The interacting effects of temperature, ground disturbance, and herbivory on seedling establishment: implications for treeline advance with climate warming

Abstract

Thermal control of treeline position is mediated by local environmental and ecological factors, making trends in treeline migration difficult to extrapolate geographically. We investigated the ecological dynamics of conifer establishment at treeline in the Mealy Mountains (Labrador, Canada) and the potential for its expansion with climate warming. Available seedbed and tree seedling emergence in the treeline ecotone were monitored, and seeds and seedlings of Picea mariana were planted along an elevational gradient from open-canopy forest through tree islands to alpine tundra. Experimental treatments included passive warming of daytime air, ground disturbance, and vertebrate herbivore exclosures. Responses in seed germination and seedling growth, damage, and mortality were monitored over two growing seasons, and re-surveyed after 5 years. While no tree seedlings were observed growing naturally above the treeline, planted seeds were able to germinate, develop and overwinter, and persist for 4 years in all habitats examined. Disturbance of the seedbed was important for seedling emergence in the forest and tree islands.While temperature enhancement alone had little impact on emergence, even moderate temperature increases had significantly disproportionate effects on emergence of seedlings in the alpine habitat when combined with soil disturbance, indicating that future climate warming could lead to treeline advance if viable seed and suitable substrate for recruitment are available. The positive effect of excluding herbivores suggests that herbivory may be an important filter modifying future species distribution. While seedbed conditions and herbivory would control the rate of individual species advance, the results indicate potential upslope migration of the treeline in the Mealy Mountains, with consequent loss of alpine ecosystems.

Treelines—Approaches at Different Scales

“…Comparatively little attention has been paid so far to the effects of mammalian herbivores, birds, and insects on treeline ecotones, with a few exceptions (e.g., [5,94,[132][133][134][135][136][137][138][139]). Animals may lastingly influence tree growth and physiognomy, regeneration, seed dispersal, soils, distribution pattern of trees, and treeline dynamics at the landscape and smaller scales (cf.…”

“…Soil material excavated by burrowing rodents and displaced over the ground surface may lastingly influence site conditions (e.g., [94,134,140,141]). Animals' effects, in particular on seedlings and saplings, should be more intensively studied [5,94,138].…”

Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators

“…In recent years, an increasing number of studies have investigated the response of treeline seedlings to changing environmental factors such as soil temperature, soil moisture, soil nutrients, light conditions, herbivory, and competition/facilitation (e.g. Maher and Germino, 2006;Anschlag et al, 2008;Batllori et al, 2009;Hofgaard et al, 2009;Kabeya, 2010;Munier et al, 2010); some studies have also explicitly addressed the relative importance of multiple factors (Barbeito et al, 2012;Zurbriggen et al, 2013).…”

Multisensor NDVI-Based Monitoring of the Tundra-Taiga Interface (Mealy Mountains, Labrador, Canada)

“…This is consistent with the advances of 3 to 11 m per year for the period of 1980-2000 in northern Russia [32], but it is much lower than the mean advance of 378 m per year (2.5 km from 2000 to 2009) reported for the pine forest line in northern Norway [77]. The variations observed for the Mealy Mountains study are attributed to site conditions, as seedling in the TTI in Labrador can be affected by the soil wetness, wind exposure, temperature [78], seedbed composition [79] and the topography [80]. The impact of these factors, individually or in combination, yet has to be investigated in relation to the information that was extracted from the satellite images.…”