Seth W. Bigelow

Habitat Associations of Small Mammals at Two Spatial Scales in the Northern Sierra Nevada

Abstract Effective management strategies require an understanding of the spatial scale at which fauna use their habitat. Toward this end, we sampled small mammals in the northern Sierra Nevada, California, over 2 years at 18 livetrapping grids among 5 forest types. Forest types were defined by overstory tree composition, and 19 microhabitat variables were measured at all trap stations. Forest type and year explained 93% of variation in abundance of North American deermice (Peromyscus maniculatus), whereas only 40% was explained by microhabitat and year. Similarly, variation in abundance of long-eared and shadow chipmunks (Neotamias quadrimaculatus and N. senex) was better explained by forest type and year (67%) than by microhabitat and year (30%). Red fir (Abies magnifica) forests supported more P. maniculatus and Neotamias species than mixed-conifer and pine–cedar forests, and more Neotamias species than mixed-fir forests. Five of 6 uncommon species were significantly associated with forest type; golden-mantled ground squirrels (Spermophilus lateralis), northern flying squirrels (Glaucomys sabrinus), and long-tailed and montane voles (Microtus longicaudus and M. montanus) were captured almost exclusively in red fir forests, whereas dusky-footed woodrats (Neotoma fuscipes) and California ground squirrels (Spermophilus beecheyi) were found in pine–cedar, mixed-fir, and mixed-conifer forests. The first 2 axes of a canonical correspondence analysis on microhabitat variables explained 71% of variation in combined small mammal abundance. Microhabitat associations varied among species but were driven primarily by canopy openness, shrub cover, and shrub richness. Although much of the small mammal fauna appeared to select habitat at both spatial scales studied, canonical correspondence analysis using forest type as a covariate revealed that microhabitat explained much less variation in small mammal abundance than did forest type.

Plant Life-Forms and Tropical Ecosystem Functioning

Life-form was said by Warming (1909) to represent the sum of adaptive characters in a species, and thus is an expression of the harmony between a plant and its environment. This colorful perspective, although too broad to be useful in classification, does highlight the essential point that life-form groupings should be ecologically relevant.

Leaf nutrients in relation to stature and life form in tropical rain forest

. To document the relationship between a plants position in the canopy and its leaf nutrient content, leaf nitrogen and phosphorus were determined for 30 species growing in mature evergreen lowland rain forest at La Selva Biological Station, Costa Rica. Species that grow either in the understory, midstory, or the canopy were selected. Species were further separated into three life forms: self-supporting monocots, self-supporting dicots, and climbers. Mass-based nutrient concentrations were expected to decrease with stature, as has been reported in studies of other forests. In fact, mass-based nitrogen and phosphorus did not vary significantly among the three adult-stature classes, although area-based values differed greatly: canopy plants averaged 60 % more nitrogen and 90 % more phosphorus per unit leaf area than understory plants. Differences in leaf characteristics were evident among the three life forms. Most notably, area-based phosphorus and leaf specific mass were lowest in climbers, intermediate in self-supporting dicots, and highest in self-supporting monocots. These results support the characterization of climbers as investing in inexpensive structures, perhaps in order to gain competitive advantage in light capture by allocating resources to maximize elongation rates.

ENHANCING NUTRIENT RETENTION IN TROPICAL TREE PLANTATIONS: NO SHORT CUTS

In the humid tropics large quantities of nutrients can be rapidly leached when the soil is unprotected by actively growing vegetation. We established experimental plan- tations of three indigenous tree species on a fertile Andisol in Costa Rica and managed them under 1- or 4-year cutting cycles with uncut stands as controls. Our goals were to test whether nutrient leaching was greatest under a regime of frequent disturbances that returned modest amounts of biomass to the soil surface (cutting and replanting on a 1-yr cycle) or less frequent disturbances that returned significantly greater amounts of plant tissues to the soil (cutting and replanting on a 4-yr cycle), and to compare those cutting cycles with nutrient leaching from uncut stands. Leaching of NO3 (over 9 yr), Ca2+, Mg2+, and K+ (over 4 yr) from upper soil horizons was monitored. Water balance was determined by linking Penman-Monteith evaporation with changes in soil water storage modeled from soil physical parameters. Drainage water for solute measurement was sampled from porous ceramic cups at 1.1 m depth in the soil. Disturbance frequency proved to be an important determinant of NO3- leaching. Average long-term NO3- leaching losses from stands on a 1-yr cutting cycle were extraordinarily large: 442 mmol_-m-2.yr-1 (62 kg-ha-1.yr-1 of N), compared to 187 mmolc.m-2.yr-1 under a 4-yr cutting cycle and 71 mmolc.m-2.yr-1 from uncut stands. Elevated NO3- leaching was primarily due to increased concentration in the soil solution (rather than increased water drainage), because cutting usually resulted in a reduction of <10% in evapotranspiration. Resilience of stands decreased with continued disturbance; under a 4-yr cutting cycle, stands tended to take longer to return to the low levels of NO3- leaching characteristic of undis- turbed stands with each episode of cutting and replanting, while NO3- losses from annually cut stands became increasingly variable over time. Due to high concentrations of soil Ca at the site, the stands proved resistant to treatment-induced losses of base cations: no increases in Ca2+ or Mg2+ leaching accompanied elevated NO3- leaching, although K+ leaching did increase under the 1-yr cutting cycle. Because of the potential for massive, sustained NO3- losses, development of land use systems for these soils should focus on minimizing frequency of disturbance.

Small mammals exhibit limited spatiotemporal structure in Sierra Nevada forests.

Abstract Forests in the Sierra Nevada, similar to those across the continent, have been substantially altered by logging, fire exclusion, and other human activities. Current forest management emphasizes maintenance or restoration of resiliency in the face of contemporary disturbance factors that include wildfire, climate change, continued urbanization, and invasive species. We evaluated responses of small mammals to forest management by monitoring a series of 12 replicate trapping grids in compositionally homogeneous forest over 8 years, and implemented 2 levels of canopy thinning. Livetrapping efforts (119,712 trap-nights) yielded 15,613 captures of 2,305 individuals of 13 species, and although forest structure was significantly influenced by canopy treatments, small mammal numbers and assemblage composition were not. To better understand this we assessed habitat associations of small mammals at 599 census points on 75 transects established in a stratified random manner throughout Plumas National Forest. We analyzed these data with 2 complementary forms of constrained ordination (canonical correspondence and canonical correlation) that extract major gradients in 1 data set (e.g., distribution of small mammal captures) and explain these in terms of measured variables from a 2nd data set (e.g., habitat and environmental measurements). Over 3 years and 57,504 trap-nights of effort we captured 1,367 individuals of 11 species. Both forms of ordination exposed significant associations between small mammals and underlying habitat metrics, but they explained remarkably little variation in these data, suggesting that small mammals are responding only modestly to habitat variation as expressed by the available environmental variables measured at each plot. We followed this with stepwise multiple Poisson regression to build models of habitat associations of these species. We applied model-averaging and employed Akaikes information criterion corrected for small sample size (AICc) to evaluate candidate models. Reflecting ordination results, competitive models (e.g., those with Akaike differences [ΔAICc] < 2.0) cumulatively explained little variation (12–36%) and regression coefficients were very low. Hence, both ordination and Poisson multiple regression suggest that the limited response by small mammals to canopy thinning primarily reflects the generalist habits of the common species in this forest. We propose that anthropogenic influences have led to structural homogenization of these forests, even across > 800 m of elevation, such that habitat specialists (e.g., old-forest–dependent species such as Myodes [Clethrionomys] californicus and Glaucomys sabrinus) have become less common due to the lack of suitable habitat. Further efforts should target nonforested habitats (meadows and riparian corridors), but results presented here suggest that managers should strive to increase heterogeneity at large spatial scales and especially to promote the development of mature forest structure.

Evidence That Soil Aluminum Enforces Site Fidelity of Southern New England Forest Trees

Abstract Tree species composition of hardwood forests of the northeastern United States corresponds with soil chemistry, and differential performance along soil calcium (Ca) gradients has been proposed as a mechanism for enforcing this fidelity of species to site. We conducted studies in a southern New England forest to test if surface-soil Ca is more important than other factors in determining survival of seedlings of six common canopy tree species. Our hypothesis was that the calcicole species Acer saccharum and Fraxinus americana would show elevated survival rates at higher Ca levels, and that the calcifuge species A. rubrum, Fagus grandifolia, Quercus rubra, and Tsuga canadensis would show lower survival at high Ca. Other factors examined were 1) exchangeable magnesium (Mg), potassium (K), and aluminum (Al); 2) understory light availability; and 3) identity of overstory tree species. In one study, seedlings were transplanted into plots fertilized with Ca or Mg sulfate and survival was measured over 2 years. In the other study, 1-year or 2-year survival of naturally established seedlings in stands dominated by mature trees of one of the six study species was followed. Fertilization with Ca or Mg did not affect survival of planted seedlings, but ambient exchangeable Al was strongly negatively correlated with survival of F. americana. Of the three species with sufficient naturally established seedlings, exchangeable Al plus a proxy for light (overstory tree identity) were the most important determinants of survival. Survival of A. saccharum declined and A. rubrum and F. grandifolia increased at higher levels of exchangeable Al. This pattern is consistent with the positions of these species along the soil gradient. We conclude that soil chemistry effects on seedling survival play a role in establishing the soil relationships characteristic of these species as adults, but that Al is more important than Ca in establishing these effects during the seedling stage.

Litterfall as a niche construction process in a northern hardwood forest

Northern hardwood forests can exhibit considerable temporal stability in their species composition, which litterfall may help to maintain by modifying the soil environment to create an ecological inheritance. We evaluated the evidence for this niche-construction perspective by carrying out a spatially explicit analysis of redistribution of calcium (Ca), magnesium (Mg), and potassium (K) via leaf litterfall of the six dominant tree species in a temperate forest in northwest Connecticut, USA. We combined stem maps with litterfall measurements to fit models of leaf litterfall for individual species, and we chemically analyzed leaves, surface soil extractions, and unweathered soil particles. Analysis of elemental oxides in unweathered soil particles suggested that species did not differ in ability to gain access to rock-bound nutrients. Nevertheless, species had a significant effect on quantity of Ca cycled in litter—the two calcicoles (species with affinity for calcium-rich soils) occurred most commonly on t...

Neighborhood-Scale Analyses of Non-additive Species Effects on Cation Concentrations in Forest Soils

AbstractTrees affect soil chemistry and nutrient availability via a broad range of processes. Effects can vary dramatically among species, whose distinctive spatial “footprints” can vary for different nutrients. Potentially overlapping effects of neighboring trees in mixed-species stands make footprint shape and interspecific interactions important: If interactions are non-additive, then not only abundance but also spatial configuration influence tree species’ effects on ecosystem properties. We used spatially explicit neighborhood-scale data on tree distributions to fit maximum likelihood models of exchangeable calcium, magnesium, and potassium in surface soils of four sites in northern hardwood forests in northwestern Connecticut, USA. The models incorporated parent material, site, and tree species or functional group configuration to predict availability of the three cations. Site had a stronger effect than species for all cations (there was no species effect for potassium), even after accounting for variation in parent material. Species’ spatial footprints extended further from the stem for calcium than magnesium, which is consistent with the relative importance of litterfall versus stemflow transfer of these nutrients. The magnitude of species effects on calcium and magnesium varied widely. Functional groups made up of species with positive or negative effects provided parsimonious models for magnesium and calcium, and the best model for calcium included a non-additive, antagonistic effect whose strength varied by site. This non-additive effect suggests that the degree of intermingling of tree species from negative- and positive-effect functional groups may influence stand-level availability of calcium, a key nutrient for forest health in these ecosystems.