Harvard Forest Symposium Abstract 2017
- Title: Simes Hemlock Removal Experiment: Tree Sapling Layer Development
- Primary Author: Audrey Barker Plotkin (Harvard Forest)
- Additional Authors: Aaron Ellison (Harvard Forest); David Orwig (Harvard Forest)
We initiated the Harvard Forest Hemlock Removal Experiment to examine forest decline and re-development following hemlock loss by girdling (to simulate mortality from hemlock woolly adelgid) vs. logging (to simulate the actions of some landowners faced with hemlock decline). Here we discuss results from ten years of forest regeneration surveys.
Before the manipulation (2004) and every other year after (2007, 2009, 2011, 2013, 2015) we tallied the number and species of sapling-sized trees (>1.3m tall but <5cm dbh) within the central 30m x 30m portion of each plot. In 2015, we measured sapling heights with a telescoping height pole. We measured all stems for uncommon species, and a random subsample (e.g. every 10th stem encountered) for frequent species. Stems that recruited into the >5cm dbh class were tagged, measured and mapped in the 2009 & 2014 tree censuses.
Robust tree regeneration occurred in all manipulated plots; by 2015, density of tree saplings (individual trees > 1.3 m tall but < 5 cm dbh) was between 10,000-27,000 stems per hectare (ie, an average of one to three saplings per m2). Net recruitment into the sapling size class is still increasing as of 2015, 10 years after the manipulations, but it is likely that these densities are nearing their peak and will soon begin to decline as the plots enter the stem exclusion phase of development. .A few of these saplings grew into the tree-sized class (dbh > 5cm) by 2014, especially in the logged plots.
The sapling cohort was overwhelmingly dominated by black birch (Betula lenta), which comprised 72-76% of the saplings in the manipulated plots in 2015. This novel forest type has also been observed in many post-adelgid stands in Connecticut and Massachusetts.
A total of 15 species of saplings were found across all treatments. Within the manipulated plots (girdled & logged), 11 sapling species occurred, mainly black birch, red maple, white pine, and hemlock, with occasional paper birch, black & pin cherries, red & white oak, and 2 glossy buckthorn. Yellow birch was abundant in plot 1 (this girdled plot is adjacent to a forested wetland) only.
In the logged plot in the ridge block, red oak may have some chance of becoming part of the dominant canopy. This plot had 133 red oak stems/ha in 2015. Oliver (1978) suggested that ~150 red oak per ha is sufficient for red oak to eventually dominate a stand, if those stems are tall/strong enough.
Yellow birch is increasing in abundance in plot 1. By 2015, there were nearly 6,000 stems/ha, or about 1/3 as many yellow birch as black birch.
Red maple recruitment was strongest in the logged plots. Sprouts from cut stems is a a major reason for its higher density in the logged vs. girdled plots.
White pine did not recruit into the sapling layer until 2011 (both logged plots) or 2013 (girdled plots), but has increased rapidly in three of these plots to densities between 1500-2000 stems/ha. Depending on how rapidly they can grow in height, they may be able to make a major contribution to the future forest canopy. They probably will do best in patches with relatively low black birch density.
Hemlock is also recruiting, particularly in the girdled plots. Some, but not all, of these are currently infested with HWA, which will limit their potential to ascend to the canopy.
Sapling height is a key predictor to which of the saplings will eventually ascend to the canopy, althought height development. We measured the heights of a random sub-sample of saplings in 2015 (10 years after the treatments were implemented) from each species and plot.
Of the four most common species, black birch was the tallest in both the girdled and logged plots (2.8m and 4.0m, respectively). Red maple, white pine, and hemlock were all in the 1.6-2.0m range in the girdled & logged plots, except for taller red maple in the logged plots (2.7m, most likely because of stump sprouts). The occasional paper birch and cherries were, on average, between 2-4m tall, whereas the occasional red oaks were only 1.5-1.7m tall. Overall, the saplings in the logged plots were 30% (0.75m) taller than those in the girdled plots, reflecting the more rapid canopy removal and subsequent regeneration in response to the logging treatment.
The rate of forest development is slower here than in the urban hemlock logging experiment at the Arnold Arboretum in Boston. Because of differences in sampling (here we included all saplings >1.3m in height but less than 5cm dbh; at the Arnold Arboretum, the minimum size for saplings was 1cm dbh and there was no upper limit for diameter), direct comparisons are not possible. However, we know that by 2014, there were only 100 –150 stems/ha that had grown >5cm dbh since the manipulations in the logged plots (and only 2-25 stems/ha >5cm dbh recruits in the girdled plots). The height of black birch saplings at the Simes Tract at Harvard Forest (4m mean height in the logged plots) is only half that at the Arnold Arboretum site (8.5-9m median height).
- Research Category: Invasive Plants, Pests & Pathogens; Large Experiments and Permanent Plot Studies