Effects of snow-melt timing on plant life in an alpine snow-bed 高山雪田環境における消雪時期が植物の生活に及ぼす影響

博士論文

Effects of snow－melt timing on alpine plants were studied in a snow－bed in the Taisetsu Mountains by setting six plots （A to F） on a slope along a snow－melting gradient， from 1988 to 1990． Chapter 1： Geological， topographical and meteorogical aspects of the Taisetsu Mountains and snow condition of study area were mentioned． Although the time of snow disappearance fluctuates from year to year， snow disappearance patterns were similar every year． The length of snow－free season were 120 - 50days in 1988， 95－45 days in 1989 and 130－60 days in 1990 （from plots A to F）． Chapter 2： Distributional pattern of plants was studied along the snow－melting gradient． Study area was divided into two habitats by microtopographic aspects within each plot； the flat habitat and the mound habitat． The number of species per plot decreased with the shortened snow－free period． Some tendencies were recognized between plant growth form and distributional pattern． At the flat habitat， dominant growth forms exchanged from long snow－free plot to short one as follows； lichens - evergreen and deciduous shrubs - forbs - graminoids - mosses． At the mound habitat， evergreen and deciduous shrubs prevailed widely along the gradient in comparison with that at the flat habitat． This indicated the ability of shrubs to exploit new habitat by creeping up exposed rocks． Since plants can get much light resource and escape from cold melting water by inhabiting on the mounds， exploitation of the mound habitat is adaptive for these shrubs． Deciduous shrubs and forbs completely disappeared in the extremely short snow－free plot， F． Forbs and graminoids shifted their distributional mode to shorter snow-free plots where shrubs decreased the vegetation cover， as a result of competitive exclusion by shrubs． Chapter 3．1： Phenological pattern of various species was studied along the snow－melting gradient． The contraction of snow－free period reduced the flowering and fruiting rates． Thephenology of plants was controlled primarily by （1） growth form，（2） the span of pre－flowering， and （3） the term for the maturity of seeds． Evergreen shrub species required a long term for the maturing of seeds． They abandoned sexual reproduction and reproduced vegetatively by layering in short snow－free plots． Whereas， forb and graminoid species opened flowers even in the short snow－free plots． Some graminoid species shifted their leaf habit from summergreen to semi－evergreen in short snow－free plots． This is considered strategy to extend the photosynthetic period． Chapter 3．2： Factors controlling flowering and fruiting time were studied with nine species． Effective cumulative temperatures for flowering and fruiting were physiologically determined in each species， and actual flowering time was affected by the timing of snow disappearance． Alpine plants could not regulate the pre－flowering and fruiting periods in accordance with habitat conditions， and the species having short pre－flowering and fruiting periods were adaptive for seed－set success under the short snow－free condition． Chapter 4．1： For five forb species， height， leaf number， flower number and fruit number were measured along the gradient．Responses of plant performance to snow－melt condition varied among species. Veronica stelleri var. longistyla and Solidago virga-aurea var. leiocarpa, which decreased flower and leaf numbers, were less tolerant for short growing season. Whereas Primula cuneifolia and Potentilla matsumurae. Which maintained their flower number throughout the gradient, had high tolerance of reproductive activity to short growing season. Although Peucedanum multivittatum had a high tolerance, long pre-flowering period prevented the seed-set in late melting plot, E. Chapter 4．2： Relationships between snow-free period and leaf characteristics of evergreen shrubs were studied for Phyllodoce aleutica and Rhododendron aureum. Along the snow-melting gradient, leaf size and the number of new leaves were reduced and while leaf longevity, leaf specific weight and nitrogen contents in leaves increased. These phenomena are considered adaptations aimed at maintaining a carbon balance under the conditions where the growing season is severely limited. In short snow-free places where the photosynthetic assimilation is extremely limited, plants need long periods of time in order to pay back leaf construction costs. Hence tough leaves having a long life－span are produced at the expense of leaf area． Chapter 5： lnterrelationship between reproductive success and pollinator activity were studied with reference to the flowering time along the snow－melting gradient． Fruiting of early flowered individuals in early snow－melt plots was restricted by pollinator limitation because of low pollinator activity under the low temperature condition of an early season．Fruiting rate obviously increased with lateness of flowering time， or increase of temperature． Since too late flowering brought about low fruiting success because of lack of fruiting period， existence of best flowering timing for seed－set was expected．