Blueberry and hemlock

Some “then and now” sliders in the last post about Glacier Bay suggested that blueberry and hemlock were spreading in the understory at two of the older study sites we visited last month. Below I am trying out another method of displaying these pairs of old (1990 or 1995) and new (last month) photos. Some new photo pairs from York Creek and Beartrack Cove have been added as well as pairs from a third site.

  • 1995, York Creek, Plot 9, SE-NE

Above: Three photo pairs from York Creek (Site 8, ice-free about 1825). Each pair includes an early photo from a Kodachrome slide (June 1990 or 1995, smaller image) and a June 2021 version of the same scene. Each pair suggests that both blueberry and hemlock have become more important in the understory.

  • 1990, Beartrack Cove, Plot 2, SE-SW

Above: Two photo pairs from Beartrack Cove (Site 9, ice-free about 1835).
Each pair includes an early photo from a Kodachrome slide (June 1990, smaller image) and a June 2021 version of the same scene. Each pair suggests that both blueberry and hemlock have become more important in the understory.

Blueberry near Plot 1 at the Beartrack Cove site. Most of the blueberry is Vaccinium ovalifolium and a few plants of V. Alaskaense are also present at some sites.

I also put together some new photo comparisons from Bartlett Lake, the oldest site (about 55 years older than York Creek or Beartrack Cove). These suggest that blueberry there was already widespread in 1995. It looks like blueberry has continued to spread, but unlike the York Creek and Beartrack Cove sites there is no indication that hemlock has become more important during the last three decades.

  • 1995, Bartlett Lake, Plot 5, NW-NE

Above: Three photo pairs from Bartlett Lake (Site 10, ice-free about 1770).
Each pair includes an early photo from a Kodachrome slide (June 1995, smaller image) and a June 2021 version of the same scene. Each pair suggests that blueberry was abundant 30 years ago and has increased some since then. Hemlock has not become conspicuously more important in the understory.

Western hemlock (Tsuga heterophylla) near Plot 6 at the York Creek site. Sitka spruce and western hemlock are two of the most common trees in many parts of the temperate rainforest in Southeast Alaska.

There is no need to speculate further about the existence or magnitude of these understory changes. The data I brought home include estimates for the percent cover of blueberry in each study plot and also the number of western hemlock seedlings, saplings, and trees in each plot. We also have these same data from all 10 of the study sites along the eastern shore of Glacier Bay.

Above: Location of 10 study sites (circled numbers). Each site has ten 10 x 15 m permanent plots (five 15 x 20 m plots at Site 1). Site 8 is York Creek, Site 9 is Beartrack Cove, Site 10 is Bartlett Lake. Dotted lines are positions of glacier termini at the indicated date.
Figure 1. Current percent cover of blueberry (Vaccinium spp.) at all 10 study sites on the east side of Glacier Bay. Cover was estimated in each of 10 study plots (10 x 15 m) at each site.

The story of both blueberries and hemlock is a story about the three oldest sites. Blueberry shrubs are common at Sites 8, 9, and 10 (York Creek, Beartrack Cove, Bartlett Lake) and rare at younger sites (Figure 1). One or two blueberry plants are present in two or three plots at Morse Creek and Sandy Cove but absent at the other sites.

Figure 2. Density (plants/ha) of western hemlock (Tsuga heterophylla) at 10 study sites on the east side of Glacier Bay. All live trees (DBH > 4 cm) in the 10 plots (10 x 15 m) per site are marked with numbered tags. Seedlings and saplings were counted in each of 10 plots at each site. Seedling counts were made while walking through each plot, a method that could undercount small seedlings (< 5 cm tall).

Western hemlock trees were absent at all sites younger than York Creek when the study plots were established (1988-1990) although a few seedlings and saplings were present at three younger sites (Fastie 1995 Table 4). Today, hemlock trees (DBH > 4 cm) are common only at the three oldest sites and present only at one additional site (Sandy Cove, Figure 2).

Data about blueberry and hemlock in each study plot have been collected three or four times in the last three decades. So we can determine the reliability of the three decades of photographic evidence of their spread through the understory at the three oldest sites. The measured percent cover of blueberry has increased several fold at each of these sites (Figure 3). Three decades ago the oldest site (Bartlett Lake) had about as much blueberry as the two younger sites have today (Figure 3). This pattern is evident in the repeat photos.

Figure 3. Plot resampling results for Vaccinium cover at three sites. Vaccinium had just begun to invade at York Creek and Beartrack Cove when the plots were established in the late 1980s (when both sites had been ice-free for about 165 years). Vaccinium was already common at Bartlett Lake in 1988 when the site was about 220 years old. Vaccinium has increased at all three sites since the study began.

The York Creek and Beartrack Cove sites are similar in age and are about 55 years younger than Bartlett Lake. The greater blueberry cover at Bartlett Lake is likely due to that site’s greater age and probably hints at the future for the two younger sites. In other words we can use these three sites as a chronosequence and predict the future of younger sites based on the current state of an older site. Or we can know the history of the oldest site by looking at the series of younger sites.

The history of western hemlock differs from that of Vaccinium at the three oldest sites. As the repeat photos at York Creek and Beartrack Cove suggest, there are more hemlock saplings now than there were 30 years ago (Figure 4). But York Creek, not Bartlett Lake, is the hemlock regeneration champ. Bartlett Lake has lots of little hemlock seedlings and old trees, but nothing in between (Figure 2). Apparently the old trees produce seeds, then seedlings establish, but few if any seedlings survive to become saplings.

Figure 4. Plot resampling results for western hemlock at three sites. Hemlock saplings are establishing at York Creek and Beartrack Cove and their numbers have been increasing. But in the last three decades there have never been a lot of hemlock saplings at Bartlett Lake.

The poor success of hemlock regeneration at Bartlett Lake is conspicuous in the detailed seedling results (Figure 5). Almost all of the hemlock seedlings are small (< 50 cm tall) and this has been true for 30 years (Figure 5). In other words there are always small hemlock seedlings but few of them have survived to become taller than 50 cm.

Figure 5. Resampling results for western hemlock seedlings at Bartlett Lake. The 10 plots were resampled four times between 1988 and 2018. Seedlings were counted in two size classes and on two substrates. Most of the little seedlings seem to establish on the moss-covered ground, not on fallen logs, but maybe that is changing as more fallen trunks and branches become potential nurse logs.

There are lots of big hemlock trees at Bartlett Lake and when the study began the site had more hemlock trees than any other site. Today, York Creek has the most hemlock trees, although most of them have just recently grown from sapling size to tree size (DBH > 4 cm). In contrast, almost no new hemlock trees have been added recently at Bartlett Lake (Figure 6). I do not have an explanation for the failure of hemlock at Bartlett Lake to reproduce while healthy saplings and small hemlock trees are common at the two younger sites.

Figure 6. Stand density of western hemlock at three sites. Recent trees are trees that have grown to tree size (DBH > 4 cm) since 1995.


The dark green bars in Figure 6 are a good approximation of the hemlock stand density in 1990 at these sites (Fastie 1995, Table 4). In 1990, most of these trees were understory or subcanopy trees and a few of these are now part of the overstory. The light green bars suggest that a second cohort of hemlock could become an important part of the understory (and maybe the future overstory) at York Creek and Beartrack Cove. There is no sign of a coming second cohort of hemlock at Bartlett Lake and also no indication that there was ever a second cohort there (Fastie 1995, Fig. 4).

It does not appear that using these three old sites as a chronosequence would lead to reliable conclusions about the stand dynamics of western hemlock. The current state of the oldest site (Bartlett Lake) does not allow us to predict the future of hemlock at York Creek or Beartrack Cove.

An overstory western hemlock near Plot 6 at York Creek. Most hemlocks at the three oldest study sites are part of the subcanopy and understory. There are probably enough big trees to provide substantial hemlock seed rain. Jun 19, 2021.

I have characterized these three old sites as being similar because of the stand dynamics of Sitka spruce and their inferred early site histories. It appears that the recent dynamics of hemlock might make these sites less similar than I thought.

In relation to all 10 study sites on the east side of Glacier Bay, these three old sites are more similar to each other than to any other site. Sites 8-10 differ from Sites 2-7 because 8-10 lack evidence for early alder dominance, and Sites 8-10 differ from Sites 1-4 because 8-10 lack evidence for early importance of black cottonwood (Fastie 1995). Although Sites 8-10 are following the same no-alder, no-cottonwood pathway, they now might be developing important differences in the dynamics of the overstory dominants spruce and hemlock.

Instead of leading to a divergence of these forest communities, these differences might be adjustments that eventually make these communities more similar to one another. For the next century or two, it is likely that all three sites will support forests with both Sitka spruce and western hemlock in the overstory and lots of blueberry spreading over the forest floor.

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