Hackberry (Celtis occidentalis) at the northeastern limit of its distribution in North America: population structure and radial growth patterns

摘要

Populations established on the limits of the geographical distribution of a species often differ in their demographic characteristics and in the growth pattern of their individuals from more centrally located populations. We studied the age structure and the radial growth patterns of hackberry populations (Celtis occidentalisL.) at the northeastern limit of the species distribution in North America, at Lake St. Paul, Quebec, Canada. There are two populations of hackberry in the area: the first one occupies an isthmus separating two small lakes (Geese Lake population), and the second one can be found on the upper section of a floodplain along a shallow river (Godefroy River population). The oldest hackberry trees on the Geese Lake site were 54 years old, but those on the Godefroy River site were much older, i.e., 114 years old. The annual ring width patterns revealed two episodes of accelerated growth corresponding to two periods of release from suppression as a result of disturbance. These periods of release occurred concurrently on both study sites, but release was more evident on the Geese Lake site. A progressive decrease in growth from 1960 to 1974 might have resulted from canopy closure following disturbance events. Radial growth rates on the study sites (1.90 and 1.13ensp;mm/year for the Geese Lake and the Godefroy River populations, respectively) were similar to those reported for populations located further south (e.g., 1.70ensp;mm/year in Illinois). We propose that the Geese Lake population recently (≈amp;ensp;1930) got established as an outpost of the Godefroy River population. The establishment would have occurred at the time when Dutch elm disease was first introduced in North America. It seems likely that hackberry extended its populations during that period into areas where previously closed forest canopy became opened.Key words: dendrochronology, Dutch elm disease, hackberry, Lake St. Paul, rare plants, releas