Forestry - current issue

Comparison of stratified and non-stratified most similar neighbour imputation for estimating stand tables

Eskelson, B. N. I., Temesgen, H., Barrett, T. M. — 2008-05-12

Many growth and yield simulators require a stand table or tree-list to set the initial condition for projections in time. Most similar neighbour (MSN) approaches can be used for estimating stand tables from information commonly available on forest cover maps (e.g. height, volume, per cent canopy cover and species composition). Simulations were used to compare MSN (using an entire database) with two stratified MSN approaches. The first stratified MSN approach used species composition to partition the population into two inventory type strata, while the second stratified MSN approach used average stand age to partition the data into two stand development stages (strata). The MSN approach was used within the whole population and within each stratum to select a reference stand and to impute the ground variables of the reference stand to each target stand. Observed vs estimated stand tables were then compared for the stratified and non-stratified simulations. The imputation within a stratum did not result in better estimates than using the MSN approach within the whole population. Possible reasons for poor performance of stratified MSN are provided.

Advance Fagus sylvatica and Acer pseudoplatanus seedlings dominate tree regeneration in a mixed broadleaved former coppice-with-standards forest

Collet, C., Piboule, A., Leroy, O., Frochot, H. — 2008-05-12

This study assessed the species composition and the development of regeneration plots in gaps created by a windstorm in a mixed-species broadleaved stand. The stand was a former coppice-with-standards characterized by a high broadleaved tree species diversity. Thirteen years after gap creation, all gaps were fully stocked and the regeneration was almost exclusively dominated by Fagus sylvatica and Acer pseudoplatanus seedlings, two species characterized by a high shade tolerance in their early stages. All other species (Quercus sp., Fraxinus excelsior, Carpinus betulus, Acer campestre, Acer platanoïdes, Sorbus torminalis, Tilia sp.) were either absent from the regeneration or completely over-topped by the two dominant species. These features were ascribed to a regime of periodic natural or anthropic disturbance of intermediate intensity, where small gaps are regularly created in the canopy. This regime results in a succession of short, open and closed canopy episodes that eventually promote shade-tolerant species. During this regime, the shade-tolerant species are able to build a strong advance regeneration that is ready to outgrow the other species when gaps are created. If the management objective is to maintain species diversity during the regeneration process, the development of this advance regeneration will have to be strictly controlled.

Decay characteristics of hazardous Tilia, Betula, and Acer trees felled by municipal urban tree managers in the Helsinki City Area

Terho, M., Hallaksela, A-M. — 2008-05-12

In order to improve the management and protection of old urban trees, the decay characteristics of trees that had been removed as hazardous according to the local management protocol were investigated. The stage and the cross-sectional extent of decay were examined on a total of 181 park and street trees in the Helsinki city area: 64 trees of Tilia spp., 58 Betula spp. and 59 Acer spp. For Tilia, hollowed heartwood with low fungal expression and advanced decay caused by Ganoderma lipsiense were the two most common characteristics of cross-section samples from the points where stem breakage was most likely. For Betula, the primary reason for tree removal was usually lowered amenity value in terms of a declined appearance of the crown. Advanced decay, mainly caused by Inonotus obliquus, Piptoporus betulinus and Cerrena unicolor, were the most potential causes for stem breakage on Betula. For Acer, internal cracks, most often due to weak fork formation, were common causes of potential hazard. Decay caused by Rigidoporus populinus often increased the risk of stem breakage of these trees. In addition, advanced decay caused by Phellinus igniarius and Kretzschmaria deusta were the most important reasons for decreased safety of Acer trees.

Rotated sigmoid structures in managed uneven-aged northern hardwood stands: a look at the Burr Type III distribution

Gove, J. H., Ducey, M. J., Leak, W. B., Zhang, L. — 2008-05-12

Stand structures from a combined density manipulation and even- to uneven-aged conversion experiment on the Bartlett Experimental Forest (New Hampshire, USA) were examined 25 years after initial treatment for rotated sigmoidal diameter distributions. A comparison was made on these stands between two probability density functions for fitting these residual structures: the Burr Type III and a finite mixture of Weibulls. All stands exhibited some form of uneven-aged structure (either reverse J or rotated sigmoid) after 25 years. The Burr distribution fit the final distributions as well as the more complicated finite mixture model in all cases.

Alternative definitions of growth and removals and implications for forest sustainability

Van Deusen, P. C., Roesch, F. A. — 2008-05-12

Alternative definitions for growth and removals from a forest being monitored over time are discussed. It is shown that the definitions that are used in practice may not be what one would expect and estimates can vary substantially under alternative definitions. This can result in conclusions about forest sustainability that may be misleading. Alternative definitions are applied to selected states using US Department of Agriculture Forest Service inventory data. Standard errors of growth over removals ratios are used to indicate potential sustainability problems for selected forest types.

Examination of forest recovery scenarios in a southern Appalachian Picea-Abies forest

Moore, P.T., Van Miegroet, H., Nicholas, N.S. — 2008-05-12

This study contrasts various forest recovery scenarios in a Picea rubens Sarg. Abies fraseri (Pursh.) Poir. forest 20 years after the onset of Adelges picea Ratz. and tests them against a 10-year data set from an intensive catchment study in the Great Smoky Mountains National Park. Standing live biomass, increment, ingrowth, mortality and net change in live biomass are analysed by species and elevation based on inventory data gathered in 1993, 1998 and 2003 at a network of 50 permanent plots stratified along nine elevation bands (1700–1900 m). Total standing live biomass at the study site remained stable between inventories (~260 Mg ha^–1). Betula showed little, if any, response to the recent set of catastrophic overstory disturbances. Biomass and increment of Picea increased somewhat; but overall, there is limited evidence that Picea is expanding. Abies showed significant increases in standing live biomass (from 3.3 to 12.7 Mg ha^–1), increment (380 to 850 kg ha^–1 year^–1) and ingrowth (320 to 610 kg ha^–1 year^–1) over time. While some scenarios have not fully played out yet, at this time, total elimination of Abies is not indicated and there is considerable evidence to support the stable Picea and Abies scenario.

Linkages between wolf presence and aspen recruitment in the Gallatin elk winter range of southwestern Montana, USA

Halofsky, J., Ripple, W. — 2008-05-12

The decline of aspen (Populus tremuloides) in Yellowstone National Park (YNP) has been attributed to conifer encroachment, climatic influences, ungulate herbivory and more recently, a lack of grey wolves (Canis lupus) via trophic cascades. The current study explores evidence of a trophic cascade on aspen decline. We conducted the study in the Gallatin elk winter range of YNP, an area where aspen have not previously been studied. We approximated aspen origination dates from an age–diameter relationship to examine potential correlations between wolf presence and absence, elk (Cervus elaphus) herbivory and aspen recruitment. A comparative analysis was also conducted between the aspen data collected in the winter range and aspen data collected within two elk exclosures. Within the elk exclosures, aspen successfully recruited since fence construction in the 1940s. Outside the exclosures, aspen recruitment into mature stems began to decline in the 1920s (during wolf extirpation), completely ceased after the 1950s and has only been observed since the 1990s (post-wolf reintroduction). While a host of interacting biophysical factors may influence aspen recruitment and growth, the correlative results between aspen recruitment and historical elk browsing activities, coincident with the presence and absence of wolves, are consistent with a top–down trophic cascade.

Spatially assessing model errors of four regression techniques for three types of forest stands

Zhang, L., Ma, Z., Guo, L. — 2008-05-12

In this study, three types of forest stands with different spatial patterns of tree locations were used to investigate the spatial autocorrelation and heterogeneity in model errors from four regression techniques: ordinary least squares (OLS), linear mixed model (LMM), generalized additive model (GAM) and geographically weighted regression (GWR). In uneven-aged stands of softwood and hardwood, trees were clustered or randomly distributed over space. Tree variables were significantly correlated, and the relationship between tree height and diameter was non-stationary across the study area. GAM fitted the data well, but it generated spatial patterns for model errors similar to OLS because GAM is no-spatial in nature. In contrast, LMM and GWR took spatial autocorrelation into account for estimating the model coefficients and the standard errors of the coefficients. Consequently, they produced more accurate predictions for the response variable, as well as more desirable spatial distributions of model errors than those derived from OLS and GAM. The model errors from GWR were not only smaller in spatial autocorrelation but also lower in spatial heterogeneity than those from LMM.

In the pine plantation, trees were more uniform in size, and regularly distributed over space. The relationship between tree height and diameter was also stationary or homogeneous across the study area. In this case, applying spatial models such as LMM and GWR may not have obvious advantages and benefits, while OLS and GAM may be appropriate for fitting the data and predicting the response variable equally well.

Working together: a reciprocal wood flow arrangement to mitigate the economic impacts of natural disturbance

Patriquin, M. N., Lantz, V. A., Stedman, R. C., White, W. A. — 2008-05-12

This study investigates the regional economic impacts resulting from the current mountain pine beetle infestation in British Columbia, Canada, and proposes a mitigation strategy to reduce the negative impacts. The strategy consists of reducing the abrupt timber supply changes in an infested region through a reciprocal wood flow arrangement with an adjacent region less affected by the beetle. Two study areas facing different levels of beetle pressure are investigated: the Quesnel Timber Supply Area (high beetle pressure) and the combined Williams Lake/100 Mile House Timber Supply Area (low beetle pressure). A computable general equilibrium model is constructed for each region and is used to simulate the sensitivity of a suite of economic indicators to various timber supply scenarios for both regions. The results indicate that the negative economic impacts attributed to beetle damage could be reduced under a reciprocal wood flow agreement between the two regions. The degree to which the impacts are reduced in each region depends largely on assumptions about forest regeneration and growth and the terms of the reciprocal wood flow agreement.