Comparing early seedling traits in Pinus albicaulis and Pinus flexilis across climates and rust resistance levels

Whitebark pine (Pinus albicaulis; PIAL) and limber pine (Pinus flexilis; PIFL) are high- elevation five-needle (high-5) white pines (Pinaceae, Pinus, Strobus), keystone and foundational subalpine species in western North America. Both species are declining due to threats including climate change and white pine blister rust. The primary restoration strategy for PIAL is outplanting rust resistant seedlings. Seedling establishment is critical for successful restoration and shapes future forest composition. However, the mechanisms underlying first-year-germinant seedling establishment are poorly understood. We measured 10 phenological and survival, 56 growth, and 18 NSC traits in <1-year-old greenhouse-grown seedlings in a common garden setup. Seedlings originated from 51 PIAL families that spanned 32 to 54 °N in latitude and -127 to -115 °E in longitude across Washington and Oregon, U.S. (USFS Region 6) and British Columbia (B.C.), and 12 PIFL families from Alberta. For Question 1, we observed that U.S. and B.C. PIAL seedlings originating from higher latitudes exhibited adaptations to colder and shorter days and growing seasons, including higher survival to each phenological stage, reaching each stage earlier, shorter height, higher biomass allocation belowground to roots than aboveground, and lower leaf and root sucrose concentrations than those originating from lower latitudes. For Question 2, PIAL seed zones 3, 4, and 5 had the highest survival to each phenological stage, but survival appeared to be driven by contrasting resource use strategies: acquisitive in zones 3 and 5, and conservative in zone 4. For Question 3, more resistant PIAL seedlings had higher survival and reached each phenological stage earlier higher biomass allocation aboveground to tough, large leaves, lower biomass allocation belowground, lower leaf starch, and higher leaf sugars than less resistant seedlings. For Question 4, PIFL exhibited a mix of acquisitive and conservative resource use traits compared to PIAL including: lower survival to emergence and needle burst, later needle burst timing, higher aboveground and lower belowground biomass allocation, higher leaf sugars, and lower leaf starch. In contrast to PIAL, PIFL exhibited only one trait difference between resistant and susceptible groups. Our results may improve restoration efforts to conserve these iconic and beloved species in the North American West.

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