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INSECT DEFOLIATION AND NITROGEN CYCLING IN FORESTS
Lovett, et al., 2002
Presented by: Chelsea Krieg
OUTLINE
• Introduction: Defoliation
• Shift in Foliar N Budget due to Defoliation
• Invertebrate N Processing Efficiency
• Frass: N Immobilization Agent
• Fate of Immobilized N
• N Export
• Factors that May Affect N Loss
• Summary
INTRODUCTION
Defoliation can cause the:
Direct Impact
• Decrease in transpiration and tree growth
• Changes allocation of C within the tree
• Decrease in seed production
• May lead to increase chemical defenses against insects
Indirect Impact
• Increase in tree mortality, light penetration to the forest floor, and water drainage
• Shifts in tree species composition within the population
• Changes in population size of insect predators
• N export through drainage network (leaching of soil-available N) and acidification down network
• Not really the case here- find tight internal cycling shift, but little loss
Defoliation differs from other disturbances generating N loss through drainage Network:
1. Trees can usually survive initial damage, so their woody tissue and structure remains intact
2. Soil minimally disturbed, so rates of erosion are not heavily altered
3. If trees are not killed, recovery is possible within time span of weeks, instead of years
Argument: Defoliation disturbance leads to N redistribution rather than N loss
INTRODUCTION
Normal N-Cycle in Hardwoods:
1. N withdrawn from tree N-stores are utilized to make
leaves
2. Leaves take part in photosynthesis, normal root
uptake
3. Change of season, tree resorbs bulk of N before
abscission of leaves
4. Any remaining N required by the tree is retrieved by
roots
Happy healthy trees
In Defoliation, N retrieval from leaves is
interrupted and tree must rely on root N uptake
SHIFT IN FOLIAR N BUDGET
In Defoliation, foliar N is diverted to insect biomass, frass
(insect excrement), and green leaf fall, instead of remaining in
tight internal tree N cycle. The tree is not able to resorb N
and must rely on root N uptake.
Gypsy Moth
LEPIDOPTERAN N EFFICIENCY
• Even with alkaline midgut Gypsy Moths (Lymentria dispar L)
produces acidified frass
• Instar nymphs likely experience ammonia volatilization in hindgut, but
effectively prevent N gas loss through ion pumping
• Loss 84% N consumed through frass (insect feces)
• Also, sloppy eaters
FRASS: IMMOBILIZATION AGENT
Labile C in Frass Increase in Microbial Growth leading to Immobilization of N,
Decrease in Mineralization (less Inorganic N available for vegetation)
Other examples:
• Elm spanworm frass immobilized inorganic N when incubated in stream water
• In NC experiment, frass additions had no effect on NO3- and PO43- availability
• Aphid honeydew suppressed N mineralization rates through similar mechanism
Gypsy moths defoliate in early summer (June-July)
• Trees need following months to adjust for defoliation damage,
so low N availability (potentially caused by immobilization) can hinder tree recovery
Litter Plot
• Most 15N stayed in undecomposed
litter, small amount of 15N mobilized
into soil
• Retained less 15N than frass plot, but
what was retained remained in surface
soils potentially being more available
to plants
FATE OF IMMOBILIZED N
<0.01% of applied 15N was leached from the systems.
Defoliation disrupts the normal soil-plant N system, but each N cycle is tightly conserved by employing very different mechanisms.
Frass Plot
• Frass dissolved quickly and moved
to all of the measured soil profile
• 99% of 15N recovered in soil
• 1% of soil N was readily available
(inorg N, microbial, or
mineralized)
N is retained in the litter and is released slowly during litter
decomposition
N moves to the subsoil and its
readily retained
N EXPORT
Other studies have shown N exports after defoliation
N export not from foliar-frass cycling, but due to:
• atmospheric deposit rates
• N mobilized by defoliation
Coweeta and the Cankerworm
• Increase in stream water nitrate, but still very low (<0.5kg/ha-1/yr-1)
• <2% insect-generated N deposition lost via leaching
Hubbard Brook and Heterocampa
• Defoliation has “no effect” on
stream N export
• ~20% of watershed was
defoliated
Virginia and Lymentria
• >90% of leaf area removed
• Previously undetectable,
Nitrate exports 4kg/ha-1/yr-1!
• Annual N deposition for this
region is 8kg/ha-1/yr-1 , so
watershed experienced net
retention of N.
FACTORS THAT MAY GENERATE N LOSS
Hydrologic Bypass
• Precipitation pulse after
defoliation frass deposit can
transport frass away from
watershed and prevent
reinvestment of frass N
• Sandy soils or soils with more
channelized structure can
experience less contact with
frass N
Tree Mortality
Death of Trees:
• Reduce uptake of N mobilized
from frass
• Reduce sink large sink for pool
of N recycled from decomposing
OM annually
• Contribute another large source
of N from decaying roots
Low N retention in Soils
• Factors producing low SOM
(Thin soils, etc) will affect the
retention of frass N
• N retention also affected by N
saturation and N need
SUMMARY
1. Defoliation leads to N redistribution rather than N loss
2. In the case of Gypsy Moths, the ecosystem loses very little N by volatilization from the insects themselves, but frass becomes an important pathway for internal N-cycling.
3. N delivered to the forest floor by frass pellet trigger rapid immobilization through microbial growth and experience long term retention in soil organic matter (SOM)
4. Trees surviving defoliation are capable of N uptake through their roots, but may have to struggle with low N availability
5. Defoliation disrupts the normal soil-plant N system, but the pre and post-defoliation N cycles are tightly conserved by employing very different mechanisms.
6. N export after defoliation can still occur after defoliation
QUESTIONS
• If it weren’t as interdisciplinary, would we have had the same level of insight?
Were there any disciplines that could have been included?
• What limitations did the plot experiment have?
• If there were a large storm that knocked down trees during a defoliation
period, how do you think that would affect immobilization and mineralization?