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Macroarthropod Community Responses to Mechanical Thinning of Ponderosa Pine Stands in
Northern Arizona, USA
Jerod Benefield
Introduction
In ponderosa pine systems of the southwestern US, fire suppression, timber harvesting,
and grazing over the last century have contributed to declines of herbaceous vegetation cover and
increases in low quality litter (Kaye & Hart 1998). Mechanical thinning and fuel removal
treatments have been used to restore forest stands to more park-like states (Stoddard et al. 2011).
While forest structure components and fire risk assessment are commonly used to interpret the
success of restoration techniques (Fulé et. al. 2001), invertebrate responses can serve as reliable
long-term indicators of post-treatment soil ecosystem response (Chen et al. 2006; Villa-Castillo
& Wagner 2002). Forest insects and their predators are critical components of ecosystem
processes such as decomposition and nutrient cycling (Grady & Hart 2006; Haack & Byler 1993;
Kajak 1995). For example, detritivorous microscopic arthropods such as Collembolans affect soil
organic matter dynamics and nutrient cycling by consuming bacteria and fungi, and by
inadvertently spreading fungal spores (Phelan 2004; Santos & Whitford 1981). Macroarthropod
(i.e. arthropods easily visible and distinguishable to an unaided observer) predators like spiders
can directly impact decomposition rates by consuming detritivores like Collembolans (Lawrence
& Wise 2000).
Mechanical thinning alters the structural and compositional complexity of the forest
understory. Depending on initial understory vegetation species composition, removing overstory
components via thinning increases growing space, as well as water and nutrient availability,
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resulting in a significant growth release for understory vegetation production (Moore et al.
2006). These changes in vegetation can change both habitat and basal resources for the entire soil
food web. The purpose of this study is to assess and quantify effects of thinning on soil and litter
macroarthropod assemblages. We will focus on the diversity and abundance of macroarthropod
communities, primary predators of soil microarthropods, as integral components of the overall
soil food web.
Observed Arthropod Responses in Ponderosa Pine Forest Systems
Insect communities may respond quickly to restoration treatments such as thinning and
prescribed burns, however those responses are variable depending on the taxa (Jansen 1997;
Chen et al. 2005; Longcore 2003; Ryndock 2012; Brennan et al. 2006). For example, Waltz and
Covington (2004) found increase in pollinator activity with increased insolation (light levels)
after thinning and burning treatments reduced stand densities. Their findings suggest an
immediate effect as the changes in pollinator assemblages occurred even before the understory
plant communities responded to the treatments (Waltz & Covington 2004). Castillo and Wagner
(2002) and Chen et al. (2006) found differences in Coleopteran community assemblages based
on the severity of disturbance created by the treatment efforts, observing much higher levels of
diversity in the more disturbed stands. These results collectively suggest that restoration of
ponderosa pine systems can have important effects on the foundational levels of forest ecosystem
biota. These restoration effects include predator-prey population responses and interactions,
understory vegetation responses and resulting microhabitat structural changes, as well as prey
species responses to increased basal resource quality within understory vegetation.
Predator-Prey Population Dynamics
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Larger arthropods such as cursorial spiders and predatory beetles represent primary
consumers of microarthropods within soil and litter food webs (Moulder & Reichle 1972). As
such, we expect predator population abundances and distributions will correlate with prey groups
and vice versa due to density-dependent relationships. Lawrence and Wise (2000) found that
decreasing spider predation within litter test plots resulted in population increases of observed
Collembola densities. Kajak (1997) found similar results after excluding foraging predatory
macroarthropods from meadow soil food webs. In a manipulation experiment by Chen and Wise
(1999), both prey and predator arthropod abundances increased when decomposer groups were
given more forage resources. However, this assumption of a functional relationship between
predators and prey becomes difficult to quantify when attempting to correlate polyphagous
predators such as spiders with a specific prey species or taxa. For example, when examining
influences on web-building spider communities in a meadow food web, Greenstone (1984) could
not identify prey availability as a significant predictor of spider species richness. Birkhofer et al.
(2011) encountered similar confounders when an alternate prey group interrupted their
evaluation of the population dynamics between spiders and Collembola as associated predators
and prey. We hope to eliminate the pit-falls of attempting to isolate species specific predatorprey relationships by examining the entirety of the forest soil and litter food web communities.
Understory Vegetation: Changes in Habitat Structure and Basal Resource Quality
Prey abundance alone cannot represent the only factor influencing predatory
macroarthropod population abundances. Through altering the understory vegetation species
composition, thinning treatments also change the structural complexity and forage quality of the
forest floor vegetation (Moore et al. 2006). Increasing the forage quality and structural
complexity of litter within a forest system has been shown to alter the overall occurrence and
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abundance of insect communities, specifically in terms of habitat availability for predators and
basal resources for their herbivorous prey (Bultman & Uetz 1982, 1984; Zahn et al. 2009). When
disturbances such as agricultural manipulation or mechanical thinning lead to structural
heterogeneity within a habitat, the resulting insect community is characterized by high species
richness and evenness (Rypstra et al. 1999). These higher levels are due to an increase in
microhabitat diversity supporting a wider range of functional groups and by reducing the
dominance of more competitive species (Bultman & Uetz 1984; Sousa 1979). The findings of
Greenstone (1984) support the value of structural heterogeneity for insect communities, as they
determined web-building spider species richness was more highly correlated with vegetation tip
height diversity than prey availability. Increasingly deep litter accumulation limits mobility for
some body forms, resulting in decreases in the abundance of cursorial spiders, yet an increase in
abundances of web-building spider taxa (Bultman et al. 1982). Birkhofer et al. (2011) found an
increase in predator guild segregation as management actions homogenized habitat structures
within agricultural fields. We expect these structural changes to be a main driver of differences
between macroarthropod community assemblages within thinned and unthinned stands.
Another determinant of macroarthropod community assemblages is the quality of basal
resources available to herbivorous prey species. Multiple studies have found significant
correlation between the quality of basal resources availability to microarthropod populations and
the cascading impacts across trophic levels of the soil and litter food web (Chen & Wise 1999;
Bultman & Uetz 1984; Phelan 2004). Unthinned stands are characterized by closed canopies and
deep accumulation of deep litter layers, severely restricting the establishment or persistence of
rich and abundant understory vegetation as a habitat component. Basal resources within
unthinned stands consist of low quality forage, mainly needle casts and woody debris. These
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resources benefit some arthropod functional groups such as mycophagous species, but leave
others without a foundation for subsistence. Thinned stands characterized by a rich understory
component offer a spectrum of higher quality basal resources via grasses and herbaceous plant
species, and are likely to support a wider variety of macroarthropod functional groups (Moore et
al. 2006; Waltz & Covington 2004; Zahn et al. 2009).
From what we know of macroarthropod responses to habitat components, we
hypothesize that; 1) the richness and abundances of predatory and prey macroarthropods within
our stands will fluctuate in response to the presence of each other, 2) the increased complexity
and diversity of vegetative habitat structures in the understories of thinned stands will result in
macroarthropod communities higher in species richness and evenness, and 3) Stands with higher
quality basal resource availability will result in bottom-up effects for prey species, and therefore
support more diverse and abundant macroarthropod community assemblages.
Methods
Study Sites
In the summer of 2010, 12 sites were located in the NAU Centennial Forest are in
northern Arizona (35°08’00” M, 111°47’00” W).Six sites had been pre-commercially thinned in
the mid-1990’s (Kalies et al. 2012), and size unmanaged sites were selected to serve as control
sites. In spring 2012, four sites were added to the study, located in Taylor Woods, Fort Valley
Experimental Station (34° 2' 56 N, 111° 5' 37" W). Two of these stands were thinned in (X-Year)
to (X-density), and two unmanaged stands were selected as controls. Sites in Centennial Forest
are approximately 0.32 hectare in size, while sites within Taylor Woods are 0.04 hectares in size.
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The elevation of the two study areas is approximately 2,200 meters. Climatic patterns and parent
material are relatively uniform across the entire study area and are not expected to affect
variation between sites. Sampling has been conducted since summer 2010 during pre-monsoon
(June) and post-monsoon (September) time periods to capture seasonal variation of arthropod
assemblages. A second sampling effort will be conducted at these same time points in 2013.
Stand Characteristics
Structural and compositional components of all stands will be quantified during fall of
2012 and spring of 2013. Overstory density characteristics will be measured using point
sampling (i.e. variable-plot radius) methods and will describe density in terms of trees per
hectare and basal area per hectare (Aver & Burkhart 2002). Forest floor characteristics such as
litter depth and downed woody debris will be collected and quantified using the line transect
methods described by Brown (1974). These components reflect habitat and nutritive resources
used by arthropod communities of interest to our study. Understory vegetation richness and
abundance will also be quantified.
Macroarthropod Sampling: Design and Implimentation
Beginning in the summer of 2010, macroarthropods were sampled using the pit–trap
collection method (Cardoso 2008; Bergmann et al. 2012). Pit-traps were designed using 16 oz.
plastic SOLO cups, with bottom diameter, height and top diameter measuring 5.7 cm, 11.4 cm,
and 3.8 cm, respectively. The cups were buried to a depth so the top opening was flush with the
surface soil layer. The cup was half filled with 6-8 oz. of distilled water diluted with unscented
Dawn dish soap to trap fallen insects. The detergent solution used in the traps was created by
mixing 0.5 teaspoons of the Dawn detergent to 3 liters of distilled water. The cup opening was
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then covered with a 15 x 15 cm2 section aluminum mesh (1.27 cm mesh size) and secured into
the surrounding substrate with four 25 cm lengths of 17-gauge electric fencing wire. The mesh
coverings were placed to act as both a deterrent to other wildlife, as well as a guard against
general debris.
Ten pit-traps were established in a regular pattern with at least 20 meter (m) spacing
between adjacent traps. At some of the smaller sites, spacing between traps was reduced to 10 m
to accommodate all sampling points. Pit-traps were filled with the soap/water solution, covered
with the mesh guards, then the solutions containing arthropods from all traps were collected after
seven days.The samples were collected in 100 mL specimen cups. The collected samples were
returned to the laboratory, preserved with 70% ethanol solution, and stored at 3.9° Celsius for
future sorting and identification.
Sample Analysis
For the purposes of efficiency, samples will be analyzed according to taxonomic guilds.
The guilds we will use are Beetles (C), Spiders (S), Wasps & Bees (W), Ants (A), Orthoptera
(O), Heteroptera (HO), Hemiptera (HE), and Miscellaneous (M). Each specimen found within
each taxonomic group will be assigned a morph (A-Z), photographed with a scale in millimeters,
and a individual of the specimen will be kept in a small exhibition jar for future reference.
Statistical Analysis
Once the samples have been analyzed according to the methods described above, we will
compare taxonomic group richness and abundances between thinned and unmanaged stands for
both premonsoon and postmonsoon sampling periods using multivariate statistical techniques.
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We will also compare taxonomic group characteristics to stand components related to habitat
quality such as understory vegetation structure and basal resource quality.
Expected Results
Based on our preliminary observations and the available literature, we expect to observe
direct correlations between the structural and compositional changes in vegetation after thinning
and the community assemblages of macroarthropods those changes will support. For example,
we expect that the increased richness and quality of the understory following thinning will
support a higher abundance and wider spectrum of prey species compared to the sparse
understories within the unmanaged control sites (Stoddard et al. 2011; Bultman & Uetz 1984;
Zahn et al. 2009). We expect those changes in prey communities will cascade across the soil
food web and directly affect the complexity and abundance macroarthropod communites.
Defining how macroarthropod community assemblages vary between thinned and unthinned
stands is important in understanding the consequences of thinning treatments for soil food webs
dynamics.
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