Lecture 12: Temporal Dynamics in Terrestrial Ecosystems

Temporal fluctuations in ecosystems

• Carbon fluxes at season and annual time scales

  • Carbon in = Gross primary production (GPP) aka photosynthesis

  • Carbon out = respiration (plants, animals, microbes) 

  • Net ecosystem production = GPP - Respiration 

  • (+) NEP : Carbon is accumulating 

• Seasonal dynamics in terrestrial ecosystems

  • Respiration peaks after GPP because it relies on the organic matter produced during photosynthesis for energy 

  • Slight delay because plants and microbes need to metabolize the stored energy after it has been made 

    • Soil takes longer to warm than air in spring

    • Microbes require labile carbon inputs to become active 

  • NEP is (+) in spring; (-) in fall 

    • Carbon does not accumulate in fall since leaves are falling and trees are barren 

• Climate drives global scale variation in species composition but local variation in species composition is driven by disturbances and how the ecosystem recovers 

Disturbance 

• Definitions:

  • Terrestrial: Discrete event that removes organic matter (plant biomass/soil)

  • Marine: Event that alters the structure of populations, communities, and ecosystems; causing changes in resource availability or physical environment  

• Disturbance regime:

Succession: directional change in ecosystem structure and functioning after disturbance (characterized by species replacement over time) 

• Primary succession: starting from parent material; following major disturbance; colonization is far from seed source 

  • Mining and wars; glaciers and volcanoes 

• Secondary succession: starting from soil + seed bank (plants can re-generate); disturbance less severe 

  • Examples: fire, agricultural clearing, pest outbreaks 

• Steady state: no directional changes 

  • Herbivores consuming biomass is not succession since no change is occurring 

• Seed size in relation to dispersion:

  • Small seeds easier to disperse and can travel further compared to larger seeds 

  • Primary succession has smaller seeds so they can reach and establish in barren lands and there are larger quantities so more chance to grow (smaller seeds = faster growth)

  • Late successional species have large seeds and are slow growing 

    • What are late successional species? - plant or animal species that dominate an ecosystem once it reaches a stable state, typically characterized by slow growth, long lifespans, and adaptations to low resource availability, appearing at the final stage of ecological succession after early successional species have modified the environment

  • Secondary succession has variety of seed sizes because they have a seed bank = faster growth rates too 

  • How would you describe the trade-off between growth rate and stress tolerance? 

    • The slower the plant grows, the more stress it is able to take on but the faster it grows, the less stress it can handle

    • In relation to primary and secondary colonizers, primary colonizers would be slow growers and high stress tolerances and secondary colonizers would have lower stress tolerances but grow faster since they already have the foundational soils 

  • Which life history traits would you expect in species dominating late in succession (as compared to early colonizers)?

    • Slow growing, large seed size; able to live a long time because in steady state 

• What drives species replacement and ecosystem change over time? 

  • Primary succession: 

    • Patterns of species abundances and soil properties where ecosystems have different amounts of time to develop 

    • Classical view: species replacement driven by facilitation (when the presence of one species benefits another) 

      • Domino effect in Glacier Bay Alaska (rapid glacial retreat in 200 years)

      • mosses and lichen add organic matter to make basic soil -> small seeds dispersed and colonize -> grows plants that fix N -> plant litter acidifies soil to favor establishment of spruce trees -> slow decomposition, organic carbon increases CEC of soil 

      • Is competition inhibiting spruce from dominating earlier? (is it slowing down succession?)

        • Experimented: treated spruce seedlings with 2 different conditions

        • 1. Planted alone with no vegetation; 2. Planted to compete with vegetation

        • Results: facilitation by neighbors common early in succession and competition was common later on 

    • Species replacement over time are driven by variation in traits of the dominant species (in the beginning, small seeds dominate and larger seeds will dominate later on since they grow slow but live longer); also driven by species interactions (in the beginning, facilitation enhances the growth, but later on as things take hold, competition slows down succession)

  • Life history parameters: trade-off between establishment and longevity through succession 

    • Early colonizers produce many seeds but they are short lived compared to later colonizers who produce less seeds but since they live a long time, they dominate later

    • Early colonizing, fast growing plants are more appealing to herbivory and since they prefer these plants, they speed up succession by removing them quicker = speed nutrient cycling 

  • Which describes traits of early vs late dominant species in primary succession on floodplains?

    • Early successional species produce many seeds with short life spans that herbivores enjoy  while the late dominant species produce very few seeds that grow to dominate later on 

  • Secondary succession: 

    • Since the disturbance is not as intense, it still has the soil and seed bank so there are less transitions 

  • What factors explain the characteristic changes of plant communities over the course of succession?

    • 1. Facilitation: helps to enhance the early stages by introducing organic matter into soil so plants can grow and nitrogen fixation can occur 

    • 2. Life-history trade-offs: small seeds = faster growing, short life spans but they dominate the early stages; later on, larger seeds will dominate since they are longer lived 

    • 3. Herbivory speeds up succession by removing the fast growing seeds to allow the slow growing to dominate sooner 

• Patterns of carbon and nutrient cycling: 

  • NPP reflect succession:

    • Forest productivity will peak mid-succession due to species replacement and N inputs 

    • Later on, NPP declines because slower growing species will dominate (have longer life spans); less nutrients because it will be bound to soil organic matter, humus.

    • Stand age: total pool of plant biomass will continue to slow increase, even as carbon flux through NPP declines 

  • Plant and soil carbon pools increase over succession; any disturbances would decrease plant carbon pool and soil pool 

    • Over time, it will return to the steady state prior to the disturbance 

  • Carbon fluxes will also vary based on when the disturbance occurred   

    • NPP is max mid succession; decomposition lags C inputs; NEP is balance between NPP and decomposition 

    • In secondary succession, NEP is negative because there is more decomposition due to the remaining organic matter 

  • How do carbon pools change following disturbance through the course of primary succession?

    • They both increase then eventually level off 

  • How would net ecosystem carbon flux (NEP) change as a forest community undergoes early to late primary succession, progressing to a steady state?

    • Early on, there would be a positive NEP value as plants are taking hold and carbon inputs increase, then as it progresses, these plants with short life spans will decompose and NEP would generally decline until it reaches 0 which is when it is at a steady state 

  • As slower growing species dominate later in succession, how are decomposition and nutrient recycling rates likely to change?

    • It decreases and slows down since they have longer life spans 

• Cation nutrient retention: accumulation of soil carbon and rising CEC over time

  • As carbon in SOM accumulates, soil develop CEC that allows the retention of nutrients

  • Nutrient recycling is greater in secondary succession because there is already soil that contains SOM unlike primary succession where you are starting from parent material 

    • After disturbances, the soil allows for a supply of nutrients but the plant must be able to maintain the retention of the nutrients

      • Plant uptake (low early in succession0

      • Microbial uptake

      • Chemical fixation (CEC)

        • If not employed, nutrients will be lost to leaching

• Water/energy balance: 

  • After disturbance = no vegetation = more runoff and less transpiration because less plants 

  • Over time, runoff will decrease as ecosystem heals from disturbance 

  • Energy balance:

    • Successional changes in forest after fire

      • Albedo would decrease because everything is burnt = darker land = absorption of light

      • It will increase mid succession as plants grow back

  • What peaks in mid succession? : carbon fluxes (NPP, GPP, respiration) and albedo