Formulation of a state and transition model


 

The formulation of a state and transition model involves identifying the vegetation states, determining which of the states are linked and describing the transitions. A workshop composed of people with expertise in a plant community is a useful means for the identification of vegetation states. Feed back from land managers and end-users can help validate and complete models designed by workshop participants.


 

Catalog of states


Vegetation type:
Blue oak (Quercus douglasii) dominated savanna and woodland. Blue oak dominant, sometimes associated with foothill pine (Pinus sabiniana), interior live oak (Q. Wislizenii) and/or valley oak (Q. lobata).

Location: lower part of the Sierra Nevada foothills and adjacent valley floor, California. This zone receives less than 600 mm of yearly precipitation on average.

State I. Annual grassland. Very few if any shrubs or trees.

State II. Savanna, 10-20% canopy closure. Understory composed almost exclusively of annual grasses.

State III. Savanna, 20-60% canopy closure. Understory composed almost exclusively of annual grasses. This is the most common hardwood vegetation state in this zone. Based on physiognomic group [28].

State IV. Savanna , 20-60% canopy. Young Quercus trees are present in the understory that is otherwise composed almost exclusively of annual grasses with few shrubs (predominately wedgeleaf ceanothus (Ceanothus cuneatus), poison oak (Toxicodendron diversilobum), or redberry (Rhamnus crocea)). This is the second most common hardwood vegetation state in this zone. Based on physiognomic group [32].

State V. Woodland, 60-100% canopy closure, few if any shrubs. Understory composed almost exclusively of annual grasses, poison oak is sometimes present but does not exceed 10% cover. This state may occur only on the more productive sites in this zone. Based on physiognomic groups [24] & [34].

State VI. Savanna, 10-20% canopy closure with some understory oaks. Understory dominated by annual grasses, with many shrubs (15-45 % cover) such as wedgeleaf ceanothus (Ceanothus cuneatus), poison oak (Toxicodendron diversilobum), redberry (Rhamnus crocea), and yerba santa (Eriodictyon Californicum). Based on physiognomic group [15].

State VII. Savanna, 20-60% canopy closure with many understory oaks. Understory dominated by annual grasses and a few shrubs (5-10% cover) such as wedgeleaf ceanothus (Ceanothus cuneatus), redberry (Rhamnus crocea), and poison oak (Toxicodendron diversilobum). Based on physiognomic group [2].

State VIII. Savannah , 20-60% canopy closure, without understory oaks. Understory dominated by annual grasses, with few shrubs (5-20% cover) such as wedgeleaf ceanothus (Ceanothus cuneatus), poison oak (Toxicodendron diversilobum), redberry (Rhamnus crocea), common manzanita (Arctostaphylos manzanita) and yerba santa (Eriodictyon Californicum). Based on physiognomic group [52].


Catalog of transitions

Transition 1. (I to II) Colonization of grassland by oak trees. This transition does not occur naturally at the time scale of interest (<100 years). In this environment, blue oak (Q. Douglasii) seedlings are seldom found further away than 30 meters from the existing tree canopy.

Planting acorns or seedlings and tending of the young trees using tested methods will achieve this transition on sites that can support trees (transition time: 30-50 years).

Transition 2. (II to I) Complete loss of existing trees without replacement. This transition is likely since natural regeneration seldom occurs at this level of canopy closure in this precipitation zone. Stump resprouting is unlikely under the following conditions: trees species is blue oak, periods of low rainfall following cutting or large tree diameter. Tree loss occurs in the following cases:

(a) Type conversion: all trees are cut and/or killed by girdling, herbicides (transition time 1 year).

(b) Tree cutting: all trees are cut and resprouting conditions are unfavorable (transition time 1-5 years).

(c) Crown fire: (although likelihood is low given the absence of shrubs and the low canopy closure) and resprouting conditions are unfavorable (transition time 1-5 years).

(d) Natural mortality: Tree mortality rate is increased during long periods of drought or following summer rains favoring fungal diseases (transition time 50 - 200 years).

Transition 3. (II to III) Increase in amount of canopy closure by increase in the number of trees.

This transition seldom occurs naturally for blue oak (Q. Douglasii) at this level of canopy closure. Advance regeneration is very rare and there is no gap effect on recruitment.

This transition is possible for Valley oak (Q. lobata) on sites that are favorable (riparian corridors or deep soils with access to the water table), if a seed source is available and browsing pressure is low (transition time 30-50 years).

Planting acorns or seedlings and tending of the young trees using tested methods will achieve this transition on sites that can support trees (transition time: 30-50 years).

Transition 4. (III to II) Loss of trees without replacement.

(a) Tree cutting: trees are thinned to below 20% to 30% canopy closure, re-sprouting conditions are unfavorable and advance regeneration is not present (transition time 1-5 years).

(b) Crown fire: (although likelihood is low given the absence of shrubs and the low canopy closure) and resprouting conditions are unfavorable (transition time 1-5 years).

(c) Natural mortality: tree mortality rate is increased during long periods of drought or following summer rains favoring fungal diseases (transition time: 50 - 200 years).

Transition 5. (III to IV) Emergence of a cohort of oak saplings in the understory.

The emergence of advance regeneration depends on:

(a) Emergence of seedlings: generally dependent on a good acorn year, low levels of acorn predation (few rodents) and above average rainfall the next season. Thinning a stand may increase acorn production.

(b) Browsing pressure is low: dependent on the density of wild browsers (deer in particular) and timing of livestock grazing (transition time 2 -10 years).

(c) Protection of seedlings: achieved by a variety of methods.

Transition 6. (IV to III) Disappearance of oak saplings from the understory.

(a) Browsing pressure is high.

(b) Lack of opening in the canopy (by mortality or thinning) will keep saplings from growing above the browse line.

Transition 7. (IV to V) Increase in amount of canopy closure by increase in number of trees.

Generally occurs only at particularly productive sites, i.e. with deeper soil, access to a water table, or a concentration of run-off.

Very low or no browsing.

Transition 8. (V to III) Loss of trees without replacement.

(a) Tree cutting: trees are thinned to below 60% canopy closure, resprouting conditions are unfavorable and advance regeneration is not present. (transition time: 1-5 years)

(b) Crown fire kills trees (likely at this level of canopy closure, although few shrubs are present to constitute a ladder fuel), resprouting conditions are unfavorable and advance regeneration is not present. (transition time: 1-5 years)

(c) Natural mortality: Tree mortality rate is increased during long periods of drought or following summer rains favoring fungal diseases (transition time: 50 - 200 years).

Transition 9. (VI to VII) Increase in densities of understory oak saplings and shrubs.

The conditions described for transition 5 will lead to this state over time. Low level of browsing or complete protection from herbivory will favor shrubs as well as tree saplings. Low intensity ground fires may favor this transition by reducing competition from annual grasses.

Transition 10. (VII to III) Disappearance of oak saplings and shrubs from understory.

Occurs under conditions similar to transition 6.   Ground fires will remove understory trees and shrubs.

Transition 11. (VII to V) Canopy closure is increased by increase in tree number. Shrub cover does not increase and may decrease as canopy closes. Woodlands with a shrubby understory are rare in this zone.

Canopy closure increases generally occur at particularly productive sites, i.e. with deeper soil, access to a water table, or concentration of run-off due to topographic position.

Transition 12. (VI to VIII) Canopy closure is increased by increase in tree number.

This transition seldom occurs naturally for blue oak at this level of canopy closure. Although some saplings are present in state VI, there is little gap effect on recruitment at this level of canopy closure (transition time 30-50+ years).

Planting acorns or seedlings and tending of the young trees using tested methods will achieve this transition on sites that can support trees (transition time: 30-50 years).

Transition 13. (VIII to VI) Loss of trees without replacement.

See transition 4.

Transition ?. (VI or VIII to I, II, III or VII). Transitions are doubtful although not impossible. There are some indications that states VI and VIII with their higher levels of shrub cover occur on sites more conducive to shrubs than the majority of sites in this low precipitation zone.


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