Tree mortality following partial harvests is determined by skidding proximity

Recently developed structural retention harvesting strategies aim to improve habitat and ecological services provided by managed forest stands by better emulating natural disturbances. The potential for elevated mortality of residual trees following such harvests remains a critical concern for forest managers, and may present a barrier to more widespread implementation of the approach. We used a harvest chronosequence combined with dendrochronological techniques and an individual-based neighborhood analysis to examine the rate and time course of residual-tree mortality in the first decade following operational partial "structural retention" harvests in the boreal forest of Ontario, Canada. In the first year after harvest, residual-tree mortality peaked at 12.6 times the preharvest rate. Subsequently, mortality declined rapidly and approached preharvest levels within 10 years. Proximity to skid trails was the most important predictor both of windthrow and standing death, which contributed roughly equally to total postharvest mortality. Local exposure further increased windthrow risk, while crowding enhanced the risk of standing mortality. Ten years after harvest, an average of 10.5% of residual trees had died as a result of elevated postharvest mortality. Predicted cumulative elevated mortality in the first decade after harvest ranged from 2.4% to 37% of residual trees across the observed gradient of skid trail proximity, indicating that postharvest mortality will remain at or below acceptable rates only if skidding impacts are minimized. These results represent an important step toward understanding how elevated mortality may influence stand dynamics and habitat supply following moderate-severity disturbances such as partial harvests, insect outbreaks, and windstorms.     

Large ontogenetic declines in intra-crown leaf area index in two temperate deciduous tree species

The widespread occurrence of age-related changes in leaf morphology and allocation suggests that the leaf area index of individual trees (intra-crown LAI) may decline late in ontogeny. We used direct, within-canopy measurements to quantify the LAI of canopy trees with exposed crowns of two temperate deciduous species. Intra-crown LAI declined from approximately 7 to 4 in Acer saccharum, and from approximately 9.5 to 6.5 in Betula alleghaniensis, as tree size increased (from 15 to 72 cm diameter at breast height [dbh]). For A. saccharum, age (which varied from 30 to 160 years) was a significantly better predictor of LAI decline than dbh. We also modeled the effect of ontogenetic declines in LAI on understory light availability and found that light transmission increases significantly as canopy trees grow and mature. Our results thus suggest that gradual declines in LAI with tree age may play an important and overlooked role in contributing to the heterogeneity of sub-canopy light regimes in mature forests.     

Analysis of changes in a shelterbelt network landscape in central Jutland,Denmark

This paper investigates the changes in a shelterbelt network between 1950 and 1997 in a study area in central Jutland, Denmark. The analysis consists of two parts. Firstly, the changes in key parameters which are of landscape ecological importance (shelterbelt density, connectivity, node type and landscape grain size) are examined. The results of this analysis show that shelterbelt density varied considerably during the period due to changes in biophysical and socioeconomic factors. A GIS analysis shows that landscape grain size decreased in 45 percent of the area. A major change of shelterbelt type occurred in the area with landscape ecological and visual scenery implications. It appears that public-subsidised shelterbelt planting programmes were the most important factors influencing changes in the shelterbelt network in the study area while differences in soil type were less important. Secondly, data from a questionnaire survey were used to examine the driving forces behind shelterbelt activities over a 10 year period. Hobby farmers were the most active farmer group in shelterbelt planting and agronomic reasons were the most important motives for shelterbelt planting in the study area during this period. Furthermore, it appeared that microclimate effects and aesthetical considerations have become important reasons for shelterbelt planting.