State 1
Reference

The Reference state consists of three dominant plant communities: The curl-leaf mountain mahogany-dominated community, the mixed shrub overstory community (1.2) which can include curl-leaf mountain mahogany, mountain big sagebrush (Artemisia tridentata ssp. vaseyana), little sagebrush (Artemisia arbuscula) and black sagebrush (Artemisia nova), and the wooded mountain mahogany community which can include Douglas-fir (Pseudotsuga menziesii), Rocky Mountain juniper (Juniperus scopulorum), and limber pine (Pinus flexilis). The understory in both communities is dominated by bluebunch wheatgrass but can include a variety of other bunch and rhizomatous grasses at a lesser canopy cover. The primary difference between the two communities is the composition and foliar cover of shrub species. Foliar cover of other shrub species increases in relation to openings not already occupied by curl-leaf mountain mahogany. Processes (both natural and anthropogenic) that result in state and community changes include fire, grazing, land use change, and the occurrence of invasive species.

Characteristics and indicators. The shift between communities in the Reference state is driven by curl-leaf mountain mahogany canopy cover. Well-established stands of mountain mahogany can be dense and prevent any future establishment of other shrub species, primarily sagebrush. Likewise, less dense canopies of mountain mahogany present less competition and create opportunities for other species to establish in their absence. Low-severity disturbances are the primary driver for decreasing the canopy cover of mountain mahogany and creating opportunities for other species to establish. Severe disturbances such as fires that are stand-replacing and destroy the seed bank could lead to a shrub community not dominated by curl-leaf mountain mahogany.

Resilience management. The Reference state is overall moderately resilient. In well-established communities, resilience towards climatic disturbances such as drought is high. Curl-leaf mountain mahogany is adapted to dry conditions and is able to take advantage of moisture funneled to cracks in the bedrock. Resilience to low-severity fire is high; however, severe stand-replacing fire, which removes any nearby seed source, can take several decades to recover from (Gucker, 2006).

Community 1.1
Mountain Mahogany and Bluebunch Wheatgrass

The Mountain Mahogany and Bluebunch Wheatgrass community is well adapted to the specific conditions found at this ecological site. This community is dominated by an overstory of curl-leaf mountain mahogany and an understory dominated by bunchgrasses, primarily bluebunch wheatgrass (Pseudoroegneria spicata). The canopy density of curl-leaf mountain mahogany of this community can vary by site but can be as dense as 40 percent. On sites where curl-leaf mountain mahogany is less dense, a mixture of other shrubs fill in the canopy gaps. These species can include mountain big sagebrush, black sagebrush (Artemisia nova), mountain snowberry (Symphoricarpos oreophilus), yellow rabbitbrush (Chrysothamnus viscidiflorus), and gooseberry (Ribes sp.). The forb population can be quite diverse and includes longleaf phlox (Phlox longifolia), spiny phlox (Phlox hoodii), granite prickly phlox (Linanthus pungens), buckwheat species (Eriogonum sp.), hawksbeard (Crepis sp.), arrowleaf balsamroot (Balsamorhiza sagittata), and stemless mock goldenweed (Stenotus acaulis). Production in this community is high and heavily weighted towards the shrub overstory, specifically, curl-leaf mountain mahogany. Production is 800 to 1,300 pounds per acre, with an average of 1,000 pounds per acre.

Resilience management. This community has low to moderate resilience. Although very resistant to drought and frequent or high-severity fires, this community has difficulty recovering when much of the overstory is eliminated. Regeneration of curl-leaf mountain mahogany occurs through seed. If stand-destroying disturbances occur, distance to the seed source can play a significant role in how long reestablishment takes. Resistance at a particular site can be partially attributed to composition and canopy cover. Sites with more bare ground and lower vegetative cover are less prone to high-severity fires. Sites with these characteristics have the oldest stands of curl-leaf mountain mahogany (Gucker, 2006).

Community 1.2
Mountain Mahogany, Black Sagebrush, and Bluebunch Wheatgrass

The Mountain Mahogany, Black Sagebrush, and Bluebunch Wheatgrass community is similar to Community 1.1, however has an increase in composition and canopy cover of plants tolerant of highly calcareous soils. This is most notable in the presence of black sagebrush (Artemisia nova) and little sagebrush (Artemisia arbuscula). Black and little sagebrush are particularly well adapted to the calcareous soil conditions that often occur at this ecological site (Fryer, 2009). Black sagebrush production in this community can reach as high as 550 pounds per acre (150 pounds per acre for Community 1.1). With the increase in black and low sagebrush canopy cover, curl-leaf mountain mahogany canopy cover decreases, staying below 25 percent. Other shrubs often present at this ecological site include mountain big sagebrush, mountain snowberry, gooseberry, and yellow rabbitbrush. The understory canopy is dominated by bunchgrasses, most notably bluebunch wheatgrass. Bluebunch wheatgrass tolerates the calcareous soil conditions usually present in this community and can often thrive in the understory (Zlatnik, 1999). Other grass species often present include Indian ricegrass, Sandberg bluegrass, and Idaho fescue (Festuca idahoensis). Forb species in this community are relatively diverse but don't occupy much of the total canopy cover. The most common forbs are pink pussytoes (Antennaria rosea), tapertip hawksbeard (Crepis acuminata), Spiny phlox, and longleaf phlox. Production in this community is variable with a large range, being highly dependent on localized conditions. Production is 450 to 1,150 pounds per acre, averaging 950 pounds per acre.

Resilience management. This community has low to moderate resilience. Although very resistant to drought and frequent or high-severity fire, this community has difficulty recovering when much of the overstory is eliminated. Regeneration of curl-leaf mountain mahogany occurs through seed. If stand-destroying disturbances occur, distance to seed source can play a significant role in how long reestablishment takes. Resistance at a particular sites can be partially attributed to composition and canopy cover. Sites with more bare ground and lower vegetative cover are less prone to high-severity fires. Sites with these characteristics have the oldest stands of curl-leaf mountain mahogany (Gucker, 2006).

Community 1.3
Wooded Mountain Mahogany and Bluebunch Wheatgrass

Community 1.3 is dominated by curl-leaf mountain mahogany in the shrub overstory and bluebunch wheatgrass in the shrub understory. Unlike Communities 1.1 and 1.2, Community 1.3 is found at or near the transition to a timber-dominated canopy. This community will have Douglas-fir (Pseudotsuga menziesii), Rocky Mountain juniper (Juniperus scopulorum), limber pine (Pinus flexilis) or a combination of the three at relatively low densities in the overstory canopy. Limber pine and Rocky Mountain juniper are more prominent on sites when soils have a higher volume of calcium carbonates or are highly calcareous. Bluebunch wheatgrass remains the primary understory. Other species with lesser canopy cover include Idaho fescue (Festuca idahoensis), Indian ricegrass (Achnatherum hymenoides) , Sandberg bluegrass (Poa secunda), and needleleaf sedge (Carex duriuscula). Other shrub species, in addition to curl-leaf mountain mahogany, include mountain big sagebrush, yellow rabbitbrush, and little sagebrush. Production in this community is lower than in the other two communities in the Reference state. Production values are 700 to 950 pounds per acre, with an average 800 pounds per acre.

Resilience management. This community has low to moderate resilience. The introduction of Douglas-fir, Rocky Mountain juniper, and limber pine into the overstory increase the frequency and potential severity of fire. Additionally, because this community exists near transitions to timber dominated overstory, this increases the likelihood of encroachment and possible transition to a forested ecological site. Curl-leaf mountain mahogany are susceptible to stand destroying fires at relatively infrequent fire return intervals. Douglas-fir has a much more frequent fire return interval at seven to 20 years (Steinberg, 2002). As encroachment occurs, curl-leaf mountain mahogany are exposed to different fire regimes than those that occur within the natural range of variability.

Pathway 1.1A
Community 1.1 to 1.2

Mountain Mahogany and Bluebunch Wheatgrass

Mountain Mahogany, Black Sagebrush, and Bluebunch Wheatgrass

The primary driver for community 1.2 is localized abiotic conditions involving highly calcareous soils. These abiotic conditions are found on Rocky Hills 15 to 19 Inch Precipitation Zone ecological sites with a higher volume of calcium carbonates within the rooting depth of the soil profile. Black and little sagebrush are well adapted to conditions where soils are highly calcareous (Tilley & St. John, 2012). Black and little sagebrush replace other shrubs and grasses in the canopy that would be present in Community 1.1.

Context dependence. The abiotic conditions that result in community transitions from Community 1.1 to 1.2 are site specific and not directly influenced by anthropogenic or biotic interactions.

Pathway 1.1B
Community 1.1 to 1.3

Mountain Mahogany and Bluebunch Wheatgrass

Wooded Mountain Mahogany and Bluebunch Wheatgrass

Shifts to community 1.3 are a combination of localized abiotic conditions and altered disturbance regimes. These communities occur at or near transition zones with timber-dominated areas. Without the close proximity to a source, seed dispersion is less likely and Douglas-fir, Rocky Mountain juniper, and limber pine are less likely to establish. Historic-low severity fire at frequent fire return intervals limits the expansion of tree species into the Rocky Hills 15-19 Inch Precipitation Zone ecological site. However, suppression of fire or lack of fuels to carry fire can aid in the expansion of these forest species.

Context dependence. In addition to local abiotic conditions, fire at frequent and low severity (or lack thereof) can influence how Douglas-fir, Rocky Mountain juniper, or limber pine can establish and expand their respective range.

Pathway 1.2A
Community 1.2 to 1.3

Mountain Mahogany, Black Sagebrush, and Bluebunch Wheatgrass

Wooded Mountain Mahogany and Bluebunch Wheatgrass

Shifts to community 1.3 are a combination of localized abiotic conditions and altered disturbance regimes. These communities occur at or near transition zones with timber-dominated areas. Without the close proximity, seed dispersion is less likely and Douglas-fir, Rocky Mountain juniper, and limber pine are less likely to establish. Historic low-severity fire at frequent fire return intervals limits the expansion of tree species into this ecological site. However, suppression of fire or lack of fuels to carry fire can aid in the expansion of these forest species.

Context dependence. In addition to local abiotic conditions, fire at frequent and low severity (or lack thereof) can influence how Douglas fir, Rocky Mountain juniper, or limber pine can establish and expand their respective range.

Pathway 1.3A
Community 1.3 to 1.1

Wooded Mountain Mahogany and Bluebunch Wheatgrass

Mountain Mahogany and Bluebunch Wheatgrass

Transition from Community 1.3 to Community 1.1 can occur through low-severity fire at a site that is in the early stages of tree establishment. Fires that result in mortality for tree species while leaving other species found in the community unharmed can facilitate transition back to community 1.1. Fuels treatments that mimic historic fire regimes, removing overstory tree species can also facilitate transition to Community 1.1.

Context dependence. Fire events that facilitate transition from Community 1.3 to Community 1.1 must be severe enough that mortality occurs within the tree species (Douglas-fir, Rocky Mountain juniper, limber pine), but leaves the other dominant species unharmed (curl-leaf mountain mahogany and sagebrush species).

Pathway 1.3B
Community 1.3 to 1.2

Wooded Mountain Mahogany and Bluebunch Wheatgrass

Mountain Mahogany, Black Sagebrush, and Bluebunch Wheatgrass

Transition from Community 1.3 to Community 1.2 can occur through low severity fire at a site that is in the early stages of tree establishment. Fires that result in mortality for tree species while leaving other species found in the community unharmed can facilitate transition back to community 1.2.

Context dependence. Fire events that facilitate transition from Community 1.3 to Community 1.2 must be severe enough that mortality is induced with the tree species (Douglas-fir, Rocky Mountain juniper, limber pine) but leaves the other dominant species unharmed (curl leaf mountain mahogany, big and dwarf sagebrush species).

State 2
Grazing Resistant

When a Rocky Hills 15-19 Inch Precipitation Zone ecological site enters the Grazing Resistant state, the primary shift occurs in the understory vegetation. Grazing pressure from livestock, native ungulates, or a combination of the two drives a shift from the cool-season bunchgrasses that are present in the Reference state towards rhizomatous wheatgrasses, short-stature bunchgrasses, and ground-covering forbs. This state is generally a result of current or historic improper grazing practices that potentially included higher stocking rates and repeated heavy grazing during the growing season.

Characteristics and indicators. In this state, total canopy cover and production values for mid to large sized bunchgrasses such as bluebunch wheatgrass will decrease. These understory grasses will be relegated to the protected areas under the sagebrush canopies where they are sheltered from grazing. As they are removed from the canopy gaps, they will be replaced with grazing-resistant short-stature bunch grasses like Sandberg bluegrass (Poa secunda) and rhizomatous wheatgrasses. Openings in the canopy gaps vacated by bunchgrasses also present opportunities for an increase in disturbance-tolerant forbs such as spiny phlox (Phlox hoodii), cushion phlox (Phlox pulvinata), vetch species (Vicia ssp.), and Indian paintbrush species (Castilleja ssp.). Opportunities for invasive annual grasses also increase as competition from native grasses is removed from the canopy gaps. Depending on how well grazing-resistant rhizomatous and bunchgrasses occupy the gaps within the canopy, an increase in bare ground cover usually occurs. This can lead to increased erosion and decreased infiltration, decreasing available moisture. Because of this, productivity and canopy cover will be lower than at the Reference state.

Resilience management. Resilience in this state is lower than in the Reference state, primarily due to the decrease in available moisture and the opportunity for invasive species recruitment. The ability to recover from disturbance in rangeland systems is in part related to local climatic conditions. As available moisture increases and temperature decreases, resilience to disturbance increases (Chambers et al., 2014). Intensive grazing that results in chronic defoliation also promotes recruitment of invasive species, which lowers site resilience.

Community 2.1
Mountain Mahogany and Mixed Bunchgrass Community

Figure 17. Exclosure at Rocky Hills site - Exterior grazed

Figure 18. Grazed Rocky Hills Site

In Community 2.1, the primary overstory shrub species is curl-leaf mountain mahogany with a possible mix of other secondary shrubs such as mountain big sagebrush, black sagebrush, and little sagebrush. As a result of grazing pressure and chronic defoliation, the understory grass species shift from that of the reference communities, which are dominated by bluebunch wheatgrass, to a more even mix of bunchgrasses and Sandberg bluegrass becoming dominant. Sandberg bluegrass is a more grazing-tolerant grass species and will increase in canopy cover under more intense grazing conditions (Howard, 1997). Grazing tolerant forb species such as spiny phlox and stemless mock goldenweed (Stenotus acaulis) also increase in canopy cover when compared to the communities in the Reference state. As a result of grazing pressure, annual production in this community is significantly reduced when compared with that of the Reference communities. Production at this is 350 to 750 pounds per acre, averaging 500 pounds per acre.

Resilience management. Resilience in Community 2.1 is moderately low as a result chronic defoliation of vegetation from grazing pressure. Conversion of the understory from a bluebunch wheatgrass dominated canopy to a mixed bunchgrass canopy dominated by Sandberg bluegrass creates more opportunity for invasive species recruitment. Sandberg bluegrass is slightly less resilient to low to moderate severity fire than bluebunch wheatgrass (Howard, 1997 & Zlatnik, 1999). Resilience to erosion and invasive species establishment would be dependent on how well other bunchgrasses, rhizomatous grasses, and forbs occupy the bare ground spaces left by the removal of bluebunch wheatgrass. Rhizomatous grasses are mat forming and, if able to establish relatively quickly, can aid in reducing erosion. Additionally, if native rhizomatous grasses and forbs are able to quickly establish, this decreases the ability of invasive annual grasses such as cheatgrass to occupy bare soil openings within the site.

State 3
Disturbed

The Disturbed state in the Rocky Hills 15-19 Inch Precipitation Zone ecological site results from any disturbance that removes the overstory vegetation. The primary disturbance for this ecological site that results in the removal of overstory shrubs is frequent or severe fire. The dominant overstory species found in the Reference state such as curl-leaf mountain mahogany, black sagebrush, low sagebrush, and mountain big sagebrush experience high rates of mortality during severe fire events. In the immediate years following the event, canopy cover increases for early seral and disturbance-tolerant species such as yellow rabbitbrush (Chrysothamnus viscidiflorus), threetip sagebrush (Artemisia tripartita), and the common bunchgrass species found in most communities of the Reference state. Historic fire return intervals for curl-leaf mountain mahogany stands in the Salmon River area ranged between 13 to 22 years. Because of grazing and fire exclusion, time between fire events has increased significantly. This has resulted in fewer, but more severe stand destroying fire events (Gucker, 2006).

Characteristics and indicators. This state is distinguished by the lack of curl-leaf mountain mahogany and sagebrush overstory. This is accompanied by an increase in bunchgrass and other early seral grass species as well as disturbance-tolerant shrub species such as yellow rabbitbrush and threetip sagebrush. Canopy cover for mahogany and sagebrush can still be present, however, it is sparse, ranging between zero to eight percent canopy cover on a site.

Resilience management. Once in the Disturbed state, resilience is moderately high. The species present are disturbance tolerant or even perpetuated by frequent or continued disturbance events. Resilience is lost however, by the opportunity for invasive species establishment. Openings in the canopy caused by a disturbance can allow for recruitment of invasive species if native species don't readily occupy those spaces.

Community 3.1
Bunchgrass

This community shifts from the shrub-dominated communities of the Reference state to a community dominated by native bunchgrass and disturbance-tolerant species. The grass species tend to be similar to that of the reference communities and include bluebunch wheatgrass, Sandberg bluegrass, and Indian ricegrass. The overstory shrub community, if present, is yellow rabbitbrush and to a lesser extent, threetip sagebrush. Production for this community is no longer weighted toward the shrub functional group, but shifts towards grasses and forbs. Production values remain relatively high, ranging from 500 to 900 pounds per acre, averaging 650 pounds per acre.

Resilience management. This community is moderately resilient. Many of the bunchgrasses present are tolerant of continued low to moderate severity disturbances. Bluebunch wheatgrass is able to survive all but the most severe fire disturbances because of its protected root system (Zlatnick, 1999). Sandberg bluegrass is also tolerant of disturbance and able to quickly recover to pre-disturbance canopy cover. Both of the primary shrubs that can be present in this community (yellow rabbitbrush and threetip sagebrush) are disturbance tolerant and can be perpetuated by a frequent disturbance regime.

State 4
Invaded

The Invaded state is represented by a shift from native grasses and forbs in the understory to an understory dominated by invasive species. Transition to this state occurs after a severe disturbance that creates openings in the canopy for invasive species to establish. The primary invasive species that becomes dominant in the Invaded state is cheatgrass (Bromus tectorum). This state is especially prevalent on steep, rocky slopes where colluvial movement is active. As the soil is disturbed through movement of material downhill, ideal conditions are presented for cheatgrass recruitment. Once established and a seed source is readily available, cheatgrass is well adapted to increasing its coverage area.

Characteristics and indicators. This state is indicated by any single invasive species occupying five percent or more of the canopy, or a group of invasive species occupying 10 percent of the canopy collectively.

Resilience management. This state exhibits low resilience due to the establishment of invasive and non-native species. Once established, non-native vegetation tends to increase in canopy cover and can often be promoted by further disturbances.

Community 4.1
Invasive Species

This community may resemble other communities within other states of the ecological site; however, will have a canopy cover of five percent or greater for a single invasive species, or 10 percent or greater for all invasive species on site.

Resilience management. This community is perpetuated by continued, frequent disturbances. The invasive species that occupy the site thrive when disturbance regimes shift outside the historic range of variability. Production can be high, however may not occur in desirable species. The production range for this community is 400 to 1,000 pounds per acre, averaging 750 pounds per acre.

Transition T1A
State 1 to 2

Reference

Grazing Resistant

The transition from the Reference state to the Grazing Resistant state is a mechanism of intensive grazing practices that result in chronic defoliation of the understory vegetation. Over time, species like bluebunch wheatgrass are not able to recover from defoliation, and total canopy cover of that species will decrease on a site. In the absence of these species, grazing-tolerant plants such as Sandberg bluegrass will increase in canopy cover and production.

Constraints to recovery. Constraints to recovery include: Continued improper grazing techniques that result in chronic defoliation of vegetation; Prolonged periods of drought that diminish regeneration of desired species; Loss of nutrient rich top soil due to increased erosion; Establishment of invasive species.

Context dependence. Restoration efforts can be greatly influenced by precipitation patterns. Successful regeneration of Reference state understory species is highly dependent on moisture received post-restoration efforts. Rates of successful establishment of bluebunch wheatgrass and Sandberg bluegrass increase when available moisture is high during critical regeneration periods (Zlatnik, 1999; Howard, 1997).

Transition T1B
State 1 to 3

The transition from the Reference state to the Disturbed state is a mechanism of frequent or severe disturbances that remove the overstory shrubs on a given site. The most common disturbance that results in the loss of the shrub overstory is high-intensity fire. Historic fire return intervals for curl leaf mountain mahogany stands were relatively frequent (13 to 22 years) and events tended to be lower severity. Since 1900, fire exclusion has decreased fire return intervals and increased the likelihood of less frequent, more severe fire events that are often stand destroying (Gucker, 2006).

Constraints to recovery. Constraints to recovery include: Frequent disturbances before recovery has completed; Prolonged periods of drought that diminish regeneration of desired species; Loss of nutrient rich top soil due to increased erosion; Establishment of invasive species; Distance from seed source.

Context dependence. The primary factor driving likelihood of restoration success is post-disturbance weather patterns. Prolonged periods of drought can slow restoration processes, whereas average to above average precipitation post disturbance can greatly increase speed and success in the re-establishment of Reference state species (Steinberg, 2002; Fryer, 2009).

Transition T1C
State 1 to 4

Reference

Invaded

Transition from the Reference state to the Invaded state is usually a result of colluvial disturbances on steep slopes. Soil movement downslope that disturbs the surface creates ideal conditions for establishment of invasive species, one of the most prominent species being cheatgrass. Once locally established, cheatgrass is able to further spread across a site.

Constraints to recovery. Constraints to recovery include: Establishment speed of invasive species; Prolonged periods of drought that diminish regeneration of desired species; Loss of nutrient rich top soil due to increased erosion; Continued establishment and spread of invasive species.

Context dependence. Restoration success is dependent on invasive species composition and level of establishment. Removal of cheat grass once significant establishment occurs can be resource intensive with low success rates.

Restoration pathway R2A
State 2 to 1

Grazing Resistant

Reference

The primary mechanism for restoration from the Grazing Resistant state to the Reference state is rest from grazing. Reference state grasses such as bluebunch wheatgrass will not be able to re-establish while grazing pressure continues. Seeding of desired species may speed up the restoration process.

Context dependence. Time frame for restoration is highly dependent on variations in localized weather patterns. Prolonged periods of drought can dramatically increase time required to return to the Reference state, whereas periods of above normal precipitation can decrease that time frame. Grazing practices that are infrequent and of light intensity, mimicking natural grazing patterns will not impact the restoration pathway. However, continuous early-season grazing at moderate to high intensity resulting in significant defoliation will perpetuate the Grazing Resistant state.

Transition T2A
State 2 to 4

Grazing Resistant

Invaded

The transition from the Grazing Resistant state to the Invaded state occurs when Reference state grass species are removed through grazing which results in chronic defoliation. This leaves openings that can often be exploited by invasive species such as cheatgrass.

Constraints to recovery. Constraints to recovery include: Continued improper grazing techniques; Prolonged periods of drought that diminish regeneration of desired species; Loss of nutrient rich top soil due to increased erosion; Establishment of invasive species.

Context dependence. Restoration success is dependent on invasive species composition and level of establishment. Removal of cheat grass once significant establishment occurs can be resource intensive with low success rates.

Restoration pathway R3A
State 3 to 1

Restoration from the Disturbed state to the Reference state is a mechanism of time without stand-destroying disturbance. Once the shrub overstory has been removed, replacement seedlings will need to reach maturity without any further disturbances.

Context dependence. Time frame for restoration is highly dependent on variations in localized weather patterns. Prolonged periods of drought can dramatically increase time required to return to the Reference state, whereas periods of above normal precipitation can decrease that time frame. Distance from a seed source of representative plants from the Reference state can impact restoration timeframes in the absence of seeding or planting.

Transition T3A
State 3 to 4

The transition from the Disturbed state to the Invaded state results when a disturbance removes native vegetation cover and is replaced by non-native or invasive species. If native vegetation is not able to repopulate a site after a disturbance occurs, opportunities exist for the recruitment of invasive species in their absence. The most common invasive species to cause this transition at the Rocky Hills ecological site is cheatgrass.

Constraints to recovery. Constraints to recovery include: Grazing following a disturbance; Prolonged periods of drought that diminish regeneration of desired species; Loss of nutrient rich top soil due to increased erosion; Establishment of invasive species.

Context dependence. Restoration efforts can be greatly influenced by precipitation patterns. Successful regeneration of Reference state understory species is highly dependent on moisture received post-restoration efforts. Rates of successful establishment of bluebunch wheatgrass and Sandberg bluegrass increase when available moisture is high during critical regeneration periods (Zlatnik, 1999; Howard, 1997).