Quaking aspen (Populus tremuloides) is the most widespread tree species in North America with populations from Alaska to Newfoundland and southward to the high elevations of central Mexico. The condition of aspen ecosystems in the western United States is of rising concern with several studies reporting considerable loss of aspen from the landscape in the past 150 years. Some aspen stand declines have been termed ‘sudden' for the rapidity of loss; southern populations of aspen appear more susceptible to this rapid loss, possibly a result of recent drought followed by fungal and insect damage. Other stands can experience much slower declines as conifers establish and suppress aspen reproduction; northern populations appear more susceptible to this slower decline. We are engaged in two parallel studies on aspen condition: one examining aspen decline related to conifer encroachment in northern Utah and another examining rapid decline in southern Utah. Here, we compare pure aspen stands in these two landscapes using GAP classification of aspen landcover and field-based assessments of stand condition. We seek to compare common ecological indicators between the two aspen systems that can be useful for assessing decline and providing input to management decisions. Results/Conclusions
Aspen populations in southern Utah are found between 1600 and 3300 m, northern Utah populations are at slightly lower elevation. In both locations, aspen spans a wide range of slope and aspect. We find considerably more biomass (15.7 vs. 94.2 Mg ha-1) and higher tree density (315 vs. 769 individuals ha-1) in northern Utah. In southern Utah, 26% of basal area was dead while ca. 3% was dead in northern Utah. Both stands had considerable regeneration in the understory (< 3000 and < 4000 individuals ha-1 in southern and northern stands, respectively). Adjacent stands in northern Utah experiencing substantial conifer encroachment have reduced regeneration (ca. 2500 individuals ha-1). Greatly lacking in southern Utah is a sizable cohort of sub-canopy trees that could replace canopy trees. The slope of the mean biomass/density relationship for both populations is below –1 suggesting these stands are mature and no longer accumulating biomass as individuals die. Much literature suggests stands in decline lack regeneration, but our finding in southern Utah does not support this contention. We suggest management to stimulate regeneration under conditions of conifer encroachment, but management to improve recruitment of suckers in stands experiencing rapid decline.
