Consumer-based limitations drive oak recruitment failure.

Area of Science:

• Ecology
• Plant Science
• Conservation Biology

Background:

• North American oaks face recruitment failure, with hypotheses pointing to consumer or resource limitations.
• Quercus garryana is a keystone savanna species in northwestern North America.
• Oak decline has been attributed to various factors, including herbivory and resource availability.

Purpose of the Study:

• To investigate the direct and indirect effects of consumers (herbivory) and resources (moisture, competition) on Quercus garryana seedling recruitment.
• To differentiate the impacts of small mammal herbivory versus deer herbivory.
• To understand how herbivory interacts with environmental factors like drought and exotic grass competition.

Main Methods:

• A factorial experiment using transplanted Quercus garryana seedlings.
• Manipulation of herbivory (protected vs. unprotected seedlings).
• Assessment of seedling survival, growth, leaf production, insect attack, and interactions with moisture and exotic grass cover.

Main Results:

• Intense winter herbivory by exotic small mammals damaged or killed 100% of unprotected seedlings.
• Herbivory significantly reduced seedling size (64%) and leaf production (75%).
• Deer herbivory had no detectable impact; however, herbivory by small mammals exacerbated mortality under drought and exotic grass competition, and increased insect attack.

Conclusions:

• A combination of factors, primarily intense small mammal herbivory, exotic grass competition, and summer drought, creates a significant barrier to Quercus garryana recruitment.
• Single factors are less critical than their synergistic effects in preventing oak seedling establishment.
• Conservation strategies must address multiple interacting stressors, particularly small mammal herbivory and invasive grass management.