PINUS SYLVESTRIS (L.) STOMATAL CONCENTRATIONS AND ASSOCIATED POLLEN FREQUENCIES: DETERMINATION OF LOCAL SPECIES PRESENCE

The abundant production of wind-dispersed pollen that can be transported over long distances by Pinus sylvestris means that the species may be considerably over-represented in the palaeoecological record. Accordingly, a minimum pollen frequency threshold of 20% has been commonly adopted by palynologists for the determination of the local presence of Pinus sylvestris at a site. Recent findings during routine palynological analyses of pine stomata, indicative of local species presence, within lake sediments containing associated pollen frequencies of less than 5% has brought the validity of the standard 20% threshold into question.

Stomatal and pollen analyses are presented from four Holocene lake sedimentary sequences in the Scottish Highlands. Detailed stomatal counts reveal the presence of pine stomata within sediments with associated pine pollen frequencies as low as 0.4%. These results support the assertion that Pinus sylvestris may be locally present at a site when pollen frequencies are in very low abundance and that the commonly adopted minimum pollen frequency threshold of 20% is too high. Minimum pine pollen frequencies within sediments containing stomata at the four analysis sites examined in this study, however, are so low, ranging from 0.4% to 3.8%, that no minimum pollen frequency threshold indicative of the local presence of Pinus sylvestris may be proposed.

Pine stomata were found to precede the initial Holocene increase of Pinus sylvestris pollen at two of the sites examined, Loch an Amair in Glen Affric and Dubh-Lochan in the Great Glen region. Stomatal analyses reveal the local presence of pine at these sites approximately 1600 and 600 years, respectively, prior to the species arrival times that would be determined from the palynological evidence. These results reveal that Pinus sylvestris may be locally present at a site for hundreds or even thousands of years in low abundance before expansion of the population, and that the time lag between initial species arrival and population expansion appears to be variable amongst sites. These findings have major implications for the determination of the initial Holocene spread of pine throughout the British Isles and for some of the inferences about pine population dynamics in the Scottish Highlands that have been drawn from the palynological evidence.