Data from: Thinning and prescribed burning increase shade-tolerant conifer regeneration in a fire excluded mixed-conifer forest

Fire exclusion and past management have altered the composition,
structure, and function of frequent-fire forests throughout western North
America. In mixed-conifer forests of the California Sierra Nevada, fire
exclusion has exacerbated the effects of drought and endemic bark beetles,
resulting in extensive mortality of fire-adapted pine species. Thinning
and prescribed fire are widely used in these forests to reduce fuels,
moderate fire behavior, and restore ecosystems. Tree regeneration
influences future forest composition and structure, and therefore future
resilience to disturbances, but long-term effects of thinning and
prescribed burning on tree regeneration after prolonged fire exclusion are
poorly understood. We measured tree regeneration one year prior to, and
periodically for 16 years following thinning and prescribed burning in a
mixed-conifer forest in the Sierra Nevada, California, USA. We asked three
questions. How did the composition and density of tree regeneration change
after thinning and prescribed burning? Did pretreatment vegetation types
influence conifer regeneration density after treatments? Did planting
after overstory thinning increase regeneration density of native pine
species? Sixteen years after treatments, combined natural regeneration of
shade-tolerant white fir (Abies concolor) and incense-cedar (Calocedrus
decurrens) averaged 2,032 trees per hectare (tph) after understory
thinning, and 7,745 tph after understory thinning combined with prescribed
burning, increases of 37% and 146% from pretreatment densities. In
contrast, combined natural regeneration of white fir and incense-cedar
averaged 497 tph after overstory thinning, 780 tph after overstory
thinning with prescribed burning, 113 tph after prescribed burning alone,
and 807 tph in untreated controls, all of which were declines from
pretreatment densities. Natural regeneration of white fir and
incense-cedar was consistently an order of magnitude greater than Jeffrey
pine (Pinus jeffreyi) and sugar pine (Pinus lambertiana), whose combined
densities 16 years after treatments averaged 37 tph across treatments and
did not significantly respond to thinning and/or prescribed burning.
Natural conifer regeneration after treatments varied by pre-treatment
vegetation type (closed canopy, Ceanothus cordulatus shrub-dominated, and
open sparse), with large increases of natural regeneration after
understory thinning in closed canopy and Ceanothus shrub vegetation types.
Planting increased sugar pine regeneration density after overstory
thinning, marginally increased Jeffrey pine regeneration after overstory
thinning combined with prescribed burning, and increased white fir
regeneration after overstory thinning with and without burning. No
treatments reduced white fir and incense-cedar natural regeneration while
simultaneously increasing natural pine regeneration, suggesting new
thinning, burning, and planting approaches may be required to meet
regeneration restoration objectives.