Field and wind tunnel assessments of the implications of respacing and thinning for tree stability

The change in wind loading on trees due to tree spacing andwind speed was examined in the field and in the wind tunnel.The field measurements were made in small blocks of Sitka sprucerespaced 15 years previously to different stand densities. Thewind tunnel measurements were made with dynamically correct20-cm tall plastic trees. The maximum bending moment on treesshowed a linear increase with the ratio of spacing to height.Estimates suggest that at the wider spacing tree diameter hasincreased sufficiently so that trees are less likely to break.However, the increase in resistance to overturning is not asrapid as the increase in wind loading and wider spaced treeswill be more vulnerable to overturning. Wind tunnel measurementswere also carried out on a range of commercial thinning practicesand showed that the critical factor in reducing stand stabilityis the size of gap made in the forest. The mechanical and dynamiccharacteristics of trees at different spacing are presented.These show that with increasing spacing the Young's modulusof trees decreases and their damping coefficient increases.This illustrates that wide spaced trees have weaker wood thanclose spaced trees but they are less reliant on the supportof neighbouring trees. The implications of the measurementson the overall stability of stands and the implications forforest managers are discussed.     

Dependence of weed flora on the active soil seedbank

Previous studies have shown that reliable predictions of the above-ground weed flora may not be obtainable by using the total number of seeds in the seedbank. The purpose of this study was to determine if the actual weed flora depends on the active soil seedbank (seeds that are germinable in the spring). In the 1995 growing season, seedling emergence was monitored within permanent quadrats established in a field. Soil cores taken from the same field were also monitored for seedling emergence in a greenhouse to estimate the number of seeds in the active soil seedbank. Significant positive relationships were observed between the above two variables based on either total weed flora or individual weed species commonly found in south-western Ontario, Canada. For relatively large-seeded species, the relationship held with a sampling depth up to 15 cm below the soil surface. For small-seeded species, a shallow sampling depth (0–7·5 cm) generated better results than deeper sampling (0–15 cm). In general, 3–7% of the seeds in the active soil seedbank were capable of producing seedlings in the field. The results suggest that the level of weed infestation in a growing season may be predicted using seeds in the active soil seedbank.     

Methane and CO2 exchange over peatland and the effects of afforestation

The exchange of CH₄ and CO₂ over extensive tracts of peatlandhas recently been measured using micrometeorological methods.Fluxes of CH₄ measured in Caithness (UK) during May and June1994 using eddy covariance yielded fluxes in the range –70to +170 µmol CH₄ m^–2 h^–1 with a mean fluxof 39 µmol CH₄ m^–2 h^–1 Landscape emissions of CH₄ from peatland are shown to vary withwater table and temperature, increasing from 25 µmol CH₄m^–2 h^–1 at 5 per cent area to 50 µmol CH₄m^–2 h^–1 with 30 per cent within the footprint occupiedby pool. A positive temperature response of 4.9 µmol CH₄m^–2 h^{–1 {ring}} C^– in the temperature range6–12^{ring}C was also observed The mean day time fluxes of CO₂ during May and June 1994, of—3.6 mmol m^–2 h^–1 were two orders of magnitudelarger than the emission of CH₄ of 40 µmol m^–2 h^–1,while at night (when net radiation is negative) the respirationflux of CO₂ averaged +2.2 mmol m^–2 h^–1approximatelytwo orders of magnitude larger than night-time CH₄ emissionsof 30 µmol m^–2 h^–1.     

Pedunculate and Sessile Oak (Quercus robur L. and Quercus petraea (Mattuschka) Liebl.): A Review of the Hybrid Controversy

Early nineteenth-century studies of variation in the genus QuercusL., particularly those relating to Pedunculate and Sessile oakby Lasch and Greville, illustrate the confused situation thatawaited the forest botanist. Attempts of that period and laterto explain this condition led to the general acceptance of atheory of unlimited hybridization and introgression betweenthe two species. Experimentation and observations between thethirties and fifties of the present century by Dengler, Høeg,Jones, and others demonstrate the weakness of this popular concept—hence the controversy. More recent investigations by Johnssonand Krahl-Urban, supported by the biometric research of Cousens,Carlisle, and Brown, have helped in the production of a moreaccurate picture of the form and extent of hybridization. Agreat deal still remains to be done to elucidate the naturalfrequency of F₁ hybrids, and backcrossea in different regions.The karyotypes of both species and hybrids have still to bedefined.     

Development of water oak stump sprouts under a partial overstory

A 28-year-old water oak (Quercus nigra L.) plantation was thinned from below to either 254 or 462 stems per hectare to determine the influence of a partial canopy on oak stump sprout development. Sprout clump survival, number of living sprouts in a clump, and height and DBH of the dominant sprout in a clump were measured in years 1–5 and 7 after harvest. By year 7, sprout clump survival under the heavily thinned canopy was 23% higher than under the lightly thinned canopy. Within-clump sprout mortality was not influenced by overstory thinning level, but by year 2 height and diameter increment were 15% and 22% greater, respectively, under the heavy overstory removal. Positive height and diameter growth of dominant sprouts continued under both canopy conditions through year 7, but early benefits of the heavy overstory removal on sprout growth diminished. Additional overstory removal or sprout clump thinning would be necessary to sustain sprout clump survival and sprout growth over extended periods.