SUMMARY

Seeds were collected from populations of Boehmeria cylindrica growing in naturally enriched concentrations of CO2. In the controlled environment plants grown from populations which are normally exposed to enriched atmospheres of CO2 (400 to 575 ppmv) show a greater CO2-sensitivity of growth than plants from ambient (370 ppmv) sites. Observed differences in height growth and dry weight partitioning could be explained in terms of the different rates of plant growth under the imposed treatments, rather than in terms of population differences in growth partitioning. Comparisons of stomatal density-CO2 responses for species from different climates and over the different time scales of centuries and millennia indicate a similar decline in density with increasing CO2 concentrations, with no evidence of any changes in the CO2 sensitivity. However, the clear species-specific nature of the stomatal density response suggests it is unlikely to be observed ubiquitously in plants growing by high-CO2 springs. Nevertheless, there is a close similarity between the stomatal density responses shown by plants growing under CO2 enrichment adjacent to springs, and the stomatal density changes recorded from fossil leaves, indicating one valuable use of high-CO2 springs in biological research.

INTRODUCTION

Measurements of the atmospheric concentration of CO2, typically from air locked up in permanent ice caps, provide time courses for a continuously varying CO2 concentration over the last 160,000 years (Barnola et al., 1987). Estimates of CO2 by indirect means also show variations of CO2 concentration over millions of years (Hays, Imbrie & Shackleton, 1976; Berner, 1990).

An interesting consequence of these variations is the potential for a CO2-controlled selection in higher plants.