Ecological impact of the mid-Holocene hemlock decline in southern Ontario, Canada

G-3. - By [approximately]3500 yr BP, pollen percentages of T. canadensis had increased again to reach predecline values. As T. canadensis populations re-expanded there was a gradual decrease in pollen influx values of P. strobus, P. banksiana/resinosa, and Quercus, whereas F. grandifolia and A. saccharum did not decrease. Pollen percentages of T. canadensis increased to predecline levels, whereas pollen influx for this taxon was lower than prior to the decline.

High Lake; H-1. - T. canadensis increased in abundance (pollen influx and percentages) at High Lake at [approximately]7000 yr BP to form mixed forest with A. saccharum, F. grandifolia, P. strobus, P. banksiana/resinosa, and Betula. As T. canadensis pollen percentages increased, there was a decrease in pollen percentages and influx values of P. strobus, P. banksiana/resinosa, Quercus, and A. saccharum. Ulmus, F. nigra, A. rugosa, and Thuja/Juniperus were also present, probably occurring in the wetter areas. Pollen percentages and influx values of T. canadensis started to decline toward the end of this zone.

H-2. - T. canadensis pollen influx values and percentages declined (from [greater than]20% to [approximately]10% of pollen sum) at [approximately]5100 yr BP, [approximately]400 yr later than at Graham Lake. As at Graham Lake, pollen percentages declined, increased briefly, and declined again. The decline (in terms of pollen percentages) was less marked than that at Graham Lake (where pollen percentages fell from 20% to [less than] 5%). Pollen influx values of Ulmus, F. grandifolia, A. saccharum, and Betula increased somewhat.

H-3. - T. canadensis increased in abundance again to reach former pollen percentages after 3600 yr BP. Pollen percentages of F. grandifolia and Betula also increased, while those of Quercus and Thuja/Juniperus declined.

Response to the hemlock decline

Graham Lake. - Rates of change increased at the time of the hemlock decline at Graham Lake as other forest taxa increased in abundance [ILLUSTRATION FOR FIGURE 6 OMITTED]. Most taxa appear to have started to increase after the initial decline of hemlock at [approximately]6000 yr BP [ILLUSTRATION FOR FIGURE 5 OMITTED], and they continued to increase once hemlock populations declined further after 5500 yr BP. Pollen influx values for several taxa increased exponentially after the decline ([ILLUSTRATION FOR FIGURE 7 OMITTED], Table 3). The increase of F. grandifolia, P. strobus, Quercus, Betula, and A. saccharum were described relatively well by the exponential equation. Pollen influx values of some tree taxa, such as F. nigra and Ostrya/Carpinus, did not increase after the hemlock decline. P. banksiana/resinosa pollen influx values were highly variable with some short-lived increases.

The ordination plot in Fig. 8a shows the pattern of vegetation change at Graham Lake over the period of the hemlock decline. The samples (individual pollen levels reflecting vegetation composition around the site at certain time intervals) fall into three clusters (as delineated by numerical zonation) that represent three phases of the decline: before (7500-5500 yr BP), during (5500-3500 yr BP), and after (3500-1000 yr BP). Forest composition changed markedly during the period of the hemlock decline, represented by the distinct cluster of samples in ordination space during this period [ILLUSTRATION FOR FIGURE 8A OMITTED]. However, when T. canadensis populations recovered and increased to former levels of abundance, after [approximately]2000 yr, pollen assemblages did not return to the pre-decline composition; F. grandifolia and A. saccharum pollen influx values were higher, suggesting they were more abundant on the landscape, and Pinus and Quercus species appear to have become less abundant than prior to the decline. The samples covering this time period also fall into a distinct cluster [ILLUSTRATION FOR FIGURE 8A OMITTED].