Vegetation Line Point Intercept Data, Owens Valley, CA USA 1984-2013

Each year, the Inyo County Water Department monitors vegetation conditions on the floor of the Owens Valley. The primary goal of this monitoring, according to the technical appendix to the Long-Term Water Agreement (Green Book) are to detect any “significant decreases and changes in Owens Valley vegetation from conditions documented in 1984 to 1987”. Vegetation live cover and species composition documented during the 1984-87 mapping effort were adopted as the baseline for comparison with each annual reinventory according to the Agreement. The reference measurements collected within individual areas mapped with similar vegetation (parcels) are referred to as baseline. The Green Book details certain decreases and changes in vegetation community types that must be avoided under the Agreement. Baseline vegetation communities in which evapotranspiration exceeds precipitation were classified as groundwater-dependent communities and are referred to as Types B, C, and D. These phreatophytic communities are dependent on shallow groundwater to maintain plant populations, as precipitation alone is inadequate to meet the water demand of evaporation and transpiration (Sorensen et al. 1991, Steinwand et al. 2006). For these parcels, according to the Green Book, “the goal is to manage groundwater pumping and surface water management practices so as to avoid causing significant decreases in live vegetation cover” and to prevent a significant amount of vegetation from changing to a “vegetation type that precedes it alphabetically (for example, Type D changing to either C, B, or A vegetation).” The goal is to manage the effect of pumping on the depth to groundwater to maintain baseline plant communities. To determine whether significant decreases and/or changes in vegetation have occurred, three criteria need to be met that are described in the Green Book: (1) measurability of vegetation change, (2) attributability of vegetation change to LADWP groundwater pumping or surface water management and (3) degree of significance defined by the magnitude, extent, duration and permanency of the change along with other factors including air quality, human health, impact to species of concern, etc. To avoid confusion, it is noteworthy to highlight that the standard dictionary definition of “measurability” is the degree to which something can be measured. Vegetation of course can be measured and thus the first criteria for determining significant decreases or changes in vegetation may seem to the reader unfamiliar with the Green Book, unnecessary. However, the Green Book assigns a different definition to the term “measurability”—defined in part as a change that is statistically significant. Thus the first criteria, measurability, can be used interchangeably with statistical significance in the context of the technical appendix (Green Book) to the Agreement. A main objective of the vegetation annual report is to evaluate the statistical significance (measurability) of vegetation change compared to baseline. The second criterion, evaluating whether a statistically significant change in vegetation is caused by water management (attributability), is beyond the scope of this report owing to the need for a comprehensive analysis on a case by case basis for each vegetation parcel. Another source of confusion may arise with the third criterion which is the “degree or significance” of environmental change. For this criterion to be met, statistical significance is necessary but not sufficient. As described above, there are several other factors in addition to statistical significance that must be demonstrated to evaluate the degree of significance for the third criterion. For an example of a comprehensive evaluation of all three criteria for an individual parcel, see the report “Analysis of Conditions in Vegetation Parcel Blackrock 94” (available at www.inyowater.org). A large proportion of groundwater-dependent parcels were mapped during baseline as Type C alkali meadows (61%), and the Agreement seeks to prevent these meadows from changing to shrub-dominated communities (Type B), a state change that is associated with increased depth to groundwater. Alkali meadows are of special concern because small increases in depth to groundwater can decouple the groundwater from the root zone of grass species (Naumberg et al. 1996, Elmore et al. 2006). Alkali meadow comprises 0.1% of the vegetation community types in California and 80% of alkali meadow communities are located within the Owens Valley (Davis et al. 1998). Local management of this community type will determine the likelihood these ecosystems persist in a changing environment. Vegetation change across the Owens Valley was evaluated at both the valley scale and for each of 111 individual parcels sampled in 2013. First, at the valley-wide scale we evaluated plant community cover and composition in parcels affected by groundwater pumping and for parcels that were relatively unaffected by groundwater pumping during the period of maximum pumping rate (1987-1993). Second, we assessed whether transpiring vegetative cover differed over time for locations influenced by pumping compared to locations not influenced by groundwater pumping. Third, we quantitatively assessed the divergence of these groups of parcels from the baseline cover values recorded from 1984 to 1987. Fourth, we assessed whether vegetation composition in wellfield or control groups had changed from baseline values. Lastly, since determination of significant impacts attributed to water management by LADWP is made on a case by case basis at the individual parcel scale, we (a) quantified the magnitude of change in perennial vegetation cover over the twenty-three year reinventory period, (b) assessed whether the relative proportion of woody vegetation (hereafter shrub), gramminoid vegetation (hereafter grass) and non-gramminoid herbaceous vegetation (hereafter herb) has changed compared to baseline and (c), quantified the temporal trends of vegetation composition for each parcel and evaluated what environmental factors explained the observed patterns in vegetation change.

每年，因约县水务局（Inyo County Water Department）都会对欧文斯谷（Owens Valley）的地面植被状况开展监测。根据《长期水资源协议技术附录》（即《绿皮书》，Long-Term Water Agreement (Green Book)），本次监测的核心目标是侦测欧文斯谷植被相较于1984至1987年记录基准状态所出现的任何"显著减少与类型变化"。根据该协议，1984至1987年植被调查中记录的活覆被率与物种组成，被定为后续每年复勘的对比基准。在植被类型一致的单个区域（地块，parcels）内采集的参考测量值，被称为基准值。《绿皮书》详细列明了本协议要求必须规避的若干植被群落类型退化与变化情形。蒸散发量（evapotranspiration）大于降水量（precipitation）的基准植被群落，被归类为依赖地下水的群落（groundwater-dependent communities），并划分为B、C、D三类。此类潜生植物群落（phreatophytic communities）依赖浅层地下水维持植物种群存续，因为仅靠降水无法满足蒸发与蒸腾的需水要求（Sorensen等，1991；Steinwand等，2006）。针对此类地块，《绿皮书》规定："管理目标为管控地下水抽取与地表水管理措施，以避免造成活植被覆被的显著减少"，同时防止大量植被转变为"按字母顺序排在其前的植被类型（例如D类植被转变为C、B或A类植被）"。其核心目标是通过管控抽水对地下水埋深的影响，以维持基准植物群落。若要判定植被是否出现显著减少或变化，需满足《绿皮书》中规定的三项判定标准：（1）植被变化的可测性（measurability）；（2）植被变化可归因于LADWP的地下水抽取或地表水管理措施；（3）变化的显著性程度，由变化的幅度、范围、持续时长与永久性，结合空气质量、人类健康、受关注物种影响等其他因素共同界定。为避免歧义，需特别说明：标准词典中"可测性"的定义为"某事物可被测量的程度"。诚然，植被本身可被测量，因此对于不熟悉《绿皮书》的读者而言，第一项判定标准似乎并无必要。但《绿皮书》对"可测性"一词赋予了不同的定义——其部分定义为"具有统计学显著性的变化"。因此，在本协议的技术附录（即《绿皮书》）语境中，第一项标准"可测性"可与"统计学显著性"互换使用。本植被年度报告的核心目标之一，是评估植被相较于基准状态的变化是否具有统计学显著性（即可测性）。第二项标准——判定植被的统计学显著性变化是否由水资源管理措施引发（可归因性，attributability）——超出了本报告的范畴，因为这需要针对每个植被地块开展逐案的全面分析。第三项标准——环境变化的"程度或显著性"——也可能引发歧义。要满足此项标准，具备统计学显著性是必要条件，但并非充分条件。如前文所述，要评估第三项标准的显著性程度，除统计学显著性外，还需论证其他多项因素。若需了解针对单个地块的三项标准全面评估示例，可参阅《植被地块布莱克洛克94号状况分析》报告（可从www.inyowater.org获取）。基准调查中，61%的依赖地下水地块被划分为C类碱蓬草甸（alkali meadows），本协议旨在防止此类草甸转变为以灌木为主的群落（即B类群落）——此类状态变化与地下水埋深增加密切相关。碱蓬草甸（alkali meadows）需得到特别关注，因为地下水埋深的小幅上升即可使地下水脱离草本植物的根区（Naumberg等，1996；Elmore等，2006）。加州境内的植被群落中，碱蓬草甸仅占0.1%，而其中80%的草甸群落分布于欧文斯谷（Davis等，1998）。此类群落的本地管理措施，将决定这些生态系统在环境变化中存续的可能性。本次研究从全谷尺度与2013年采样的111个单个植被地块两个层面，对欧文斯谷的植被变化开展了评估。首先，在全谷尺度下，我们评估了地下水抽取速率峰值期（1987-1993年）内，受地下水抽取影响的地块与相对未受影响的地块的植物群落覆被率与组成。其次，我们对比了受抽水影响区域与未受地下水抽取影响区域的蒸腾植被覆被随时间的变化差异。第三，我们定量评估了这两类地块相较于1984至1987年记录的基准覆被值的偏离程度。第四，我们评估了井场组与对照组的植被组成是否相较于基准值发生变化。最后，由于判定LADWP的水资源管理措施是否造成显著影响需以单个地块为基础逐案开展，因此我们完成了以下工作：（a）量化了23年复勘周期内多年生植被覆被的变化幅度；（b）评估了木本植被（以下简称灌木）、禾草类植被（gramminoid vegetation，以下简称草本）与非禾草类草本植被（non-gramminoid herbaceous vegetation，以下简称杂类草）的相对占比是否相较于基准值发生变化；（c）量化了每个地块的植被组成的时间趋势，并解析了哪些环境因素可解释观测到的植被变化模式。