Aboveground interacions - day 2 Flashcards
What is a disturbance?
A disturbance is defined as a sudden event in time that
1. removes biomass,
2. disrupts ecosystems, communities or population structure and,
3. causes a rapid release or reallocation of environmental resources such as light, water or soil nutrients.
What is a disturbance characterised by?
- size - local or regional
- frequency - once every few years or multiple times in a season
- intensity - tree branch falling or large scale fire
What is the difference between monocyclic and polycyclic harvesting systems?
Polycyclic (selective) - only a limited
proportion of the stems is cut; the stand will be revisited when the remaining young small trees that were there before logging (the advanced regeneration or seedling bank) have grown sufficiently.
Monocyclic - all the standing timber is cut at once; future harvests depend on regeneration from the seed bank or from the seed rain which often depends on animals for dispersal or remnant seed tree
Why is the control of canopy size important for regeneration?
- An excessive opening –> regeneration problems especially in exposed conditions where soils dry out rapidly and nutrient loss through run-off is common.
- occurrence of herbaceous vegetation can interfere with regeneration and hinders forest recovery
- areas with abundant regrowth may attract herbivores which may damage regrowth and maintain open areas
What are the two different perceptions of heterogeneity?
- Anthropocentric - measured heterogeneity, what ecologist measure –> product of their informed perspective to assay the environment
- Phytocentric - functional heterogeneity, heterogeneity that the local entities actually perceive and respond to
What are the three environmental factors for plant growth?
- Resources: consumable substances required by plants for maintenance, growth and reproduction - nutrients, light, water
- Conditions: environmental factors that are NOT consumable but affect the efficiency of resource use - physical, chemical and biological attributes of the environment (soil pH, air temp.)
- Signals: conditions that influence plant development but not via resource use - blue light, R:FR ratio, phytochrome receptors (signal for local low light conditions)
What is the conceptual framework (6 ways) that environmental heterogeneity can be described by?
- Quantity - total cumulative or average level of a resource or factor, integrated across units of space or through periods of time –> Average or total values of most environmental factors = highly correlated with ecological factors
- Frequency distribution - Frequency distributions of factor levels within a seedling or patch across space or time
- Duration of time above thresholds - ecological processes may be significantly regulated by the duration of time or spatial proportion of seedlings or patches above critical thresholds for biological processes –> proportion of patches exposed to unusually high temperatures likely influences spatial patterns of seedling survivorship
- Timing or spatial location - The specific timing or spatial locations of threshold events may also be ecologically important (occurrence of a drought early in the growing season when first-year seedlings are small and shallow-rooted)
- Pattern - Spatial or temporal patterns within a patch or entity may also be an ecologically
important component of heterogeneity - Congruency of environmental factors - ost work on environmental heterogeneity has focused on single factors, but plant performance depends on multiple environmental factors
What are the management implications in regards to heterogeneity?
- Improve growing conditions - resources, conditions, or signals limit the performance of your target species allows to apply silvicultural treatments to remove the limitation and increase growth.
- Environmental heterogeneity increases diversity in structure and species - different establishment and growth responses of individual
plants and species + increases diversity in structure and spp –> forest with a higher conservation value. - Reducing environmental heterogeneity increases wood quantity and quality - grow timber species in monospecific stands = competition increases height growth + similar growing conditions = similar wood quality within and across trees
Why is light important?
Light is especially a limiting factor in closed vegetation types, and especially in tall and dense vegetation such as forests. Light interception is therefore key for forest plants, which has resulted in a whole array of adaptations to deal with light capture and shade
tolerance.
What are the three aspects of light?
- Light quantity - For many plants just a minor part of the radiation is available: in tropical rain forests generally only 1-2 % of the incident radiation reaches the forest floor,
due to the light interception by successive leaf layers.
2- Light quality - Dense shading by green leaves reduces the quality of light –> the R:FR declines when light is intercepted (R:FR sunlight 1.15 but in forest can range from 0.10 below shade tolerant spp to 0.76 below light demanding spp).
- Light periodicity - different scales: seasonal, daily etc (depends on the region studied), temporal and spatial scales of light variation are key factors in evaluating the range of physiological and ecological processes affected.
Sunflecks are temporal patches of direct radiation due to unobstructed sky (important for daily plant carbon gain in tropics)
What are the three cases that can happen with incident radiation on a plant?
- Reflectance - backscattering of the radiation that falls on a leaf
(average 6-10% of the incoming PAR), determined by leaf surface
properties (dense cover of leaf hairs increase reflectance - to reduce damage or heat load); - Absorptance - the radiation that is absorbed by the leaf (absorb 60-
80% of the incoming PAR), increased high leaf chlorophyll
concentrations, stack several parenchyma layers, and have a cell structure that internally backscatters the light –> increasing the path length of the light = higher chance of light absorptance - Transmittance - radiation that passes through the leaf and is not absorbed, decreases with leaf thickness –> flexible leaves transmit 10-20%, very thin leaves transmit 40%, thick leaves transmit almost no radiation at all (<3%)
What are the implications for management in regards to light?
- Lag times in the effect of silvicultural treatments - plants are continuously lagging behind a changing environmental reality, that they are mall-adapted, and there is a continuous mismatch between plants and their current environment.
What are the three major environmental gradients?
- Horizontal gradient from forest understorey to gap centre;
- Vertical gradient from forest floor to forest canopy;
- Gradient from centre of the forest patch to its edge.
Describe the heterogeneity in the horizontal gradient.
- Gradient from the understory to the gap centre - light gradient can vary from 1-2% of full light in the understory, up to 25% of full light in large gaps, short and unpredictable, as gaps are formed at different
moments and places + they close at different speeds, air and soil temp increases, mineral nutrients, leaf and stem litter vary in this gradient. - Root, bole, and crown provide different regeneration niches - Root exposes mineral soil = ideal germination bed for small-seeded species; degrading bole = germination bed at 0.5-1 m above the forest floor and also nutrients; crown zone –> increased litter input of fallen leaves = enhanced nutrient levels + branches = protection from herbivores.
- South-north gradient due to the position of the sun - N hemisphere = sun is always in the south. S part of the gap–> sunlight is blocked by canopy trees, but the N side of the
gap receives direct radiation for a longer part of the day –> More light = faster tree growth + taller trees - warmer temperatures = faster litter decomposition + higher nutrient levels in the N side of the gap. - West-east gradient due to timing of sunlight, with congruency of resources in the morning and stress in the afternoon - sun rises, the W side of gap gets direct light, temperature is still low + relative
humidity and CO2 conc are high = optimal growth conditions
vs
the afternoon the E side of the gap gets direct light, vapor pressure deficit is high = plant stress + stomatal closure + little carbon gain. == seedlings in W side twice biomass than E side.
Describe the heterogeneity of the vertical gradient.
Along the height gradient:
↑ light availability, air temperature, vapor pressure deficit, windspeed.
↓ relative humidity.
Total absorption depends on Leaf Area Index (total area of leaves (in m 2 ) above a certain ground area (in m 2)).
Difference of light level depends on the density foliage of the forest, arrangement of the leaves, inclination fo the leaves to incoming light.
CO2 concentration along the vertical gradient: Due to respiration of plant roots and soil organisms the daily means of CO2 concentrations at the forest floor
may be quite large, that is up to 1000 ppm.
