Applied Soil Ecology 14 2000 269–281
Soil response to canopy position and feral pig disturbance beneath Quercus agrifolia on Santa Cruz Island, California
Aaron Moody
a,∗
, Julia Allen Jones
b
a
Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 3220, USA
b
Department of Geosciences, Oregon State University, Corvallis, USA Received 10 September 1999; accepted 28 November 1999
Abstract
We assessed the effects of canopy position and feral pig disturbance on soil moisture, pH, and inorganic nitrogen con- centrations beneath canopies of Quercus agrifolia and surrounding grasslands on Santa Cruz Island, California. Percent soil
moisture, pH, nitrate-N, and total inorganic nitrogen varied significantly with canopy position. Soil moisture decreased, and pH increased along transects from the subcanopy trunk position to the open grassland outside the canopy. Nitrate and total
nitrogen increased from the trunk position to midcanopy and then decreased from midcanopy to the open grassland position. The ratio of nitrate to ammonium varied with canopy position, even after controlling for the effects of soil moisture and
pH, both of which strongly influence rates of nitrogen mineralization. Temporally, soil pH, soil moisture, nitrate-N, and total carbon all varied significantly between drought and non-drought years. Soils from fenced exclosures that removed feral pig
disturbance did not exhibit significant differences in any soil property compared to control plots where pig access was permit- ted. The results of our analysis suggest that the primary factors influencing spatial and temporal variability in oak woodland
soils on Santa Cruz Island were subcanopy position and climate variability, respectively. Biotic disturbance by feral pigs did not cause a significant cause in soil properties over the spatial and temporal domains examined in this study. © 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Quercus agrifolia; Soil nitrogen; Soil moisture; Pigs; Canopy
1. Introduction
Soil characteristics are influenced by factors that range from the scale of the individual ped, to global-
scale climate patterns. As such, coherent spatial vari- ability in soil properties can occur at multiple spatial
scales Burrough, 1983. In a given ecosystem, some of these scales of variability may dominate over oth-
ers. For example, ‘resource islands’ e.g. soil patches with relatively high nitrogen or moisture content are
∗
Corresponding author. Tel.: +1-919-962-5305; fax: +1-919-962-1537.
E-mail address: aaronmemail.unc.edu A. Moody
a salient small-scale feature under trees or shrubs in many arid and semiarid ecosystems Padien and
Lajtha, 1992; Belsky et al., 1993; Ryel et al., 1996. Although the processes that form resource is-
lands are not well understood Hook et al., 1994; Schlesinger et al., 1996 their existence under trees
and shrubs suggests that individual long-lived plants can exert a significant influence upon soil properties
in arid and semiarid environments. Moreover, the spatial patterns of these properties might persist in
the face of other influential factors such as biological disturbance or climate variation. Alternatively, phys-
ical and chemical disturbance of soils by introduced animals may alter biogeochemical and physical soil
0929-139300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 0 0 0 0 0 5 3 - 6
270 A. Moody, J.A. Jones Applied Soil Ecology 14 2000 269–281
properties Barrett, 1982; Singer et al., 1984 and thus modify subcanopy spatial patterns in soil resources.
The relative dominance of plantsoil relationships, biological disturbance, and climate dynamics as
drivers of soil spatial variability is poorly understood.
Our aim is to shed light on this issue by attempting to decouple the effects of plantsoil interactions and
biological disturbance on soil resource distribution beneath the canopy of Quercus agrifolia. We exam-
ined spatial patterns in soil resources under 10 trees on Santa Cruz Island, California over a 5-year period.
Feral pigs Sus scrofa, whose rooting is a major bio- logical disturbance under oak canopies on the island,
were excluded from a 36 m
2
experimental plot beneath each tree. We quantified and compared variability in
soil nitrogen, acidity, and moisture content associated with subcanopy spatial position, the exclusion or pres-
ence of feral pigs, and interannual variation in rainfall.
We focused on nitrogen and soil moisture because of the importance of these resources in controlling
oak regeneration and subcanopy herbaceous species in oak woodland ecosystems in California Parker,
1977; Dunn, 1980; Parker and Muller, 1982; Callaway, 1990. Soil pH, as it influences nutrient exchange rates
and soil biotic communities, is also an important prop- erty. In addition, these variables can respond rapidly
to factors such as removal of pigs, or climate fluctua- tions. More conservative soil properties, such as tex-
ture, which are also important for plant growth, are less responsive to environmental change over short
time periods e.g. less than 10 years.
Our goals in this paper are to characterize the spa- tial patterns of soil properties beneath Q. agrifolia in
an open woodland setting, and to determine whether foraging by feral pigs has an influence on these prop-
erties. An understanding of the relative magnitude and duration of these influences may benefit ecosystem
restoration in areas with populations of large intro- duced animals. The extent to which disturbance inter-
feres with the spatial dynamics of the soilplant system may also have implications for the subcanopy plant
community.
2. Background
