Challenge: Granivorous (i.e., seed eating) animals, such as rodents, birds, and harvester ants, can be detrimental to seeding efforts by consuming large amounts of freshly applied seed. Small mammals can also damage seedlings and saplings by direct herbivory and soil damage through burrowing. Larger herbivores such as deer may also damage restoration sites through herbivory.

Solution: There are a variety of approaches for protecting seeds and plants. Seeds applied with binding or covering materials such as mulch or binders can limit access to seeds by seed-eating animals. Different seed-coating technologies are also being explored to deter seed predators, such as coating with cayenne pepper powders [1]. Careful timing of seed-based restoration can help minimize negative impacts of granivores. If seeds are delivered to the site in fall just before germination-triggering rains, granivores are less likely to have a detrimental effect [2].

Installing chicken wire fencing or protecting seedlings or saplings with plastic tree guards or biodegradable guards (e.g. rigid seedling protection tubes or Eco Gard) can limit access of herbivores to restoration materials in the field. Caging can thereby enhance growth and survival of restored native plant species [3]. For larger areas, or sites that are subject to intense herbivory from deer, livestock, or other animals, fencing the entire area may be required [4].

Installing (where appropriate) barn owl boxes or raptor perches in and around restoration sites (approximately 1 per acre) can also reduce small mammal populations and protect restoration sites. Seeding less expensive but granivore-favored seed (“sacrificial seed”) close to the target restoration seeding area has also shown utility in enhancing seedling establishment [5]. More labor intensive but perhaps more effective controls can include active baiting and trapping of small mammals. Finally, in areas where herbivores are known to congregate, choose plants that are resistant to herbivory.

Challenge: Water availability is a key factor influencing the success of restoration in arid and semi-arid ecosystems. Although many of California’s native plant species are adapted to drought, both short-term seasonal drought and prolonged multi-year droughts can significantly limit restoration outcomes. For perennial species, the first few years following planting, and particularly the first summer, often represent a critical bottleneck for establishment.

Seed-based restoration efforts are unlikely to succeed without sufficient rainfall to support germination and early seedling survival. Similarly, outplanting nursery-grown individuals during dry conditions typically requires supplemental irrigation to ensure successful establishment. Drought can also limit the effectiveness of passive restoration, as reduced water availability may inhibit seed production, slow or prevent the establishment of new individuals, and inhibit the growth of existing vegetation.

Solutions: Several restoration strategies can be used to improve plant establishment even under drought conditions. The obvious solution is to supply restoration sites with supplemental water through irrigation or hand watering. Site preparation can also help increase soil moisture. For example, constructing berms around transplanted individuals can improve irrigation efficiency, capture additional rainfall, and direct water toward plant roots.

Some species may perform better under dry conditions, so prioritizing these in restoration plant palettes during drought may increase success [1]. In addition, certain populations (i.e., ecotypes) may be better adapted to arid conditions. Using these locally adapted sources could enhance restoration outcomes (though be mindful of potential risks of using non-local seed) [2]. In addition to choosing drought tolerant plant species and populations, it may also be possible to condition plants to drought by exposing seedlings to moderately severe water limitation prior to outplanting. This has been shown to increase drought tolerance and improve performance of some native plant species during restoration in California [3].

Challenge: Just as weed control is an important component of site preparation, ongoing weed management is often required for restoration success. The invasion of weedy plant species into restoration sites can severely hinder restoration success, particularly when native perennials are still establishing [1]. Even when initial weed management is thought to be successful, weedy plant species may still emerge from the soil seedbank or readily colonize from nearby sites. Furthermore, some management interventions may successfully control one species or functional group, but create a window of opportunity for secondary invasion by other species [2].

Solution: Ongoing monitoring and adaptive management will help ensure that any emerging invasive plant species are detected and controlled before they have a detrimental impact. This is especially important during the first year of restoration. Closely monitor restoration sites for emerging weed populations and promptly remove them through hand-pulling or targeted herbicide application. Restoring native plant species in and of itself can be a method for weed suppression if established plant communities are able to outcompete invasives and resist reinvasion. Establishing higher densities of mature perennials [3] and including a diversity of native species [4] may be the most effective approaches in this regard. Planting native plant species with similar traits as invasives is another proposed method to increase invasion resistance [5].

Challenge: Air pollution contains substantial amounts of nitrogen, which eventually settles onto the earth’s surface, effectively fertilizing native ecosystems. This process of nitrogen deposition is an underappreciated threat to biodiversity conservation and may also present a significant barrier to restoration, as it tends to favor invasive annuals over native species. For example, in southern California’s coastal sage scrub ecosystems around Los Angeles, elevated nitrogen deposition promotes invasive species, which in turn reduces native plant diversity [1], increases drought-related mortality of native shrubs [2], elevates fire risk [3], and hinders post-fire recovery [4]. Similarly, in the serpentine grasslands of the San Francisco Bay Area, nitrogen deposition has been linked to increased invasion of annual grasses, threatening both native plant diversity and the pollinators that depend on it [5].

Solutions: Ultimately, the only way to prevent the ecological impacts of nitrogen deposition is to reduce emissions. However, there are land management practices that may help mitigate these impacts and advance restoration of N-impacted ecosystems [6]. Biomass removal through targeted grazing may be one way to reduce the dominance of invasive annual grasses and remove nitrogen from the system [5]. Cutting and removing biomass through mowing may also help deplete nitrogen pools in grasslands. Prescribed fire may also be beneficial for native plant establishment and reducing nitrogen pools in some ecosystems [6]. Carbon amendments have been explored as a potential intervention to immobilize soil nitrogen and promote natives over invasives, but there is little evidence to support the efficacy of this approach [7]. Allocating sufficient resources for weed control in areas subjected to high levels of nitrogen deposition may also be necessary for successful restoration.