Photosynthesis and light-use efficiency by plants in a Canadian boreal forest ecosystem

Measurements of the photosynthetic response to midsummer irradiance were ma de for 11 species representing the dominant trees, understory shrubs, herba ceous plants and moss species in an old black spruce (Picea mariana (Mill.) B.S.P.) boreal forest ecosystem. Maximum rates of photosynthesis per unit foliage area at saturating irradiance, A(max) were highest for aspen (Popul us tremuloides Michx.), reaching 16 mu mol m(-2) s(-1). For tamarack (Larix laricina (Du Roi) K. Kock) and P. mariana, A(max) was only 2.6 and 1.8 mu mol m(-2) S-1, respectively. Values of A(max) for understory shrubs and her baceous plants were clustered between 9 and I I mu mol m(-2) S(-)1, whereas A(max) of feather moss (Pleurozium schreberi (Brid.) Mitt.) reached only 1 .9 mu mol m(-2) s(-1). No corrections were made for differences in shoot st ructure, but values of photosynthetic light-use efficiency were similar for most species (70-80 mmol CO2 mol(-1)); however, they were much lower for L . laricina and A mariana (15 mmol CO2 mol(-1)) and much higher for P. schre beri (102 mmol CO2 mol(-1)). There was a linear relationship between A(max) and foliage nitrogen concentration on an area basis for the broad-leaved s pecies in the canopy and understory, but the data for P. mariana, L laricin a and P. schreberi fell well below this line. We conclude that it is not po ssible to scale photosynthesis from leaves to the canopy in this ecosystem based on a single relationship between photosynthetic rate and foliage nitr ogen concentration.