Species-specific responses of three coastal dune plants to drought
conditions were examined to investigate the causal relationship
between drought tolerance and plant distribution in coastal dune
regions. Measurements of net photosynthetic rates were pursued
in two meteorologically contrasting years of 1989 and 1990; the
summer of 1990 was characterised by long rainless periods, whereas
the meteorological conditions of summer months in 1989 did not
differ significantly from the average values. The reductions
of net photosynthetic rates by the 1990-summer aridity differed
widely among the three species. Net photosynthetic rate of
Ischaemum anthephoroides, a C4 plant,
distributed in hind dunes, fell to only 33% - 43% of the values
in the same season in 1989. In contrast, those of Carex kobomugi
and Calystegia soldanella, which were generally growing
in more seaward sites, were 67% - 87% and 76% - 97%, respectively.
In July 1990, Pn of Ischaemum anthephoroides
were significantly lower than those of the other two species.
Ischaemum anthephoroides showed the most pronounced reduction
of leaf water potential as low as -2.5 MPa, while the reductions
of Calystegia soldanella and Carex kobomugi remained
around -1.5 and -2.0 MPa, respectively in 1990 summer period.
The low tolerance of I. anthephoroides to a severe drought
was correlated with shallow distribution of the root system, and
is considered to be one of the reasons why the distribution of
I. anthephoroides is restricted to the inland area where
drought conditions are milder than in seaward areas.