1 | Introduction
Suburban landscapes consist
of a mix of human-created infrastructure and fragmented natural
communities. In the temperate deciduous forest biome of northeastern
North America, small forest stands are common components of the many
parks, preserves, and private holdings in suburban areas. In conjunction
with the lawns and fields that also commonly occur in suburbia, this
landscape provides ideal habitat for white-tailed deer (Odocoileus
virginianus ) . Hunting is very limited in suburbia; consequently, deer
densities can be extremely high . The influence of high deer pressure on
suburban forest species is of particular interest because of the huge
extent of urbanizing landscapes; suburban forests now contain a large
share of many regions’ biodiversity .
Many consequences of very high deer densities for forest plants and
their natural communities are well documented. For example, herbivory by
deer causes tissue loss that can lead to increased mortality rates or
decreased reproduction; they may eat entire plants in the form of
seedlings or seeds like oak acorns; seedlings can be trampled by deer,
and deer can otherwise disturb the forest floor as they move across it
and bed down at night (reviewed in . These effects vary with the level
of deer browse pressure and among plant species . In very severely
browsed forests, there may be extremely low abundance of any plants
below the browse line, which increases sunlight penetration and
recruitment opportunities for less shade tolerant plants, and has a
cascade of other indirect effects in the forest community . Defense and
stress responses of long-lived, woody plant species also are likely to
be strongly affected by high deer browse pressure, but this possibility
has not been well studied. Here, we compared the metabolomic profiles of
indigenous and nonindigenous woody species that had been exposed to or
protected from deer in a forest community in suburban New Jersey, USA.
Woody plants have characteristics particularly attractive and vulnerable
to deer. They are chronically exposed to repeated browsing, they have
green foliage throughout the growing season or even year-round for
evergreens, and their buds provide highly nutritious forage throughout
the winter. Thus, high deer browse pressure is associated with depletion
of the woody component of forest understories .
Not surprisingly, woody species invest in various mechanical and
chemical defenses that deter browsing . Deer browse has been
demonstrated to be greater when defense levels are lower , and induced
defense responses can occur within individuals in response to deer
browse . Additionally, the physiological costs of producing defenses
against browse can affect plant fitness , resulting in evolutionary
responses at the population level over time . In some instances, on the
other hand, plant chemical defense may be unaffected by deer browse or
may even decrease . In suburban forests, where deer browse pressure is
particularly high, effects on chemical defenses of long-lived woody
plants are likely strong and, as demonstrated for the more commonly
studied woody plant defenses against insect herbivory , defenses against
deer browse could also influence allocation of resources to plant growth
and reproduction .
Advances in metabolomic research have increasingly enabled investigation
of metabolome-wide responses of plants to stressors , with growing
application of metabolomics to plants in natural ecological communities
. There has been limited ecometabolomic research on woody plants so far,
but the studies have been wide-ranging, including investigations of
abiotic drivers of population or species variation in plant chemistry ,
the role of plant defenses in phylogenetic diversification , metabolomic
niche differentiation among sympatric tropical species , ontological
variation in foliar chemistry , and disease resistance . However, there
has been very little attention paid to plant metabolomics associated
with deer herbivory, with just one study, showing that white-tailed deer
browse less frequently on nonindigenous invasive plants that are
chemically dissimilar to indigenous plants in the community .
Significant quantitative and qualitative variations are commonly
observed in defense responses of different plant species; the final
defense phenotypes result from genetic factors and/or environmental
(light, nutrient availability, geography, etc.) gradients .
Additionally, the type of defense strategy that a plant species employs
is optimized by the severity of herbivore pressure and the feeding mode
of the attacking herbivore (chewing herbivores vs piercing-sucking
herbivores) , as well as the plant’s mode of growth; i.e. annuals versus
perennials or fast-growing versus slow-growing species . Therefore,
variation among species’ chemical defense responses to abundant deer is
expected, and could have important consequences for forest community
structure. Species that are less preferred or even avoided by deer
altogether may have an ecological advantage beyond avoidance of tissue
loss. Many secondary metabolites, especially those rich in nitrogen, are
costly for plants to produce , so if upregulation of defense-related
chemistry is a consequence of heavy, chronic deer browsing, browsed
plants could experience added stress. If severe enough, these metabolic
costs could contribute to shifts in species dominance within the
community. Since some nonindigenous invasive plant species are less
preferred by deer compared to indigenous species , it is even possible
that costly, metabolome-wide responses to deer in browsed plants could
facilitate invasion by nonindigenous plant species that deer avoid.
Indeed, deer are acknowledged as invasion facilitators in some cases ,
but the possible role of metabolomic responses to deer in this
facilitation has not been considered until recently . Despite the
multiple hypotheses put forward to explain the variability in defense
phenotypes in plant communities and the growth-defense tradeoff ,
studies that compare the global metabolomic responses and/or
constitutive/induced defense responses of indigenous and nonindigenous
species are lacking.
A first step in addressing the consequences for plant communities of
metabolome-wide responses to high deer pressure is to document the
metabolomic profiles of a variety of species in a community. In our
study, we tested four long-lived, woody or semi-woody species that occur
in one forest preserve, both in plots that had been fenced for over five
years to exclude deer and in plots that were open to deer. Two species
are indigenous (Nyssa sylvatica and Lindera benzoin ), and
two are nonindigenous, invasive species (Rosa multiflora andEuonymus alatus ). The forest is situated in a suburban landscape
(more information given in the Materials and Methods section) with very
high deer density, and all four species experience deer browse, but at
different rates.
We predicted that fenced and
unfenced plants would differ in their global metabolite profiles and
signaling pathways involved in defense. We also expected that the
metabolomic divergence between fenced and unfenced plants would be more
pronounced for species that are more highly preferred and affected by
deer.