Understanding and Managing Sudden Oak Death in California

Seed production

Problem

Seed production varies widely between individual trees and by year.

Oaks and other outcrossing, wind-pollinated species may not set seed reliably if trees are widely scattered.

Remedy

Plant if seed production in target area is inadequate.

Collect seed and plant over multiple years and/or larger areas.

Seed or propagule dispersal

Problem

For heavy seeds such as acorns, dispersal is primarily by gravity and most seed is deposited near the parent tree. Many species with small seeds have adaptations that allow seed to be dispersed longer distances via wind.

Animals can move seeds relatively long distances, but resulting dispersal patterns are  inconsistent.

Remedy

Plant if needed to obtain desired species mix.

Seedbed conditions

Problem

Acorns, lose viability upon drying, so lack of organic debris on the soil surface may greatly reduce germination and seedling establishment.

Some species with small seeds are adapted to germinate on bare mineral soils and will not tolerate deep duff layers

Soil compaction can reduce root growth, leading to poor establishment in many species.

Remedy

Adjust seedbed cover (e.g., increase or decrease litter layer) as needed to favor target species.

Till soil in areas where compaction is excessive.

Damage from herbivores

Problem

Livestock, deer, rodents, and insects can damage seeds and seedlings. 
In areas with high herbivore pressure, regeneration of preferred palatable species can be completely suppressed while reproduction of less palatable species is favored.

Impacts of herbivory are typically most serious on small seedlings and saplings.

Remedy

Use fencing or individual plant protection (wire cages, treeshelters, etc) to exclude herbivores.

Modify or suspend livestock access to restoration site until tops of plants are well above browse line (the height that grazing animals can reach).

Modify habitat to disfavor damaging agents and increase populations of natural enemies.

Diseases

Problem

P. ramorum has potential to reduce seedling growth and survival for species that are subject to foliar and twig blighting (e.g., tanoak).

Soilborne plant pathogens and some foliar diseases have the potential to reduce seedling growth and survival.  

Remedy

Avoid use of species susceptible to diseases present on site.

Use only nursery stock that is certified or tested to be free of Phytophthora spp. and other significant pathogens.

Increase species diversity to minimize overall losses due to a single pathogen.

Soil moisture

Problem

Levels of soil moisture, especially in late summer, limit species survival.

Species that are relatively tolerant of low soil moisture when mature (e.g., many oaks) may be killed by drought during the first few seasons of establishment.

Drought cycles or successive years of heavy precipitation may strongly influence plant establishment.

High plant densities increase competition for limited amounts of soil moisture.

The total amount of plant-available water in soils varies by soil type. Available water is lowest in coarse or sandy soils, greatest in loam and clay- loam soils. 

Available water increases with increasing soil depth (to bedrock or impermeable layers) and increasing organic- matter content.

Shallow water tables or surface flows can increase plant available water.

Organic mulch on the soil surface helps retard evaporation.

Remedy

Match species' moisture needs to the local soil properties.

Restock to take advantage of wetter years.

Control weeds to maximize available soil moisture.

Thin to conserve soil moisture.

If  practical, maintain an organic mulch/litter layer to conserve soil moisture.

Soil erosion

Problem

Soils prone to erosion may be too unstable for establishment of slow-growing species.

Excessive topsoil loss reduces soil- water- holding capacity and available plant nutrients.  Severe erosion may render the site unsuitable for species that previously occupied the site.

Remedy

Use temporary vegetation, mulch, and other soil stabilization methods to minimize erosion while target species are becoming established.

Modify species selection to match species tolerance to current site conditions.

Solar radiation

Problem

Minimum amounts of light needed for seedling establishment and growth differ by species.  Some species establish better under partial to complete canopy cover, others require full sunlight.

Species tolerance of shading can vary with age.  Many species show less shade tolerance as they mature.

Water use (evapotranspiration) is reduced in shaded areas, and may compensate to some degree for limited soil moisture.

Solar radiation is lowest on north-facing slopes and greatest on south- and west-facing slopes.

Remedy

Select species with light tolerance that matches site conditions.

Plan for successional changes in vegetation as available  light changes over time due to tree growth.

Start restoration activities before all tree canopies are killed to provide shaded environment for seedling establishment.

Thin or prune as needed to maintain appropriate light levels for target species.