Reforestation of high elevation pines: Direct seeding success depends on seed source and sowing environment

Forest persistence in regions impacted by increasing water and temperature
stress will depend upon species’ ability to either rapidly adjust to novel
conditions or migrate to track ecological niches. Predicted, rapid climate
change is likely to outpace the adaptive and migratory capacity of
long-lived isolated tree species, and reforestation may be critical to
species’ persistence. Facilitating persistence both within and beyond a
species’ range requires identification of seed lots best adapted to the
current and future conditions predicted with rapid climate change. We
evaluate variation in emergent seedling performance that leads to
differential survival among species and populations for three
high-elevation five-needle pines. We paired a fully reciprocal field
common garden experiment with a greenhouse common garden study to a)
quantify variation in seedling emergence and functional traits, b) ask how
functional traits affect performance under different establishment
conditions, and c) evaluate whether trait and performance variation
demonstrates local adaptation and plasticity. Among study species –
limber, Great Basin bristlecone, and whitebark pines – we found divergence
in emergence and functional traits, though soil moisture was the strongest
driver of seedling emergence and abundance across all species. Generalist
limber pine had a clear emergence advantage as well as traits associated
with drought adaptation, while edaphic specialist bristlecone pine was
characterized by low emergence yet high early survival once established.
Despite evidence for edaphic specialization, soil characteristics alone
did not explain bristlecone success. Across species, trait-environment
relationships provided some evidence for local adaptation in
drought-adapted traits, but we found no evidence of local adaptation in
emergence or survival at this early life stage. For managers looking to
promote persistence, sourcing seed from drier environments is likely to
impart greater drought resistance into reforestation efforts through
strategies such as greater root investment, increasing the probability of
early seedling survival. This research demonstrates, through a rigorous
reciprocal transplant experimental design, that it may be possible to
select climate- and soil-appropriate seed sources for reforestation.
However, planting success will ultimately rely on a suitable establishment
environment, requiring careful consideration of interannual climate
variability for management interventions in these climate- and
disturbance-impacted tree species.