Abstract
We used data collected from >1400 plots by a national forest inventory to quantify population-level indicators for a tree species of concern. Whitebark pine (Pinus albicaulis) has recently experienced high mortality throughout its US range, where we assessed the area of land with whitebark pine present, size-class distribution of individual whitebark pine, growth rates, and mortality rates, all with respect to dominant forest type. As of 2016, 51% of all standing whitebark pine trees in the US were dead. Dead whitebark pines outnumbered live ones—and whitebark pine mortality outpaced growth—in all size classes ≥22.8 cm diameter at breast height (DBH), across all forest types. Although whitebark pine occurred across 4.1 million ha in the US, the vast majority of this area (85%) and of the total number of whitebark pine seedlings (72%) fell within forest types other than the whitebark pine type. Standardized growth of whitebark pines was most strongly correlated with the relative basal area of whitebark pine trees (rho = 0.67; p < 0.01), while both standardized growth and mortality were moderately correlated with relative whitebark pine stem density (rho = 0.39 and 0.40; p = 0.031 and p < 0.01, respectively). Neither growth nor mortality were well correlated with total stand basal area, total stem density, or stand mean diameter. The abundance, extent, and relative growth vs. mortality rates of whitebark pine in multiple forest types presents opportunities for management to encourage whitebark pine recruitment in mixed-species stands. The lodgepole pine forest type contained more whitebark pine seedlings (35%) than any other forest type, suggesting that this forest type represents a potential management target for silvicultural treatments that seek to facilitate the recruitment of whitebark pine seedlings into larger size classes. National forest inventories in other countries may use a similar approach to assess species of concern.
Highlights
Whitebark pine (Pinus albicaulis Engelm.) is a foundational tree species that has experienced dramatic declines in recent decades [1]
15% of the area with whitebark pine presence was classified as whitebark pine forest type; the lodgepole pine, Engelmann spruce/subalpine fir, and subalpine fir forest types each contain more area than does the whitebark pine forest type (Table 2)
The distribution of whitebark pine trees in other forest types raises two questions: First, how much whitebark pine is present in these other forest types? The interquartile range of live whitebark pine basal area among all other forest types is below the interquartile range in the whitebark pine type (Figure 2), confirming that whitebark pine density is higher in the whitebark pine forest type
Summary
Whitebark pine (Pinus albicaulis Engelm.) is a foundational tree species that has experienced dramatic declines in recent decades [1]. Given the widespread nature of the mortality agents [5,6,13], it is reasonable to assume that such mortality has occurred throughout whitebark pine’s geographic range, but information for range-wide quantitative assessments is limited. It is possible that purposive sampling has overlooked the full potential range of sites where whitebark pine may occur. While it is reasonable to assume that most tree species are most abundant and robust in sites where they are relatively dominant—excepting successional transitions—the dramatic decline of whitebark pine calls for thinking outside the box, or thinking outside the whitebark pine forest type, and if possible expanding assessments to the species’ entire range
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