We were unable to evaluate the independent effect of soil saturation on canopy openness, but the fact that canopy openness was a significant predictor of shore pine and mountain hemlock sapling occurrence suggests the important roles of soil conditions and light in determining which species are more likely to regenerate. Both species are known to have preferences for wet soils and scrubby open forests , and canopy openness in forests affected by decline has two driving components: soil saturation and crown deterioration caused by yellow cedar death . Young mountain hemlock seedlings, for example, grow best in partial shade , likely explaining why this species regenerated relatively well as saplings in recent mortality before canopy openness increased further. In contrast, western hemlock is known to tolerate a wide range of soil and light conditions for establishment and growth and seeds prolifically, as does Sitka spruce . Species can also respond to varying light conditions with differential growth responses. Western hemlock reached maximum growth rate when exposed experimentally to relatively high light intensities, whereas bunch berry responded most strongly to relatively low light intensities . Although Sitka spruce was the second most important species in sapling regeneration in the affected forests, seedling starter pot this species played a less important role in the conifer community composition in subsequent stages of stand development.
Spruce has been found to have lower regeneration than western hemlock on disturbed sites but higher survival rates; however, western hemlock regeneration that existed prior to disturbance formed a significant and dominant position in the future stand . In old mortality, the relatively high density of small, dead trees, low density of big, live trees, and high percentage of dead basal area suggest that although the Sitka spruce regenerates on plots less open, the species may be outcompeted by western hemlock over time. At the fine-scale, species traits can play important and even predictable roles in determining the establishment and growth of individuals under changing site conditions. These fine-scale interactions collectively shape community responses to the species dieback.Our study documented significant losses in the yellow-cedar population but not extirpation. These directional changes likely extend beyond the temporal period of our chronosequence due to the diminished regeneration. The temporal dynamics of decreased yellow-cedar importance in each tree size class across the chronosequence suggest that yellow-cedar decline may be more likely to affect smaller trees first, whereas larger tree mortality occurs in a staggered process. Previous research has shown that surviving yellow-cedar trees in declining stands can produce larger growth rings but with greater interannual variability after the onset of decline , and that climate thresholds for the survival and reproduction of individuals can vary across life-history stages .
Reductions in yellow-cedar sapling occurrence and abundance in forests affected by decline indicate significant, long-term reductions in species abundance across larger size classes. Whether these reductions are caused by seed limitation, changes in seedling germination conditions, herbivory, or other mortality mechanisms is beyond the scope of our study. Although we did not count individuals in the 10–99 cm size class or dead saplings, our seedling and sapling findings indicate reductions in yellow-cedar over time and consistent rank-order of other conifer species competing in sapling stage in the affected forests. Ramage et al. similarly found tanoak unlikely to regenerate successfully in forests affected by sudden oak death, a disease disturbance. Yellow-cedar appear maladapted to forests affected by decline for the foreseeable future. We observed a process of stand development similar to forests affected by host-specific insect or disease disturbance, distinguished by an increase in regeneration while surviving trees release and saplings advance into the overstory. The results of our comparisons of total sapling density across the chronosequence indicate a reinitiation phase that occurs post-decline. We found no significant difference between canopy openness in live forests compared with old mortality, significant increases in canopy openness in recent and midrange mortality, and an overall steady increase in stand density across the chronosequence. These results document stand advancement toward canopy closure over time, indicating development of a relatively mature forest distinguished by changes in conifer community composition.
We were unable to date the precise onset of widespread mortality at each plot; our estimates of temporal dynamics of stand re-initiation and advancement are informed by time-since death estimates for snags observed at each plot. Snag class estimates suggest that widespread mortality began 81.4 6 22.0 years ago for the old mortality cedar decline status and indicate approximately 50–100 years for stand advancement toward canopy closure. Consistent with our hypotheses, we found an increase in the importance of western hemlock across all size classes as the importance of yellow-cedar was greatly reduced over time in the affected forests. It has been previously hypothesized that declining yellow-cedar forests on the northern extent of the yellow-cedar population distribution may convert to scrub forest or open bog, as western hemlock is not able to exploit some soil conditions favorable to yellow-cedars . In contrast but consistent with prolific hemlock regeneration found in old-growth hemlock-spruce forests affected by stand-replacing disturbances and minor wind throw disturbance , our results show that western hemlock regenerates vigorously on plots affected by decline, and changes in community composition, as described by IVs, show a turnover to western hemlock-dominated forests. However, because our sampling frame was restricted to plots with a total live and dead basal area of 35 m2 /ha, our results are not generalizable to dynamics on poorly drained, acidic soils that may be more likely to convert to scrub forests. Chronosequences and associated space-for time substitutions serve as a useful tool for studying temporal dynamics of plant communities that occur across time . Despite critiques as to whether there are predictable links between sites and at what rates characteristics actually change over time , Walker et al. assert that chronosequences are well-suited for studying plant communities that have low biodiversity, rapid species turnover, and low frequencies and severity of disturbance. We applied this methodology to coastal temperate forests with relatively low vascular plant diversity and restricted our sites to those where widespread mortality was evident as a significant disturbance , avoiding wind throw. Based on current understanding of the pathway to decline, varying snowpack conditions were likely the cause of the spatial pattern of mortality observed across the study area ; old mortality occurred in the more southerly plots and live further north . We acknowledge that there may have been confounding factors, such as variability in deer populations, snow conditions, or relative abundance of trees species associated with the spatial distribution, that affected the patterns we observed across the chronosequence. Specifically, seedling and sapling abundance at sites located in GLBA may have been positively affected by geographic location at the northern limit of Sitka black-tailed deer . The observed increase in volume of forage species post-decline may also have a negative feedback on yellow-cedar regeneration, as deer browsing can be a barrier to seedling performance . As there were no affected forests identified in GLBA by aerial survey, round nursery pots plots measured increased our sample size of live forests and extended our chronosequence further north across a relatively limited spatial extent. Our biophysical controls used for site selection helped reduce the likelihood of other factors driving variation in forest structure and community composition among the cedar status categories.Our study provides evidence of a dynamic understory response that occurs as forests become affected by decline. Changes in community composition were elucidated by the increase in Shannon diversity for functional groups and the dissimilarities in functional group abundance across the chronosequence : live forests were primarily differentiated by bryophyte abundance, recent mortality by graminoids, and old mortality by shrubs.
We found forests affected by decline can lead to increased forage production over time, potentially increasing deer carrying capacity. As variations in snow-depth are known to effect browse availability , reduced snowpack that triggers tree mortality may also expand winter habitat for deer. An extensive literature on widespread mortality of a single dominant tree species from other types of disturbances indicates that overstory loss typically alters understory plant communities . Recent studies on the effects of a species dieback associated with climate change on understory vegetation showed: increases in species richness and abundance seven years after a major drought-induced mortality event of Juniperus monosperma ; shifts in community composition with decreases in herbaceous species, cover, and volume; and increases in shrub abundance, cover, and volume over a two-year time period in stands affected by sudden aspen decline . Typically characterized by frequency, duration, severity, size and spatial pattern, disturbance regimes provide critical information to understand stand formation and subsequent development . Timing and intensity of yellow-cedar mortality plays a critical role in determining plant community responses. Although we observed an increase in the vascular plant taxa identified across the chronosequence, our ability to make inferences to changes in species richness and diversity as forest become affected by decline was limited by taxonomic resolution . We did not identify sedges , grasses , clubmosses , horsetails , and bryophytes at the species level, but the abundance of the associated functional groups differed substantially across the chronosequence. As such, taxonomic richness and diversity for these vascular plant taxa and the bryophytes group would play an important role in overall dynamics of species diversity. Our ability to identify and quantify the trajectories of ecological communities is important for monitoring and maintaining biodiversity, as well as other ecosystem services. Because of the remoteness and inaccessibility of the Archipelago in southeast Alaska, monitoring and active forest management, such as harvesting or planting, may occur only on a small portion of forest experiencing yellow-cedar decline . We documented far-reaching effects of climate change on protected wilderness lands, affirming that areas once set-aside for conservation may be insufficient for species preservation in a changing climate . Given expected climate change in the current century, many vegetation types and individual species may lose representation in protected areas . Effective conservation strategies require understanding the changing plant community dynamics and assessing habitat where species are more likely to survive on both managed and protected lands. Managers and conservation planners operating in other ecosystems may need to consider impacts of climate change on plant communities in protected areas to evaluate broad-scale implications of activities on actively managed lands. Whether this species decline will expand northward into the live forests in Glacier Bay National Park and Preserve is currently unknown; this study’s plots offer opportunities for directly monitoring future changes in affected and unaffected forests to date. The changes observed across the chronosequence can have a range of cascading effects on ecosystem services. Conversion to western-hemlock dominated forests represents a long-term reduction in culturally valued trees and a loss of the cultural services these trees provide. Forests affected by decline may sequester less carbon in the long-term, given western hemlock’s relatively shorter lifespan and wood deterioration rates . Researchers are just beginning to understand the influence of dead cedars on watershed nutrient export . By replacing yellow-cedar trees with the most abundant tree species in the region , yellow cedar decline can lead to a loss of conifer diversity at the landscape level, yet may provide increased forage availability for deer hunted throughout the region. Although our research occurred on protected lands at the northern reaches of yellow-cedar decline, dynamics observed can provide forest managers on the Tongass National Forest with a better understanding of the processes of stand development and conifer species most likely to dominate impacted forests over time. In consideration of salvage activity or thinning on managed lands affected by decline, our results suggest that managers should recognize that western hemlock is more likely to outcompete other species, and that favoring spruce or mountain hemlock individuals may help maintain conifer diversity. Further research could evaluate whether forests affected by decline support a greater deer population or if shifting hunting pressures to these areas could have a positive impact on yellow-cedar regeneration. Our study also underscores the importance of maintaining yellow-cedar populations at higher elevations, given the decreased likelihood for yellow-cedar regeneration once low-elevation, coastal forests become affected by the dieback. Alfalfa hay is a relatively cheap, high-quality forage which is high in protein and thus an ideal feed for ruminant animals .