Jay Sexton

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.

Decomposition of coarse woody debris originating by clearcutting of an old-growth conifer forest

ABSTRACT Decomposition constants (k) for above-ground logs and stumps and sub-surface coarse roots originating from harvested old-growth forest (estimated age 400–600 y) were assessed by volume–density change methods along a 70-y chronosequence of clearcuts on the Wind River Ranger District, Washington, USA. Principal species sampled were Tsuga heterophylla and Pseudotsuga menziesii. Wood and bark tissue densities were weighted by sample fraction, adjusted for fragmentation, then regressed to determine k by tissue type for each species. After accounting for stand age, no significant differences were found between log and stump density within species, but P. menziesii decomposed more slowly (k = 0.015·y−1) than T. heterophylla (k = 0.036·y−1), a species pattern repeated both above- and below-ground. Small-diameter (1–3 cm) P. menziesii roots decomposed faster (k = 0.014·y−1) than large-diameter (3–8 cm) roots (k = 0.008·y−1), a pattern echoed by T. heterophylla roots (1–3 cm, k = 0.023·y−1; 3–8 cm, k = 0.017·y−1), suggesting a relationship between diameter and k. Given our mean k and mean mass of coarse woody debris stores in each stand (determined earlier), we estimate decomposing logs, stumps, and snags are releasing back to the atmosphere between 0.3 and 0.9 Mg C·ha−1·y−1 (assuming all coarse woody debris is P. menziesii) or 0.8–2.3 Mg C·ha−1·y−1 (assuming all coarse woody debris is T. heterophylla). Including coarse roots increases these loss calculations (averages of all decomposition classes for the study year) to 0.5–1.9 Mg C·ha−1·y−1 or 1.0–3.5 Mg C·ha−1·y−1, respectively. Our results support substitution of log k in C flux models when stump k is unknown. Substitution of log k for coarse root k could, however, substantially overestimate C flux back to the atmosphere from these forests.

Water balance of conifer logs in early stages of decomposition

Seasonal and long-term changes in the water balance of conifer logs during the first 8 years of decomposition were studied in an old-growth Pseudotsuga/Tsuga forest in the Oregon Cascade Mountains. Measurements were made of the moisture content of outer bark, inner bark, sapwood, and heartwood and of the flow of water into and out of logs of four species (Abies amabilis, Pseudotsuga menziesii, Thuja plicata, and Tsuga heterophylla). After the logs had decomposed from 1 to 2 years, 38–47% of the canopy throughfall landing upon them ran off the surface, 29–34% leached from the bottom, and 21–30% was absorbed and evaporated. After 8 years of decomposition, water entering and then leaching from logs increased 1.3 times while runoff decreased a similar amount. The proportion of water stored by and evaporated from logs in this study indicates that in old growth forests they may intercept 2–5% of the canopy throughfall to the forest floor and that, even in early stages of decomposition, they may affect the hydrological cycle of Pacific Northwest old-growth forests.

Differences between standing and downed dead tree wood density reduction factors: A comparison across decay classes and tree species

Woody detritus or dead wood is an important part of forest ecosystems and has become a routine facet of forest monitoring and inventory. Biomass and carbon estimates of dead wood depend on knowledge of species- and decay class-specifi c density or density reduction factors. While some progress has been made in determining these parameters for dead and downed trees (DD), there are very few estimates of these key parameters for standing dead trees (SD). We evaluated indicators of decay to relate subjective SD and DD decay classifi cations then compared SD and DD density and density reduction factors by decay class for a total of 19 tree species at nine sites in the United States and Russia. Results indicate that SD density declined with decay class for all examined species. By applying these results, a new set of SD density reduction factors was developed for 260 species inventoried by the U.S. Forest Services Forest Inventory and Analysis program in forests of the United States.

Decomposition of fine woody debris in a deciduous forest in North Carolina1

Abstract We examined the effect of position with respect to the soil surface, species, and piece size on the decomposition rate of fine woody debris (< 15 cm diameter) in a North Carolina forest disturbed by hurricane. To examine year-to-year trends, pieces of two species (Carya tomentosa ((Lam.) Nutt.) and Quercus alba (Lam.)) in four size classes were placed on the forest floor and collected annually for ten years. In addition, to examine position effects samples of the same species and sizes were suspended in the air and buried underground at a depth of 20 cm and collected at years 2, 4, and 8. Nine other species were placed on the forest floor and collected at years 2, 4, and 8 to determine the range of variability among species. Decomposition was slower the first year than subsequent years, therefore the lag exponential equation was used to determine time trends and an integrated decomposition rate-constant (kI) reflecting the overall decomposition rate-constant was calculated. The kI for C. tomentosa and Q. alba ranged from 0.17–0.25 year−1 with a significant interaction between species and size. The buried and suspended samples generally decomposed more slowly than the samples on the surface and kI ranged from 0.11–0.24 year−1 and from 0.10–0.18 year−1, respectively. There was a significant interaction between position and size; while drying limited decomposition of suspended pieces regardless of size, high moisture may have limited decomposition in the largest buried pieces. The kI for all eleven species and sizes averaged over all size classes ranged from 0.06–0.33 year−1. There was a highly significant interaction between species and size with the smaller sizes tending to decompose faster than the larger sizes and in general species with the most decay-resistant heartwood having the largest response to increases in size. Our experiments and comparison to other studies suggests that the interactions between species, size, and position relative to soil surface are highly complex and dependent on site climate.

Evaluation of techniques for determining the density of fine woody debris

Evaluated various techniques for determining the density (i.e., bulk density) of fine woody debris during forest inventory activities. It was found that only experts in dead wood inventory may be able to identify fine woody debris stages of decay. Suggests various future research directions such as development of a 2-class fine woody debris decay class system.

Water Dynamics in Conifer Logs in Early Stages of Decay in the Pacific Northwest, U.S.A.

Abstract Water dynamics in decaying conifer logs of four species (Abies amabilis [Pacific silver fir], Pseudotsuga menziesii [Douglas-fir], Thuja plicata [western red cedar], and Tsuga heterophylla [western hemlock]) were studied in the Coast Range of Oregon. Measurements were made of throughfall, leachate, runoff, and absorption for logs during their 6th through 8th year of decay. During this period 47–70% of the throughfall landing on the logs evaporated, 18–35% flowed through the log and leached out, 3–29% ran off the surface, and absorption accounted for 3–11%. Together absorption and evaporation intercepted 60% of the throughfall impacting the logs. Although the second year of the study had twice as much precipitation as the first, the partition of the fluxes was essentially identical. Direct measurement of the changes in log weight allowed calculation of water stores and the evaporative component; the latter proved to be the largest fraction of the water balance, with the majority of losses during the cool, wet, winter period.