Range Shift

Elevation Shift

Many plants and animals have been shifting their distribution (Chen et al., 2011), and there has been evidence that Arceuthobium has been as well. Many types of mistletoe are limited in maximum elevation by minimum temperature conditions throughout the year. As the climate warms, these minimum temperature conditions move up in elevation, allowing mistletoe to move up mountains correspondingly (Dobbertin, 2005).

Adapted from Dobbertin, 2005. Journal of International Biometeorology. 

Latitudinal Shift

While most research up to this point has only provided examples of mistletoe elevation shifts, one could speculate that a latitudinal shift will also occur in correspondence to a similar shift in their conifer hosts. Studies have in fact found, since climate is the limiting factor in mistletoe distribution, that their range might expand up to 757% in some parts of the Pacific Northwest (Barret et al., 2012), although there is a high level of uncertainty in this. In addition, it is unlikely that the velocity of conifer range shift will exceed that of Arceuthobium significantly in relation to current distributions, primarily due to the dispersal mechanisms of the Dwarf Mistletoe. Their dispersal mechanism is quite unique, as during the ripening of the fruit, hydrostatic pressure builds up to such a point that a single, sticky seed is shot out at speeds of up to 50 miles/hour, which enables considerable dispersal range (Hawksworth, 1996). Furthermore, as one can imagine, this dispersal mechanism is more effective when the disperser has a height advantage relative to the target host plant due to the nature of gravity. This makes mistletoe particularly effective in forests with an uneven-age stand, where seeds dropping from mature trees can easily hit younger understory trees (Mathiasen, 1996). By extension, as conifers expand their range northward, all the saplings will be considerably smaller than the neighboring forest, providing a situation advantageous to Dwarf mistletoes should they already occur in that area.

The fact that certain species of Dwarf Mistletoe also receive assistance in dispersal from aerial vectors (eg. birds) mean that under certain circumstances they can disperse farther than hydrostatic pressure. The fact that this genus has multiple dispersal mechanisms of varying distance, combined with a moderately short generation time, suggests that they will not be significantly affected by inter-decadal climatic variability that would otherwise cut off climate paths necessary for range shifts (Early & Sax, 2011).

Life cycle of Dwarf Mistletoe. Note the two routes available for dispersal. Original Figure.