Lecture 5: Defoliation

Question 1

Disturbance

Answer 1

Mechanisms that limit plant biomass by causing its partial or total destruction

Question 2

Types of disturbance

Answer 2

Fire, wind, herbivory (grazing, browsing, insect herbivory, granivory, fossorial)

Question 3

Immediate removal

Answer 3

Mowing or grazing, where plant biomass is removed from the system

Question 4

In situ disturbance

Answer 4

Frost or senescence, where the plants die and the biomass is removed through decomposition

Question 5

Intensity

Answer 5

The severity of the effects, with a measurable impact

Question 6

Frequency and duration

Answer 6

How often, and how long a disturbance lasts

Question 7

Scale

Answer 7

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

Question 8

Herbivory

Answer 8

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

Question 9

Effects of grazers

Answer 9

Defoliation, trampling, and nutrient inputs. All of these alter the environment, nutrient availability, plant biomass production, and plant community richness and composition

Question 10

Selective grazing

Answer 10

Animals tend to be selective in the vegetation they consume, impacts the composition even if grazing is small

Question 11

Diet selection

Answer 11

Landscape -> plant community -> patch -> feeding station -> plant

Question 12

Basic principles of defoliation

Answer 12

• disturbances affect meristem conditions
• most plants have evolved or adapted to periodic herbivory
• management strives to balance animal use with plant recovery

Question 13

Ongoing plant survival

Answer 13

• continual synthesis and storage of ps
• maintain healthy root system
• periodic reproduction

Question 14

Avoidance

Answer 14

Plants use physical means to deter herbivory. Can be mechanical or chemical

Question 15

Spatial avoidance

Answer 15

Reduced access to plants, like growing in the protective understory of other plants

Question 16

Temporal avoidance

Answer 16

Reduced exposure to herbivores, such as rapidly growing

Question 17

Tolerance

Answer 17

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

Question 18

Compensatory growth

Answer 18

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.

Question 19

Resistance to grazing: forbes

Answer 19

Produce many seeds, delay elevation of growing points, and chemical resistance

Question 20

Resistance to grazing: shrubs

Answer 20

Mechanical, chemical, branches, woody growth not palatable, removal of apical meristem may stimulate axillary bud

Question 21

Resistance to grazing: grasses

Answer 21

Fewer shoots, longer delay in elevation of apical buds, more likely to sprout from apical dome, higher ratio of vegetative to reproductive stems

Question 22

Grasses and grazing, adaptations include…

Answer 22

Hypsodonty (thick tooth enamel), ruminating, and hooves

Question 23

Compensatory Growth 2

Answer 23

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

Question 24

Growing points

Answer 24

Places where the cells divide and elongate. In s vegetative grass they are located near the ground, and are raised as the stem elongates

Question 25

Vegetative tillers

Answer 25

Growing point close to the ground, tiller continues to grow.

Question 26

Reproductive tillers

Answer 26

Growing point elevated, must grow a ne tiller from a dormant bud.

Question 27

Bunch grass growing point

Answer 27

Elevated growing point early, more susceptible to late spring to early summer grazing

Question 28

Rhizomatous grasses growing point

Answer 28

Growing points close to or below ground for most of the growing season. More resistant to close continuous grazing

Question 29

Light defoliation effects on grasses

Answer 29

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

Question 30

Moderate defoliation in grasses

Answer 30

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.

Question 31

Severe defoliation effects in grasses

Answer 31

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.

Question 32

Death defoliation effects in grasses

Answer 32

Unrecoverable loss of leaf area and CHO reserves. This varies with plant species, herbivores, growing conditions, and other disturbances.

Question 33

Plant recovery after defoliation

-endogenous (plant-based) factors

Answer 33

Condition of the plant, for example the leaf area and meristem, and plant growth form and species, for example genetics and the CHO cycle

Question 34

Soft grasses

Answer 34

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

Question 35

Hard grasses

Answer 35

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.

Question 36

Plant recovery after defoliation, exogenous (non-plant based) factors

Answer 36

• 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

Question 37

Grass growth cycle: major stages

Answer 37

Spring vegetative growth, lat spring/early summer reproductive growth, and fall/winter
-this varies by elevation, slope, and aspect

Question 38

Early spring vegetative growth

Answer 38

Initially relies on stored carbs for growth, growing point at ground level, new leaves begin to photo synthesize, and potential for regrowth.

Question 39

Spring/summer reproductive growth

Answer 39

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

Question 40

Fall/winter

Answer 40

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