Title
Abstract
Bottomland hardwood wetland forests along the Atlantic Coast
of the United States have undergone significant changes over
the past two decades. One of the most prominent
transformations is the rise in tree mortality rates, particularly in
coastal forests. Using data collected from 2009 to 2019 in a
bottomland hardwood forest in North Carolina, this study
evaluates trends in tree mortality and identifies the key drivers
Hydrologic
—hydrological (e.g., sea-level rise, groundwater table depth),
Perturbation Is a Key
biological (e.g., leaf area index), and climatic (e.g., solar
Driver of Tree
radiation, air temperature) factors—through a structural
Mortality in
equation modeling (SEM) framework. The results show that
Bottomland
tree mortality rates increased from 1.64% in 2009 to 45.82%
Hardwood Wetland
over ten years. Hydrological variables were identified as the
Forests of North
primary drivers (R² = 0.65), followed by biological (R² = 0.37)
Carolina, USA
and climatic (R² = 0.10) variables. Prolonged inundation, sealevel rise, and other stressors have initiated the early stages of
'ghost forest' formation, even in areas previously considered far
from coastal influence. This research advances the
understanding of coastal ecosystem transitions amidst rising sea
levels and underscores the necessity of incorporating these
findings into ecosystem modeling frameworks to better predict
and manage the impacts of climate change.
This study explores the viability of American sycamore as a
bioenergy feedstock grown under short-rotation coppice (SRC)
systems with minimal inputs. Over two rotation cycles (2010–
2014 and 2015–2019), trees were planted at varying densities
(1250, 2500, 5000, and 10,000 trees per hectare) on degraded
agricultural land in North Carolina. Biomass productivity was
proportional to planting density, with the highest productivity
Productivity of LowInput Short-Rotation recorded at 10,000 tph (23.2 ± 0.9 Mg ha−1 in the first rotation
Coppice American
Sycamore (Platanus
occidentalis L.)
Grown at Different
Planting Densities as
and 39.1 ± 2.4 Mg ha−1 in the second). Biomass partitioning
a Bioenergy
Feedstock Over Two
Rotation Cycles
patterns showed that stem wood allocation increased with
density and over time, with significant reductions in allocation
to branches. These findings demonstrate the potential of lowinput SRC sycamore systems on marginal lands for sustainable
biomass production.
Coastal forested wetlands provide crucial ecosystem services in
the southeastern United States but are increasingly threatened
by anthropogenic and natural disturbances. This study
investigates species composition, mortality, aboveground
biomass, and carbon content in natural pine forests of the North
Millennial-Scale
Carolina Lower Coastal Plain. Comparisons were made
Carbon Storage in
between a declining "ghost forest" near a drainage canal and a
Natural Pine Forests
"healthy forest" with natural hydrology. Using radiocarbon
of the North Carolina
dating of soil organic carbon (SOC), the study documents
Lower Coastal Plain:
millennial-scale carbon storage and its vulnerability to modern
Effects of Artificial
sea-level rise (SLR). Key findings include higher tree density
Drainage in a Time
but significantly greater mortality in ghost forests, leading to
of Rapid Sea Level
transitions dominated by woody shrubs. Total SOC stocks were
Rise
similar in both forest types, but ghost forests exhibited slower
peat accumulation rates that could not keep pace with SLR. The
results highlight the threat posed by accelerated SLR and
hydrological disturbances to carbon sequestration in these
ecosystems.
Forest water use efficiency (WUE), defined as the ratio of gross
primary productivity (GPP) to evapotranspiration (ET), is
critical for understanding the coupling between carbon and
Ecosystem
water cycles. This study investigates WUE in managed loblolly
Productivity and
pine plantations of varying ages in North Carolina, utilizing
Evapotranspiration long-term data. Annual GPP, ET, and WUE increased with
Are Tightly Coupled stand age until stabilization at approximately 10 years. Younger
in Loblolly Pine
plantations showed higher variability in WUE compared to
(Pinus taeda L.)
mature plantations. Seasonal trends in GPP and ET were driven
Plantations along the by radiation and air temperature. Drought enhanced GPP (19%
Coastal Plain of the in mature plantations and 11% in younger plantations) while
Southeastern U.S.
reducing ET (7% and 19%, respectively), leading to a higher
WUE (27–32%). The results demonstrate tight carbon-water
coupling in loblolly pine ecosystems and emphasize the
importance of considering trade-offs between water and carbon
cycling in forest management.
Evapotranspiration (ET) is a critical link among water, energy,
and carbon balances, significantly influenced by climate and
land-use changes in the southeastern U.S. This study
synthesizes 162 site-years of eddy covariance data across 24
AmeriFlux sites, covering six ecosystems (croplands,
grasslands, savannas, wetlands, evergreen needleleaf forests,
Energy Availability
and deciduous broadleaf forests). The study assessed daily,
and Leaf Area
seasonal, and annual ET variability, climatic and biological
Dominate Control of
controls, and the predictive power of machine learning-based
Ecosystem
ET models. Results reveal that energy availability (potential
Evapotranspiration in
evapotranspiration, PET) and vegetation structure (leaf area
the Southeastern U.S.
index, LAI) are the primary drivers of ET variability. Machine
learning models, including artificial neural networks (ANN),
outperformed traditional regression models in predicting ET.
The study underscores the need for land-cover-specific ET
models and offers critical insights into the ecohydrology of
humid, energy-limited ecosystems.
Coastal wetlands are globally significant carbon (C) sinks but
face challenges from climate change, sea-level rise (SLR), and
other disturbances. This study investigates carbon fluxes and
balance in a natural bottomland hardwood forest in North
Carolina over 11 years (2009–2019). Utilizing an eddy
covariance flux tower, the study measured gross primary
productivity (GPP), ecosystem respiration (RE), and net
ecosystem exchange (NEE), along with their climatic and
hydrological drivers. The site transitioned from a net carbon
The Unabated
Atmospheric Carbon
Losses in a Drowning
sink in 2009 (NEE = −368 g C m² yr⁻¹) to a persistent net
Wetland Forest of
North Carolina: A
Point of No Return?
carbon source in subsequent years, with NEE ranging from 87
to 759 g C m² yr⁻¹. Major drivers included declining
groundwater levels, SLR, prolonged hydroperiods, and tree
mortality. The findings emphasize the vulnerability of coastal
wetlands to hydrological stress and highlight the potential for
these ecosystems to reach a "point of no return" in their carbon
balance trajectory.
Forested wetlands in the southeastern U.S. are critical for
regulating hydrology and climate, but their functions are
affected by land-use changes and climatic extremes. This study
quantified the impacts of converting natural forested wetlands
Effects of Land-Use to managed loblolly pine plantations, examining decadal-scale
Change and Drought water balance and evapotranspiration (ET) from 2005 to 2018.
on Decadal
Using eddy covariance techniques, researchers compared four
Evapotranspiration sites: two young plantations, a rotation-age plantation, and a
and Water Balance of natural hardwood wetland forest. The findings revealed that
Natural and Managed rotation-age plantations exhibited the highest ET (933 ± 63
Forested Wetlands
mm), driven by canopy structure and age, while young
along the
plantations showed greater sensitivity to drought. Land-use
Southeastern US
change, such as drainage systems in managed plantations,
Lower Coastal Plain altered water dynamics by increasing drainage and lowering
ET. The study underscores the complex interplay between land
management, climate, and hydrological processes, offering
insights for managing forested wetlands under changing
environmental conditions.
Forested wetlands are critical carbon (C) sinks but face
significant pressures from land-use changes and climate
variability. This study evaluates the carbon dynamics of
managed loblolly pine plantations and natural bottomland
hardwood forests in the coastal plain of North Carolina over 13
Long-term Carbon
years (2005–2017). Using eddy covariance flux data from four
Flux and Balance in sites of varying ages and management histories, the study
Managed and Natural assessed gross primary productivity (GPP), ecosystem
Coastal Forested
respiration (RE), and net ecosystem exchange (NEE). Results
Wetlands of the
showed that young plantations were net carbon sources for the
Southeastern USA
first 5–8 years post-harvest, while rotation-age plantations were
consistent carbon sinks. Conversely, natural forests transitioned
to weak carbon sources over the study period, linked to tree
mortality and hydrological stress. These findings highlight the
contrasting carbon trajectories of managed versus natural
forests, emphasizing the need for adaptive management to
mitigate carbon losses in coastal ecosystems.
Short-rotation woody crops (SRWCs) are increasingly
recognized for their potential to provide renewable biomass
energy and improve soil health on degraded lands. This study
Root Biomass
examines the effects of planting density (10,000, 5,000, and
Distribution and Soil
2,500 trees per hectare) on root biomass distribution and soil
Physical Properties
physical properties in a nine-year-old American sycamore
of Short-Rotation
plantation in North Carolina. Results show that fine root
Coppice American
biomass was highest at the highest planting density (10,000
Sycamore (Platanus
tph), while coarse root biomass was higher at lower densities.
occidentalis L.)
High planting densities improved soil macroporosity and
Grown at Different
saturated hydraulic conductivity but reduced plant-available
Planting Densities
water. The findings indicate that SRWCs can enhance soil
structure and gas exchange, providing a sustainable approach
for managing degraded or marginal lands.
Introduction
Context and Importance:
Bottomland hardwood wetland forests are vital ecosystems
providing physical, chemical, and biological functions. These
ecosystems are increasingly vulnerable to climate change,
particularly to threats like sea-level rise (SLR), saltwater
intrusion, and prolonged inundation. Coastal wetlands globally
are experiencing rapid ecosystem transitions due to these
perturbations.
Problem Statement:
Tree mortality in these forests has drastically increased in recent
decades, leading to the emergence of “ghost forests.” Despite
some predictive models, the drivers of this transition remain
poorly understood, particularly the interplay between
hydrological, biological, and climatic factors.
Objectives and Hypotheses:
Analyze long-term tree mortality trends (2009–2019).
Examine the interaction of hydrological, biological, and
climatic variables using a structural equation model (SEM).
Investigate species-specific vulnerabilities.
The primary hypothesis posited hydrological perturbations as
the main driver of tree mortality.
Background:
The study focuses on renewable energy sources like bioenergy,
highlighting their role in reducing greenhouse gas (GHG)
emissions and supporting climate goals such as the Paris
Agreement. Woody biomass, particularly from short-rotation
woody crops (SRWCs), is considered a sustainable and lowinput bioenergy feedstock.
Problem Statement:
Most bioenergy research has focused on herbaceous crops and
perennial grasses, while woody species like American sycamore
have been underexplored, especially for marginal agricultural
lands. SRWCs offer advantages, including vigorous coppicing,
deep rooting systems, and minimal input requirements.
Research Objectives:
This study evaluates the productivity and biomass partitioning
of American sycamore (Platanus occidentalis L.) across two
rotation cycles under different planting densities, with the
hypothesis that:
Productivity increases with planting density.
High planting densities favor stem biomass partitioning.
Productivity remains stable across rotations, ensuring
sustainability.
Coastal wetlands store significant global carbon and provide
diverse ecosystem services.
These ecosystems are threatened by climate change (e.g., sealevel rise, extreme storms) and human activities (e.g., drainage,
land conversion).
The study hypothesizes that ghost forests near drainage canals
have lower carbon stocks and are more vulnerable to
hydrological stress than healthy forests.
Context:
Water use efficiency (WUE) serves as a metric for ecosystem
trade-offs between carbon assimilation and water loss.
Understanding WUE helps to model forest resilience under
climate change.
Research Focus:
This study examines GPP, ET, and WUE in young (2–8 years)
and mature (15–28 years) loblolly pine plantations, focusing on
the influence of age, seasonal dynamics, and drought effects.
Context:
ET is essential for understanding ecosystem responses to global
changes, linking water, energy, and carbon cycles. In the
southeastern U.S., ET dynamics are influenced by complex
interactions of climatic and biological drivers.
Objectives:
Assess ET variability across daily to annual scales in diverse
ecosystems.
Investigate ET responses to climatic and biological drivers (e.g.,
PET, precipitation, LAI).
Develop predictive models for ET using machine learning
approaches.
Characterize regional ET patterns using the Budyko framework.
Context:
Coastal wetlands provide critical ecosystem services, including
carbon storage and storm protection, but are increasingly
threatened by SLR, climate change, and human activities.
Objective:
To assess long-term carbon flux trends, climatic and
hydrological influences, and the role of SLR in driving
ecosystem changes, particularly the formation of "ghost
forests."
Context:
Wetland forests play a vital role in regulating ecohydrology,
water quality, and productivity. However, large-scale drainage
and land-use changes have converted many natural wetlands
into commercial forests.
Objective:
To understand how land-use changes (e.g., converting natural
forests to plantations) and climatic factors (e.g., drought) affect
the water balance and evapotranspiration of wetland forests over
decadal timescales.
Context:
Coastal wetlands store significant carbon stocks and provide
ecosystem services, but land-use changes, such as conversion to
plantations, and climate impacts (e.g., drought, rising sea levels)
are altering their carbon dynamics.
Objectives:
To compare carbon fluxes and balances in natural and managed
coastal forests, focusing on the impacts of age, management
practices, and environmental variability.
2. Methods
Study Sites:
Four sites in North Carolina:
Newly established loblolly pine (YP2–6, 2–6 years old).
Young loblolly pine (YP2–8, 2–8 years old).
Rotation-age loblolly pine (MP, 15–27 years old).
Natural bottomland hardwood forest (BHF, >100 years old).
All sites were monitored using eddy covariance flux towers.
Data Collection:
Carbon fluxes: GPP, RE, and NEE.
Environmental parameters: soil moisture, groundwater depth,
radiation, temperature.
Analysis:
Statistical models were used to evaluate age-related trends and
climatic drivers of carbon fluxes.
Context:
SRWCs are sustainable systems that offer bioenergy and
ecosystem services, including carbon sequestration and
improved soil health.
Marginal lands, characterized by poor soil quality and
waterlogging, present challenges that SRWCs can address.
Objectives:
Assess the impact of planting density on root biomass (fine and
coarse).
Evaluate soil physical properties, such as porosity, water
retention, and hydraulic conductivity, under different planting
densities.
Data and method
Study Site:
The research was conducted at the Alligator River
National Wildlife Refuge in North Carolina,
covering ~99,347 hectares. Chronic inundation,
driven by SLR, has significantly affected this
region, altering vegetation dominance and leading to
habitat transitions.
Data Collection:
1.Vegetation Surveys:
Conducted annually between 2009 and 2019.
Focused on tree mortality rates, species
composition, and density.
2.Hydrological Variables:
Measured groundwater table depth (GWT),
precipitation, evapotranspiration (ET), and potential
evapotranspiration (PET).
Data on SLR obtained from NOAA tide gauges.
3.Biological Variables:
Leaf Area Index (LAI) from satellite datasets.
4.Climatic Variables:
Air temperature and net radiation recorded using
onsite sensors.
Analysis Framework:
Structural Equation Models (SEM) were used to
evaluate causal relationships between variables.
Trends were analyzed using nonparametric Mann
Study Site:
Located in Granville County, North Carolina, the
site features degraded soils typical of marginal
agricultural lands. The climate has an annual
precipitation of ~1400 mm and mean temperatures
ranging from 7.8 to 21.6°C.
Experimental Design:
Four planting densities (1250, 2500, 5000, and
10,000 trees per hectare) were tested in a
randomized block design with three replicates.
Seedlings were planted in 2010, and rotation cycles
included:
First Rotation (FR): 2010–2014
Second Rotation (SR): 2015–2019
Minimal inputs (no fertilizers or irrigation) were
applied, and biomass was harvested via coppicing at
the end of each cycle.
Data Collection and Analysis:
Biomass productivity was estimated using
allometric equations.
Aboveground net primary productivity (ANPP) was
calculated as the difference in biomass between
consecutive years
Study Site:
Located in the Alligator River National Wildlife
Refuge, North Carolina. The site includes both ghost
and healthy forest plots at ~30 cm above sea level.
Data Collection:
Vegetation biomass and carbon content were
estimated using allometric equations.
Soil samples were collected up to 2.3 m depth for
SOC analysis.
Radiocarbon dating was applied to peat layers for
assessing long-term carbon storage and
accumulation rates.
Analysis: Statistical comparisons were made
between forest types for biomass, SOC, and species
composition.
Study Area:
Two loblolly pine plantation sites in North Carolina,
managed under similar conditions, provided the
chronosequence for the study.
Data Collection:
Eddy covariance flux towers measured CO2, H2O
fluxes, and environmental variables (e.g., radiation,
temperature, groundwater depth). Drought years
(2007–2008) were identified via soil water stress
indices.
Analysis:
WUE was calculated as the ratio of GPP to ET.
Regression models and cluster analyses assessed
environmental impacts on fluxes.
Data Sources:
Data from 24 AmeriFlux sites spanning six
ecosystem types (croplands, grasslands, savannas,
wetlands, evergreen forests, deciduous forests).
162 site-years of eddy covariance data analyzed for
ET, PET, and related variables.
Modeling Approaches:
Machine learning models (ANN, RF, GAM, MLR)
used to predict ET.
Budyko framework applied to analyze evaporative
and dryness indices (ET/P and PET/P).
Study Site:
Location: Alligator River National Wildlife Refuge,
North Carolina.
Ecosystem: >100-year-old bottomland hardwood
forest with natural drainage, impacted by
hydrological changes and SLR.
Data Collection:
Measurements: GPP, RE, NEE, groundwater table
depth (GWT), air temperature, precipitation, and
radiation using an eddy covariance flux tower.
Temporal Scope: 2009–2019.
Analysis:
Seasonal and interannual variations were analyzed.
Relationships between carbon fluxes and
environmental drivers were assessed using
generalized additive models.
Study Sites:
Four sites in North Carolina:
YP2–7: Young loblolly pine (2–7 years old).
YP2–8: Young loblolly pine (2–8 years old).
MP: Rotation-age loblolly pine (15–28 years old).
BHF: Natural bottomland hardwood forest (>100
years old).
Data Collection:
ET, precipitation, and drainage were measured using
eddy covariance systems and hydrological
monitoring.
Gross primary productivity (GPP) and other climatic
variables (e.g., radiation, soil moisture) were
analyzed.
Water Balance Equation:
Precipitation = ET + Drainage + Change in Soil
Storage (ΔS).
Study Sites:
Four sites in North Carolina:
Newly established loblolly pine (YP2–6, 2–6 years
old).
Young loblolly pine (YP2–8, 2–8 years old).
Rotation-age loblolly pine (MP, 15–27 years old).
Natural bottomland hardwood forest (BHF, >100
years old).
All sites were monitored using eddy covariance flux
towers.
Data Collection:
Carbon fluxes: GPP, RE, and NEE.
Environmental parameters: soil moisture,
groundwater depth, radiation, temperature.
Analysis:
Statistical models were used to evaluate age-related
trends and climatic drivers of carbon fluxes.
Study Site:
Location: Piedmont region of North Carolina, USA.
Characteristics: Marginal, eroded soils with high
sand content (62%) and a bulk density of 1.52 g/cm
³.
Experimental Design:
Planting densities: 10,000, 5,000, and 2,500 trees
per hectare.
Measurements: Root biomass (fine and coarse), soil
porosity, water retention, and hydraulic
conductivity.
Soil cores were analyzed for root biomass and soil
physical properties.
Statistical Analysis:
ANOVA and linear models were applied to evaluate
differences among treatments.
Result
3.1.Species Composition and Tree Density
Seven major species were identified, with Nyssa spp. (black gum) being the most
dominant (140 trees/ha).
Vulnerable species like loblolly pine had the highest mortality rates (69%).
3.2. Interannual Tree Mortality Trends
Mortality increased sharply from 1.64% in 2009 to 45.82% in 2019.
Both standing dead and downed trees contributed significantly, with downed trees
accounting for the majority of cumulative mortality.
3.3. Drivers of Tree Mortality
Hydrological Factors:
Prolonged inundation and rising groundwater tables (GWT) strongly correlated with
increased mortality.
SLR contributed indirectly by impeding drainage and increasing hydroperiods.
Biological Factors:
Declining LAI, indicating reduced photosynthesis and tree vigor, was associated with
higher mortality rates.
Climatic Factors:
Net radiation and temperature had minimal impacts compared to hydrological and
biological variables.
3.4. Structural Equation Model Findings
Hydrological variables explained 65% of the variance in tree mortality, while
biological and climatic drivers explained 37% and 10%, respectively.
Key hydrological contributors: SLR (R² = 0.83) and GWT (R² = 0.81).
Biological factors like declining LAI were significant but secondary to hydrology.
3.1. Biomass Productivity
Productivity was proportional to planting density:
FR: 10,000 tph produced 23.2 ± 0.9 Mg ha−1.
SR: 10,000 tph reached 39.1 ± 2.4 Mg ha−1, an 11% increase over FR.
Lower densities (1250 tph) yielded significantly lower biomass in both rotations.
3.2. Aboveground Net Primary Productivity (ANPP)
The highest ANPP occurred in the third year of each rotation, peaking at:
12.7 Mg ha−1 yr−1 (FR) and 11.1 Mg ha−1 yr−1 (SR) for 10,000 tph.
Productivity declined in later years due to resource competition.
3.3. Biomass Partitioning
In FR, higher densities allocated more biomass to stems (44–59%) compared to
Tree Density and Mortality:
Ghost forests had higher tree densities (687 trees/ha) but nearly 10× higher mortality
than healthy forests (265 trees/ha).
Species composition shifted from pond pine dominance in healthy forests to swamp
bay and shrubs in ghost forests.
Biomass and Carbon Stocks:
Ghost forests had higher total aboveground biomass (55.9 Mg/ha) than healthy forests
(27.9 Mg/ha) due to dead standing trees.
Aboveground carbon was also higher in ghost forests (33.98 Mg C/ha) compared to
healthy forests (24.7 Mg C/ha).
Soil Carbon and Accumulation:
SOC stocks to 2.3 m depth were comparable between forest types (~800 Mg C/ha).
Radiocarbon dating revealed peat accumulation rates of 1.11–1.13 mm/year,
insufficient to keep pace with SLR (~2.1–2.4 mm/year).
Inter-Annual and Seasonal Variations:
GPP and ET increased with stand age, peaking at 10 years before stabilizing.
Seasonal variations showed higher GPP and ET in spring-summer and lower in fallwinter.
Drought Effects:
GPP rose during drought years, while ET declined, increasing WUE by 27–32%.
Mature plantations exhibited more stable water-carbon dynamics under drought
conditions compared to younger stands.
Environmental Influences:
Radiation and temperature strongly influenced GPP and ET but had negligible effects
on WUE.
Groundwater depth had minimal impact due to shallow fluctuations.
ET Variability:
Daily ET varied significantly, with grasslands showing the lowest rates (1.36 mm/day)
and savannas the highest (2.30 mm/day).
Seasonal ET peaked in summer across all ecosystems, consuming 58–82% of
precipitation.
Climatic and Biological Controls:
PET and LAI emerged as dominant drivers of ET variability.
Precipitation had minimal influence, suggesting the southeastern U.S. is primarily
energy-limited.
Model Performance:
ANN models achieved the highest predictive accuracy (84% explained variance),
followed by RF.
Ecosystem-specific models performed better than generalized models.
Budyko Framework:
Forested ecosystems (e.g., evergreen forests) had the highest ET/P ratios, indicating
efficient water use.
Savannas displayed signs of potential water limitation with high PET/P ratios.
Carbon Flux Trends:
The site transitioned from a carbon sink in 2009 to a persistent carbon source by 2010.
GPP and RE showed clear seasonal patterns, peaking in summer and declining in
winter.
RE consistently exceeded GPP from 2010 onwards, resulting in net carbon emissions.
Hydrological and Climatic Drivers:
GWT and air temperature were the strongest predictors of GPP and RE variability.
Lower GWT levels enhanced GPP and RE but led to higher overall carbon emissions.
Tree Mortality and LAI Decline:
Mortality rates increased sharply, with LAI declining by ~20% over the study period.
Prolonged hydroperiods stressed vegetation, contributing to mortality and ecosystem
shifts.
Inter-Site ET and Water Dynamics:
Rotation-age plantations had the highest annual ET (933 ± 63 mm), followed by young
plantations and BHF.
Managed sites showed higher drainage due to ditching, with young plantations draining
122–216 mm more than mature plantations.
Stand Age and ET:
ET increased with plantation age, stabilizing at ~10 years when canopy cover matured.
Drought Sensitivity:
Severe droughts (2007–2008) reduced ET by 30–43% in young plantations but only 8–
11% in mature plantations.
Drought-induced reductions in soil water and drainage highlighted young plantations'
vulnerability.
Climatic Drivers:
Net radiation and canopy structure (LAI) were the primary drivers of ET variability.
Mature plantations showed better resilience due to deeper root systems and higher
water use efficiency.
Carbon Flux Trends:
Young plantations (YP2–6, YP2–8) were net carbon sources immediately post-harvest
(NEE = +1133 to +897 g C m² yr⁻¹).
Rotation-age plantations (MP) were consistent carbon sinks (NEE = −369 to −1131 g
C m² yr⁻¹).
Natural forests (BHF) transitioned from a carbon sink (NEE = −368 g C m² yr⁻¹) to a
weak carbon source (+87 to +759 g C m² yr⁻¹) due to tree mortality and hydrological
stress.
Stand Age Effects:
Carbon uptake (GPP) increased with stand age, peaking at rotation-age plantations.
Ecosystem respiration (RE) also increased but at a slower rate, leading to sustained net
carbon uptake in older plantations.
Environmental Drivers:
Root Biomass Distribution:
Fine Roots: 10,000 tph had significantly higher fine root biomass (316.7 g/m³)
compared to 5,000 tph (238.9 g/m³) and 2,500 tph (208.8 g/m³).
Coarse Roots: Lower planting densities had more coarse root biomass, contributing 65
–67% of the total root biomass.
Soil Water Retention:
10,000 tph exhibited lower water retention at field capacity (0.21 m³/m³) and plantavailable water compared to lower densities.
High planting densities increased soil aeration and drainage.
Porosity and Hydraulic Conductivity:
10,000 tph had higher saturated hydraulic conductivity (Ksat) due to improved
macroporosity.
Total porosity did not differ significantly among planting densities.
Discussion
Conclusion
Key Findings:
Hydrological changes, primarily
4.1. Species-Specific Vulnerabilities
driven by SLR, are the dominant
Loblolly pine was the most vulnerable, likely due to its moderate
cause of increased tree mortality
flood tolerance and susceptibility to pests like the southern pine
and the transition to ghost
beetle.
forests.
Bald cypress and tupelo exhibited better resilience to flooding but
Biological and climatic factors,
still faced significant mortality under prolonged inundation.
while less significant, contribute
4.2. Role of Hydrological Drivers
to the complexity of this process.
SLR-induced increases in GWT and hydroperiods were identified
Implications:
as the primary drivers of mortality.
Ecosystem models need to
Prolonged inundation disrupts oxygen availability for tree roots,
incorporate these drivers for
leading to stress and death.
more accurate predictions of
4.3. Biological Contributions
ghost forest formation.
Reduced LAI reflects declining canopy health and productivity,
Management strategies should
making trees more susceptible to other stressors.
prioritize flood-tolerant species
The study highlights the need to consider additional plant traits for
to enhance resilience.
understanding resilience mechanisms.
Future Directions:
4.4. Climatic Contributions
Monitor long-term salinity
While climatic factors were less significant, they may amplify stress
impacts and biogeochemical
in conjunction with hydrological and biological changes.
changes.
Temperature and radiation effects were minimal but could become
Investigate physiological
more prominent as canopies decline.
adaptations in flood-tolerant
species.
Primary Contributions:
Key Findings:
Demonstrated the potential of
High planting densities enhance productivity and favor biomass
American sycamore for lowpartitioning to stems.
input bioenergy systems on
Coppicing promotes sustainable productivity across multiple
marginal lands.
rotations by leveraging established root systems.
Confirmed productivity
Comparison with Other Species:
sustainability across two
Sycamore outperformed several other SRWC candidates like
rotations.
sweetgum and poplar, particularly on degraded soils and under low- Future Directions:
input regimes.
Further studies to evaluate longPractical Implications:
term trends in biomass
5000 tph may be the most economical density, balancing costs and productivity and disease
yields.
resistance.
Sycamore's resilience to environmental stress makes it a strong
Exploration of optimal
candidate for bioenergy production.
harvesting ages and economic
trade-offs.
Species Transition:
The dominance of swamp bay and increased shrub cover indicate
an ecosystem transition driven by hydrological stress.
Carbon Sequestration:
Although ghost forests maintain high SOC, ongoing SLR and
hydrological disruption threaten long-term carbon storage.
Hydrological Impacts:
Artificial drainage exacerbates saltwater intrusion and prolongs
inundation, increasing mortality and driving forest decline.
Ghost forests exemplify the
vulnerability of coastal wetlands
to climate and anthropogenic
stressors.
Millennial-scale carbon storage
is at risk due to accelerated SLR
exceeding peat accumulation
rates.
Effective wetland management
should prioritize mitigating
hydrological impacts and
conserving high-carbon
ecosystems.
Key Findings:
WUE stabilizes after ~10 years,
aligning with canopy and root
Coupling of GPP and ET:
system maturity.
Tight coupling reflects the interdependence of water and carbon
Drought periods decouple
fluxes in loblolly pines. Younger plantations showed higher
carbon and water fluxes, with
variability due to developing canopies and root systems.
GPP less affected than ET.
Management Implications:
Recommendations:
Stable WUE in mature stands suggests predictable productivity and Forest management should
water use, critical for designing resilient forest systems under
integrate WUE dynamics for
climate change.
optimizing carbon sequestration
and water conservation,
particularly in drought-prone
regions.
Key Findings:
ET is tightly coupled with energy availability and vegetation
structure (LAI) in the humid southeastern U.S.
Machine learning approaches provide robust tools for scaling ET
predictions across diverse ecosystems.
Management Implications:
Vegetation management, such as optimizing LAI, can influence
regional hydrology and enhance ecosystem resilience.
Future climate change may shift ecosystems toward water-limited
conditions, particularly in savannas.
Primary Contributions:
Demonstrated the energy-limited
nature of southeastern U.S.
ecosystems, where atmospheric
demand outweighs water supply
in driving ET.
Developed robust, ecosystemspecific models to inform
regional water management and
climate adaptation strategies.
Recommendations:
Incorporate additional variables
(e.g., topography, soil
properties) to improve model
accuracy.
Address data gaps for
underrepresented ecosystems and
extreme climatic conditions.
Key Findings:
SLR and hydrological changes
Hydrological Stress and Ecosystem Transition:
are critical drivers of the
Rising SLR and prolonged inundation have reduced ecosystem
transition from carbon sink to
productivity and resilience. Tree mortality and declining LAI signal source in coastal wetlands.
a transition toward a shrub-dominated ecosystem.
Without intervention, these
Carbon Losses and "Point of No Return":
ecosystems risk reaching a "point
The continuous carbon source condition suggests that the ecosystem of no return."
may be approaching a tipping point, with ghost forest formation
Recommendations:
accelerating.
Effective management strategies,
Implications for Management:
including reforestation,
Adaptive strategies, such as facilitating wetland migration and
hydrological restoration, and
introducing salt-tolerant vegetation, are essential to mitigate carbon adaptive planting, are crucial to
losses and preserve ecosystem functions.
preserving carbon storage and
ecosystem services in coastal
wetlands.
Hydrological Impacts of Land-Use Change:
Key Findings:
Land-use change and
Conversion of wetlands to plantations alters hydrology, reducing
management practices
ET in young stages and increasing drainage.
significantly influence wetland
Mature plantations partially recover hydrological functions due to
hydrology and ET, with young
canopy development.
plantations being highly sensitive
Drought Effects and Resilience:
to drought.
Mature plantations demonstrate
Young plantations exhibit significant reductions in ET and drainage
greater hydrological stability and
during drought, while mature forests maintain water balance.
carbon-water coupling.
Root system depth and soil water availability are key to mitigating
Recommendations:
drought impacts.
Future forest management
Management Implications:
should integrate climate
adaptation strategies, such as
Adaptive management should prioritize mixed-age plantations to
preserving natural wetlands and
stabilize water cycles.
diversifying plantation age
Maintaining natural wetlands alongside plantations can buffer
classes.
hydrological disruptions.
Managed vs. Natural Forests:
Managed plantations recovered carbon sink status 5–8 years after
harvest and offset post-harvest carbon losses within 8–14 years.
Natural forests were more sensitive to hydrological changes, with
increasing tree mortality and declining carbon uptake.
Implications for Management:
Adaptive strategies should address hydrological restoration and
carbon loss mitigation, particularly in natural forests.
Key Findings:
Managed plantations are
effective short-term carbon sinks
but require ongoing management
to sustain carbon sequestration.
Natural forests are at risk of
becoming net carbon sources
under increasing environmental
stressors.
Recommendations:
Protect mature forests to
maintain long-term carbon
storage.
Implement hydrological
restoration and mixed-age
management in plantations for
resilience against climate change.
Key Findings:
Planting density significantly
influences root biomass
Root Effects on Soil Properties:
distribution and soil properties.
Fine roots at high densities improved soil structure but reduced
High densities enhance drainage
water retention, favoring aeration and drainage.
and gas exchange but reduce
Coarse roots dominated in lower densities, improving soil stability
water retention.
and carbon storage.
Recommendations:
Management Implications:
Tailor planting density to siteHigh-density planting is suitable for wet, marginal sites needing
specific goals, such as improving
improved drainage.
drainage or increasing water
Lower densities may be preferred for maximizing water retention
availability.
and carbon sequestration.
Further research is needed on
root turnover and deeper soil
profiles.
年份期刊
Ciatation
Importance
Journal of Geophysical Research:
Atmospheres
124卷 19期