Woody plant secondary chemicals increase in response to abundant deer and arrival of invasive plants in suburban forests

• Published in: Ecology and evolution Vol. 12; no. 4; pp. e8814 - n/a
• Main Authors: Morrison, Janet A., Roche, Bernadette, Veatch‐Blohm, Maren
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01.04.2022; John Wiley and Sons Inc; Wiley
• Subjects: Allelopathy; Antioxidants; Beech; Chemical defense; Chemical Ecology; Chemicals; Chemistry; Community Ecology; Competition; Competitiveness; Conservation Ecology; Deer; Defense; Fagus grandifolia; Flavonoids; Forests; Fraxinus pennsylvanica; Herbivory; Herbs; Hypotheses; Influence; Invasion Ecology; Invasive plants; Juveniles; Light levels; Mathematical models; Microstegium vimineum; multiple stressors; Multivariate statistical analysis; Native species; Phenols; plant defense; Soils; Stress response; Structural equation modeling; Suburban areas; suburban forest; Trees; Trophic Interactions; Understory; Water availability; white‐tailed deer; Woody plants; United States > US; New Jersey; suburban forest; plant defense; structural equation modeling; Microstegium vimineum; multiple stressors; white‐tailed deer
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.8814

Plants in suburban forests of eastern North America face the dual stressors of high white‐tailed deer density and invasion by nonindigenous plants. Chronic deer herbivory combined with strong competition from invasive plants could alter a plant's stress‐ and defense‐related secondary chemistry, especially for long‐lived juvenile trees in the understory, but this has not been studied. We measured foliar total antioxidants, phenolics, and flavonoids in juveniles of two native trees, Fraxinus pennsylvanica (green ash) and Fagus grandifolia (American beech), growing in six forests in the suburban landscape of central New Jersey, USA. The trees grew in experimental plots subjected for 2.5 years to factorial treatments of deer access/exclosure × addition/no addition of the nonindigenous invasive grass Microstegium vimineum (Japanese stiltgrass). As other hypothesized drivers of plant secondary chemistry, we also measured nonstiltgrass herb layer cover, light levels, and water availability. Univariate mixed model analysis of the deer and stiltgrass effects and multivariate structural equation modeling (SEM) of all variables showed that both greater stiltgrass cover and greater deer pressure induced antioxidants, phenolics, and flavonoids, with some variation between species. Deer were generally the stronger factor, and stiltgrass effects were most apparent at high stiltgrass density. SEM also revealed that soil dryness directly increased the chemicals; deer had additional positive, but indirect, effects via influence on the soil; in beech photosynthetically active radiation (PAR) positively affected flavonoids; and herb layer cover had no effect. Juvenile trees’ chemical defense/stress responses to deer and invasive plants can be protective, but also could have a physiological cost, with negative consequences for recruitment to the canopy. Ecological implications for species and their communities will depend on costs and benefits of stress/defense chemistry in the specific environmental context, particularly with respect to invasive plant competitiveness, extent of invasion, local deer density, and deer browse preferences. We tested the effects of overabundant white‐tailed deer and the nonindigenous, invasive grass Microstegium vimineum on foliar secondary chemistry of juveniles of the trees Fraxinus pennsylvanica and Fagus grandifolia, in six suburban forests of central New Jersey, USA. We analyzed leaves sampled from individuals growing in plots in a factorial experiment of deer exclosure presence/absence × M. vimineum addition/no addition. Using univariate mixed models and structural equation modeling, we found that in some cases these two common stressors in suburban forests increased juvenile trees’ secondary chemicals involved in defense and stress responses, deer generally had stronger and/or more consistent effects than M. vimineum, and additional influences on the trees’ secondary chemistry were revealed by the SEM, that is, light and soil moisture.