INTRODUCTION

BACKGROUND/METHODS

FINDINGS

CONCLUSIONS

Students from the Yale School of Forestry and Environmental Studies in courses FES575a (Patterns and Processes in Terrestrial Ecosystems) and 530a (Introduction to Soil Science) collected tree cores from ten  study sites across Connecticut and New Hampshire between September and November 1999. From this collection, we mounted 45 tree cores representing 19 species, including conifers and hardwoods from a variety of forested ecosystems. We then used a 30x microscope to analyze each tree’s age and year-by-year growth, based on the number and width of annual growth rings. We also scanned many of the tree cores, used an image editing program to label their salient features (including some of the annual rings), and converted the images to a web-friendly format. Please find hyper links to these images in the Background/Methods section, below.

The study of tree growth rings, or dendrochronology, can yield information about discrete historical environmental events affecting tree growth as well as more subtle environmental variations occurring over time. Focusing on the period 1950-1999, we compiled growth data from 45 tree cores and then looked for inter- and intra-species growth patterns, as well as regional tree growth trends. We also looked at regional drought indices to see if drought conditions were reflected in the growth data.

Growth rates varied considerably within and across species in our sample. The impact of regional climate was also evident. We plotted the Palmer Drought index against an index that calculated the proportion of trees that grew better in any given year than in the previous year, and found a statistically significant correlation (see Regional Growth Trends, below).  The growth data also suggest that Hurricane Gloria, in 1985, had a beneficial impact on surviving trees, and suggest that drought in 1999 limited tree growth. Please see the Findings section, below, for a detailed discussion of the data analysis.
 
 

Sites visited by the students in FES575a and 530a were chosen largely for their ability to illustrate processes controlling/affecting various types of New England ecosystems. Similarly, tree cores collected from these sites were intended primarily to date historical disturbances and/or to show the relative ages of different forest stands. In this study, we used the tree cores to compare and contrast tree ring characteristics across the 19 study species, to record tree ages and growth ring widths, and to identify any trends and patterns evident in the growth data. Brief study site descriptions and a discussion of internal tree structure, as it relates to growth rings, are also presented below.
 
 

The eleven study sites from across New Hampshire and Connecticut are described below. Trees cored taken from each site are also identified. Click on sites highlighted in blue to see a map of the site.
 

1. Floodplain, North Haven, CT (9/8/99): Mixed hardwood forest growing in sandy alluvial soil on Quinnipiac River floodplain. Tree cores – shagbark hickory (Carya ovata), white ash (Fraxinus americana), red oak (Quercus rubrum).

2. Bittner Park, Guilford, CT (9/15/99): Three-tiered site featuring early-, mid-, and late-successional forest growing on kame terrace deposits and glacial till. Tree cores – red oak, white ash (2).

3. Hosley Avenue Bog, CT (9/18/99): Historically beaver-controlled swamp converted to mixed hardwood forest after partial draining. Site features deep peat sediments interspersed with lacustrine/glacially deposited soils. Tree cores – red maple (Acer rubrum), white ash (2)

3. Keene, NH (9/24 and 10/8/99): Diverse, mature conifer/mixed hardwood community on a west-facing hillside in a Yale University forest. Tree cores – hemlock (2) (Tsuga canadensis), white pine (Pinus strobus), black birch (Betula lenta), red oak, big-toothed aspen (Populus grandidentata), red maple.

4. Mount Moosilauke, NH (9/25/99): Balsam fir community growing at midslope in northern New England. Tree cores – balsam fir (3) (Abies balsamea).

5. Hubbard Brook, NH (9/25 and 10/9/99): Mature, mixed hardwood forest with a well-developed understory growing in shallow, sloping soils atop glacial till. Tree cores – white ash, sugar maple (Acer saccharum), beech (Fagus grandifolia).

6. Mount Kineo Ridge, NH (10/9/99): Mixed coniferous/hardwood forest on classic spodosol soils. Frequented by moose and impacted by clouds and ice. Tree cores – yellow birch (Betula lutea), white birch (Betula papyrifera), red spruce (Picea rubens).

7. Thornton Pines, NH (9/26 and 10/10/99): Mixed, mature conifer community on a sandy glacial delta, showing impact of agricultural disturbances. Tree cores – red spruce (3), white pine.

8. North Madison Cedar Swamp, CT (10/13/99): Mature (19^th century) Atlantic white cedar community in north-draining swamp/bog bounded by glacial drumlins. Tree cores – Atlantic white cedar (6) (Chamaecyparis thyoides).

9. Railroad Point, North Haven, CT (10/27/99): Frequently disturbed (fire and wind) mixed hardwood community growing on deltaic sand adjacent to Quinnipiac River tidal marsh. Tree cores – black oak (Quercus velutina), sassafras (Sassafras albidum), white oak (Quercus alba).

10. Pitch Pine Forest, North Haven, CT (11/10/99): Relic pitch pine forest on deltaic sand, with rapidly growing mixed hardwood (predominantly oak) understory. Tree cores - pitch pine (4) (Pinus rigida), black oak, white oak
 
 

The structural elements of wood that create tree rings differ in coniferous and hardwood species. Coniferous trees have relatively simple internal structure, with the same types of cells serving as vascular tissues and providing structural support. In white pine, for example, 90% of the wood is made up of longitudinal tracheids – cells which provide rigidity and carry fluids vertically through the tree (other tissues, known as wood rays, transport fluids radially, but these account for only a small percentage of the wood). Variation in the radial (side-to-side) diameter of tracheid cells results in the distinctive coloration of coniferous tree rings. The cells are wide and thin walled in the spring, when the tree is growing rapidly. Toward the end of the growing season, the cells become narrower and thicker walled, producing a dark ring.

The anatomy of hardwoods is somewhat more complex than that of conifers. Tracheid cells make up a significant portion of hardwoods, but the function of these cells is primarily structural. Hardwoods also possess vessel elements, or pores, that are the primary vascular tissues. The pores are created when certain cells join one another, causing the shared walls to disintegrate. In ring porous hardwood species, the pores are large early in the year and smaller toward the end of the growth season. Ring porous species thus have distinct growth rings similar to those seen in conifers. Examples of ring porous genera are oak, hickory, and ash. The size of vessels in diffuse porous trees, on the other hand, varies only slightly throughout the growing season, making counting and measurement of growth rings difficult in diffuse porous tree cores. Birch, maple, and aspen are examples of diffuse porous genera.

Some hardwood species have characteristics that are intermediate between diffuse and ring porous, and other species may display different structural elements in different parts of their natural range. Our hardwood species, however, fit neatly into either the ring porous of diffuse porous categories.
 

Below is a list of the tree species studied, separated into three structural categories : conifers, and ring porous/diffuse porous hardwoods. For species appearing in blue text, please follow the hyper link to an annotated image of the tree core.

Conifers

Atlantic white cedar(Chamaecyparis thyoides)
Balsam fir (Abies balsamea)

 

Hemlock(Tsuga canadensis)

    Pitch Pine (Pinus rigida)
    Red spruce (Picea rubens)
    White pine (Pinus strobus)

Hardwoods

    Ring Porous Species :

We examined the growth of 45 New England trees over the period 1950-1999. Tree growth is dependent on a host of factors – light availability, nutrient availability, soil conditions, precipitation, temperature, elevation, herbivory, disease, human and natural disturbances (fire, ice storms, hurricanes, pollution, soil disturbance), and other variables. Rather than attempting to correlate growth of individual trees with one or more of these variables, we sought to capture the net affect of their complex interactions by simply measuring year-to-year growth for all trees sampled. The data do appear, however, to reveal the impact of certain natural disturbances and climatic trends, as well as inter- and intra-species variation in average growth rates and degree of growth variation from year-to-year.

Regional Growth Trends

The trees sampled at ten study sites in Connecticut and New Hampshire grew an average of 1.74 millimeters per year for the last 50 years. Average growth was somewhat higher during the period 1975-1999 (1.74 mm/yr) than 1950-1974 (1.68 mm/yr). This result is somewhat surprising, because trees tend to grow more slowly as they mature, and 35 of our 45 specimens show more than 50 years of growth, with 10 specimens dating back to the 19^th century.

Decade-by-decade growth showed somewhat more variation (see Table 1, Average Growth by Decade). The 1970s saw the most rapid growth, with tree ring widths averaging 1.75 millimeters per year during the Nixon-Ford-Carter era. The 1960s was the worst growth decade, with an average of ring width of 1.64 mm/yr. Drought suffered by New England in the mid-1960s may be partly responsible for the poor tree growth in this decade.

Table 1: Average Tree Ring Growth by Decade
 

Decade	Average Ring Growth (mm/yr)
1950s	1.74
1960s	1.64
1970s	1.75
1980s	1.74
1990s	1.68
1950-1999	1.74

To examine the impact of drought on tree growth, we first calculated whether a given tree grew better in any given year than in the previous year.  We then converted these tree-by-tree figures to a regional index, showing the percentage of trees, out of our sample of 45, that grew better in any given year than the previous year. Finally, we plotted this index versus the Palmer Drought Index for the years 1950-1994 (see Tree Response to Drought, below).  Linear regression analysis showed a significant relationship between drought and our regional growth index. In any given year, whether a given tree grew better or worse than in the previous year was correlated to the severity of drought -- that is, more intense drought was correlated with a higher percentage of trees having a "bad" growth year.
 

The best growth years were 1974-1977, when regional tree growth averaged between 1.95 and 2.01 mm/yr. Another good year was 1985, with an overall average growth of 1.90 mm. Hurricane Gloria ripped through southern and central New England on a north-westerly track in April 1985, knocking down a large number of trees, clearing the way for understory regeneration, and enhancing light and nutrient availability for surviving trees. These phenomena were evident at the Railroad Point site in North Haven, CT, where a number of downed oaks point to the northwest, and where a network of vines and understory trees took hold 15 years ago. Black oak and white oak cores from the Railroad Point site also show accelerated growth in 1985 and for several years thereafter.

The worst growth years were 1970 and 1999. New England saw very little rainfall and record heat in the summer of 1999, which may have negatively affected the trees.

Species/Individual Growth Characteristics

On a species/individual tree level, the growth data were far more variable. The two most rapid growers between 1950 and 1999 were a white pine (3.92 mm/yr) from the Keene site and a sugar maple from Hubbard Brook, although both of these specimens were outpaced by a black oak from North Haven that grew an average of 4.70 mm/yr from 1961-1999. As a species, the red oaks were the fastest growers at an average of 2.96 mm ring growth per year (see Chart below and Table 2, Species Average Growth Rates).

In contrast, a hemlock from Keene (0.57 mm/yr) and a white pine from Thornton Pines (0.59 mm/yr) were the slowest growers. Hemlock, or Tsuga canadensis, is a shade-tolerant, late-successional species that can grow slowly for many years as an understory tree. White pine, on the other hand, is shade intolerant, and typically grows quickly to beat its competitors to the sunlight. The slow growth of the Thornton Pines white pine over the period 1950-1999 suggests that red spruce, creeping up from the understory at this site, will eventually out compete the pines.

Table 2: Species Average Growth Rates
 

Species	Avg. Growth (mm/yr): 1950-1999
Red Oak	2.96
Sugar Maple	2.82
Black Oak	2.71
Yellow Birch	2.06
White Ash	2.02
White Pine	1.99
Pitch Pine	1.93
White Oak	1.84
Beech	1.82
Shagbark Hickory	1.77
Aspen	1.66
Red Maple	1.44
White Birch	1.40
Atl. White Cedar	1.30
Red Spruce	1.25
Hemlock	1.23
Balsam Fir	1.13
Sassafras	0.90
Black Birch	0.63

Some species were relatively consistent growers, in terms of year-to-year ring widths, while average ring width for other species varied widely from tree to tree. As noted above, average growth of white pine ranged from 0.59 to 3.92 mm/yr. Both trees grew in New Hampshire. The faster grower, from the Keene site, dates back to 1941, while the slower one from the Thornton Pines shows growth rings dating to 1927. Growth rate variation for these trees could relate to the difference in their ages (white pines tend to grow rapidly when young), and/or to a host of other factors.

Atlantic white cedar grew at a more consistent rate. Growth of the six Atlantic white cedar specimens varied between 0.96 and 1.54 mm/yr. These trees are all residents of the cedar swamp in North Madison, CT, so it is unclear from the data whether growth of this species is inherently even, or whether similar environmental conditions controlled the relatively constant growth rates among these six trees.

Year-to-Year Variation

We also gauged tree growth variability, based on year-to-year ring width fluctuations. The average change in growth from year-to-year for all 45 specimens was approximately 24%. Growth of pitch pine and big-toothed aspen, however, was considerably more variable, with year-to-year changes averaging nearly 46% for big-toothed aspen and over 41% for pitch pine (see Table 3, Average Year-to-Year Growth Change.  This high degree of variability could indicate a high species sensitivity to environmental changes, or could stem from extreme environmental variation at the site in question. On the other end of the spectrum, average growth of shagbark hickory, red spruce, beech, and sugar maple varied less than 20% from year-to-year.

Table 3: Species Average Year-to-Year Growth Change
 

Species	Avg. Year-to-Year Growth Change
Aspen	45.91%
Pitch Pine	41.38%
Red Maple	38.41%
White Birch	37.68%
Balsam Fir	28.28%
White Oak	26.46%
Yellow Birch	26.42%
Red Oak	25.25%
Black Birch	24.92%
Atl. White Cedar	24.33%
White Pine	22.28%
Hemlock	21.98%
Black Oak	21.18%
Sassafras	20.95%
White Ash	20.60%
Shagbark Hickory	19.61%
Red Spruce	19.53%
Sugar Maple	17.97%
Beech	17.30%

CONCLUSIONS

Growth data from 45 trees, representing 19 species, suggest that New England trees have grown at a relatively even pace for the past 50 years. It appears that the various environmental conditions affecting tree growth have remained roughly in balance since 1950. Average tree growth over the last five decades ranged from 1.68 to 1.75 mm/yr, representing a maximum variation from the mean of only 3.5%, with slow growth in the 1960s coinciding with a prolonged regional drought.  In any given year, whether a given tree grew better or worse than in the previous year was correlated to the severity of drought -- that is, more intense drought was correlated with a higher percentage of trees having a "bad" growth year. Growth rates clearly varied widely from species to species, and, in certain cases, from tree to tree within the same species. This variability points out the control exerted by microhabitat and microclimate over tree growth.