P-IIIC. Forest Ecosystems
Summary: Carbon is the primary focus of our regional studies because we have developed a moderate understanding of regulators of pools and fluxes in both terrestrial and aquatic systems, yet there are still major uncertainties in carbon, nutrient, and water cycling responses to environmental heterogeneity and disturbance.
In previous years, we established a series of plots in undisturbed watersheds which span an environmental and species composition gradient. We have been characterizing pools and fluxes of carbon, nutrients, and water and associated driving variables in these plots (Table 6 and Figure 3). The objectives of the
research
were to understand the role of environmental heterogeneity in regulating
ecosystem function and the impacts of "natural disturbance" (e.g. drought,
herbivory, etc.) and stress on biogeochemical cycling and ecosystem processes. A
unique aspect of this research is that a suite of process and pool size measurements have been made at the same place and time, and a
research infrastructure is in place to facilitate long-term measurements.
Because pools and processes often respond slowly to all but dramatic variation
in environmental conditions, the value of the data collected on these plots will
increase tremendously over time. Equally important, these measurements and
subsequent understanding provide the foundation for scaling measurements to the
watershed, landscape, and region via modeling.
To remove some of these uncertainties, we are building and
expanding upon our previous studies by focusing on three specific questions:
1) How do pools and
fluxes of carbon, water, and nutrients vary across a heterogeneous
landscape?
2) What is the relative
contribution of species composition vs. environmental gradients in
regulating variation in pools and fluxes?
3) How are pools and
cycles affected by disturbance and stress?
Question 3) is addressed specifically in our study of regional C pools and flux response to land use change and is inherent in our long-term studies of ecosystem processes on gradient plots.
For more
detailed information on the following topics, visit the links below:
P-IIIC1. Variation in pools and fluxes of carbon
P-IIIC2. Variations in pools and fluxes of nutrients
P-IIIC3. Variations in pools and fluxes of water
P-IIIC4. Linkages among carbon, nutrients, and water cycling
P-IIIC5. The
role of species vs. environmental heterogeneity in regulating ecosystem
processes
P-IIIC1. Variation in pools and fluxes of carbon
Summary:
Carbon pools and fluxes, as well as controlling mechanisms, require
quantification to fully understand C cycling processes and potential impacts
of disturbance and stress.
Many aspects of the
carbon (C) cycle remain poorly understood (Tans et al. 1990).
Part of our uncertainty is due to substantial spatial and temporal
variation in terrestrial C pools and fluxes. C amount and distribution vary
among and within ecosystems, and we have not adequately sampled or modeled
this range of environmental and biotic conditions. For example, small-scale
variation in soil conditions from dry ridge to streamside may result in an
order of magnitude increase in belowground C. These fine scale variations
are juxtaposed on climatic variation, often due to macro- and meso-scale
changes in elevation and latitude. We are measuring and modeling the C cycle
at multiple scales (e.g., plot to region).
Our approach is to measure key carbon inputs, internal pools, and outputs.
Modeling will be used to integrate and extrapolate measurements spatially
and temporally.
Carbon inputs and outputs
Canopy access towers and
walkways are located in three of the five gradient plots and in a low
elevation, xeric oak community type in WS 2. These towers have been used to
collect leaf -level physiological data, such as photosynthesis (Sullivan et
al., in press) and respiration rate (Vose and Bolstad, in preparation) by
species. In addition, we have used these towers to determine the vertical
distribution of environmental driving variables (i.e. light, temperature,
relative humidity, leaf area index, and amount and vertical distribution by
species (Vose et al. I995). We have expanded the network of canopy
towers (i.e. towers located at all gradient plots) to increase spatial
sampling and include more sites and species, and to continue measuring leaf
photosynthesis and respiration at those sites.
Plant respiration losses can account for as much as 50% of the gross carbon fixed in forest ecosystems. In 1995, Our initial data for leaf and stem respiration indicated a wide variation in respiration rates among species and measurement periods. From 1996-2001 we expanded these measurements to include more species and sample periods to better understand this variation and associated driving variables. In 1996-97, litter decomposition was measured using three species replicated three times per month at each of the five gradient plots. Soils contain a considerable proportion of ecosystem carbon; fluxes from the soil are a major component of the ecosystem carbon budget. To quantify the contribution of soils, we have measured diumal soil CO2 evolution from gradient plots in spring, summer, fall, and winter using an automated infrared gas analyzer measurement systems (Vose et al. I995) from 1996-2001. In the other months, we used a static absorption technique which correlates well with the IRGA based system at the plot level. With both techniques, flux estimates include the contribution from forest floor.
Inter- and
intra-annual variation in carbon pools
For inter-annual variation in wood production, trees on
gradient plots will be re-measured to determine growth and mortality rates
by species and diameter (converted to biomass with locally developed
allometric equations). For intra-annual variation, dendrometer
bands will continue to be measured on a sub-set of trees spanning
variation in tree size and species. We continue to estimate monthly
leaf area index on gradient plots using light interception and locally
developed extinction coefficients (Vose et al. 1995), and litterfall
continues to be measured monthly. Phenological development of overstory
species, shrubs, and herbaceous vegetation are quantified using weekly
observations of phenological events (e.g. bud break, flowering, etc.).
Forest floor mass (separated by L, F, and H layers) will be re-measured in
years 1, 3, and 6 using destructive sampling from locations on the edges of
the plots. In addition, as the forests at Coweeta age, we anticipate that
coarse woody debris will increase in amount and importance in the overall
carbon budget. Hence, coarse woody debris (by stage of decomposition) will
be measured within the plots (volume measurements converted to mass using
CWD density) in years I and 5 m.
We have expanded our belowground sampling to better quantify fine root biomass, turnover, and linkages to aboveground phenological activity and environmental conditions. Using existing root observation boxes we are able to monitor long-term fine root growth phenology across all gradient plots. This work has been expanded to include video images of each window prior to budbreak, immediately after I00% expansion, during midsummer, at the onset of leaf senescence and immediately after 100% leaf fall. These images are digitized manually with the ROOTS (Hendrick and Pregitzer 1992) image analysis program, and the individual dimensions, development and fate of the roots are determined. Fine root biomass are determined from soil cores taken at the beginning and end of the growing season, and used in conjunction with the dynamics data to calculate belowground productivity. In addition to quantifying the relationship between above and below ground phenology across community types and environmental gradients, these data will help establish the magnitude of inter-annual variation in the timing and amount of root growth and mortality. Soil carbon (to a 15 cm depth) has continued to be measured on all gradient plots concurrent with in situ N mineralization and nitrification (see section II.D.3.b.2).
Carbon modeling
A significant new
component of the research is development, validation, and
application of carbon cycling models within the Coweeta basin. Data
collected to date, as well as data proposed to be collected in this grant,
provide a unique opportunity for developing and testing models with complete
and long-term data sets across a gradient of species and environmental
driving variables. Modeling activities include continuation of
development and validation of micro- and meso-scale environmental models
(i.e. soil temperature, air temperature, within and below-canopy
micro-climate, soil moisture) which will be as drivers of C cycling rates.
In addition, we have continued development of a physiologically-based basin
scale model of net primary productivity and soil carbon cycling (Figure
17). Recent work has detected strong relationships between landscape
position, vegetation type, and belowground carbon properties such as
fine-root biomass and total soil carbon. We are currently modeling
belowground processes and variables driven by landscape position, vegetation
type, and soil temperature and moisture. The integration of sub-models,
combined with additional sub-component carbon flux measurements and more
detailed mapping of temporally static driving variables will be a major
focus of our carbon modeling. The primary goal of these activities is to
develop and validate a spatially-explicit carbon cycling model for the
Coweeta basin.
Regional carbon pools and fluxes
We expanded our study of carbon pools and fluxes within the Coweeta basin to the southern Appalachian region. In addition to quantifying
carbon budgets across an expanded environmental gradient, a major objective
of the regional study is to understand the effects of land-use change on
carbon pools. We have hypothesized substantial variation in C pool size, C
distribution, and C-cycling rates among land cover classes.
Two major
activities are used to quantify the impacts of land-use change on C pools and fluxes
in the southern Appalachians:
1) We measure C pool and flux estimates from a network of field sites over a
representative range of biotic and abiotic conditions.
2) We use recent historical and land-use/landcover
characterization for the region and C cycling models to scale pool and flux
measurements to the southern Appalachian region.
Our long-term goal is to replicate measures of fluxes and pools for a range of biotic conditions in each land cover class. We have sampling for a subset of southern Appalachian ecosystems, and are extending the sampling in years 1 through 5 both categorically (to include more land cover types) and spatially (to better sample within type variation) as a long-term activity. By sampling I0 land cover classes, we can represent over 90% of the southern Appalachian land surface: old-growth, mature, and early successional forest types at each of ridge, slope, and cove positions, and pasture in low cove or valley conditions.
A suite of extensive measurements have been made to determine C pools at all sites (Table 7). Beginning in 1995, measurements to estimate fluxes and to more finely partition C pools were made at all sites which includes direct soil CO2 measurements using a recently developed system (Vose et al. 1995), comprised of surface cuvettes, connected through null-balance pumps to an infrared gas analyzer. Fine (2 mm) and coarse root (2-5 mm) biomass are measured at the intensive sites coincident with each flux measurement, following locally developed (McGinty 1976) and regionally validated soil and root sampling protocols. Measurements also include stem diameter and height, coupled with allometric equations to determine live vegetation C pools and recent ( < 5 years) above-ground C accretion.
Tree species and size were selected based on initial plot survey data by stratifying tree species density and size. Instantaneous tree respiration rates are measured on a range of tree species and size classes. Sapwood temperature is measured with thermocouples concurrent with the stem respiration measurements. To understand effects of phenological and environmental variation, stem respiration is measured bi-monthly throughout the year to encompass periods of stem growth (summer), carbohydrate and water re-translocation (spring and fall), and dormancy (winter). Since phenology varies significantly between sites and species, six annual sampling periods allow sampling each species at least once during each primary phenological condition.
A two-step process is used for estimating regional C pools and fluxes. Our first approach involves delineating "homogeneous" geographic units (HGU's sensu Brand et al. 1991) and estimating state variables for each HGU such as land cover, soil properties, thermal and moisture regimes, and carbon stocks. HGU's are developed via spatial overlay of relevant biotic and abiotic variables, some already developed (elevation, soils, temperature, precipitation), and some yet to be assembled (geology, landcover, current biomass). Once HGU's are identified, we run point process models and the Coweeta C cycling model, with appropriate state variable estimates for each HGU.
P-IIIC2. Variations in pools and fluxes of nutrients
Summary:
Previous research at
Coweeta has documented differences in nutrient cycling processes across the
elevational gradient (Swank and Waide 1988) and results have shown that large differences exist in pool sizes and cycling
rates of many nutrients among the gradient plots. We continue to
measure response variables to detect long-term changes that are used as
drivers of other processes, such as carbon cycling.
Nutrient
inputs and outputs
Hydrologic and nutrient
budget studies continue to provide the framework for linking terrestrial and
aquatic processes and responses to disturbance. We continue to make
long-term hydrologic and associated chemistry measurements for baseline and
disturbed forested watersheds at Coweeta. In addition, the long-term record of stream water chemistry
at Coweeta includes measures of DOC concentrations in streams draining watersheds 7 (clear-cut in 1977) and 14
(low elevation undisturbed) since
1979 and
watershed 27 (high elevation undisturbed). The data set documents
recovery from clearcutting as well as differences in streams draining high
and low elevation watersheds. Throughfall chemistry
has been measured on all gradient plots for a period of two years.
Inter- and intra-annual variation in nutrient pools and fluxes
To determine variation in
nutrient pools and fluxes a suite of measurements must be done. Foliar
chemistry was measured from border trees on all gradient plots in 1993. We
re-sampled all previously measured trees in years 1 and 6. Nutrients in the
forest floor were measured
in spring and fall of 1994. We have also re-measure litter chemistry on
samples used to determine forest floor mass in years 1,3, and 6.
Exchangeable base
cations (quarterly measurements) have been measured on the gradient
plots for the past five years. Porous cup lysimeters are located at 15 cm and 60 cm depths and
soil chemistry has been measured for the past two years. We have analyze the
data for inter-annual temporal trends. Measurement will be suspended if
these analyses indicate minimal inter-annual variation.
Nutrient modeling
Revised nitrogen and
cation cycling models for undisturbed mixed hardwood forests are completed
and published. These revised models will synthesize a broad body
of knowledge of cycling processes developed at Coweeta over the past years.
The N cycling data will also be used to validate a N-saturation model (PnET-CN/CHESS)
currently used by J. Aber, C. Driscoll, and M. Mitchell for northern
hardwood forests.
P-IIIC3. Variations in pools and fluxes of water
Summary:
Water is a major driver
of ecosystem processes in the southern Appalachians.
Abundant rainfall
(between 1800 to 2300 mm per year) supports high terrestrial productivity,
increases the importance of riparian zones, and results in numerous streams
ranging from first to fourth order within the Coweeta basin. Despite the
high average annual inputs, inter- and intra-annual variation is often large
and ecosystems respond to this variation in subtle (e.g., growth reduction;
Vose and Swank 1995) and obvious ways (e.g., tree mortality; Clinton et al.
1993, Smith 1991).
Hydrologic
inputs and outputs
The network of climatic
stations and rain gage locations have continued to be used to estimate
precipitation input for each gradient plot. Both
watersheds containing
gradient plots are gaged and are used for estimating streamflow and
evapotranspiration (i.e. ET = precipitation - streamflow). In
addition, models used for carbon cycling have an evapotranspiration component which
is used to predict ET across the
gradient.
Inter- and
intra-annual variation in soil moisture
Gradient plots are measured weekly for
soil moisture at 5 and 20 cm depth using Time Domain Reflectometry (TDR).
Soil moisture measurements Along with stream flow and climatic data, soil moisture data from these
measurements (monthly beginning Oct 94) are being used to parameterize and
calibrate a terrain-based hillslope hydrology model for watersheds spanning
the Coweeta basin (Yeakley 1993; Yeakley et al. in prep). Model output of
lateral and vertical soil moisture distributions at approximately 10 m x 10
m spatial scales, at temporal resolution as fine as hourly, have been
developed. These data will be linked with models which require soil moisture
as a driving variable (i.e., soil CO2, NPP) and with studies
assessing tree regeneration patterns in gradient plots.
Hydrologic modeling
Long-term hydrologic
studies at Coweeta are utilized to further develop a hydrologic model (IHACRE)
for predicting daily stream flow for ungaged forested catchments. This
research is a collaborative effort with Tony Jakeman and colleagues at
the Australian National University and Wayne Swank at Coweeta. Application
of the rainfall-runoff model to Coweeta catchments will provide insights
into refinement of methods needed to quantitatively predict how differences
in physical attributes of catchments affect their hydrologic response. In
another collaborative effort, led by the H.J. Andrews LTER, we will continue
to participate in an intersite synthesis of hydrologic data and modeling for
baseline and disturbed forest ecosystems.
P-IIIC4. Linkages among carbon, nutrients, and water cycling
Summary: One of our primary
objectives is to understand the linkages among the ecosystem attributes
using long-term data sets being developed (Table
6). In addition to these analyses of long-term data, we propose to
establish shorter-term studies to determine specific linkages.
Functional role of coarse woody debris
Functional roles (i.e. as a water or nutrient source) of
coarse woody debris have been determined by sampling highly decomposed CWD to
determine the quantity of roots occupying CWD on each site. At the time of
collection, sub-samples of roots are placed in buffered solutions where
uptake of Ca and K is monitored over a 1.5 hour period. The uptake
rates are measured twice during the growing season, during a dry and wet
period.
Relationships between fine root phenology, canopy processes,
and the soil environment
If fine root phenology
is regulated by environmental factors to the extent that canopy development
is, then the timing of root growth and mortality might be expected to change
across climatic gradients, in an altered climate, or during extreme weather
events. However, we are currently unable to detect these changes because of
our inability to quantify the effects of the soil environment on fine root
activity, or to discriminate between inter-annual variation and
environmentally induced changes in fine root demography. Periods of high
water demand might also be expected to be closely related to root
production, especially relatively deep in the soil. Unfortunately, root
dynamics at depth > 30 cm have received little attention.
To quantify the relationships between fine root dynamics, soil moisture, and canopy physiology we use the xeric oak and northern hardwood gradient plots, and establish two more plots at each forest type. These sites represent reasonable contrasts in overall soil moisture regimes, probabilities of growing season soil water deficits, and a balance between forest types in which root dynamics are relatively well (northern hardwoods) and poorly (xeric oak) understood.
We have installed five minirhizotrons (5 cm diameter x 2 m length) in each plot. Minirhizotron images are collected monthly during the growing season on Hi-8 videotape (with a digital imaging camera) along a vertical transect in each minirhizotron inscribed with individually numbered image frames. Additional images are produced during periods of drought or after significant rain events. The production and fate of each individual root appearing in the frame will be measured with ROOTS (Hendrick and Pregitzer 1992). Canopy water demand and balance are determined from weekly measurements of leaf water potential and conductance using existing towers at the previously established gradient plots. TDR is used to measure soil moisture at all plots, and soil temperature has been installed at 0, 15, and 75 cm depths. Relationships between soil temperature, soil moisture, canopy transpiration, and root growth dynamics are then able to be quantified.
P-IIIC5. The
role of species vs. environmental heterogeneity in regulating ecosystem
processes
Summary:
By
using transplant experiments we are able to determine the relative
importance of species vs. environmental effects on organic matter
decomposition and N-mineralization and nitrification.
Community types (i.e.
species composition) vary considerably in response to the environmental
heterogeneity which occurs across the gradient (Figure
13). Patterns of ecosystem processes such as
nitrogen mineralization (Figure
18) and leaf decomposition (Figure
19) has generated questions about the relative importance of biotic vs.
abiotic drivers of ecosystem processes. For example, the northern
hardwood community soils and litter are coolest and, if temperature is a
strong regulator of processes, we would expect process rates to be lowest in
the northern hardwood community. However, this was not the case for many
ecosystem processes. The soil transplant
experiment is not a direct test of species effects per se, because of
differences in soil type among the plots. However, large differences in soil
carbon (ranges from 3.3 % on xeric oak/pine to 9.9 % on northern hardwood)
and nitrogen (ranges from 0.1 % on xeric oak/pine to 0.7 % on northern
hardwood) indicate that differences in species composition may be
influencing soil characteristics.
Decomposition
Nine species (Pinus
rigida, Acer rubrum, Acer saccharum, Liriodendron tulipifera, Betula lutea,
Quercus coccinea, Quercus rubra, Quercus prinus, and Carya glabra)
are representative of the dominant overstory composition among the gradient plots.
In order to determine wood decomposition, trees are felled from areas
outside the plots, and log segments 25-40 cm in diameter and 0.75 to 1 m in
length and are cut. Eyebolts (30 cm long) are placed in the ends of logs
to facilitate weighing. Four randomly selected logs of each of the nine
species are placed at random locations in
each gradient plot. Logs are re-measured every year. Nitrogen and C content
are determined on wood from
increment core samples, and logs are visually scored for decay class.
Small branch (< 1 cm) and leaf decomposition is measured for the same
species using litter bag techniques.
Nitrogen
Cycling
We
have focused on three gradient plots: [northern hardwood (527), high elevation mesic oak (427), and xeric oak/pine (118)] representing extremes in
environmental conditions and historical soil N cycling rates. To determine
cycling rates we use
closed PVC cores to sample soil from ten random locations in each plot. Five
of the cores are removed (with the soil intact inside the cores) and
transplanted in the following manner: northern hardwood to xeric oak/pine,
northern hardwood to mesic oak, xeric oak/pine to northern hardwood, xeric
oak/pine to mesic oak, mesic oak to xeric oak pine, and mesic oak to
northern hardwood. The other five cores remain on each site.
Twenty-eight day in situ incubations carried out for 7 months
during the growing season (April through October).
Investigators and Collaborators:
Paul
Bolstad
James Clark
Dave Coleman
D.A.
Crossley
Bruce
Haines
Ronald Hendrick
Brian Kloeppel
Jennifer Knoepp
Judy Meyer
Wayne Swank
James Vose
J. Alan Yeakley