Timothy D. Schowalter

Canopy arthropod communities in relation to forest age and alternative harvest practices in western Oregon

Abstract Arthropod community structure and herbivory were compared in replicate Douglas-fir and western hemlock canopies in intact old-growth (> 400-year-old), and Douglas-fir only in partially harvested old-growth, natural mature (150-year-old) stand, and regenerating plantations (10–20-year-old) in a 15 000 ha area including the H.J. Andrews Experimental Forest in western Oregon. Species diversity and abundances of several taxa, especially predators and detritivores, were significantly lower in plantations compared to older forests. Mature, old-growth, and partially harvested stands showed few significant differences, but principal components analysis suggested some differences in community structure and indicated that old-growth was least variable (tighter clustering) in arthropod diversity and abundance, whereas partially harvested stands were most variable. Defoliation was higher in the mature stands, probably because these stands were composed of relatively dense and pure Douglas-fir. Although old-growth appeared to be the source of greatest arthropod biodiversity in these forests, arthropod communities in Douglas-fir canopies may largely recover old-growth structure by 150 years, and partially harvested stands retain substantially greater arthropod diversity than do regenerating plantations.

Respiration from coarse woody debris as affected by moisture and saprotroph functional diversity in Western Oregon

Abstract Decomposing coarse woody debris (CWD) is a conspicuous and important component of forest ecosystems. Seasonal temperature and precipitation patterns influence heterotroph activity, which determines the rate of CWD decomposition. We tested the hypothesis that moisture content and heterotroph community composition influence carbon flux in freshly-cut Douglas fir (Pseudotsuga menziesii) logs. To evaluate the effects of physical penetration of bark and wood and transmission of basidiomycete compared with ascomycete fungi by insects, 360 experimental logs were assigned to five replicate sites, each with 12 heterotroph×moisture treatment combinations in 1995. Half of the logs in each heterotroph treatment received normal rainfall and half were placed individually under elevated clear plastic tents to reduce water inputs. Respiration was measured every 1–3 months. In 1996 and 1997 a different log representing each treatment combination was harvested from each replicate and analyzed for the presence of inoculated and colonizing fungi. Logs inoculated with decay fungi had higher respiration than uninoculated logs but this effect only approached significance (P=0.08) during the first season. Respiration was significantly higher in sheltered than in exposed logs. Our results indicate that respiration and wood decomposition rates may be depressed by high moisture content in the wet forests of the coastal Pacific Northwest.

Phytophage effects on primary production, nutrient turnover, and litter decomposition of young Douglas-fir in western Oregon

Abstract We tested the effect of defoliating and sap-sucking phytophages on young Douglas-fir at the H.J. Andrews Experimental Forest in western Oregon. Experimental trees were subjected to manipulated abundances of a sap-sucking insect at 0–1 insect g−1 foliage or a defoliating insect at 0 –0.06 g−1 foliage. Tree mass, throughfall, litterfall, litter decomposition, and N, K and Ca turnover were measured for each tree over a 3 year period. Herbivore abundance had no effect on calculated tree growth or nutrient content. These data suggest compensatory growth and replacement of lost nutrients. Herbivory also did not significantly affect decomposition rate for exogenous Douglas-fir needle litter. Throughfall volume, N, K and Ca content, and litterfall mass were positively related (P The results of this study support results from an eastern deciduous forest. Our study relates nutrient turnover rates to herbivore abundances, a prerequisite for modeling phytophage effects on nutrient flows.

Insect Responses to Major Landscape-Level Disturbance

Disturbances are abrupt events that dramatically alter habitat conditions and resource distribution for populations and communities. Terrestrial landscapes are subject to various disturbance events that create a matrix of patches with different histories of disturbance and recovery. Species tolerances to extreme conditions during disturbance or to altered habitat or resource conditions following disturbances determine responses to disturbance. Intolerant populations may become locally extinct, whereas other species respond positively to the creation of new habitat or resource conditions. Local extinction represents a challenge for conservation biologists. On the other hand, outbreaks of herbivorous species often are triggered by abundant or stressed hosts and relaxation of predation following disturbances. These insect responses can cause further changes in ecosystem conditions and predispose communities to future disturbances. Improved understanding of insect responses to disturbance will improve prediction of population and community dynamics, as well as ecosystem and global changes.

Invertebrate Community Structure and Herbivory in a Tropical Rain Forest Canopy in Puerto Rico Following Hurricane Hugo

The effect of hurricane disturbance on canopy invertebrate communities was studied at the Luquillo Experimental Forest in Puerto Rico. Invertebrate community structure and herbivory were compared among tree species co-occurring in intact forest (standing trees) and canopy gaps. Foliage-bearing branches were collected using a long-handled insect net fitted with a closeable plastic bag. Invertebrates were tabulated and foliage was dried, weighed, and measured for area missing. Lepidoptera, predaceous beetles, and decomposers were significantly more abundant in standing trees; whereas, some sap-sucking arthropods were more abundant in canopy gaps, indicating that these taxa responded to changes in microclimate and/or host condition resulting from disturbance. Herbivory (estimated as leaf area missing) and numbers of several taxa differed significantly among tree species, likely reflecting host preferences based on foliage nutritional quality. Species diversity and the gradient of disturbance may have masked disturbance effects.

Decomposition of Greenfall vs. Senescent Foliage in a Tropical Forest Ecosystem in Puerto Rico

ABSTRACT In many forest ecosystems, green leaf deposition (greenfall) constitutes an enrichment over background levels of litterfall nutrients and may therefore influence key ecosystem processes. This study examined the litter quality and decomposition rates of green leaves compared to senescent litterfall for four dominant tree species (Dacryodes excelsa, Manilkara bidentata, Guarea guidonia, and Cecropia schreberiana) in a lower montane rain forest at El Verde Field Station, Luquillo Experimental Forest, Puerto Rico. Green leaves from the canopy and freshly senesced leaves from the forest floor were analyzed for carbon, nitrogen, and fiber and placed in litterbags in the field for up to 16 weeks. Green leaves displayed significantly higher rates of decomposition than did senescent litter among all four species. Green leaves also had significantly higher nitrogen concentrations and lower lignin to nitrogen ratios compared to senescent leaves. These results suggest that greenfall may have a major influence on decay processes and nutrient cycling in forests that experience large-scale green foliage removal.

Early patterns of heterotroph activity in conifer logs

Findings from the first two years of a long-term study of conifer log decomposition are presented. Log decomposition is regulated by the physical and chemical states, and development of decomposer foodwebs. The functional group with the greatest initial effect on the log is the channelisers, represented in our study by ambrosia and bark beetles. They not only create multitudes of channels into the logs but vector the initial decomposer community. Ambrosia beetles exclude certain elements of the decomposer community from channels until they vacate the log, at the end of their reproductive phase. The foodweb during the early stages of decomposition includes nitrogen-fixing and other bacteria, fungi, protozoa, nematodes, and arthropods. Seasonal fluctuations of temperature and moisture are hypothesised to work in tandem to modulate the activities of the decomposer community.

LITTER INVERTEBRATE RESPONSES TO VARIABLE DENSITY THINNING IN WESTERN WASHINGTON FOREST

We evaluated the response of forest floor invertebrates to variable density thinning (VDT) of the Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest overstory at Ft. Lewis, Washington, during 2000 and 2001 (7–8 years post-thinning). We placed pitfall traps at 8–12 random grid points in each of four thinned units and four control (unthinned) units in each of two sites (blocks) representing different management histories. Most taxa showed significant seasonal trends, with peak abundances during summer. ANOVA indicated strongly significant effects of site for 9 of 39 species and combined taxa, probably reflecting factors associated with management history; only two taxa showed significant responses to the thinning treatment or to block × treatment interactions. Indicator analysis revealed three spider species as potential indicators of thinning treatment. Detrended correspondence analysis (DCA) and cluster analysis for 85 species indicated that the thinning treatments altered the invertebrate assembla...

Effects of manipulated herbivore inputs on nutrient flux and decomposition in a tropical rainforest in Puerto Rico

Forest canopy herbivores are known to increase rates of nutrient fluxes to the forest floor in a number of temperate and boreal forests, but few studies have measured effects of herbivore-enhanced nutrient fluxes in tropical forests. We simulated herbivore-induced fluxes in a tropical rainforest in Puerto Rico by augmenting greenfall (fresh foliage fragments), frassfall (insect feces), and throughfall (precipitation enriched with foliar leachates) in replicated experimental plots on the forest floor. Background rates of greenfall and frassfall were measured monthly using litterfall collectors and augmented by adding 10× greenfall or 10× frassfall to designated plots. Throughfall fluxes of NH4, NO3 and PO4 (but not water) were doubled in treatment plots, based on published rates of fluxes of these nutrients in throughfall. Control plots received only background flux rates for these compounds but the same minimum amount of distilled water. We evaluated treatment effects as changes in flux rates for NO3, NH4 and PO4, measured as decomposition rate of leaf litter in litterbags and as adsorption in ion-exchange resin bags at the litter–soil interface. Frass addition significantly increased NO3 and NH4 fluxes, and frass and throughfall additions significantly reduced decay rate, compared to controls. Reduced decay rate suggests that nitrogen flux was sufficient to inhibit microbial decomposition activity. Our treatments represented fluxes expected from low–moderate herbivore outbreaks and demonstrated that herbivores, at these outbreak levels, increase ecosystem-level N and P fluxes by >30% in this tropical rainforest.

CANOPY ARTHROPOD RESPONSE TO DENSITY AND DISTRIBUTION OF GREEN TREES RETAINED AFTER PARTIAL HARVEST

We measured canopy arthropod responses to six contrasting green-tree re- tention treatments at six locations (blocks) in western Oregon and Washington as part of the Demonstration of Ecosystem Management Options (DEMO) study. Treatments were lOOgo retention (uncut), 75% retention with three 1-ha harvested gaps, 40% dispersed retention, 40% aggregated retention with five 1-ha uncut aggregates, 15% dispersed reten- tion, and 15% aggregated retention with two 1-ha uncut aggregates. Arthropods were sam- pled from upper, mid-, and lower crown levels of one overstory Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and from three understory vine maple (Acer circinatum Pursh) in each treatment unit during June and August each year to assess seasonal variation in abundances. Pretreatment data were collected in 1996 and posttreatment data in 1999- 2000. Arthropods showed little evidence of response to treatments, but the abundance of arthropods on both plant species showed significant variation among blocks, reflecting responses to environmental gradients at a regional scale. Arthropod abundance also varied significantly over time in unmanipulated (control) treatments, suggesting sensitivity to annual changes in weather. Our results suggest that disturbance at this intensity or scale has little influence on canopy arthropods in the short term. Future sampling will be necessary to evaluate the extent to which arthropods respond to changes in environmental conditions created by these treatments over longer time periods.