Trees in transition: The role of standing dead trees in forest carbon dynamics

Background/Question/Methods

As a direct result of well-documented increases in forest morbidity and mortality, standing dead trees are becoming ever more important players in forest carbon dynamics. Tree death marks a crucial transition in the carbon cycle when individual trees shift from growing carbon sinks to decaying carbon sources. These trees in transition play essential roles in the biogeochemistry and biodiversity of forests. In Sierra Nevada old growth forests, tree mortality rates have more than doubled in recent decades. Regionally increased mortality rates foreshadow the forest degradation expected throughout the North American West as warmer, dryer climates drive additional mortality. In this study, we quantify the role of standing dead trees in forest carbon dynamics. We assessed carbon content for six dominant species across a gradient of decay in Sierra Nevada mixed conifer forests. We used dimensional analysis of individual trees (n = 109) to develop species-specific density reduction functions that describe how tree characteristics change with advancing decay. Our approach is explicitly designed for use with the structural decay stages used by the Forest Inventory and Analysis program. The resulting estimates of standing dead tree carbon pools will be used to refine greenhouse gas budgets to better reflect observed ecosystem conditions.

Results/Conclusions

Our extensive measurement and volumetric reconstruction of individual tree tissues has shown that declines in wood density drive decreases in the total carbon density of standing dead trees. Simultaneously, structural changes cause a decrease in total volume. In our analysis of white fir, first-stage standing dead trees contained 90% of the carbon found in live trees of the same volume; fifth-stage individuals contained only 42.7%. Across Sierra mixed conifer species, we observed that wood basic specific gravity (SG) declines as decay stage advances. Trees in the first stage had mean SG of 0.37, similar to live trees. Those with advanced decay had declined by 29% to mean SG of 0.26. Additionally, SG of standing dead trees is more varied than that of live trees, as follows from the diversity of weathering and decomposition processes underway. Preliminary analysis indicates that carbon content increases with decay stage, rising from 50.1% to 54.4%. The species-specific density reduction functions derived from these measurements predict standing dead tree carbon characteristics as they progress through stages of decay. Our initial results suggest that current estimates of carbon stored in standing dead trees based on simpler models of live tree characteristics are typically overestimates.