Native annual forb and vegetation data for ten paired grazed and ungrazed prairies in coastal California grasslands at two time periods 15 years apart

Livestock grazing is often used as a land management tool to maximize vegetation diversity in grassland ecosystems worldwide. Prior research has shown that cattle grazing benefits native annual forb species in California’s coastal prairies, but drought and increasing aridity may alter this relationship. In 2016 and 2017, we resurveyed the vegetation structure, native annual forb cover, and native annual forb richness in ten grazed and ungrazed prairies that were originally measured in 2000 and 2001 along a 200-km gradient from Monterey to Sonoma counties in California. We found that grazed prairies continued to have significantly lower vegetation height and thatch depth than ungrazed prairies, and that shrub encroachment over the 15-year period was significantly greater in ungrazed prairies. Furthermore, grazed prairies continued to have greater native annual forb richness (4.9 species per site) than ungrazed sites (3.0 species per site), but that native annual forb richness declined by 2.8 species per site in grazed prairies and 0.1 species per site in ungrazed prairies between survey periods. We suggest that severe drought and increasing aridity may be driving declines in native annual forb richness in grazed prairies. The species we recorded only in earlier surveys were disproportionately wetland-associated and had higher average specific leaf area than species that remained through the second survey period. Finally, cover of native annual species increased regardless of whether prairies were grazed, suggesting that the high precipitation in 2017 may have benefitted the native annual forb species that persisted at sites between surveys. Our study shows that weather conditions affect the outcomes of land management strategies.

Methods
In 2000–2001, we surveyed 26 paired grazed and ungrazed coastal prairie sites across 400 km of coastline between San Luis Obispo and Mendocino counties in California, USA. In 2016–2017, we resampled ten sites along 200 km in the central area of the prior survey extent (Monterey to Sonoma Counties) where we could obtain permission from landowners for access and grazing patterns had been maintained since 2000-2001.

At all sites, paired plots were less than 2 km apart, on similar slopes and aspects, and transects were placed at least 5 m from fence lines to avoid edge effects. Cattle were the only domestic grazing animals present at the sites, there was no evidence of prior mechanical soil disturbance, and soils were deeper than 40 cm and not ultramafic (serpentine). We sampled from April through May, traveling from southern sites to northern sites to follow the peak flowering phenology of most species. At each site, we relocated T1 50-m line transects using a Garmin eTrex 20 GPS. There were five transects each in the grazed and ungrazed portions at each site. 

We quantified community composition by recording the identity of each species that intersected a 1.8-mm-diameter pin at 1-m intervals along the transects. We recorded each species that intercepted the pin once. Our nomenclature and species origin information follow the Jepson Manual and online Jepson eFlora database. At 5-m intervals, we measured vegetation height using a piece of paper dropped onto foliage and recording the lowest point, and thatch depth (i.e., build-up of dead plant biomass) by pushing a 1.8-mm pin to the soil surface and measuring the tallest standing dead biomass. We calculated shrub cover as the number of woody shrubs intercepts along each transect as a percent of the 50 intercepts total along the transect. Finally, we carefully searched for and quantified the cover of all native annual forbs (to the nearest cm2) present within a 1-m belt transect centered over the 50-m transect.

We compiled specific leaf area (SLA, leaf area/leaf dry mass) for 40 of the 55 native annual forbs species from the TRY Plant Trait Database and data collected by others. We also collected data from species present in and near our sites during spring and summer 2018. Most of these data came from grazed plots, since this is where the species were more prevalent, though we selected individuals that showed no evidence of recent grazing. When data for a single species was available from multiple sources, we averaged the values, which are available here. We also gathered the US Army Corps of Engineer wetland indicator status of the native annual forbs.

畜牧放牧常被用作全球草地生态系统中提升植被多样性的土地管理手段。已有研究表明，牛群放牧对加利福尼亚海岸草原的本土一年生非禾草草本植物（annual forb）有益，但干旱与日益加剧的干旱化可能会改变这一关系。2016年与2017年，我们对2000年及2001年曾调查过的10个放牧与未放牧海岸草原样地进行了重新调查，这些样地沿加利福尼亚州从蒙特雷县至索诺马县的200公里梯度分布。本次重调查的指标包括植被结构、本土一年生非禾草草本植物盖度及物种丰富度。

研究结果显示，放牧样地的植被高度与枯落物厚度始终显著低于未放牧样地，且在15年的调查周期内，未放牧样地的灌木入侵程度显著更高。此外，放牧样地的本土一年生非禾草草本植物丰富度仍高于未放牧样地（分别为每个样地4.9种与3.0种），但两次调查期间，放牧样地的本土一年生非禾草草本植物丰富度下降了2.8种/样地，未放牧样地仅下降0.1种/样地。我们认为，严重干旱与日益加剧的干旱化可能是导致放牧样地本土一年生非禾草草本植物丰富度下降的原因。仅在首次调查中被记录到的物种，其与湿地相关的占比更高，且比第二次调查中留存的物种拥有更高的平均比叶面积。最后，无论样地是否经过放牧，本土一年生植物的盖度均有所提升，这表明2017年的高降水量可能对两次调查间仍存活于样地的本土一年生非禾草草本植物有利。本研究表明，气候条件会影响土地管理策略的实施效果。

研究方法
2000–2001年，我们在美国加利福尼亚州圣路易斯奥比斯波县至门多西诺县的400公里海岸沿线，调查了26对配对的放牧与未放牧海岸草原样地。2016–2017年，我们在原调查范围的中部区域（蒙特雷县至索诺马县）的200公里范围内重新采样了10个样地：这些样地已获得土地所有者的通行许可，且放牧模式自2000–2001年以来未发生改变。

所有样地的配对样方间距均小于2公里，且坡度与坡向相似；样线设置位置至少距离围栏5米，以避免边缘效应。样地内仅存在牛类家畜放牧，未发现机械土壤扰动的痕迹，土壤厚度大于40厘米且非超镁铁质（蛇纹岩）土壤。采样时间为4月至5月，我们从南部样地向北推进，以跟随多数物种的开花物候峰值。在每个样地，我们使用Garmin eTrex 20 GPS重新定位2000–2001年设置的50米样线；每个样地的放牧与未放牧区域各设置5条样线。

我们通过沿样线以1米间隔使用直径1.8毫米的针扦记录与之相交的每个物种的身份，来量化群落组成：每个物种被针扦截获一次即记录一次。物种命名及起源信息遵循《Jepson植物志》（Jepson Manual）及在线Jepson植物区系数据库（Jepson eFlora database）。在每5米间隔处，我们将一张纸片放置于植被上以测量植被高度并记录最低点；同时通过将1.8毫米的针扦推至土壤表面，测量直立死亡植物生物量的最大高度，以此得到枯落物厚度（即死亡植物生物量的累积量）。灌木盖度通过每条样线上木本灌木被针扦截获的次数占总50次截获量的百分比计算得出。最后，我们在以50米样线为中心的1米宽样带内，仔细搜寻并量化所有本土一年生非禾草草本植物的盖度（精确至1平方厘米）。

我们从TRY植物性状数据库（TRY Plant Trait Database）及他人已发表的数据中，获取了本次调查涉及的55种本土一年生非禾草草本植物中40种的比叶面积（specific leaf area, SLA，即叶面积与叶干重的比值）数据。同时于2018年春夏收集了样地内及周边出现物种的相关数据：由于多数物种在放牧样地中更为常见，因此多数数据来自放牧样地，但我们选择了无近期放牧痕迹的个体。当单一物种存在多来源数据时，取其平均值，相关数据可在此处获取。此外，我们还收集了本土一年生非禾草草本植物的美国陆军工程兵团湿地指示物种状态信息。