Dataset from: Significant mortality of old trees across a dry forest landscape, Oregon, USA

<p dir="ltr">This dataset consists of an unbiased sample of tree species and tree ages from 11 randomly located sites across unlogged roadless areas combined with a targeted sample of the older age classes of trees present at each site. This network of 1,617 trees on the Malheur National Forest was originally part of two separate research projects conducted in 2012-2013 and each tree was revisited in 2022-2023 to examine mortality rates of old and mature trees. Four of the sites experienced wildfire in 2014, 2015, or 2021.</p><p dir="ltr">During initial data collection in 2012-2013 we extracted a 5.15 mm diameter core from each tree containing the pith or within five rings of the pith allowing us to determine the age of each tree. All tree cores collected in the field were sanded with progressively finer grits of sandpaper, crossdated, and measured to 0.001 mm precision. We verified the accuracy of crossdating using COFECHA software and adjusted dating as necessary (Holmes 1983).</p><p dir="ltr">This data set contains tree information including locations, status, species, diameter at breast height, age, tree height, scaled basal area increment for 5, 10, 25, and 50 years prior to initial measurement; site information including potential vegetation type, competition metrics such as basal area and stems per hectare; and environmental variables including VPD, temperature, precipitation, net primary productivity, heat load, slope, and aspect. For more detailed information see manuscript and supplemental online materials.</p><p dir="ltr">We investigated recent forest mortality dynamics by revisiting a network of 1,617 trees in unlogged roadless areas in the Blue Mountains of eastern Oregon for which we have precise age and growth data. There was extensive mortality of the oldest trees in stands—a quarter of trees ≥300 years of age died over a ten-year period. Trees with below average basal area increment were more likely to die in stands that have not experienced wildfire in the last 130 years. Smaller trees were more likely to die in stands that experienced wildfire during the ten-year observation period. Douglas-fir and western juniper experienced a lower rate of mortality than ponderosa pine, western larch, or grand fir in sites that have not experienced wildfire. There was little evidence for a difference in mortality between species in sites that experienced wildfire. We describe a novel simulation model which demonstrated that succession of young trees is unlikely to replace recent losses of old trees. The confluence of a rare insect defoliator outbreak, drought, and past management decisions that increased competitive stress and fuel connectivity is the most likely explanation for the elevated mortality we observed. The protected areas where we collected data are not achieving old tree conservation objectives. Active management to reduce forest competition outside of protected areas is the most realistic pathway for old tree conservation in dry forests.</p><p dir="ltr">Model and code are available on github: https://github.com/AmandaBrackett/Malheur-OldTree-Mort</p>