Sustained mahogany (Swietenia macrophylla) plantation heartwood increment

In a search for an increment-based rotation for plantation mahogany (Swietenia macrophylla King), heartwood volume per tree was regressed on DBH (trunk diameter outside bark at 1.4 m above the ground) and merchantable height measurements. We updated a previous study [Wadsworth, F.H., González González, E., Figuera Colón, J.C., Lugo Pérez, J., 2003. Fifty-nine year performance of planted big-leaf mahogany (S. macrophylla King) in Puerto Rico. In: Lugo, A. Figueroa Colón, J.C., Alayón, M. (Eds.), Big-leaf Mahogany, Genetics, Ecology, and Management. Springer-Verlag, New York, pp. 343–357] and determined heartwood volume increment in a plantation between the 40th and 69th years. Although DBH growth rate declined with age, that for heartwood increased throughout the entire 69 years. This, with expected growing value per unit of heartwood volume with tree size, presents incentives for long rotations.     

Natural and human disturbances affect natural regeneration of Swietenia macrophylla: Implications for rainforest management

In this paper we document how changes in forest structure mediated by natural (hurricane) and human (logging) disturbances affect natural regeneration of Swietenia macrophylla via its effects on seed predation, germination, and early recruitment. Work was carried out in two ejidos within the Mayan zone of Quintana Roo, differing with regard to their exposure to the effect of the hurricane Dean 2008 (Naranjal Poniente; high hurricane impact) and Señor (with no evidence of hurricane effects). We experimentally set S. macrophylla seeds in both hurricane conditions and contrasting logging effects, in three treatments: (1) open access, (2) rodent and insect access and (3) rodent and insect exclusion. We measured three dependent variables: (i) number of seeds predated, (ii) number of seeds germinated and (iii) number of seedlings established. We found that the three variables measured were differentially affected by logging and hurricane Dean. Seed predation was higher in areas with hurricane effects; germination showed no differences, but early recruitment was differentially affected by the two conditions, via an increasing of herbivory and sun exposition. Both types of disturbances studied occurred periodically in Yucatan Peninsula and elsewhere in tropics, so it is necessary to implement specific actions in order to face changes in mahogany natural regeneration stocks, and supporting the future management of tropical forests.     

Post-logging loss of genetic diversity in a mahogany (Swietenia macrophylla King,Meliaceae) population in Brazilian Amazonia

Big-leaf mahogany (Swietenia macrophylla King, Meliaceae) is the most valuable widely traded timber species from the Neotropics. Mahogany has been severely overexploited throughout its natural range, which stretches from southern Mexico through Central and South America as far south as the Bolivian Amazon. We investigated effects of selective logging on the genetic diversity of a mahogany population in eastern Amazonia. Using eight hypervariable microsatellite loci, we estimated the number of alleles per locus (A), mean expected and observed heterozygosity (H_e, H_o), the number of distinct multilocus genotypes (G_o), and the inbreeding coefficient (f) for pre- and post-logging cohorts (adults and seedlings) collected over identical spatial scales. We found a significant reduction in the number of alleles, observed heterozygosity, and distinct multilocus genotype number in the post- compared to the pre-logging cohort. The loss of genetic diversity is likely to have occurred due to a reduction in the effective population size as a consequence of logging, which leads to the loss of alleles and limits mating possibilities. The results raise concerns about the conservation genetics of logged mahogany populations where a high proportion of adults are removed from the system. We suggest initiatives to promote the maintenance of genetic diversity as an integral part of the production system, allowing genetically sustainable use of this valuable timber resource.     

Genetic (AFLP) diversity of nine Cedrela odorata populations in Madre de Dios,southern Peruvian Amazon

Cedrela odorata L., one of the most important neotropical timber species, is threatened by deforestation and unsustainable logging in many parts of its natural range. Information on patterns of genetic variation is useful in informing both reforestation and genetic conservation activities. However, to date, no such information is available in Peru or elsewhere in South America. In the present study, genetic diversity between and within nine Peruvian populations of the species, based on amplified fragment length polymorphism (AFLP) markers, is reported. Overall diversity level was high (Ht = 0.22), as expected for a widespread, long-lived tropical species, and consistent with previous studies carried out in Central America. Levels of intrapopulation diversity were higher than those previously reported for the species (Hs = 0.13–0.21). Analysis of molecular variation revealed genetic differences between two population groups located on different rivers and between populations located on the same rivers. Differences between groups were greater than those within groups. Genetic and geographical distances were significantly correlated. The relatively strong genetic differences between populations may be related to the riparian, essentially one-dimensional spatial distribution pattern of the populations studied. No difference was found in percentage of polymorphic loci between relatively undisturbed and logged populations. The existence of appreciable genetic differentiation over a relatively small part of the species range in the Peruvian Amazon suggests the need for caution in use of seed outside its zone of origin. For genetic conservation purposes, it would probably be prudent to sample (ex situ) or conserve (in situ) populations in each of the major watersheds of the Peruvian Amazon.     

What loggers leave behind: Impacts on big-leaf mahogany (Swietenia macrophylla) commercial populations and potential for post-logging recovery in the Brazilian Amazon

The sustainability of current harvest practices for high-value Meliaceae can be assessed by quantifying logging intensity and projecting growth and survival by post-logging populations over anticipated intervals between harvests. From 100%-area inventories of big-leaf mahogany (Swietenia macrophylla) covering 204 ha or more at eight logged and unlogged forest sites across southern Brazilian Amazonia, we report generally higher landscape-scale densities and smaller population-level mean diameters in eastern forests compared to western forests, where most commercial stocks survive. Density of trees ≥20 cm diameter varied by two orders of magnitude and peaked at 1.17 ha⁻¹. Size class frequency distributions appeared unimodal at two high-density sites, but were essentially amodal or flat elsewhere; diameter increment patterns indicate that populations were multi- or all-aged. At two high-density sites, conventional logging removed 93–95% of commercial trees (≥45 cm diameter at the time of logging), illegally eliminated 31–47% of sub-merchantable trees, and targeted trees as small as 20 cm diameter. Projected recovery by commercial stems during 30 years after conventional logging represented 9.9–37.5% of initial densities and was highly dependent on initial logging intensity and size class frequency distributions of commercial trees. We simulated post-logging recovery over the same period at all sites according to the 2003 regulatory framework for mahogany in Brazil, which raised the minimum diameter cutting limit to 60 cm and requires retention during the first harvest of 20% of commercial-sized trees. Recovery during 30 years ranged from approximately 0 to 31% over 20% retention densities at seven of eight sites. At only one site where sub-merchantable trees dominated the population did the simulated density of harvestable stems after 30 years exceed initial commercial densities. These results indicate that 80% harvest intensity will not be sustainable over multiple cutting cycles for most populations without silvicultural interventions ensuring establishment and long-term growth of artificial regeneration to augment depleted natural stocks, including repeated tending of outplanted seedlings. Without improved harvest protocols for mahogany in Brazil as explored in this paper, future commercial supplies of this species as well as other high-value tropical timbers are endangered. Rapid changes in the timber industry and land-use in the Amazon are also significant challenges to sustainable management of mahogany.     

The utility of the predictive decapitation test as a tool for early genetic selection for Hypsipyla tolerance in big-leaf mahogany (Swietenia macrophylla King)

The plantation silviculture of mahogany (Swietenia spp.) has been severely limited by the attack of the mahogany shoot borer (Hypsipyla spp.), which greatly reduces or nullifies the commercial timber value of attacked trees by causing forking low on the stem. Selection for resistance, including tolerance (the capacity to retain apical dominance after attack), is widely considered to have potential as one element in an integrated pest management system. It has been suggested that the response of young seedlings to a predictive decapitation test might be useful as a means of detecting genotypes with high tolerance. This paper describes a pilot provenance/progeny experiment aimed at exploring this hypothesis. The number of shoots produced in response to attacks in the first year of field growth by Costa Rican material was found to be significantly correlated with response to decapitation in the nursery, on both individual tree and family mean bases. It appears, therefore, that the decapitation test could have a role in early selection for tolerance of Hypsipyla in mahogany. However, the presence of within-provenance genetic variation in response to Hypsipyla attack was not established.     

Relative susceptibility of four species of African mahogany to the shoot borer Hypsipyla robusta (Lepidoptera: Pyralidae) in the moist semideciduous forest of Ghana

We examined the relative susceptibility of four mahogany species, Khaya ivorensis, Khaya anthotheca, Entandrophragma angolense, and E. utile, to Hypsipyla robusta attack. Seeds were obtained from one to three parent trees for each species. The research was conducted in the moist semideciduous forest zone in Ghana and used a randomized complete block design. Tree height and diameter and height to first branch were measured until 24 months after out-planting in the field. H. robusta damage was assessed by counting the numbers of shoots attacked, branches, and dead shoots. Khaya spp. grew better but experienced more attack than Entandrophragma spp. The relative susceptibility to H. robusta attack, from most to least, of the four species was: K. anthotheca > K. ivorensis > E. angolense > E. utile. At 24 months, the mean number of shoots attacked per tree ranged from 1.0 for an E. utile seed source to 3.6 on for a K. anthotheca seed source. At 15 months, K. anthotheca and K. ivorensis started branching at about 1.5 m, but height of clear trunk increased over time due to self-pruning. As K. anthotheca grew taller, the number of H. robusta attacks per tree declined. This suggested that selection of genotypes and species that are tolerant of H. robusta attack based on infestation of young plants may not be appropriate. Genetic factors more completely reflecting the response of different species and genotypes to H. robusta attack may manifest themselves at later growth stages.     

Enrichment of big-leaf mahogany (Swietenia macrophylla King) in logging gaps in Bolivia: The effects of planting method and silvicultural treatments on long-term seedling survival and growth

To insure adequate regeneration and future timber yields of mahogany (Swietenia macrophylla King), many logged forests will have to be restocked through enrichment planting and managed using silvicultural techniques that maintain this species’ long-term survival and growth. This study compared the effects of planting method and two silvicultural treatments on the survival and growth of mahogany seedlings in logging gaps in Bolivia. We tested the hypotheses that survival and growth will be higher among transplanted seedlings than seedlings established from sown seeds and higher in silvicultural treatments that reduce competing vegetation and increase light. The first silvicultural treatment consisted of gaps logged 6 months prior to planting, gaps logged just prior to planting, and gaps treated with herbicide prior to planting. The second treatment, applied 12 months after planting, consisted of manual vegetation cleaning around mahogany seedlings in half of the gaps. The first hypothesis was supported in terms of initial seedling growth but not survival, which was similar between planting methods during the 12-92 months after planting. Transplanted seedlings grew significantly faster than those established from sown seeds during the first year, but this growth advantage disappeared by the second year. Although transplants were 84 cm taller than seed-sown seedlings by the end of the study, this height gain was probably not worth the cost of growing and transplanting seedlings. The second hypothesis was supported in terms of both survival and growth. A significantly greater proportion of seedlings survived in herbicide (62%) compared to 6-month-old (46%) and recent gaps (18%) and in cleaned (51%) versus control gaps (39%). Seedlings initially grew faster in herbicide and recent gaps than in 6-month-old gaps. These differences among silvicultural treatments were largely explained by canopy cover, which, throughout the study, was at least 14% lower in herbicide gaps and 9% lower in cleaned gaps relative to their respective alternatives. By 64 months growth diminished to near zero and no longer differed among gap treatments, despite lower canopy cover in herbicide gaps. By 92 months, saplings in herbicide gaps were only 145 and 77 cm taller than those in recent and 6-month-old gaps, respectively. To maximize survival and growth of mahogany seedlings in logging gaps while minimizing costs, silvicultural strategies should focus on direct seed sowing and appropriately timed interventions (i.e. manual cleaning) to control competing vegetation.     

Effects of active pasture,teak (Tectona grandis) and mixed native plantations on soil chemistry in Costa Rica

In this study soil samples were taken from the O/A and B horizons of undisturbed forest, active pasture, and 8- to 12-year-old teak and mixed native plantations. Samples were analyzed for K, Ca, Mg, soil organic carbon, pH, exchangeable acidity, bulk density, and compared with a fertility equation. Bulk density was significantly lower in the undisturbed forest than other land uses, suggesting that after approximately 10 years of growth neither plantation lowered bulk density significantly from that of the active pasture. Teak plantations had significantly higher Mg and K (B horizon) and Ca (O/A horizon) concentrations than the undisturbed forest. This trend suggests that exchangeable base concentrations increase when land use changes from undisturbed forest to pasture, then pasture to plantation, with the most pronounced effect of this in teak plantations exhibiting more high fertility plots than other land uses. Soil organic carbon concentration was similar for all land uses except for a significantly lower concentration in teak plantations than in active pasture (O/A horizons). These results suggest that teak plantations may be advantageous for increasing soil fertility but, with respect to restoration of undisturbed forest conditions, present significant deviations in soil chemistry. Options for improved plantations soil management are discussed.     

Biomass conversion factors (density and carbon concentration) by decay classes for dead wood of Pinus sylvestris, Picea abies and Betula spp. in boreal forests of Sweden

For estimating the amount of carbon (C) in dead wood, conversion factors from raw volume per decay class to dry weight were developed using three different classification systems for the species Norway spruce (Picea abies L. Karst), Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth and B. pubescens Ehrh) in Sweden. Also the C concentration in dead wood (dry weight) was studied. About 2500 discs were collected from logs in managed forests located on 289 temporary National Forest Inventory (NFI) sample plots and in 11 strips located in preserved forests. The conversion factors were based on an extensive data compilation with a wide representation of different site-, stand-, species- and dead wood properties and were assumed to represent the population of fallen dead wood in Sweden. The density decreased significantly by decay class and the range in density for decay classes was widest for the NFI decay classification system, suggesting this to be the most suitable. The C concentration in dead wood biomass increased with increasing decay class and in average Norway spruce (P. abies) showed a lower C concentration than Scots pine (P. sylvestris). The average dead wood C store of Swedish forests was estimated to 0.85 Mg C/ha.     

Changes in light levels and stream temperatures with loss of eastern hemlock (Tsuga canadensis) at a southern Appalachian stream: Implications for brook trout

The exotic invasive insect, hemlock woolly adelgid (Adelges tsugae Annand), is causing mortality in eastern hemlocks (Tsuga canadensis [L.] Carr.) throughout the eastern U.S. Because hemlocks produce dense shade, and are being replaced by hardwood species that produce less shade, their loss may increase understory light levels. In the southern Appalachians, increases in light could increase stream temperatures, threatening species such as brook trout (Salvelinus fontinalis). We studied changes in light and stream temperature with eastern hemlock decline at a headwater southern Appalachian brook trout stream. Our results indicate that stream light levels have increased significantly with adelgid infestation. Leaf-on light levels are currently significantly higher (P < 0.02) in plots containing high basal areas of hemlock (mean global site factor (GSF)(SE) = 0.267(0.01)) compared with plots containing no hemlock (mean GSF(SE) = 0.261(0.01)), suggesting that increases in light have occurred with hemlock decline. The Normalized Difference Vegetation Index (NDVI), a remotely sensed metric of vegetation density, decreased with hemlock decline from 2001 to 2008. In 2001, NDVI showed no relationship (R² = 0.003; F = 0.14; P = 0.71) with hemlock basal area, but by 2008, there was a significant negative relationship (R² = 0.352; F = 19.55; P < 0.001) between NDVI and hemlock basal area. A gap experiment showed that light levels may increase by up to 64.7% more (mean increase in GSF = 27.5%) as hemlocks fall, creating gaps in the canopy. However, stream temperatures did not increase with hemlock decline during the study period, and we found that ground water inputs have a stronger influence on water temperature than light levels at this site. Linear regression showed a significant negative relationship between water temperature and proximity to ground water sources (R² = 0.451; F = 13.14; P = 0.002), but no relationship between water temperature and light levels (R² < 0.02; P > 0.05). In addition, by comparing light levels between plots containing hemlock and those containing only hardwoods, we found that if hemlocks are replaced by hardwoods, light levels under an all-hardwood canopy (mean GSF(SE) = 0.240(0.005)) are unlikely to be higher than they are under the current forest (mean GSF(SE) = 0.254(0.007)). These results suggest that loss of hemlock along southern Appalachian headwater streams could have short-term impacts on light levels, but that long-term changes in light levels, increases in water temperature, and adverse effects on brook trout may be unlikely.     

A comparison of riparian plant community response to herbivory by beavers (Castor canadensis) and ungulates in Canada's boreal mixed-wood forest

Herbivores, such as beavers (Castor canadensis) and ungulates, can dramatically alter the species composition and structure of riparian vegetation communities. We examined how four different levels of beaver presence and ungulate density influenced the structure and composition of riparian vegetation communities in the mixed-wood boreal region of east-central Alberta, Canada. Beavers, as central-place foragers, focus on similar species utilized by the six ungulate species in our study area. However, we found that vegetation responded differently relative to the percent cover and heights of emergent, shrub, herbaceous dicots, and tree species depending on the varying combinations of beaver presence and ungulate densities. There was an obvious negative effect of ungulate herbivory on shrub heights and cover; however, we were unable to find any significant effects on the overall composition and structure of vegetation communities that were exclusively related to beaver activities. In areas with no beavers and lower ungulate densities there was a positive growth response in preferred species (e.g., Salix spp., Prunus spp.), which clearly demonstrated the degree of combined influence that these herbivores have on riparian communities. In these areas, Salix spp. was four times shorter than in areas where beavers were not present and ungulate densities were lower. In addition percent cover was lowest for emergent vegetation and herbaceous dicots in areas with higher ungulate densities. Many studies focus on single-species effects on forage resources, yet our research shows that the study of combined effects of multiple herbivores can provide a new insight into these complex systems.