Pinus albicaulis Engelm. (Whitebark Pine) in Mixed-Species Stands throughout Its US Range: Broad-Scale Indicators of Extent and Recent Decline

Abstract

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.

Designer Niches Promote Seedling Survival in Forest Restoration: A 7-Year Study of Whitebark Pine (Pinus albicaulis) Seedlings in Waterton Lakes National Park

“…A study using paired burned and unburned sites in Wyoming found that the density of whitebark pine seedlings regenerating naturally was actually higher in most unburned areas [47]. A recent large range-wide study found that 75% of naturally regenerating whitebark pine seedlings are now found in forest types other than whitebark pine, and 35% of these are in lodgepole pine forests [48]. Results may reflect comparatively less regeneration in ghost forests since 50% of the whitebark pine assessed was dead.…”

“…In natural settings, where whitebark pine exists in forests, natural regeneration has been reported to be prevalent in Vaccinum scoparium and rare in Xenax tenuifolium (beargrass) [11,26,48]. In our study, after seven years of planting, whitebark pine seedlings in beargrass root mats in the burned areas reduced seedling survival by 15.1%, X 2 (1, n = 685) = 16.11, p <0.001 and cluster survival by 16.3%, X 2 (1, n = 231) = 8.85, p = 0.003).…”

Effects of Disturbance on Tree Community Dynamics in Whitebark Pine (Pinus albicaulis Engelm.) Ecosystems

“…Our study is one of only a handful that address tree species composition and regeneration in whitebark pine stands based on repeated field measurements (but see [13,14,19,27]). While one-time observations provide important insight into current stand conditions, measurements over time are necessary to assess successional dynamics.…”

“…Despite evidence that mountain pine beetle and white pine blister rust could contribute to successional replacement of whitebark pine by more shade-tolerant conifers in some areas [10][11][12][13], the occurrence, rate, and extent of this replacement across the tree's range is unknown. Moreover, there is little understanding of how interacting disturbances such as bark beetles and blister rust may differentially affect sub-mesic whitebark plant associations, where whitebark is typically co-dominant, versus xeric whitebark associations, where whitebark is often the dominant or only tree species (but see [14]). Even with a high degree of interest in whitebark pine conservation and restoration, long-term data on tree community dynamics in relation to disturbance are lacking.…”

“…Additionally, theoretical studies using computer models have been used to predict trends [10,37,48]. Few studies have directly assessed changes in field plots over time (but see [13,14,19,27]). We were able to take advantage of permanent plots in whitebark pine stands in the eastern Cascade Mountains of Washington State (USA), an area where whitebark pine is understudied, to assess successional changes in tree species abundance and response to disturbance over a 22-year period.…”