Forestry - current issue

Pennsylvania sedge cover, forest management and Deer density influence tree regeneration dynamics in a northern hardwood forest

Powers, M. D., Nagel, L. M. — 2009-06-25

Pennsylvania sedge (Carex pensylvanica Lam.) is a native species that can form dense mats in the understory of northern hardwood forests. Resource managers suspect Pennsylvania sedge mats could limit tree regeneration, leading to changes in forest structure and composition. A number of other factors could alter the understory plant community, so any attempt to understand the effects of sedge populations should also account for these other processes. This study considered relationships between Pennsylvania sedge cover, management history, deer density and tree regeneration in a northern hardwood forest. All the factors considered had significant effects on saplings, but few significant relationships with tree seedlings. Stands managed using even-aged silvicultural systems had lower sapling densities for several species and species groups and lower species diversity than unharvested second-growth stands. High deer densities were associated with low levels of both sapling density and sapling species diversity. Pennsylvania sedge cover was negatively correlated with the density of secondary species and sapling species diversity in stands with moderate deer densities. Results from this study suggest a number of factors are influencing northern hardwood tree regeneration in these forests, and further research involving multifactor approaches is needed to clarify the mechanisms driving forest vegetation dynamics.

Carbon stock and stock changes across a Sitka spruce chronosequence on surface-water gley soils

2009-06-25

We assessed age-related alterations in carbon (C) stocks and sequestration rates of first rotation Sitka spruce (Picea sitchensis (Bong.) Carr) plantations on predominantly surface-water gley soils. Sites were selected to represent a typical Sitka spruce chronosequence following land use transition from grasslands dominated by surface-water gley soils. Based on inventory, eddy covariance, physiological and modelling assessments of net ecosystem productivity (NEP), we show that afforested stands are a C sink at 10 years, and possibly earlier, followed by an increase to a maximum of 9 t C ha^–1 year^–1 before the first thinning cycle. NEP subsequently declined from 9 t C ha^–1 year^–1, at closed canopy, to 2 t C ha^–1 year^–1 in older and thinned stands. Reductions in the C sequestration rate of older stands were coupled with a decrease in gross primary productivity, increases in maintenance/growth respiration and decomposition losses following harvest. We suggest that the high sequestration potential of these forests may be associated with the high net primary productivity of these plantations in Ireland, a high allocation of assimilates and litter into the belowground C pool and accumulation of C in mineral gley soils following afforestation.

Modelling the anchorage of shallow-rooted trees

Achim, A., Nicoll, B. C. — 2009-06-25

We present here a mechanistic model (the ‘Resistance’ model) that uses easily measurable or predictable input parameters describing tree, soil and root–soil plate characteristics to predict the anchorage strength of Sitka spruce (Picea sitchensis (Bong.) Carr.) trees. Results show that for a given soil group and rooting depth, the anchorage moment can be modelled as being proportional to the square of the root–soil plate spread. Models are also developed of root–soil plate spread in relation to soil group and tree characteristics and are applied with the Resistance model to provide predictions of anchorage based on the diameter at breast height. To examine the relative sensitivity of trees on different soil groups and with different rooting depths, the models are applied here to an example where the leeward ‘hinge’ radius is constrained to a maximum of 1 m. The results are discussed in relation to forest wind risk modelling and stand management.

Deep shade alters the acclimation response to moderate water stress in Quercus suber L.

Jimenez, M. D., Pardos, M., Puertolas, J., Kleczkowski, L. A., Pardos, J. A. — 2009-06-25

The interactive effects of shade and drought on different morphological and physiological traits were addressed on Quercus suber L. seedlings. In our experiment, limited light treatment (1 per cent) represented the main factor constraining cork oak seedlings growth. Maximal photochemical efficiency (F_v/F_m) with light ≥15 per cent exhibited a midday fall, but under deep shade (1 per cent), F_v/F_m remained constant (<0.8 values) throughout the day. The quantum efficiency of photosystem II (PSII) was lower under moderate drought only in deeply shaded plants. The drought also interacted with light through the increase of the soluble sugar content at 100 per cent light, but not under shade. Under deep shade, soluble sugar content tended to be even lower under moderate drought conditions. The chlorophyll content was not the highest under deep shade as could be expected, while the efficiency of PSII was the lowest under deep shade. We concluded that cork oak can acclimate to moderate shade (15 per cent light), but deep shade impairs some of the physiological responses to cope with low light conditions under moderate drought stress. Plants growing under deep shade were very sensitive to moderate water stress in terms of loss of carbon fixation capacity.

Vegetation response to midstorey mulching and prescribed burning for wildfire hazard reduction and longleaf pine (Pinus palustris Mill.) ecosystem restoration

Brockway, D. G., Outcalt, K. W., Estes, B. L., Rummer, R. B. — 2009-06-25

Dense midstorey vegetation, developed during fire exclusion, not only reduces understorey plant diversity and increases the risk of damaging wildfire but also impedes efforts to safely restore prescribed burning in longleaf pine (Pinus palustris Mill.) ecosystems. Our study examined the effects of midstorey reduction on stand structure and plant diversity in a forest treated by mulching alone and also when followed by prescribed fire during the winter, spring or summer. For trees ≥5 cm diameter at breast height (d.b.h.), mulching reduced stand density (1220–258 trees ha^–1) and basal area (24–17.7 m² ha^–1) and increased mean d.b.h. (12.8–29.2 cm), with the largest reductions in loblolly pine (Pinus taeda L.), sweetgum (Liquidambar styraciflua L.) and oaks (Quercus spp. L.). Removing hardwoods and smaller pines resulted in a decline in tree species richness (8.9–4.4). Despite a modest increase in evenness (0.72–0.79), tree species diversity (H' = 1.32–0.84) dynamics were largely driven by changes in richness. While the cover of tree seedlings initially declined from 32.4 to 16.9 per cent, rapid regrowth of hardwoods led to recovery by end of the second growing season. This, along with gains by shrubs, vines, grasses and forbs, resulted in a near doubling of understorey plant cover. Although tree seedling increases were not related to fire season, peak responses occurred for shrubs and vines after winter fire and spring fire, grasses following winter fire and forbs after summer fire. An increase in species richness (18.7–24.5) and decline in species evenness (0.86–0.70) produced only a small increase in understorey species diversity (H' = 2.31–2.45). The greater number of understorey species following treatment were less equitably distributed as a result of differential rates of plant growth. While mulching led to a short-term increase in woody and herbaceous understorey plants, prescribed fire is needed to curtail redevelopment of the woody midstorey and further increase grasses and forbs.

A volume change index for forest growth and sustainability

Van deusen, P. C., Roesch, F. A. — 2009-06-25

A volume change index is suggested that is derived from growth components that can be estimated from remeasured plots. The new index incorporates more information than the traditional growth over removals ratio. The new index directly indicates whether the standing volume will be increasing or decreasing if current conditions persist, whereas the ratio of growth over removals obscures the impact of mortality. The details of estimating the components of growth are discussed with regard to interpreting the new index. The effect of spatial scale on the index is explored and a variance estimator is suggested. The new index is estimated from US Department of Agriculture Forest Service annual inventory data for two example applications. The first application looks at spatial trends in the index. The second application applies the index to a mill working circle.

Development and long-term evaluation of harvesting patterns to reduce windthrow risk of understorey spruce in aspen-white spruce mixedwood stands in Alberta, Canada

Macisaac, D. A., Krygier, R. — 2009-06-25

The Hotchkiss River Mixedwood Timber Harvesting Study developed new approaches to harvesting systems for western Canada's boreal mixedwood forests. Conventional harvesting equipment was used to test 11 harvesting and silvicultural systems over a 530-ha site. These include one- and two-pass modified uniform shelterwoods, two- and three-pass strip shelterwoods, two-pass alternate strip shelterwoods and four-pass progressive strip shelterwoods. These were used to test varying levels of wind protection designed to protect and minimize wind damage to understorey (immature) white spruce residuals following harvest of the aspen overstorey. Ongoing monitoring of wind dynamics and associated windthrow patterns since 1992 have provided clear management practice guidelines for reducing windthrow of immature spruce and residual aspen following harvesting. There are thresholds related to spruce height and distance from aspen residuals, beyond which windthrow damage increases significantly. The influence of topography, timing of harvest and the spatial configurations of multiple harvests on windthrow dynamics have been clarified through this research. These results have provided valuable information to help forest planners to utilize harvesting and silvicultural systems that best reduce windthrow damage to understorey spruce, following harvest of overstorey aspen.

The effect of duration of vegetation management on broadleaved woodland creation by direct seeding

Willoughby, I., Jinks, R. L. — 2009-06-25

The need for vegetation management when practising direct seeding, and the potential for using the technique to establish a range of tree and shrub species used for new native woodland creation, was investigated on a good quality, fertile ex-agricultural site in lowland Britain. After 4 years, an average of nearly 20 000 stems ha^–1 of all species had established, amounting to 20 per cent of viable seed sown, with the trees from the fastest growing species being over 2.6 m in height on average, with the tallest trees reaching over 3.7 m. Weeding using selective herbicides improved survival by up to 50 per cent and growth rates by up to 80 per cent. Weeding for 1 year gave a significant benefit to overall survival, but weeding for a maximum of 3 years gave the most improvement to survival and growth of the remaining trees. Tree form was improved by increased stocking densities. Direct seeding proved to be a viable technique for creating a mixed woodland of ash (Fraxinus excelsior L.), cherry (Prunus avium L), field maple (Acer campestre L), oak (Quercus robur L.), hawthorn (Crataegus monogyna Jacq.), hazel (Corylus avellana L.), dogwood (Cornus sanguinea L.), blackthorn (Prunus spinosa L.), guelder rose (Viburnum opulus L.), spindle (Euonymus europaeus L.), wayfaring tree (Viburnum lantana L.), sweet chestnut (Castanea sativa Mill.) and sycamore (Acer pseudoplatanus L.). The work confirmed the importance of achieving good weed control immediately before sowing and for at least the first growing season thereafter, to reduce seedling mortality and increase the chances of achieving successful woodland creation by direct seeding.