5.4 Plant succession and disturbance

Plant succession is the gradual change in plant communities over time. It involves the colonization of new areas by pioneer species, followed by intermediate stages, and eventually reaching a stable climax community. This process is influenced by factors like soil development, climate, and biotic interactions.

Disturbances, both natural and human-induced, can reset or alter succession. Understanding these processes helps predict ecosystem responses and inform management strategies. Succession theory has important applications in ecological restoration, invasive species management, and sustainable land use practices.

Types of plant succession

• Plant succession refers to the gradual changes in plant community composition over time, often following a disturbance or the creation of new habitat
• Succession is a key concept in understanding how plant communities develop, change, and recover from disturbances

Primary vs secondary succession

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• Primary succession occurs on newly formed or exposed substrates (bare rock, sand dunes, lava flows) where no soil or vegetation previously existed
• Secondary succession takes place in areas where soil remains and some vegetation may persist after a disturbance (abandoned agricultural fields, forest clearings, fire-affected areas)
• Primary succession is a slower process as it involves the gradual development of soil and colonization by pioneer species, while secondary succession often proceeds more rapidly due to the presence of soil and remnant vegetation

Autogenic vs allogenic succession

• Autogenic succession is driven by internal factors within the plant community, such as biotic interactions and modifications to the environment by the plants themselves (soil development, shading)
• Allogenic succession is influenced by external factors, such as changes in climate, geology, or disturbance regimes (fire frequency, flooding)
• Both autogenic and allogenic factors can interact to shape the course of succession in a given ecosystem

Stages of succession

• Succession typically proceeds through a series of stages, each characterized by distinct plant communities and environmental conditions
• The stages of succession can vary depending on the ecosystem and the type of disturbance, but generally include pioneer, intermediate, and climax stages

Pioneer species

• Pioneer species are the first plants to colonize a newly exposed or disturbed area, often characterized by fast growth, high dispersal ability, and tolerance to harsh conditions (lichens, mosses, annual plants)
• Pioneer species play a crucial role in stabilizing the substrate, initiating soil development, and facilitating the establishment of later-successional species
• Examples of pioneer species include fireweed (Chamaenerion angustifolium) in post-fire environments and beach grass (Ammophila breviligulata) on coastal dunes

Intermediate stages

• Intermediate stages of succession are characterized by a greater diversity of plant species and more complex community structure compared to the pioneer stage
• During intermediate stages, biotic interactions (competition, facilitation) become increasingly important in shaping the plant community
• Examples of intermediate stage species include shrubs (Rubus spp., Salix spp.) and fast-growing trees (Betula spp., Populus spp.)

Climax community

• The climax community represents the final, relatively stable stage of succession, characterized by long-lived, shade-tolerant species and a complex community structure
• The composition of the climax community is largely determined by the regional climate and soil conditions, and is considered to be in equilibrium with these factors
• Examples of climax species include shade-tolerant trees such as sugar maple (Acer saccharum) in eastern North American forests and redwood (Sequoia sempervirens) in coastal California

Factors influencing succession

• Several key factors influence the course and rate of succession in plant communities, including soil development, climate and microclimate, and biotic interactions
• Understanding these factors is essential for predicting successional trajectories and managing plant communities

Soil development

• Soil development is a gradual process that occurs alongside plant succession, involving the accumulation of organic matter, weathering of parent material, and changes in soil structure and nutrient availability
• Pioneer species contribute to soil development through litter accumulation, root growth, and interactions with soil microorganisms (nitrogen fixation, mycorrhizal associations)
• Soil development can influence the establishment and growth of later-successional species, which may have different nutrient and moisture requirements than pioneer species

Climate and microclimate

• Regional climate, including temperature and precipitation patterns, plays a significant role in determining the potential climax community for a given area
• Microclimate factors, such as aspect, slope, and elevation, can create local variations in temperature, moisture, and light availability, influencing the establishment and growth of different plant species
• Changes in climate over time (climate change) can alter successional trajectories and shift the composition of climax communities

Biotic interactions

• Biotic interactions, such as competition, facilitation, and herbivory, can significantly influence the course of succession
• Competition for resources (light, water, nutrients) can lead to the exclusion of some species and the dominance of others, shaping community composition
• Facilitation, where one species enhances the growth or survival of another, can promote the establishment of later-successional species (nurse plants providing shade or shelter)
• Herbivory by animals can selectively remove certain plant species, altering competitive dynamics and successional trajectories

Mechanisms of succession

• Several key mechanisms underlie the process of succession, including facilitation, inhibition, and tolerance
• These mechanisms describe the different ways in which plant species interact and influence each other during succession

Facilitation

• Facilitation occurs when one species enhances the establishment, growth, or survival of another species
• Examples of facilitation include nurse plants providing shade or shelter for seedlings, nitrogen-fixing plants increasing soil fertility, and mycorrhizal fungi improving nutrient uptake for host plants
• Facilitation can accelerate succession by promoting the establishment of later-successional species

Inhibition

• Inhibition refers to the negative effects of one species on the establishment or growth of another species, often through competition for resources or allelopathy (chemical inhibition)
• Early-successional species may inhibit the establishment of later-successional species through shading, nutrient depletion, or the release of allelopathic compounds
• Inhibition can slow down succession by preventing the establishment of certain species or maintaining the dominance of early-successional species

Tolerance

• Tolerance describes the ability of a species to persist in the presence of competitors or stressors
• Late-successional species often exhibit high tolerance to shade and resource limitation, allowing them to establish and grow beneath the canopy of earlier-successional species
• Tolerance allows for the gradual replacement of early-successional species by later-successional species as the community develops

Disturbance in plant communities

• Disturbances are events that disrupt plant communities, often removing or damaging vegetation and altering environmental conditions
• Disturbances can be natural or human-induced, and their frequency, intensity, and scale can significantly influence the structure and composition of plant communities

Natural disturbances

• Natural disturbances include fires, windstorms, floods, droughts, and pest or pathogen outbreaks
• These disturbances are often an integral part of ecosystem dynamics, maintaining diversity and creating opportunities for regeneration
• Examples include periodic wildfires in fire-adapted ecosystems (chaparral, ponderosa pine forests) and gap formation in old-growth forests due to windthrow or tree mortality

Human-induced disturbances

• Human activities can cause significant disturbances to plant communities, often differing in frequency, intensity, and scale from natural disturbances
• Examples of human-induced disturbances include deforestation, agricultural land conversion, urbanization, and the introduction of invasive species
• Human-induced disturbances can lead to the loss of native species, alteration of successional trajectories, and the creation of novel ecosystems

Effects of disturbance on succession

• Disturbances can have profound effects on the course and rate of succession, depending on their type, frequency, and intensity
• Understanding the effects of disturbance on succession is crucial for predicting ecosystem responses and informing management strategies

Resetting succession

• Severe disturbances that remove most or all of the existing vegetation can reset succession to an earlier stage, such as the pioneer stage
• Examples include volcanic eruptions, glacial retreat, and clear-cutting of forests
• Resetting succession can provide opportunities for new species to establish and alter the trajectory of community development

Altering successional pathways

• Disturbances can alter the pathway of succession by changing environmental conditions, species composition, or biotic interactions
• For example, frequent low-intensity fires in grasslands can maintain the dominance of fire-adapted grasses and prevent the establishment of woody species
• Altered successional pathways can lead to the development of alternative stable states or novel ecosystems that differ from the historical climax community

Resilience and resistance

• Resilience and resistance are important concepts in understanding how plant communities respond to disturbances and environmental change
• Resilience refers to the ability of a community to recover from disturbance, while resistance describes the ability to withstand change

Resilience to disturbance

• Resilience is the capacity of a plant community to absorb disturbance and reorganize while maintaining its essential structure, functions, and feedbacks
• Factors that contribute to resilience include species diversity, functional redundancy, and the presence of species with different life history strategies
• Resilient communities are more likely to recover from disturbances and maintain their ecological integrity over time

Resistance to change

• Resistance is the ability of a plant community to withstand change and maintain its structure and composition in the face of disturbance or environmental stress
• Factors that contribute to resistance include the presence of long-lived, stress-tolerant species, and adaptations to specific disturbance regimes (fire resistance, drought tolerance)
• Resistant communities may be less likely to undergo significant changes in response to disturbances or environmental fluctuations

Succession in different ecosystems

• The patterns and processes of succession can vary significantly among different ecosystems, depending on factors such as climate, soil conditions, and disturbance regimes
• Understanding succession in different ecosystems is important for predicting community responses to change and informing management strategies

Forest succession

• Forest succession often follows a pattern of pioneer species (shade-intolerant trees, shrubs) being gradually replaced by intermediate and late-successional species (shade-tolerant trees)
• The rate and trajectory of forest succession can be influenced by factors such as seed dispersal, gap formation, and herbivory
• Examples of forest succession include the development of temperate deciduous forests in eastern North America and the regeneration of tropical rainforests after disturbances

Grassland succession

• Grassland succession is often characterized by the replacement of annual or short-lived perennial grasses by longer-lived perennial grasses and forbs
• The course of grassland succession can be influenced by factors such as grazing, fire, and soil moisture
• Examples of grassland succession include the development of tallgrass prairies in central North America and the recovery of grasslands after agricultural abandonment

Wetland succession

• Wetland succession involves changes in hydrology, soil development, and plant community composition over time
• Wetland succession can occur through the gradual filling of water bodies with sediment and organic matter (hydrosere) or the development of peatlands through the accumulation of partially decomposed plant material
• Examples of wetland succession include the development of freshwater marshes, bogs, and swamps

Applications of succession theory

• Understanding the principles and processes of succession has important applications in fields such as ecological restoration, invasive species management, and sustainable land management
• Applying succession theory can help guide management decisions and predict ecosystem responses to disturbances and environmental change

Ecological restoration

• Ecological restoration seeks to assist the recovery of degraded, damaged, or destroyed ecosystems
• Knowledge of successional processes can inform the selection of appropriate restoration techniques, such as the planting of native pioneer species or the manipulation of disturbance regimes
• Successful restoration projects often aim to accelerate or mimic natural successional processes to promote the development of diverse, resilient communities

Invasive species management

• Invasive species can significantly alter the course of succession by outcompeting native species, modifying ecosystem processes, and creating novel ecological conditions
• Understanding the role of invasive species in succession can help guide management strategies, such as targeted removal or the introduction of native competitors
• Effective invasive species management often requires a long-term, adaptive approach that considers the successional context and potential community responses

Sustainable land management

• Sustainable land management practices aim to balance human use of ecosystems with the maintenance of ecological integrity and biodiversity
• Incorporating succession theory into land management can help predict the effects of different management practices on plant communities and guide the development of sustainable use strategies
• Examples include the use of rotational grazing in grasslands to maintain species diversity and the implementation of selective logging practices in forests to promote natural regeneration and maintain late-successional habitats