Weak local adaptation to drought in seedlings of a widespread conifer

Tree seedlings from populations native to drier regions are often assumed
to be more drought tolerant than those from wetter provenances. However,
intraspecific variation in drought tolerance has not been well
characterized despite being critical for developing climate change
mitigation and adaptation strategies, and for predicting the effects of
drought on forests. We used a large-scale common-garden drought-to-death
experiment to assess range-wide variation in drought tolerance measured by
decline of photosynthetic efficiency, growth, and plastic responses to
extreme summer drought in seedlings of 73 natural populations of the two
main varieties of Douglas-fir (Pseudotsuga menziesii var. menziesii and
var. glauca). Local adaptation to drought was weak in var. glauca
and nearly absent in menziesii. Var. glauca showed higher tolerance to
drought but slower growth than var. menziesii. Clinal variation in drought
tolerance and growth species-wide was mainly associated with temperature
rather than precipitation. A higher degree of plasticity for growth was
observed in var. menziesii in response to extreme drought. Genetic
variation for drought tolerance in seedlings within varieties is
maintained primarily within populations. Selective breeding within
populations may facilitate adaptation to drought more than assisted gene
flow.