Basics
Tahoe Yellow Cress (Rorippa Subumbellata)
Tahoe Yellow Cress
Action
Program
Each Unit (number)
Tahoe Yellow Cress (Rorippa Subumbellata)
The number of population sites that are maintained as suitable habitat for sensitive plant species (as determined by a qualified expert).
This Indicator is reported in the following LT Info areas:
Accomplishments

No Subcategories for this Indicator.

Program Tahoe Yellow Cress Monitoring

Approach

Knowledge of TYC distribution has been developed through shorezone surveys since 1979. Before 2000, surveys followed a general protocol and were completed at various times during the summer. Since 2001, surveys are conducted the first week of September following a standardized protocol. During the first survey in 1979, 32 TYC sites were surveyed; this has since grown to 55 sites. A survey “site” is defined as a stretch of public beach, adjacent private parcels, or adjacent parcels under a combination of private and public ownership. Surveys include stem count estimates as a measure of TYC abundance because clonal growth makes it impossible to distinguish individuals. The amount of available shorezone habitat for TYC fluctuates widely with changes in lake level, with high lake levels leaving little habitat. On average, over 70% of surveyed sites are occupied when the lake is below 6,225 ft. in September, but less than 40% are occupied when the lake level is above 6,228 ft.

To download all of the tahoe yellow cress data on this page please see Tahoe Open Data.

Partners

Associated Programs data not provided.

View Threshold Evaluation

Tahoe Yellow Cress (Rorippa Subumbellata) is include in the Threshold Dashboard. Threshold Indicators are evaluated against Threshold Standards every 4 years. Thresholds are environmental goals and standards for the Lake Tahoe Basin that indirectly define the capacity of the Region to accommodate additional land development.

Background
Relevance

Tahoe yellow cress (TYC, Rorippa subumbellata) is a small perennial plant in the mustard family (Brassicaceae) known only from the shores of Lake Tahoe in California and Nevada. Impacts from recreation and development led to conservation concerns as early as 1974 (Smithsonian Institute 1974). In 1982, TYC was listed as endangered by the State of California and as critically endangered by the State of Nevada. Those levels of protection are the highest of any plant species in the Lake Tahoe Region. TYC is also a U.S. Forest Service Sensitive Species. The U.S. Fish and Wildlife Service (USFWS) placed TYC on the candidate list under the Endangered Species Act several times. The first was in 1980, but the USFWS removed TYC from the candidate list in 1996 after a prolonged regional drought exposed large expanses of shoreline habitat and lake-wide surveys indicated high rates of site occupancy. In 1999, after a period of sustained high lake levels in which TYC habitat was inundated and occupied sites declined, USFWS again placed TYC on the candidate list. In October 2015, the USFWS announced a “not warranted” finding and removed TYC from the federal candidate list due to the successful implementation of the Tahoe Yellow Cress Conservation Strategy (Pavlik et al. 2002, Stanton et al. 2015).

Human and Environmental Drivers

Knowledge of TYC distribution has been developed through systematic lake-wide surveys that have been completed in targeted parts of the Lake Tahoe shorezone since 1979 (Knapp 1980, CSLC 1994, Pavlik et al. 2002, Stanton et al. 2015). The primary driver of TYC distribution and abundance is the level of Lake Tahoe. The amount of available shorezone habitat for TYC fluctuates widely with changes in lake level such that large amounts of shorezone habitat are exposed at the lowest lake levels, and as Lake Tahoe rises, these areas are inundated due to the geometry of the filling Region (Pavlik et al. 2002). The natural rim of Lake Tahoe occurs at 6,223.0 feet (1,896.8 meters) and the high water line at 6,229.1 feet (1,898.6 meters) Lake Tahoe Datum (LTD). TYC has been found at elevations lower than the natural rim, but occurrences above the high water line are rare (Stanton and TYCAMWG, 2015). Although lake level is controlled in part by the operation of the dam at the outlet of Lake Tahoe in Tahoe City, California, lake level is primarily controlled by environmental factors that increase water input (tributary stream discharge and precipitation) or cause water loss (evaporation and outflow to the Truckee River) (Reuter and Miller 2000). Successive years of high lake levels have the potential to seriously reduce the presence and abundance of TYC as was observed between 1995 and 2000 when the number of occupied Tahoe yellow cress sites declined from 35 in 1993 to only eight in 1995-96, prompting concerns of imminent extinction of the species (Pavlik et al., 2002). The effect of climate change on TYC depends on how climate changes affect the level of Lake Tahoe. The climate-related scenario with the greatest threat to TYC would be a drought-induced period of sustained low lake level followed by a rapid rise in lake level which inundates TYC plants across the entire elevation range of the species (Stanton and TYCAMWG, 2015). If this occurred, species viability would depend entirely on recruitment from the seedbank and re-sprouting of submerged rootstocks after the lake receded. Recreation and land management practices on the beaches of Lake Tahoe are the primary human drivers of TYC distribution and abundance and constitute the greatest manageable threat to TYC and its habitat (Stanton and TYCAMWG, 2015). Trampling from human foot traffic and dogs may directly destroy plants, roots, and/or seeds and inhibit germination and recruitment of seedlings. Beach raking to remove debris and vegetation can directly destroy plants and decrease the amount of suitable habitat. These human-caused impacts are intensified when the level of Lake Tahoe is high (greater than 6,226 feet) and use is concentrated on smaller amounts of shoreline. Although significant development in the shorezone occurred prior to the adoption of the TRPA Regional Plan in 1987, current TRPA regulations strongly limit the types and amount of development that can occur in the shorezone of Lake Tahoe and the threat to TYC from future development of additional boat launch facilities in the shorezone is expected to remain relatively small (Stanton and TYCAMWG, 2015).

Standards

Numeric

Rorippa subumbellata – Tahoe yellow cress (26)

Indicator Status, Trend, and Confidence
View Evaluation Legend
Status: Considerably Better Than Target
Trend: Moderate Improvement
Confidence: High

Status, Trend, and Confidence Details

Status

Considerably better than the target. As of 2015, there are 50 survey sites and each has been surveyed between 10 and 28 times in the 36-year period from 1979 to 2014 (Stanton and TYCAMWG, 2015). During the survey period, the number of occupied TYC sites fluctuated inversely with the level of Lake Tahoe in September (Figure 2 of this indicator sheet). The number of occupied TYC sites declined significantly with increasing lake levels during the period (Figure 3 of this indicator sheet, Spearman’s rank correlation is -0.80, p<0.0001). The line in Figure 3 shows an average loss of nine sites for every two-foot rise in lake level (i.e., from 41 sites at 6,222 ft. to 32 sites at 6,224 ft. LTD.). With respect to TYC, the level of Lake Tahoe is characterized as low (=6,224 ft. (1,897 m) LTD), in transition (6,225 to 6,226 ft. (1,897–1,897 m) LTD), or high (6,227–6,228 ft. (1,898–1,898 m) LTD). The current dataset from 1979 to 2015 includes 28 years when more than 60 percent of the known population sites were monitored and is balanced with an equal number of years of low (11) and high lake level years (11), with six transition years. During this period the average number of occupied sites at low lake levels was 34.5, in transition years it was 26.7, and at high lake levels it was only 13.2 sites. These occupancy rates indicate that it is highly unlikely that the current threshold standard can be met at high lake levels because the greatest number of occupied sites when the lake has been high was 21 in 2006 and 23 in 2011 (it was 15 or less in all other high lake level years). However, the standard is attainable at most lake levels. Across the entire period, the average number of sites occupied is 24.4, lower than the target of 26. However, since the implementation of the conservation strategy in 2002, an average of 34 sites have been occupied by TYC, which is 131 percent of the standard. Furthermore, it was concluded in 2002 that extirpation of TYC populations had occurred three times as often as colonization during the survey period from 1979 to 2000 (Pavlik et al., 2002). The continued collection of data from lake-wide surveys has shown that the number of colonizations is now equal to or greater than extirpations and suggests the species is resilient to fluctuations in lake elevation, by either persisting or re-colonizing when conditions become favorable (Stanton and TYCAMWG, 2015). Therefore, the indicator is considerably better than the target. The location of 55 Tahoe yellow cress survey sites with land ownership are depicted on the map to the right. Nineteen sites are in Nevada and 36 are in California. Twenty-four sites are under private ownership and 31 sites have public/mixed ownership. Five of the sites are considered historical and are no longer surveyed (Stanton and TYCAMWG, 2015).

Trend

Moderate Improvement. The trend for the species in any given period of time depends on fluctuations in lake level. Over the last five years, Lake Tahoe fluctuated from a high level in 2011 to transition in 2012, and has remained low since 2013. The number of population sites occupied by TYC rose from 23 to 29 to 38 (Figure 2). The previous five-year period from 2006 through 2010 showed the same pattern with the number of occupied sites rising from 21 under the high lake level in 2006 to 42 under the low lake level in 2009. The longer term trend for TYC occupancy since the adoption of the conservation strategy in 2002 has shown rapid improvement. The occupancy rate is calculated for each year as a proportion of the number of sites that were surveyed. During the period from 1979 to 2000, on average 32.1 sites were surveyed each year and 17.8 of those sites were occupied (55 percent occupancy). From 2001 to 2014, the average number of surveyed sites climbed to 46.7 and 33.3 of those were occupied (71 percent occupancy). Since there was no change in the short term trend, but a rapid improvement in the longer term trend, the overall trend for Tahoe yellow cress is showing moderate improvement.
The number of occupied Tahoe yellow cress sites surveyed from 1979 to 2014 as a function of lake level, as measured in September (USGS Tahoe City gage 103370000). Spearman’s rank correlation is -0.80, p<0.0001. Fifty sites were surveyed. Six years have no survey data, and two years with less than 60 percent survey were excluded. Source: Stanton et al. 2015.

Confidence

Status: High. There is a high degree of confidence in the status and trend based on the longevity of the monitoring program and the quality of the data collected.

Trend:

Overall:

Methods

Implementation and Effectiveness
Programs and Actions Implemented to Improve Conditions

In response to the placement of TYC on the candidate list under Endangered Species Act in 1999, a multi-agency and private interest group task force was formed to develop and implement a conservation strategy to promote the recovery and conservation of TYC through adaptive management and cost sharing. The Conservation Strategy for Tahoe yellow cress (Pavlik et al., 2002) was finalized in 2002, and in January 2003, the 13 entities listed as monitoring partners above signed a memorandum of understanding/conservation agreement (MOU/CA), agreeing to cooperatively implement the conservation strategy on a voluntary basis. The 2003 MOU/CA expired on January 29, 2013, and a new MOU/CA was signed on June 1, 2013, by all 13 original entities (Stanton and TYCAMWG, 2015). The current MOU/CA is active for 10 years, with an expiration date of June 1, 2023. In 2012, Region executives approved a revision of the 2002 conservation strategy and the revised Conservation Strategy for Tahoe yellow cress (CS2015) was completed in October 2015 (Stanton and TYCAMWG, 2015). The AMWG continues to meet on a quarterly basis to coordinate and manage ongoing implementation of the revised strategy. The revised conservation strategy builds upon the previous strategy and represents both a synthesis and significant expansion of TYC information and includes sections on TYC ecology, threats, conservation history, management goals and actions, the stewardship program, and regulatory framework. A field research program from 2003 to 2010 increased understanding of TYC ecology and identified the optimal planting techniques, plant characteristics, habitat conditions, and logistical factors that influence restoration/mitigation success. The suite of management and restoration actions described in the revised conservation strategy provides options for avoiding, minimizing, and mitigating impacts to TYC and its habitat on public and private lands. It also recognizes the critical role of private landowners in ensuring the long-term survival of TYC, and presents the TYC Stewardship Program, which is aimed at gaining landowner participation and implementing strategies that respect private property rights. TYC management goals and objectives in the revised conservation strategy are:

Goal 1: Protect TYC plants and habitat on public lands
Goal 2: Promote stewardship, protection, and awareness of TYC on private lands
Goal 3: Manage TYC populations to promote persistence
Goal 4: Utilize key management questions to direct research that supports management and conservation
Goal 5: Continue long-term monitoring using an adaptive survey strategy
Goal 6: Utilize an adaptive management framework

The stewardship program has been operating under guidance of the AMWG as a cooperative effort of the Tahoe Lakefront Owner’s Association, the Nevada Tahoe Conservation District (NTCD), and the Nevada Division of Forestry since 2009 (Stanton and TYCAMWG, 2015). It provides lakefront landowners an opportunity to choose from a range of TYC conservation measures and create a completely customized plan for TYC on their property. Elements of a stewardship plan include a site assessment, approved conservation practices, habitat restoration measures, and monitoring. NTCD has been the primary entity engaging with private property owners. Any lakefront landowner may request a TYC site assessment from NTCD to develop a stewardship plan. Stewardship plans are voluntary and information is kept confidential. TRPA will consider stewardship plans in the permitting process for private landowners with a project that occurs in the shorezone. Information on the stewardship program and other aspects of TYC conservation and management may be found at www.tahoeyellowcress.org.

Effectiveness of Programs and Actions

The first conservation strategy in 2002 was developed with the specific intent of precluding the need to list TYC under the ESA. On October 8, 2015, the USFWS published a 12-month finding that listing TYC under the ESA was not warranted, largely based upon the lengthy track record of the MOU signatories in successful, ongoing implementation of conservation actions that are managing, avoiding, or mitigating identified impacts to TYC and its habitat (80 FR 60834). The 2013 MOU and implementation of CS2015 are intended to continue to ensure long-term conservation of TYC, such that USFWS will not have to re-evaluate the status of TYC under the ESA. Actions to downlist or remove TYC from the endangered species list have not been considered in California or Nevada, but could be pursued in the future. In 2011, NTCD completed 37 stewardship plans and outplantings of TYC on eight properties in Nevada. NTCD also expanded its Backyard Conservation Program to include TYC education and outreach. In 2013, NTCD completed 10 stewardship plans, completed plantings at four of the properties, held volunteer group plantings at four additional locations (NTCD 2013). NTCD led volunteer groups which collected seeds at the Upper Truckee Marsh enclosure and Baldwin Beach (NTCD 2013). Survival of the plantings in 2013 varied from 0 to over 60 percent.

Interim Target

None, threshold is currently in attainment.

Target Attainment Date

None, threshold is currently in attainment.

Recommendations
Analytic Approach

The number of occupied TYC sites has been assessed in the lake-wide surveys of approximately 50 sites from 1979 to 2014. The measured variables in the dataset include stem counts (Stanton and TYCAMWG, 2015) and lake level in the first week of September. The relationship between lake level and number of occupied TYC sites can be described using a monotonic function, where the number of occupied TYC sites decreases as lake level increases (Figure 2). The analysis approach is highly statistically significant and appropriate. The stem count data has not been utilized in the analysis here, but the revised conservation strategy presents analysis of the relationship between mean stem counts for a site over the survey period, populations persistence (number of years TYC was present/the number of surveyed years*100), and lake level (Stanton and TYCAMWG, 2015). For the period from 1979 to 2014, TYC sites with higher stem counts tended to be more persistent, and sites supported higher stem counts under lower lake levels when there is more habitat available. However, there is an unknown relationship between stem count and population size because of the clonal growth of TYC. Although populations with larger stem counts could be more resilient in the face of fluctuating lake levels, recreation patterns on the beaches probably dampen these relationships because TYC may be trampled under all lake levels. The revised conservation strategy ranks TYC survey sites for purposes of conservation and restoration based on a numeric formula that utilizes persistence, stem counts, and variation in stem count for the dataset (1979 to 2014) to calculate a site viability index. In 2015, 45 of the survey sites were ranked: six core, 11 high, 11 medium, 10 low, and seven ephemeral sites. The ranking categories reflect important differences in the biological character of TYC populations. Core sites have the highest conservation priority because they support relatively large, invariant, and persistent populations of TYC that play an important role in maintaining the species. All six core sites are located at the mouths of large creeks where a high degree of topographic diversity consistently provides favorable habitat conditions across a wide range of lake levels. Many of the high sites have lower recreational pressure and/or high topographic diversity and are capable of supporting large numbers of stems in some years. In contrast, most of the low sites only have habitat in low lake level years, some are very heavily used, and trampling may be an important factor in the variability of stem counts. The revised conservation strategy recommends that the AMWG continue to utilize this analytical approach to assess TYC survey sites.

Monitoring Approach

In 2010, the AMWG adopted an adaptive survey strategy that emphasizes high lake level monitoring. Surveys are now completed every year when Lake Tahoe is at or above 6,226 feet (1,897.7 meters) LTD, but only every other year at lower lake levels. This approach is adequate for assessing the numeric standard of the number of sites occupied by TYC. Goal 5 in the revised conservation strategy is to continue the adaptive survey strategy with the following objectives: 1) maintain this adaptive survey strategy; 2) continue to utilize the survey data to maintain site viability rankings; 3) develop a monitoring strategy to evaluate geomorphic beach processes, especially those at creek mouths or outflows that form berms and swales; and 4) develop a monitoring strategy to evaluate impacts to TYC plants and habitat from recreation.

Modification of the Threshold Standard or Indicator

The number of TYC occupied sites is an appropriate indicator as long as the level of Lake Tahoe is considered in the analysis. Standard review should consider inclusion of a lake level adjusted target for number of occupied sites.

Attain or Maintain Threshold

Maintaining habitat and promoting the persistence of existing TYC populations will require ongoing implementation of the TYC Conservation Strategy and the participation of TRPA and other partners in the AMWG. Successful implementation of the conservation strategy may continue to preclude the need to list TYC under the ESA and may provide grounds for changing the legal status of the species in California and Nevada. The six goals presented in the original conservation strategy were modified in the revised strategy to incorporate results from the field research program, information derived from a longer survey record, and the professional knowledge of independent researchers and the AMWG members that have been the day-to-day practitioners of TYC conservation for over 12 years. The revised goals and objectives in the revised strategy are not intended to alter the current regulatory requirements of any agency or negatively affect the protection afforded this species through existing policies and guidelines. The six goals have between two to five objectives each that provide measureable targets for the conservation and management of TYC within an adaptive management framework. Many of the 23 total objectives can be implemented within a site-specific management context and may or may not require dedicated funding, depending on the agency landowner. However, funding to implement the conservation strategy ended in 2015 and additional funding is needed to support agency participation in the adaptive management process and also to meet several specific objectives. Funding for the stewardship program ended in 2013 and additional funding is needed to implement the program and also to maintain a supply of TYC seed and container-grown TYC for plantings for population enhancement or creation. Additional funding is also needed to continue the adaptive survey strategy and develop new monitoring strategies that evaluate geomorphic beach processes and impacts from recreation. Finally, the AMWG used a key management question framework to focus the research phase of the TYC adaptive management program from 2003 to 2010 (Pavlik and O'Leary 2002). The process should be initiated again upon adoption of the revised conservation strategy to develop key management questions to address knowledge gaps for TYC decision-making over the next 10 years.

Map
Description

Tahoe Yellow Cress survey sites

Additional Figures
Historic 2011 Threshold Evaluation
View Evaluation Legend

Status Considerably Better Than Target
Trend Moderate Improvement
Confidence High