Next Generation Sequencing Identifies Population Structure and Signatures of Local Adaptation in Red Alder (<i>Alnus rubra</i> Bong.)

Abstract

Red alder (Alnus rubra Bong.) is the dominant hardwood tree species in the Pacific Northwest. Red alders are only found in western North America, generally within 200 km of the coast and below 1000 m in elevation--although there are several disjunct populations in the northern Rocky Mountains in Idaho. Commonly a riparian species, the shade intolerant red alder generates most of the leaf litter in the streams and rivers it occupies which has been shown to greatly influence the decomposer communities. This, in turn, has a cascading effect throughout the entire riparian ecosystem greatly influencing multiple levels of the food chain. Red alders also aid in stream cooling by shading areas with its canopy which has been shown to reduce diel temperature swings. This reduction improves survivability in ecologically and economically important salmonid fish species (e.g. Coho Salmon, Rainbow Trout) in which the fry require cool freshwater streams and rivers for juvenile growth and development. When not occupying riparian habitat, red alder is a pioneer species that is well suited to occupy recently disturbed areas (e.g. timber harvesting, fire damage), or areas with poor soil conditions where essential nutrients may be limited. The success of red alder as an early successional species is facilitated by the symbiotic relationship it forms with the N2-fixing actinorhizal bacteria Frankia alni. In this mutualism, Frankia bacteria fix nitrogen for red alder and in return they receive photosynthetic products produced by their symbiotic host. The process of bacterial nitrogen-fixation can be very expensive for the host plant since as much as 40% of all photoassimilates produced are funneled to the Frankia symbionts for this process. Red alders usually associate with specific strains of Frankia--which exhibit low diversity over large special scales--with the choice being controlled primarily by the host. Resultantly, alders are able to improve soil conditions which has been shown to improve growth and success of other valuable tree species in the Pacific Northwest (e.g. Douglas fir, big leaf maple). Initially considered a nuisance weed or a pest species by local logging operations due to the enumerable red alder seedlings that occupy freshly logged areas, red alder has become one of the most commercially valuable species in the Pacific Northwest. The quick growth, short lifespan (< 100 years), and beautiful red tinted wood--paired with red alder's ability to fix nitrogen thereby eliminating the need for chemical fertilizers--has made red alder one of the most