Canopy Conductance of Coniferous Forests Related to Climate

Canopy conductance, G , and specific canopy conductance, g , were estimated from Bowen ratio measurements above a sparse Scots pine forest. Both conductances increased with increasing solar radiation, R g , with the highest sensitivity in the low R g range. Above 400–500 W m −2 they were practically constant with respect to R g . During typical daytime situations the conductances depended primarily on vapor pressure deficit, VPD . For R g above 500 W m −2 , g decreased from 11 mm s −1 at a VPD of 300 Pa down to 5.5 mm s −1 at 800 Pa, then more slowly down to 3 mm s −1 at 2000 Pa. The diurnal variation of g was characterized by a rapid increase in the morning up to about 6 mm s −1 , then g slowly decreased until noon and recovered somewhat in the afternoon to decrease again down to about 2.5 mm s −1 in the evening. From May to August the projected needle area index varied from 2.0 to 2.6, whereas total canopy conductance increased by 100% due to variations in climate (50%), needle area (30%), and stomatal efficiency (20%). Comparison between three forests indicated that it was more important to understand the mechanisms behind the seasonal variation of G than to assess differences in G between forests, these latter differences being remarkably small.