Transcript Successional processes Hypothesis: Climate influences the

Successional processes
Hypothesis: Climate influences the rate and trajectory of
succession by altering disturbance regime and the abundance of
key species.
How do plant, animal and microbial communities change through
succession and what are the consequences for ecosystem
processes?
Task S7
Characterize soil microbial community
composition among successional stages and
seasons in floodplain and upland
ecosystems.
Studying plant-microbial interactions in the cycling
of soil C and N
(The black box approach)
Soil C and N levels are determined
by the balance between organic
matter inputs and losses due to
decomposition, erosion and
leaching.
Plant inputs
– Litterfall
– Root turnover
– Exudation
Soil microbial community
Schimel et al 2006
– Decomposition
– Formation of organic matter
Microbial contributions to soil C storage
What role does microbial
community composition play in
soil C sequestration?
• Microbial growth efficiency
• Recalcitrance of microbiallyderived organic matter
How does community composition
change across successional
development?
• Substrate availability
• Substrate quality
Six et al 2006
Proposed research
Assess soil microbial composition and biomass along
floodplain and upland chronosequences using PFLA
analysis.
WHY?
In order to develop and test hypotheses about the role of
soil microbes in C cycling in forested ecosystems of
interior Alaska, we need to have empirical observations
of how community structure varies over time and space.
PLFA
• Unlike CF methods, PLFA is useful as a proxy for living
and possibly active biomass
– Phosphate group is quickly consumed upon cell death
– Not found in storage products
– Found in relatively constant proportion of the biomass
• Great structural diversity, coupled with high biological
specificity
Taxonomic groups
Fatty Acid
Microbial Group
15:0i, 17:0i, 15:0a, etc..
cy17:0, cy19:0, 18:1D11c
Gram positive bacteria
Gram negative bacteria (also
cy19:0 gm+)
Actinomycetes
10 Me18:0, 10 Me17:0, 10
Me16:0
18:2w6,9, 18:1w9c
20:4 w6
16:1 w5
18:1w8c
Fungi
Protozoan
Arbuscular mycorrhizal fungi
Methanotrophs
Experimental design
• All major stages of succession
in FP (n=5) and UP (n=3)
communities
• 3-5 replicate stands per stage
• 3 sampling periods
– May
– Mid July
– Late September
• 2 horizons
– O (integrated organic)
– A (mineral)
• 50 cores composited from
each 30m x 30m plot
• 2+ years??
Predictions
Broader patterns
• Microbial biomass ↑ along the
chronosequence.
• FP: Microbial community shifts
from bacterial-dominated to
fungal-dominated over
succession. UP: ↓ B:F.
• Potential for vertical stratification
in community structure as a
function of substrate availability
and water-filled pore space.
Seasonal patterns
• Bacterial:Fungal ↓ seasonally.
Successional processes
Hypothesis: Climate influences the rate and trajectory of
succession by altering disturbance regime and the abundance of
key species.
How do plant, animal and microbial communities change through
succession and what are the consequences for ecosystem
processes?
Task S8
Determine the direct and interactive effects of
soil resources, microclimate, and microbial
symbionts on the cumulative nitrogen fixation
through succession by alder in floodplain and
upland ecosystems.
Physiological ecology of the Alnus-Frankia-EMF
tripartite
A. tenuifolia – a key player in the N
economy of floodplain forest
ecosystems in interior AK
Persists throughout successional
development
How important are coordinated
changes in ectomycorrhizal
and Frankia associations of
alder in enabling species
persistence and N fixation
capacity throughout
succession?
Objectives
2.
Identify EMF composition and
functional traits in Alnus tenuifolia
across a 200 year floodplain
chronosequence
Characterize the ecophysiology of
host selection for EMF in response
to N and P fertilization in field plots,
and in response to controlled
partner choice experiments in the
greenhouse.
-0.5
-1.0
Leaf 15N (‰)
1.
-1.5
-2.0
-2.5
10
15
20
25
Leaf N:P Ratio
30
35
Hypothesis
Alder shifts associations with
ectomycorrhizal species based on
variation in plant demand for N and P,
combined with the availability and forms of
these nutrients in soil.
Objective 1: Describe EMF community composition and
functional traits across succession
Prediction: Successional nutrient gradients favor selection of different
fungal species across successional stages.
Task 1 - Extract DNAs from randomly subsampled EM root tips (control
plots) and identify fungal associates through PCR and sequence
analysis of the ITS region
 Seasonality of mycorrhizal development
Task 2 - Determine whether the activities of key enzymes related to
nutrient acquisition vary among fungal associates and successional
stages
 Acid phosphatase and phytase activity in single root tips using
methylumbelliferone (MU)-labelled fluorescent substrate analogues.
Objective 2: Characterize host selection of EMF in
response to N and P fertilization
Prediction: N fertilization will have the greatest effect on N-mobilizing
EMF species and enzymes in late succession, while P fertilization will
down-regulate acid phosphatase activity primarily in early succession
Task 1 - Extract and sequence DNAs from randomly subsampled EM
root tips across N and P ammended plots
Task 2 – Controlled greenhouse experiment to examine the capacities of
the dominant alder EMF species to mobilize different forms of P, organic
vs. inorganic.
Field study
3 successional stages
 Alder, balsam poplar, white spruce
3 sampling periods
 June, mid-July, early September
3 stand replications
 20m x 20m plot divided into
16 5m x 5m subplots
Results (to date)
• Overall EMF diversity appears low
– Strong core to core as well as site to site
variation
– Most sites ‘appear’ to be dominated by <6
morphotypes with several ‘rare’ morphotypes
mixed within.
• Fine root development delayed in alder
relative to other taxon