Assessing the sustainability of harvest of Pterocarpus angolensis in Rukwa Region, Tanzania

Pterocarpus angolensis is being cut for construction and medicinal purposes through much of southern Africa. To assess the sustainability of this offtake, we estimated current harvest practice, population size structure, growth rates and seed production in protected and unprotected areas of Rukwa Region, western Tanzania. Results showed that adult P. angolensis are being removed very rapidly given its slow growth rate as determined from tree ring data, and that regeneration both inside and outside the protected area is very low. A simple model indicates that current harvest is unsustainable raising serious concerns about the long-term viability of this important hardwood species in miombo woodland.     

Growth and structure development of semi-natural larch-spruce-fir (Larix olgensis–Picea jezoensis–Abies nephrolepis) forests in northeast China: 12-year results after thinning

Analyzing and understanding the structure and growth dynamics of semi-natural plantations is useful for their management. Since 1987, 16 plots with 4 treatments (CT: control; LT: light thinning; MT: medium thinning; and HT: heavy thinning) by 0, 20, 30 and 40% of basal area removal, respectively, and four replications were established in semi-natural larch-spruce-fir forests in northeast China. The structure and growth dynamics of semi-natural larch-spruce-fir stands and the effects of thinning on the growth, structure and diversity were examined. A mixed model repeated measures analysis of variance (RMANOVA) was used to test the effects of treatment and time.

Results showed that differences in periodic annual increment (PAI) of stand basal area and volume and the individual diameter and volume among treatments changed over time in a complex statistical interaction. Thinning, however, had a significant effect on growth at tree and stand levels 12 years after thinning while the PAI of the diameter, basal area and volume was positively correlated with thinning intensity. No significant differences were found in the total stand yield among treatments. Composition of tree species group (larch, other conifers and deciduous trees) during monitoring years did not change significantly. Moreover, no significant differences were observed in tree species and size diversity among treatments in the years following thinning. Both thinning and control plots had similar understory plant diversity after the 12 year period. Univariate point pattern analysis revealed that clumped and random distributions were dominant for tree species groups in this study. The current species composition and regeneration dynamics within these semi-natural plantations suggested a development towards mixed coniferous and broad-leaved forests. Management implications for the transformation from larch plantations towards mixed broad-leaved Korean forests with a more diverse structure, the climax vegetation in this region, were discussed.     

Hollows in jarrah (Eucalyptus marginata) and marri (Corymbia calophylla) trees: I. Hollow sizes, tree attributes and ages

One hundred and fifty-four jarrah (Eucalyptus marginata) and 85 marri (Corymbia calophylla) trees were measured and assessed, and the numbers and sizes of hollows in these trees were determined by destructive sampling; 665 hollows were located and measured. The relationship between tree diameter and tree age was determined from counts of annual growth rings on 162 of these trees. Large trees and trees with moderately senescent crowns individually bear the most hollows. Although the number of hollows found in individual trees increased with tree diameter, the distribution of tree diameters in the forest is skewed and the large number of small trees with diameters between 40 and 80 cm contribute approximately 50% of all hollows in the jarrah forest. The distributions of entry size, and of hollow depth, are highly skewed, with small hollows occurring more frequently than large hollows. Although jarrah trees bear more hollows than marri trees and the distribution of entry sizes is similar for both tree species, the hollows in jarrah are significantly smaller than the hollows in marri. Most hollows are cylindrical in shape, vertically oriented and occur in dead wood in the tree crown. Relatively few hollows (14%) occur in the tree bole or at crown break. Counts of hollows made from ground level are inaccurate as estimates of the actual number of hollows in trees.     

Density-dependent effects of northern hardwood competition on selected environmental resources and young white spruce (Picea glauca) plantation growth, mineral nutrition, and stand structural development – a 5-year study

The effects of competition from red raspberry (Rubus idaeus L.) and northern hardwood tree species on white spruce (Picea glauca (Moench) Voss) seedlings were examined on a clearcut site of the boreal mixedwood forest of the Bas-Saint-Laurent region of Quebec, Canada. A controlled experiment involving a gradient of five vegetation densities on the basis of the leaf area index (LAI) was established in a completely randomized plot design with six replications. Each of the five levels of vegetation cover (including vegetation-free plots) were examined to evaluate how they affected environmental factors (quantity and quality of light reaching the spruce seedlings, and soil temperature), spruce growth (height, basal diameter, volume index, and above-ground biomass), spruce mortality, browsing damage, spruce foliar mineral nutrition, as well as the stand structural development, during the first 5 years after seedling planting.

Each spruce growth variable analyzed in this study, according to a RMANOVA procedure, followed a negative hyperbolic form of density dependence of competitive effects. Loss of growth in young white spruce plantations in competition with northern hardwoods is likely to occur with the first few competitors. In cases where higher levels of competing vegetation were maintained over time, loss of spruce growth was extremely severe, to an extent where the exponential growth character of the young trees has been lost. At the end of the fifth year, spruce growing with no interference were larger in mean total above-ground biomass by a factor of 9.7 than those growing with the highest level of vegetation cover. Spruce did not develop a strategy of shade avoidance by increasing tree height, on the contrary. Spruce mortality differed among treatments only in the fifth year, indicating that early evaluation of spruce survival is not a strong indicator of competitive effects, when compared to diameter growth. Spruce foliar N and Ca contents were significantly reduced by the first level of competing vegetation cover, while K increased with the density of the vegetation cover, and P and Mg were not affected. Nitrogen nutrition of young white spruce planted on recently disturbed sites is discussed in relation to the potential root discrimination of this species against soil nitrate, a reaction observed by Kronzucker et al. [Kronzucker, H.J., Siddiqi, M.Y., Glass, A.D.M., 1997. Conifer root discrimination against soil nitrate and the ecology of forest succession. Nature London 385, 59–61]. The effects of hardwood competition indicate a prevalence of competition for light over a competition for nutrients, as revealed by the substantial increase in the h/d ratio of white spruce. Two indicators, h/d ratio and the quantity of light received at the tree seedling level, are suggested as a basis for the management of hardwood competition in a white spruce plantation.

Analysis of the stand structural development indicates that spruce height distribution was affected only by moderate or dense cover of vegetation, while diameter distribution, when compared to competing vegetation-free plots, was affected by the lowest level of vegetation cover. This study shows that competition influenced the stand structural development in the same way as genetic and micro-site factors by aggravating the amplitude of size inequality. The impact of hardwood competition is discussed in view of reaching an equilibrium between optimal spruce plantation growth and benefits from further silvicultural treatments, and maintaining hardwood species known to improve long term site quality, within a white spruce plantation.     

Temperature sensitivity and recruitment dynamics of Siberian larch (Larix sibirica) and Siberian spruce (Picea obovata) in northern Mongolia’s boreal forest

This study sought to clarify the recruitment dynamics and growth of Siberian larch (Larix sibirica) and Siberian spruce (Picea obovata) in relation to changing temperatures in northern Mongolia. These tree species are the primary forest species found in the closed-canopy boreal forest of north-central Mongolia. Mongolia’s boreal forests exist along the southern terminus of the Siberian boreal system in both pure and mixed species stands. I collected tree cores and cross-sections as well as site and tree stature parameters from 118 forest plots in the Darhad valley of north-central Mongolia. Principle components analysis of 130 L. sibirica tree ring series informed the construction of two composite chronologies for this species. A chronology for P. obovata was developed using 24 tree ring series. Correlation analysis between tree ring indices and temperature data showed two distinct growth signals: a positive response to growing season temperatures was exhibited by one L. sibirica chronology and a negative response to spring temperatures was exhibited by a second L. sibirica chronology. The P. obovata chronology exhibited strong negative correlations with mean monthly and mean maximum monthly growing season temperatures. Multiple analyses of variance (MANOVA) indicated that tree stature (dbh, height) and site parameters (latitude, longitude, slope, aspect, elevation) did not significantly predict growth response or species. Forest recruitment events appear episodic for both species. Synchronous establishment of saplings, based on approximate root collar age, suggests an initial floristic model for mixed composition stands likely due to supra-annual variations of fire, land-use and climate. Forest management activities in the region should consider the diverging growth response to temperature shown here by prioritizing protection forests and the various ecosystem services provided by forests in arid ecosystems. In addition, promoting selection harvests over clear-felling would maximize future alternatives under conditions of rapidly changing climate. Care should be taken in new forest management planning activities until adequate information exists on the likely trajectory of this system due to climate-induced forest change.     

Nitrate dynamics after clear felling monitored by in vivo nitrate reductase activity (NRA) and natural N abundance of Deschampsia flexuosa (L.) Trin.

The N dynamics following clear felling, focusing on NO₃⁻ turnover, were studied at four forested sites in southern Sweden. Two different methods were used to study N availability: (i) an in vivo nitrate reductase activity (NRA) bioassay and (ii) measurements of natural abundance of stable N isotopes in leaves of the grass species Deschampsia flexuosa, and in organic soil horizons. At each of the four sites, six plots were established and each year, for 5 consecutive years (1989–1993), one plot per site was felled. Thus, in 1993 there were five plots with different ages since clear felling and one control (closed forest) plot at each site. NRA was analyzed three times annually during the years 1989–1993. Samples for grass and soil analysis of δ¹⁵N, total N and soil pH were taken in 1993 only. NRA rapidly increased after the felling and remained high throughout the studied period. This suggests that there was an increased pool of plant-available soil NO₃⁻ more than 5 years after clear felling. Despite differences in site productivity and N deposition between the four sites, no significant differences in NRA were found between the sites. There were also rapid changes in δ¹⁵N in leaves of D. flexuosa, coinciding with the increases in NRA, during the first 3 years after felling. In contrast to NRA, shoot δ¹⁵N decreased 3–4 years after the felling at three out of four sites. Variations in the δ¹⁵N figures between sites may have been largely due to between-site differences in field-layer retention of N. At two of the sites, where NO₃⁻ leaching was also measured, a correlation was found between the NO₃⁻ concentration in the water and the difference in δ¹⁵N between D. flexuosa leaves from felled and closed forest plots. The data presented here suggest that NO₃⁻ leakage after clear felling is a rapid process, which is influenced by the development of field-layer biomass after the felling. Furthermore, losses of NO₃⁻ through leaching rapidly change the natural abundance of the plant available N pools in the soil.     

Growth analysis on the competition–density effect in Chinese fir (Cunninghamia lanceolata) and Masson pine (Pinus massoniana) stands

The competition–density (C–D) effect for Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) and Masson pine (Pinus massoniana Lamb.) stands was analyzed using the reciprocal equation of the C–D effect, i.e. 1/w=Aρ+B, where w and ρ, respectively, represent mean stem volume and stand density. The C–D curve, which is given by the reciprocal equation on logarithmic coordinates, shifted upward with the progress of time. The coefficients A and B included in the reciprocal equation were calculated at each growth stage. The biological time τ defined as the integral of λ(t), the coefficient of growth in the general logistic growth curve, with respect to physical time t, tended to increase to an asymptote with increasing t. With increasing τ, the coefficient A, the reciprocal of which means the asymptote of yield (=wρ), increased abruptly up to a maximum value and then tended to decrease gradually to a constant level. On the other hand, the coefficient B, the reciprocal of which means the asymptote of mean stem volume, decreased exponentially and tended to close to zero with increasing τ. The λ(t) decreased with increasing stand age, whereas the final yield Y(t) defined as W(t) ρ, where W(t) is the asymptote of w in the general logistic growth curve, tended to increase gradually with increasing stand age.