Biomass and Coarse Woody Debris Dynamics in Old

Report
Long-term Dynamics of Living and Dead Biomass Pools in an Old-Growth Northern Hardwood Forest
Kerry D. Woods, Bennington College, Bennington, VT
I use long-term data from permanent plots in old-growth hemlock-northern hardwood forests to estimate
dynamics of the living, aboveground biomass pool and input rates to the CWD pool. I combine these
data with a one-time measurement of CWD to assess:
1) Trends in living biomass density with respect to stand properties;
2) Patterns over time in CWD inputs due to tree mortality;
3) Relationships between biomass dynamics and current CWD pools.
Analysis: I estimated aboveground living biomass for all trees >12.5 cm for each measurement year,
using diameter-based allometric equations, choosing equations on the basis of region, and number and
maximum size of trees used to fit equation; trees in the current study reached sizes larger than those used
to fit equations in all cases (Chapman and Gower (1991) for Acer saccharum, Young et al. (1980) for
Tsuga, Morrison (1990) for Betula, Perala and Alban (1993) for Tilia, Crow and Erdmann (1983),
Hocker and Earley (1983), Harding and Grigal (1985) for minor species).
Following Woods (2000), plots were grouped by composition and site properties (Fig. 1):
1) Acer-dominated: five plots on deep, alluvial soils with relatively high pH and cation
concentrations; canopy mixed-age and > 300 yr old.
2) Tsuga-dominated: two plots on soils of relatively low pH and cation concentrations. Age > 250
yr, but inadequate data for establishing age structure.
3) mixed: nine plots on soils of varying texture and depth, but relatively low soil pH and cation
concentration; mixed-age with trees > 300 yr old
4) burned: three plots from an area burned ca. 1830. Canopy is even-aged and strongly Tsugadominated. Soils similar to those for 'mixed' stands.
Dead wood measurements from 2007 were converted to volume for down wood by treating measured
segments as frusta of cones. Standing dead stems and stumps were treated as cones or frusta. Volumes
were converted to mass using relationships specific to decay class and species from Liu et al. (2006).
Fig. 1: Great Lakes region
(above) with inset showing study
area. Habitat photos for four plot
groups: (clockwise from top right)
burned; Acer-dominated; mixed;
Tsuga-dominated.
CWD inputs were calculated from mortality between measurements using stem-only biomass equations
with last recorded diameter. This neglects contributions of large branches, but these constitute a small
proportion of both living and dead biomass. Any diameter growth between last live measurement and
tree death is also discounted, which may be of consequence only for the long 1967-1989 interval.
RESULTS:
1. Biomass densities are exceptionally high: Aboveground living biomass densities averaged across all
plots ranged from 367 to 406 Mg/ha across measurement dates, and from 340 to 430 Mg/ha among
stand-types (Figs. 3,4). Tsuga and Acer-dominated stands reached similar total biomass with higher
values than mixed stands. These exceed most other values reported for old-growth forests of
northeastern North America (Mroz et al. 1985, Morrison 1990, Curzon and Keeton 2010, Keeton et al.
2011) and are comparable to values for southern Appalachian cove forests (Busing 1993).
THE STUDY:
Data-set: In 1962, Eric Bourdo (Michigan Technical Univ.) established 19 0.2 acre (~800 m2) circular
permanent plots in mesic old-growth stands, labeling and measuring all trees > 12.5 cm dbh. Bourdo
remeasured plots in 1967. In 1989, and every five years through 2009, I remeasured the same plots,
documenting mortality and establishment.
Biomass density (Mg/ha)
Site: The Huron Mountain Club in northern Michigan (Fig. 1) includes ca. 4000 ha of never-logged, oldgrowth forest. Mesic stands are dominated by Acer saccharum and Tsuga canadensis, with varying
representation of Betula alleghaniensis and Tilia americana, and are typically mixed-age and > 300
years old. Composition varies with soil properties, with Acer dominance increasing with soil pH and
mineral nutrient availability (Woods 2000).
500
Fig. 3: Stacked bars divide
total living, above-ground
biomass density by species
contribution for first (1962, left
in each pair) and latest (2009,
right) plot measurements, for
each of four plot groups (19
plots total).
400
300
200
Other
Tilia
Betula
1962 2009
Tsuga
100
Acer
0
Tsuga
(n=2)
Acer
(n=5)
mixed
(n=9)
Plot group
burn
(n=3)
425
Acer plots (n=5)
400
375
mixed (n=9)
Living Biomass
1.0
1.0
0.8
Biomass
2009
0.6
Stems
2009
Biomass
1962
0.4
Stems
1962
0.2
0.0
CWD Input
Mass
1962-1989
0.8
0.6
Stems
1962-1989
0.4
Mass
1989-2009
Stems
1989-2009
0.2
0.0
100
350
Cumulative proportion of total
Fig. 4: Lines connect
estimates of total,
aboveground biomass
density, over time, for four
composition and site-defined
plot groups.
5: Large trees increasingly dominate both living biomass and CWD input : Stems > 50 cm dbh
accounted for 40% of living biomass in 1962, but 59% in 2009 while total tree density decreased from
520 to 406 stems/ha (Fig. 7). Trees > 50 cm dbh contributed 50% of CWD inputs before 1989 and 62%
after 1989. Average mortality rate 0.6%/yr for the first 27 years of the record, , increasing to 0.9%/yr
for the last 20 yrs. (Patterns are not altered with exclusion of burned plots.)
Cumulative proportion of total
Tsuga plots (n=2)
450
80
60
40
20
Diameter at breast height (cm)
100
80
60
40
20
Last living diameter (cm)
325
Fig. 7: Cumulative contributions by diameter (from largest) to living biomass and stem
density (left), and to CWD and dead stem contributions through mortality (right).
burn (n=3)
300
1960
1970
1980
1990
2000
2010
SUMMARY AND SPECULATION:
Sample Year
3. Total CWD densities are high: Total dead biomass averaged 151.4 m3/ha or 46.0 Mg/ha over all plots
in 2007 (Fig. 5). Standing dead trees (average density = 32.5/ha) accounted for 16% of this total.
Accounting for differences in methods, values are similar to the highest values recorded in other studies
in hemlock-northern hardwood forests (Tyrrell and Crow 1994, Hale et al. 1999, McGee et al. 1999,
Stewart et al. 2003, Hura and crow 2004, Angers et al. 2005). CWD pools were greatest in Acerdominated stands (Fig. 5), but were not otherwise different among plot groups and canopy types
(p>0.05, Kruskal-Wallis test, post hoc Mann-Whitney pair-wise tests with Bonferroni correction). Most
advanced decay classes were more prominent in Acer-dominated stands.
60
200
50
Most
Decayed
(Class 5)
40
30
20
Least
Decayed
(Class 1)
10
0
Acer
Tsuga
mixed
CWD (m 3 /ha), 2007
Biomass dynamics in late-successional forests have generally been inferred from chronosequences,
conceptual and simulation models, or extrapolations from dynamics of early-successional stands. Direct
measurements over significant time periods in old-growth forests are rare. One-time measurements of
dead biomass (or coarse woody debris, CWD) are available for a number of old temperate forests, but
assessment of CWD dynamics is impossible without long-term input rates or remeasurements.
Biomass density (Mg/ha)
Fig. 2: Mapping CWD
(left); a decay-class 5 Tilia
americana log (right).
2. Biomass increased slightly over time, but with large fluctuations: Overall biomass density increased
by about 5.5% (4.5% excluding burned plots) over 47 years (Fig. 4), an average increase of 0.43
Mg/ha/yr (0.35 Mg/ha/yr without burned plots). All plot groups showed net increases in biomass density
with declining biomass for some sampling intervals. Biomass density and trends showed no consistent
relationship with dominant species or soil properties. The younger, burned plots had lower initial
biomass but were similar to mixed stands in recent decades; these plots had much higher stem density
but smaller maximum sizes. In all cases, living biomass pools were increasingly dominated by alreadydominant species (Fig. 2, Woods 2000).
CWD (Mg/ha), 2007
Long-held models of forest succession assume biomass density follows a saturating curve over time and
net ecosystem production approaches zero in late succession (Odum 1969, Whittaker et al. 1974). The
influential model of Bormann and Likens (1979) suggests maximum biomass should be achieved by
about 200 years after stand origin in northeastern forests. Several recent studies, however, suggest that
living biomass may continue to accumulate at much greater successional ages (Keeton et al. 2011).
Conventional notions of late succession also assume inputs to the pool of dead biomass are balanced by
decay rates in very late succession, producing overall carbon neutrality in 'old-growth' forests.
In 2007, we measured all dead wood (CWD) > 10 cm in diameter (standing and down) for the study
plots (Fig. 2). We used a Lasercraft Contour XLRic to map ends of all down wood in relatively straight
segments, and measured end-diameters of each section. We also measured height, basal diameter (above
basal swell) and top diameter (for stumps and broken stems) of all standing dead stems. All sections of
dead wood were assigned to species where possible and to one of five decay classes (following Harmon
and Sexton 1996, Liu et al. 2006).
180
Decay
Class 5
160
140
120
100
80
60
40
0
burn-Tsuga
Decay
Class 1
20
Acer
Plot Group
Tsuga
mixed
burn-Tsuga
Plot Group
Fig. 5: Stacked bars divide total CWD density (left) and volume (right) by decay class
contribution for each grouping of plots. Decay class 1 includes stems with no evident decay;
decay class 5 includes sections with little or no solid wood or structure.
4. Inputs of dead biomass increased over time, but were not consistently related to stand properties:
CWD input varied widely among measurement intervals and ordering of plot groups was not consistent
(Fig. 6). Overall average inputs were 2.1 Mg/ha/yr for 1962-1989, increasing to 3.1 Mg/ha/yr for 19892009. Underestimates for 1967-1989 due to post-1967 growth and stems that both recruited and died
within the interval are likely small; growth rates average ~20 mm/yr (Woods 2000) and CWD mass is
dominated by large stems. Assuming a 20-yr ‘half-life’ for CWD (Vanderwel et al. 2008), these input
rates would produce steady-state CWD densities of approximately 60 and 90 Mg/ha – substantially
greater than those observed – after about 150 yr.
Average CWD input (Mg/ha/yr)
INTRODUCTION:
8
Fig. 6: Lines indicate
average annual inputs of
CWD (standing and down
combined) via tree mortality
by measurement interval and
plot group. Values are for
stem or bole only, estimated
from last living diameter
measurement.
Acer
6
mixed
burn
Tsuga
4
2
0
1960
1970
1980
1990
Year
2000
2010
• Densities of living and dead biomass in these old-growth forests are among the highest reported for
eastern North American forests.
• Living biomass density increased modestly over 47 yr, but average increments are small and trends are
inconsistent among sub-intervals.
• CWD input rates increased by about half between the first half of the study period and the second half;
recent rates appear to be consistent with substantially greater CWD pools than observed.
• Increasing dominance of large size classes in both living and dying trees and decreasing tree density
are consistent with ongoing successional processes even in very old, mixed-age forests.
• Observed patterns could be driven by long-persistent effects of significant, area-wide ‘intermediate’
disturbances sufficient to inaugurate substantial tree cohorts but not generating an even-aged stand
(Woods 2000,2004).
• This interpretation is supported by changes in stand composition and by increasing rates of CWD input
possibly reflecting senescence of large cohorts of stems.
• 47 years remains a small portion of typical tree life-expectancies in these stands. If this model is
accurate, I anticipate relatively small changes in standing live biomass as demography ‘equilibrates’, but
potentially large increases in CWD pools.
• Inadequacy of existing biomass equations for large stems and possible biases introduced by the long
1967-1989 may influence these equations, but probably not enough to alter observed patterns.
ACKNOWLEDGEMENTS:
This research has been supported by grants and logistical support from the National Science Foundation, the Andrew J.
Mellon Foundation, and the Huron Mountain Wildlife Foundation. Some of the analysis was done during a Fellowship
at the National Center for Ecological Analysis and Synthesis. Many Bennington College undergraduates have played an
essential role in the field and lab.
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