d leaves during the treatment period.
Young P. australis plants require shallow water levels without long lasting
submergence to grow and survive. Tolerance to submergence increases with a
ge. These processes contribute to define the conditions for colonisation vi
a seeds in P. australis. (C) 2001 Elsevier Science B.V. All rights reserved
.