Red alder defense mechanisms against western tent caterpillar defoliation

Red alder (Alnus rubra Bong.) is a tree of high economic and ecological importance but subject to severe defoliation during episodic outbreaks of tent caterpillars (Malacosoma spp.). We evaluated variation in western tent caterpillar (M. californicum Packard, 1864) (WTC) resistance among and within red alder populations and clones, and investigated potential defense mechanisms. Bioassay feeding trials were conducted with WTC on 20 red alder clones from 10 provenances (two clones per provenance). Phenology and quality of red alder leaves were analyzed to determine if budburst, leaf chemical content, water content or physical traits are determinants of WTC preference. Alder clones differed in percent leaf area eaten by WTC and in leaf defense traits. The concentrations of total phenolics, condensed tannin and the diarylheptanoid oregonin negatively correlated with the percent leaf area eaten by the caterpillars, and a potential threshold was observed, above which the concentration of each of the chemicals appeared to reduce WTC feeding. Particularly, foliar concentrations of oregonin greater than 20 % of leaf dry weight were consistently associated with reduced feeding. The effects of oregonin concentration in red alder leaves on tent caterpillar feeding is a novel finding.

Plant Water Stress and Soil Depletion in Variable-Density, Red Alder/Western Hemlock Coastal Oregon Plantations

Riparian ecosystems provide critical habitat and functions while being some of the most productive areas in forests. Both conifers and hardwoods contribute to maintenance of habitat and function. To determine the impact of water stress on growth of red alder (Alnus rubra Bong.) and western hemlock (Tsuga heterophylla [Raf.] Sarg.), we installed Nelder type 1a combined with replacement series plots on three Oregon Coast Range sites. Densities ranged from 988 to 85,400 trees/hectare, with ratios (hemlock:alder) of 100:0, 75:25, 50:50, 25:75, and 0:100. In the first 4 years after planting, alder used water in the growing season at greater depths earlier than western hemlock. Higher densities resulted in greater water stress later in the growing season in weeded areas (maintained by herbicide applications), but stress was similar across densities in unweeded areas. Water stress at early ages was correlated with decreased size 14 or 24 years after planting for both species, but these correlations were confounded with other effects of density. Increasing water availability in areas with low summer precipitation could enhance growth of red alder and western hemlock, even in highly productive riparian areas.