1. Fluctuating asymmetry (FA) of birch leaves collected along four pol
lution gradients in NW Europe was assessed by measuring the difference
in width between right and left halves of the leaf. 2. FA of leaves w
as not related to tree diameter, and values for long and short shoots
of the same tree were correlated. 3. FA of silver birch Betula pendula
and two subspecies of B. pubescens, white birch B. pubescens ssp. pub
escens and mountain birch B. pubescens ssp. tortuosa, decreased hyperb
olically with distance away from four sources of aerial emission which
differed in amounts and composition of pollutants. 4. The extent of t
he zone of increased FA was higher around polluters which produced mor
e emissions. 5. FA of leaves sampled around two copper-nickel smelters
was positively related to foliar nickel concentrations, and regressio
ns from these two sites had similar slopes and intercepts. 6. The data
suggest that FA of birch leaves may represent a convenient indicator
for rapid assessment of environmental quality.