Cherry birch (Betula lenta) COSEWIC assessment and status report: chapter 9

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Population sizes and trends

• Search effort
• Abundance
• Fluctuations and trends
• Rescue effect

Search effort

The two adjacent sites at 15 Mile Creek were surveyed and each tree was located and measured; results were compared with the writer’s 1984 observations at the northerly property. The adjacent lakeshore property was searched in 2005 and no survivors were found, other than the seedling that the owners planted from their recently lost tree (trees were individually lost through the 1990s, up to the last one falling into the lake in 1998 [Fallding, 1998; University of Guelph Arboretum native tree records]). A recent report (Thompson, 1992) of a single tree in the slope forest of nearby 16 Mile Creek could not be found after about 6 person-hours of searching; one of the party, George Meyers, observed it here in the 1980s. A report (Soyka et al., 1977) of this species at the lakeshore park west of 4 Mile Creek was followed up with a visit but could not be found after about 8 person-hours of searching; the look-alike Prunus avium was abundant. Other likely sites were searched: the Niagara Glen (where reported in 1943) and Navy Island. One Natural Heritage Information Centre (NHIC) record with a valid specimen no longer had any cherry birch; another NHIC recorded sighting turned out to be Prunus avium.

In total, about 27 person-hours were spent searching and documenting recorded sites and another 20 person-hours searching potential sites within the region. In addition, a two-day expedition to western New York State was undertaken with four people in 2004, to observe the species in three different sites in order to have a better concept of typical cherry birch habitat and ecology.

Abundance

Fourteen naturally occurring trees, 13 to 62.5 cm dbh (diameter at breast height), currently exist at the 15 Mile Creek sites (Table 1). Six were fruiting. In addition, the northerly owners have had four additional trees planted from the local seed source where others have been lost. It is possible that an additional few trees exist on the forested slopes around the bay.

Fluctuations and trends

In 1984, nine trees 14-95cm dbh were recorded by the author at the northerly 15 Mile Creek bay slope site (1a in Table 1). Thompson (1992) reported 14 trees 10-93 cm dbh at this site and an adjacent lakeshore site to the northeast, 1c (but not including the southwesterly property, 1b; Thompson, pers. comm.). The lakeshore site, above the Lake Ontario shoreline, three properties to the northeast from 1a, was reported in the St. Catharines Standard (Fallding, 1998). A few trees were noted as having recently been there but only one remaining at the time of the article; it fell into the lake later that year. Herbarium records for the general area of this set of three adjacent sites date back to 1898 (McCalla, Ontario Agricultural College [OAC]). The first quantitative record was in 1967; Hosie (1979) reported that:

“There are approximately 50 trees of different ages and sizes at this location. Several medium-sized trees are dead, a few others appear to be dying, but a good number are healthy. Two or three of the probable originators have been growing along the top of the slope for at least 75 years.”

It is assumed that this report was of a stand extending from the bay slopes to the shores of Lake Ontario, including all of the above three sub-sites. Although there has been no apparent decline in numbers between 1992 and 2004, despite the loss of the 3 largest trees and the loss of all of the lakeshore trees, this is due to the discovery of additional trees to the southwest along the bay. However, the occurrence of new naturally occurring saplings is a positive indication, while the open location lacking protection from lake storms is a continuing threat.

The 38 year period (1967-2005) represents approximately one generation (assuming an average age of 40 years for mature trees). It is likely that the greatest decline occurred in the last 40 years and one can only speculate on the decline over the past three generations--about 120 years. This is an area of early European settlement, beginning in the 1780s (Marsh, 1985), thus the largest trees observed in the last 20 years probably dated back to this time (see age calculations under Special Significance of the Species). Early moderate clearing may have actually opened additional habitat for seedling recruitment, but as land clearing intensified in this significant tender-fruit growing region, and more recently, as lakeshore residential development intensified, the rate of decline certainly increased rapidly. Thus, one could speculate that the population was stable through to the end of the 1800s and early 1900s, but began to decline in the early to mid-1900s (Hosie noticed several dead and declining trees in 1967) and continued declining to near present times.

In summary, the population had been in an apparent decline at the time of Hosie’s record in 1967 and continued to decline until Thompson (1992) recorded only 14 of the previous count of about 50. However, since eight additional trees were discovered in 2005, the 1992 number does not appear to represent all the trees existing in that year. Seven or eight (four at 1a, a ‘few’ at 1c and 1d uncertain) trees are known to have been lost in the last thirteen years, so the apparent lack of current decline is misleading. A potential decline from 1967 to 2005 (about 1 generation), taking into account previously undocumented trees, appears to be about 72% (14 trees currently remaining out of a possible 50 in 1967).

Rescue effect

The three populations visited in adjacent western New York appeared healthy and some regeneration was observed. Climatic conditions are similar and it is expected that progeny from these sources would do well in Ontario, should stock from outside of Ontario be needed. It is unlikely that propagules from these sites would make it into Ontario through natural migration pathways in less than a geological timescale.