Data from: Impacts of growing-season climate on tree growth and post-fire regeneration in ponderosa pine and Douglas-fir forests

We studied the impacts of climate variability on low-elevation forests in
the U.S. northern Rocky Mountains by quantifying how post-fire tree
regeneration and radial growth varied with growing-season climate. We
reconstructed post-fire regeneration and radial growth rates of Pinus
ponderosa and Pseudotsuga menziesii at 33 sites
that burned between 1992 and 2007, by aging seedlings at the root-shoot
boundary. We also measured radial growth in adult trees from 12 additional
sites that burned between 1900 and 1990. To quantify the relationship
between climate and regeneration, we characterized seasonal climate
before, during, and after recruitment pulses using superposed epoch
analysis. To quantify growth sensitivity to climate, we performed moving
regression analysis for each species and for juvenile and adult life
stages. Climatic conditions favoring regeneration and tree growth differed
between species. Water deficit and temperature were significantly lower
than average during recruitment pulses of ponderosa pine, suggesting that
germination-year climate limits regeneration. Growing degree days were
significantly higher than average during years with Douglas-fir
recruitment pulses, but water deficit was significantly lower one year
following pulses, suggesting moisture sensitivity in two-year-old
seedlings. Growth was also sensitive to water deficit, but effects varied
between life stages, species, and through time, with juvenile ponderosa
pine growth more sensitive to climate than adult growth and juvenile
Douglas-fir growth. Increasing water deficit corresponded with reduced
adult growth of both species. Increases in maximum temperature and water
deficit corresponded with increases in juvenile growth of both species in
the early 20th century but strong reductions in growth for juvenile
ponderosa pine in recent decades. Changing sensitivity of growth to
climate suggests that increased temperature and water deficit may be
pushing these species towards the edge of their climatic tolerances. Our
study demonstrates increased vulnerability of dry mixed-conifer forests to
post-fire regeneration failures and decreased growth as temperatures and
drought increase. Shifts towards unfavorable conditions for regeneration
and juvenile growth may alter the composition and resilience of
low-elevation forests to future climate and fire activity.