Lecture 5: Defoliation Flashcards
Disturbance
Mechanisms that limit plant biomass by causing its partial or total destruction
Types of disturbance
Fire, wind, herbivory (grazing, browsing, insect herbivory, granivory, fossorial)
Immediate removal
Mowing or grazing, where plant biomass is removed from the system
In situ disturbance
Frost or senescence, where the plants die and the biomass is removed through decomposition
Intensity
The severity of the effects, with a measurable impact
Frequency and duration
How often, and how long a disturbance lasts
Scale
The area affected by a disturbance. This can be used to rank ecological impact, and size usually is used to set management and research priorities
Herbivory
Consumes more than 50% of above ground and more than 20% of below ground plant productivity. Small mammals and invertebrates account for 10-15% of this consumption
Effects of grazers
Defoliation, trampling, and nutrient inputs. All of these alter the environment, nutrient availability, plant biomass production, and plant community richness and composition
Selective grazing
Animals tend to be selective in the vegetation they consume, impacts the composition even if grazing is small
Diet selection
Landscape -> plant community -> patch -> feeding station -> plant
Basic principles of defoliation
- disturbances affect meristem conditions
- most plants have evolved or adapted to periodic herbivory
- management strives to balance animal use with plant recovery
Ongoing plant survival
- continual synthesis and storage of ps
- maintain healthy root system
- periodic reproduction
Avoidance
Plants use physical means to deter herbivory. Can be mechanical or chemical
Spatial avoidance
Reduced access to plants, like growing in the protective understory of other plants
Temporal avoidance
Reduced exposure to herbivores, such as rapidly growing
Tolerance
Adapted to survive being defoliated. May protect growing points near ground, re sprout freely after defoliation, or have lots of root storage.
- reproduce via rhizomes or stolons
- store high CHOs in roots and stems to facilitate rapid re growth
- have lots of tillers, and elevate seed heads rapidly.
- may exhibit compensatory growth
Compensatory growth
Grazing that increases primary production over the ungrazed state. This is most likely in moist, nutrient rich areas with grazing tolerant grasses and where re growth potential is high.
Resistance to grazing: forbes
Produce many seeds, delay elevation of growing points, and chemical resistance
Resistance to grazing: shrubs
Mechanical, chemical, branches, woody growth not palatable, removal of apical meristem may stimulate axillary bud
Resistance to grazing: grasses
Fewer shoots, longer delay in elevation of apical buds, more likely to sprout from apical dome, higher ratio of vegetative to reproductive stems
Grasses and grazing, adaptations include…
Hypsodonty (thick tooth enamel), ruminating, and hooves
Compensatory Growth 2
Response will vary by season, environment, species of plant. Positive response is due to reallocation of stores, increased light and nutrients, increased carbon fixation, and delayed senescence.
This is NOT beneficial in environments that get repeatedly grazed, as it will use up all its stores regrowing just to be defoliated soon after
Growing points
Places where the cells divide and elongate. In s vegetative grass they are located near the ground, and are raised as the stem elongates
Vegetative tillers
Growing point close to the ground, tiller continues to grow.
Reproductive tillers
Growing point elevated, must grow a ne tiller from a dormant bud.
Bunch grass growing point
Elevated growing point early, more susceptible to late spring to early summer grazing
Rhizomatous grasses growing point
Growing points close to or below ground for most of the growing season. More resistant to close continuous grazing
Light defoliation effects on grasses
Reduction in leaf area temporarily slows or stops root growth. Plant respiration easily met, with new leaf growth promptly initiated. Upward mobilization of CHOs lasts briefly with prompt replacement
Moderate defoliation in grasses
Plant respiration barely met, some root death occurs beyond maintenance. Extended slowdown in new leaf development and moderate CHO depletion. Reduction in total forage production.
Severe defoliation effects in grasses
Major reduction or stoppage in growth. Widespread root death and depletion of CHO reserves. Normal phenological functions are impaired, for example root growth, flowering, and seed set.
Death defoliation effects in grasses
Unrecoverable loss of leaf area and CHO reserves. This varies with plant species, herbivores, growing conditions, and other disturbances.
Plant recovery after defoliation
-endogenous (plant-based) factors
Condition of the plant, for example the leaf area and meristem, and plant growth form and species, for example genetics and the CHO cycle
Soft grasses
Resistant to growing season grazing, do not maintain forage quality when ungrazed, high regrowth potential, should be grazed during the growing season to utilize regrowth potential and maintain quality, most tame grasses
Hard grasses
Not resistant to growing season grazing, but well adapted to dormant season grazing, maintain forage quality when ungrazed, low regrowth potential, should be grazed during the dormant season to maintain vigour, no advantages to grazing during the growing season, most native grasses.
Plant recovery after defoliation, exogenous (non-plant based) factors
- Intensity of defoliation, or the proportion of leaf area and meristem removal, relative to growth stage.
- Growing conditions during recovery, such as additive water and nutrient stress. This can be exacerbated by root loss and loss of insulating litter.
- Season of defoliation
- Type of herbivore, horse and sheep have closer grazing than cows
- Method of defoliation, selectivity, hoof impact, salivation, and nutrient removal and redistribution
Grass growth cycle: major stages
Spring vegetative growth, lat spring/early summer reproductive growth, and fall/winter
-this varies by elevation, slope, and aspect
Early spring vegetative growth
Initially relies on stored carbs for growth, growing point at ground level, new leaves begin to photo synthesize, and potential for regrowth.
Spring/summer reproductive growth
Plant produces enough energy to support itself, and a period of rapid growth begins. Environmental cues signal change to reproductive growth, energy allocated to seed production and storage, growing points are elevated, and summer slow down/dormancy
Fall/winter
Enter partial dormancy, plants are never truly dormant as they respire all winter, if adequate fall moisture is available, new tillers may develop from dormant buds
