Assisting ‘warm-adapted’ native cottonwoods to migrate to cooler sites to beat rising temperatures – a bridge too far?

Coping with the trend towards a warmer and drier climate in the western US can be a struggle for native vegetation. One solution tested in a study published recently by Kevin Grady and his team in the journal Restoration Ecology is to give plants a helping hand to migrate to cooler areas - by physically moving them there.

Warm-adapted genotypes should be more likely to survive in currently cool but warming locations, as temperatures rise. But there haven’t been many trials of such ‘assisted migrations’ – to see if warm-adapted varieties can cope with cooler climates … and pathogens present at transplant sites. Until now.

Riparian ecosystems are priority targets for such warm-adapted restoration because they harbor very diverse communities of plants and animals. So Grady and coworkers at NAU have been carrying out a provenance trial with Fremont cottonwood (Populus fremontii) – an important foundation species in many Arizona riparian ecosystems.

At research sites including several in the SEGA site network and working with replicated genotypes from 19 Fremont cottonwood populations researchers transplanted warm-adapted genetic stock to a cold site. Then they measured the genetic variations in growth rates, mortality, and resistance to shoot blight fungi (Venturia species). Populations of cottonwoods from cool sites grew up to 4 times faster, had 3 times higher survival rates and 8 times higher resistance to the Venturia fungi than populations from warm sites - evidence of local adaptation to both climate and pathogenic fungi in the ‘warm’ genotypes.

But … these trials have also shown that the predicted 6∘C increase in average annual temperatures predicted by climate change models is a ‘bridge-too-far’ even for cottonwoods with the warm-adapted genes. What that means in practice is that any assisted migration will require an intermediate transfer stage to a location with a predicted temperature increase of less than 3∘C - so that the trees can cope. Because moving even warm-tolerant plants too far too soon will likely result in them dying from cold before the warming happens.

To read the full paper - A bridge too far: cold and pathogen constraints to assisted migration of riparian forests. Kevin C. Grady et al. 2015. Restoration Ecology Vol. 23, No. 6, pp. 811–820.