Traffic patterns, more than adjacent land use, influence element content of roadside forbs for insect pollinators

Roadsides are targeted for restoration of pollinator-friendly plants. Yet, roads are sources of macronutrient, micronutrient, and heavy metal pollution that may contaminate roadside plants. Adjacent landscape features such as railroads and agriculture provide additional macronutrient and heavy metal pollution that may exacerbate traffic effects. However, we lack perspective on how roads combine with rural landscape features to influence nutrition of roadside plants, which could have implications for pollinator health. We surveyed roadsides across Minnesota USA and measured foliar levels of dietary macronutrients (nitrogen, phosphorous, and potassium), a micronutrient (sodium), and metals (iron, zinc, copper, chromium, nickel, lead, aluminum, and cadmium) in six abundant roadside forb species used by insect pollinators: Asclepias syriaca, Dalea purpurea, Monarda fistulosa, Ratibida pinnata, Solidago spp., and Trifolium pratense. We aimed to determine (1) how road variables (traffic volume and distance from road) combine with adjacent land use (railroad and agriculture) to influence element content of roadside forbs and (2) whether some forb species show consistent differences in their accumulation of potentially toxic heavy metals, which could inform selection of species to plant along roadsides. We found that foliar concentrations of nine elements increased with greater traffic volume (nitrogen, phosphorous, iron, zinc, copper, chromium, nickel, lead, and aluminum), and concentrations of six elements declined with distance from the road (nitrogen, phosphorous, potassium, iron, zinc, and copper). Leaves collected adjacent to railroad had less phosphorous, potassium, iron, nickel, and aluminum than leaves collected from sites not adjacent to railroad. Additionally, leaves collected from sites adjacent to agriculture had lower copper levels than leaves from sites without adjacent agriculture. We found no evidence that forb species along roadsides consistently rank higher than other species in their accumulation of heavy metals. Our results show that traffic alters more elements in roadside plants than does adjacent landscape context, alleviating concerns that landscape features exacerbate pollutant levels in roadside pollinator habitat. However, nutrient contamination of most roadside plants is unlikely to reach toxic levels for insect pollinators. This work is consistent with the positive conservation potential of low to moderate traffic roadsides for pollinators. This file contains data on leaf and soil chemistry from roadside sites across Minnesota.

路边生境常被用作传粉友好型植物的修复重建场地。然而道路本身会产生大量营养素（macronutrient）、微量营养素（micronutrient）以及重金属（heavy metal）污染物，可能对路边植物造成污染。邻近的铁路与农田等景观要素，会带来额外的大量营养素与重金属污染，可能加剧道路交通带来的污染影响。然而目前我们尚不明确道路与乡村景观要素如何共同作用，影响路边植物的营养状况，而这一问题可能对传粉昆虫的健康产生潜在影响。我们针对美国明尼苏达州全域的路边生境开展了系统调查，并测定了6种常见且被昆虫传粉者利用的路边非禾本科草本植物（forb）的叶片元素水平：其中包括膳食相关大量营养素（氮、磷、钾）、一种微量营养素（钠）以及多种金属元素（铁、锌、铜、铬、镍、铅、铝与镉），供试物种分别为普通马利筋（Asclepias syriaca）、紫色达利草（Dalea purpurea）、拟美国薄荷（Monarda fistulosa）、羽状松果菊（Ratibida pinnata）、一枝黄花属植物（Solidago spp.）以及红车轴草（Trifolium pratense）。本研究旨在阐明两个核心科学问题：（1）道路交通变量（车流量与距道路距离）与周边土地利用类型（铁路周边与农田周边）如何共同作用，影响路边非禾本科草本植物的元素含量；（2）是否存在部分草本植物物种在潜在有毒重金属累积方面表现出一致的差异，该结果可为路边植被修复的物种筛选提供科学依据。研究结果显示，9种元素的叶片浓度随车流量增加而升高（包括氮、磷、铁、锌、铜、铬、镍、铅与铝），另有6种元素的叶片浓度随距道路距离增加而降低（包括氮、磷、钾、铁、锌与铜）。相较于非邻近铁路的采样点，邻近铁路的采样点所采叶片中的磷、钾、铁、镍与铝含量更低。此外，邻近农田的采样点所采叶片的铜含量，同样低于非邻近农田的采样点叶片。本研究未发现路边非禾本科草本植物物种在重金属累积能力上普遍优于其他物种的证据。研究结果表明，相较于周边景观背景，道路交通对路边植物元素含量的影响更为广泛且显著，这缓解了此前学界的担忧：周边景观要素会加剧路边传粉昆虫生境的污染物水平。不过，多数路边植物的营养盐与重金属污染水平，不太可能达到昆虫传粉者的中毒阈值。本研究结果与“低至中等车流量的路边生境对传粉昆虫具有积极的保护潜力”这一观点相符。本数据集包含美国明尼苏达州全域路边生境的叶片与土壤化学性质相关数据。