COS 198-2 - Reconstructing frequent-fire Jeffrey pine forests in the eastern Sierra Nevada

Abstract: In dry conifer forests of the western United States, wildfire size and severity has significantly increased due to heavy fuel loads from a century of fire suppression. Before that management practice was instituted, frequent low-intensity fires maintained low tree densities, limited surface fuels, and caused tree spatial patterns that were more resilient to fire damage than modern forests. The historical baseline for fire-adapted forests can be described through spatially explicit studies of past forest conditions, but such work has predominantly focused on ponderosa pine and mixed conifer forests of the western Sierra. To date, historical frequent-fire conditions for eastern Sierra Jeffrey pine (Pinus jeffreyi) forests have not been studied. Describing the departure from fire-adapted conditions in this ecosystem is a novel contribution to western forest fire ecology.

We established six 1-ha plots at two eastern Sierra sites in August 2018, and collected tree cores from 200 trees, along with size/species/condition data for ca. 1000 stems. Cores were sanded, aged, and cross-dated to provide estimates of tree age vs. diameter at breast height (DBH).

We used digital cross-dating to produce a reference chronology for the first of our two sites based on 94 tree cores. Modern (2018) stand metrics were produced using mapped locations of all stems >5cm DBH. From each stem we subtracted 77 years’ worth of growth, and calculated the size and position of living trees inferred from dead snags and logs, to reconstruct the forest as it appeared in 1941 (end of frequent fire period). We compared stand metrics between historical and modern forest conditions. Stand metrics were calculated for live trees only for the 2018 dataset, because dead trees cannot be inferred for historical reconstruction. From 1941 to 2018, average tree size increased 87.8%, from 32.2 to 60.6 cm DBH (sd = 28.5). Live stems per hectare decreased 29.2%, from 115.3 to 81.7; basal area increased 79%, from 16.0 to 28.7 m2/ha. Preliminary results are surprising given the patterns reported in Western Sierra forests, where fire-suppressed forests tend to have more stems per hectare and a smaller average DBH.

This study is the first spatially explicit stand reconstruction conducted in eastern Sierra Jeffrey pine forests. In addition to illuminating the impacts of fire on this ecosystem, our results provide forest managers with restoration targets for promoting forest health and reducing fire risk near human populations.