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CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration Changes
in forest soils through succession and restoration: nutrients,
organic matter, and microbial organisms (Department
of Biological Science, State University of New York – Binghamton,
Binghamton, NY 13902, USA) Abstract:
Restoring degraded ecosystems to their natural statuses is an urgent
need in human dominated earth landscape where human land use, global
climate change, global alteration of key biogeochemical cycles
(carbon and nitrogen, for example), and broad scale exotic species
invasion are threatening the integrity of various terrestrial and
aquatic ecosystems and negatively affect the ecosystem services
provided by these systems. The successful restoration of a degraded
ecosystem is a true test (acid
test) of our ecological understandings, and thus provides real
challenges and opportunities to the academic advancement.
Restoration means the re-establishment of both biological
structures, including original species composition and trophic
interactions, and the re-establishment of ecosystem functions,
including key ecosystem processes and various biogeochemical pools
comparable to the originals. An integrated, ecosystem based approach
has been gradually adopted in the practice of restoration ecology,
over more traditional species based approach (focus on target
species only) or simply habitat reclamation. Restoration practices should be
based on the thorough understanding of ecosystem development,
especially the rich literatures available in the study of
succession. Restoration processes share many similarities with
natural successions. Depending on the severity of the previous site
disturbance, ecosystem restoration can mimic either secondary
succession (where soils are more or less intact and beneficial soil
organisms mostly exist) or primary succession (where original soils
are lost and heavy human assistance is needed). A key question here
is how to fasten such naturally occurring processes in restoration
projects in which observable results are demanded in limited time
frame. Soil is the foundation of all terrestrial ecosystems and
affects every ecosystem component and process. The degradation of
soil in terrestrial landscape is common and very detrimental to the
restoration of plants, and heterotrophic organisms hereafter.
Understanding the change of soil organisms, organic matter
accumulation, and the alteration of key biogeochemical processes
thus is fundamental to the success of ecosystem restoration. Soil
restoration is crucial to plant growth, while sustained plant growth
contributes to long-term soil development. Plant-soil interactions
include plant uptake of soil nutrients, litter return of organic
matter to the soil, soil mineralization of organic matter to release
mineral nutrients, and long-term soil development. The interactions
also back the coupling of primary production and decomposition, two
most basic ecosystem processes that any earth ecosystem must have,
and upon which animal trophic structure can be built. Soil organic matter (SOM) is the
basis of all soil biological processes, in which all soil organisms
(bacteria, fungi, and soil fauna) live. SOM contributes directly to
soil structure, particle size partition, water holding capacity,
nutrient turnover, cation ion exchange capacity (CEC), and base
saturation. SOM can be very low at the early stages of restoration,
as in systems subjected to severe disturbance (landslide, mining,
etc). In such cases restoration follows more closely the trajectory
of primary succession where early accumulation of SOM is very
important. The long-term accumulation of SOM is ultimately
determined by plant primary production, but healthy plant growth is
affected strongly by initial soil fertility. Nutrient cycling is
affected by both SOM quantity and quality. Sufficient nutrient
supply is especially important at the early stages of restoration
and frequently need human assistance. The long-term buildup of soil
fertility, however, depends on the integrative relationships among
plants, soil detrivores, and soil fauna. Early colonizing plants,
especially those with N fixation symbiosis (legume species, Frankia
plants) can greatly enhance site fertility and organic matter
accumulation in otherwise degraded soils. With the initial buildup
of soil fertility and the accumulation of SOM, later restoration
stages should contain intensive internal nutrient cycling (N
mineralization for example, should increase dramatically) instead of
relying on continuous nutrient input. The buildup of microbial
biomass contributes directly to mineralization and organic matter
accumulation, while symbiotic mycorrhizal fungi enhance plant
nutrient uptake. Numerous factors, including initial plants selected
for restoration, chemistry of soil parent material, and management
choices (fertilization, irrigation, controlled fire, etc.), control
the accumulation of soil organic matter, buildup of microbial
biomass and diversity, and alteration of nutrient supplies, and thus
affect the consequences of restoration. While SOM and initial soil
fertility in some restorations may not necessary low (returning
agricultural fields to natural forests, a case of secondary
succession), the lack of comparable beneficial soil microbial
organisms to the desired plants frequently prevents the success of
these restoration projects. A diversified plant community not only
ensure a more diversified above-ground insect and animal
communities, but also contribute to the establishment of a more
diversified below-ground microbial and fauna communities, and limits
the pathogenic and insect pests outbreak frequently noticed in
single-plant restoration sites. Real world restorations may not
always achieve the goal of full ecosystem restoration, both
structurally and functionally (and bear the different names such as
reclamation, rehabilitation, etc.). However, long term
sustainability of restored sites, after initial human assistance, is
a necessary requirement (and the goal) for every restoration
projects. To achieve the long-term sustainability of restoration
projects, strategies addressing short-term nutrient supply and
long-term soil development, and emphasizing intimate plant-soil
feedbacks are needed. Such strategies will not only ensure the
success of restoration projects, but will contribute to the
fundamental understanding of ecosystem development.
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