Transforming even-aged coniferous stands to multi-aged stands: an opportunity to increase tree species diversity?

Transforming even-aged coniferous stands into multi-aged ones is attracting growing interest in Europe. However, applying this silvicultural treatment, maintaining a continuous cover and relying on natural regeneration require a deep understanding of the factors driving interspecific competition in the understorey. In particular, knowledge of species-specific response to different light conditions is needed to plan silvicultural treatments and forecast long-term stand composition. In this context, we assessed regeneration (±10–400 cm in height) and light conditions (±1–40 per cent of transmittance) in nine coniferous stands with ranging stand age (±20–120 years) and species composition (Norway spruce, Douglas fir, larch, silver fir and western hemlock) in Belgium. We then modelled interspecific differences in regeneration height growth to forecast the outcome of interspecific competition in different light conditions. Controlling understorey light seems an efficient way to control the interspecific competition, but with some limits, and taking into account sapling size. Maintaining low light conditions (transmittance < 15 per cent) probably reduces interspecific competition as it allows small saplings (height < 100 cm) of most species to grow at a comparable rate. Maintaining higher light conditions might allow a few species to rapidly overgrow the others. Species ranking in height growth changed across the studied light range only between spruce and larch, suggesting that the competition between these two species can be driven through the control of understorey light. On the other hand, controlling canopy openness was found to be insufficient, for example, to promote an advanced regeneration (height ≥ 200 cm) of fir over advanced regeneration of spruce, to promote any species over western hemlock or to promote Douglas fir. Western hemlock, a very shade-tolerant species, was found to grow three times faster than the other species in all the observed conditions (PACL = 5–20 per cent). Douglas fir saplings showed weak growth and marked defoliation, which we hypothetically relate to the recent outbreak of Contarinia pseudotsugae in Western Europe.

The role of advanced regeneration at time of partial harvest on tolerant hardwood stands development

In tolerant hardwood forests of eastern North America, multiple-aged silvicultural systems rely on advanced regeneration to restock the forests. Evaluation of the long-term influence of advanced regeneration on the mature stand is critical for improving management practices. We used a retrospective approach to evaluate the influence of advanced regeneration present at the time of harvest on the current (2012) stand structure and the quality of the growing stock. The study was carried out in partially harvested stands in northwestern New Brunswick, Canada. Trees were sampled from stands with varying degrees of harvest intensities, times since harvest, and site characteristics. Pre-existing advanced regeneration contributed the bulk of trees in the 10–19 cm diameter class across the stands. In stands with low-intensity harvest, the overstory was dominated by sugar maple (Acer saccharum Marsh.) that originated as advanced regeneration. In stands with high-intensity harvest, however, yellow birch (Betula alleghaniensis Britt.) pre-existing advanced regeneration dominated the overstory. The probability of sugar maple and yellow birch being acceptable growing stock peaked at a diameter at breast height (DBH) of about 30–40 cm, while other species combined peaked at a DBH of around 20–25 cm. Our results suggest that harvest intensities based on the dominant advanced regeneration composition and harvesting systems that minimize the risk of physical damage to advanced regeneration are required to achieve partial harvesting objectives.

Asignación de recursos e influencia de los cotiledones en el crecimiento de plántulasde Nectandra ambigens (Blake) C. K. Allen (Lauraceae) en una selva tropical húmeda

In the tropical rain forest, those species having cotyledons, endosperm, or both, which function as storage of parental resources, have a higher probability of surviving as part of the regeneration process. In this study, the influence of cotyledons on the growth of Nectandra ambigens seedlings was analyzed, as well as patterns of resource allocation to shoot, roots and leaves. After germination, seeds where grown under the forest canopy. Five surveys were made at two month intervals. The seedling and cotyledon biomass were significantly different along the time. A negative and significant correlation between cotyledon and seedling biomass was observed. An increase in photosynthetic biomass was measured getting resource from stem biomass and not from roots. This study shows the dependence of Nectandra ambigens seedlings on parental resources for the initial 250 days, which makes them successful in persisting on the forest floor and becoming part of the advanced regeneration.

Regeneración natural y diversidad de especies arbóreas en selvas húmedas

One of the major biological mysteries still to be explained is the maintenance of the enormous local tree species diversity in tropical rain forests .This review explores the relationship between the dynamics of natural regeneration and the evolutionary and ecological processes and mechanisms involved in the origin and maintenance of such extraordinary diversity. First, 1 review ideas on the origin of tree species diversity in the tropics. This review suggests that: i) historical, evolutionary and biogeographical phenomena have a paramount influence on local species richness, and ii) tropical rain forest tree communities are species unsaturated, suggesting that newly originated species may freely migrate across a regional landscape. Second, I describe the forest regeneration process. Gap dynamics, promoted by branch and tree falls, is a fundamental component of the forest canopy renewal. Small gaps (caused by branch falls) facilitate the establishment and survival of seedlings and saplings in the shaded understory (advanced regeneration), whereas large gaps (caused by tree falls) enable trees to reach mature sizes. Gap creation and tree maturation are the extremes of a process of tree and species replacement in the forest canopy. Third, I explore relationships between the tree replacement process and the population and community mechanisms that facilitate maintenance of species diversity at a local scale of a few hectares. I argue and document that factors that promote high species diversity in the advanced regeneration favor high probabilities of heterospecific replacements among canopy trees. Hence, these factors facilitate the maintenance of species diversity in the forest canopy. Frugivores, by promoting diversity in the seed rain community, and biotic agents of seed, seedling and sapling mortality by operating mainly on abundant species, are key factors in facilitating diversity. Furthermore, the existence of trade offs in tree life history attributes (such as seed dispersal capacity, survivorship in the shade and growth under gap conditions) contributes to diversity maintenance by promoting heterospecific replacements. This review does not support ú1e idea that maintenance of tree species diversity in tropical rain forest depends on random processes, as some authors have claimed. instead, I conclude that ecological phenomena have a paran1ounl role on the possibility that a species gains a membresy in such highly diverse forests.

NET PRECIPITATION IN A SEMIDECIDUOUS FOREST FRAGMENT IN VIÇOSA CITY, MG

ABSTRACT We aimed to evaluate the net precipitation and rainfall interception in a fragment of semideciduous forest at both early and advanced stages of regeneration in Viçosa city, Minas Gerais state, southeastern Brazil, from January 2012 through July 2013. Six plots were stablished, three in each regeneration stage area. For throughfall and stemflow quantification, 25 rain gauges and collectors were installed in trees having circumference higher than 15 cm, in each plot. An additional rain gauge was installed in an open area to quantify gross precipitation. Gross precipitation in the studied period was 1934 mm. On average, 79.3% and 72.6% of gross precipitation reached the soil via net precipitation in the areas at early and advanced regeneration stages, respectively. Rainfall interception by the forest canopy was higher in the advanced-regeneration area, corresponding to 25.8% of gross precipitation. In the early- regeneration area, on the other hand, rainfall interception accounted for 20.2% of gross precipitation. This suggests that more densely populated areas intercept more rainfall, and consequently have lower net precipitation. Thus, our study shows that rainwater distribution in forest environments changes according to their regeneration status. The replacement of species at different successional stages renders the water to follow different pathways, such as interception, stemflow, and throughfall. This information helps us understand that the succession process can be slow, yet it is the natural way of forest regeneration.