Jiban Chandra Deb

The impact of climate change on the distribution of two threatened Dipterocarp trees

Abstract Two ecologically and economically important, and threatened Dipterocarp trees Sal (Shorea robusta) and Garjan (Dipterocarpus turbinatus) form mono‐specific canopies in dry deciduous, moist deciduous, evergreen, and semievergreen forests across South Asia and continental parts of Southeast Asia. They provide valuable timber and play an important role in the economy of many Asian countries. However, both Dipterocarp trees are threatened by continuing forest clearing, habitat alteration, and global climate change. While climatic regimes in the Asian tropics are changing, research on climate change‐driven shifts in the distribution of tropical Asian trees is limited. We applied a bioclimatic modeling approach to these two Dipterocarp trees Sal and Garjan. We used presence‐only records for the tree species, five bioclimatic variables, and selected two climatic scenarios (RCP4.5: an optimistic scenario and RCP8.5: a pessimistic scenario) and three global climate models (GCMs) to encompass the full range of variation in the models. We modeled climate space suitability for both species, projected to 2070, using a climate envelope modeling tool “MaxEnt” (the maximum entropy algorithm). Annual precipitation was the key bioclimatic variable in all GCMs for explaining the current and future distributions of Sal and Garjan (Sal: 49.97 ± 1.33; Garjan: 37.63 ± 1.19). Our models predict that suitable climate space for Sal will decline by 24% and 34% (the mean of the three GCMs) by 2070 under RCP4.5 and RCP8.5, respectively. In contrast, the consequences of imminent climate change appear less severe for Garjan, with a decline of 17% and 27% under RCP4.5 and RCP8.5, respectively. The findings of this study can be used to set conservation guidelines for Sal and Garjan by identifying vulnerable habitats in the region. In addition, the natural habitats of Sal and Garjan can be categorized as low to high risk under changing climates where artificial regeneration should be undertaken for forest restoration.

Climatic-Induced Shifts in the Distribution of Teak (Tectona grandis) in Tropical Asia: Implications for Forest Management and Planning

Modelling the future suitable climate space for tree species has become a widely used tool for forest management planning under global climate change. Teak (Tectona grandis) is one of the most valuable tropical hardwood species in the international timber market, and natural teak forests are distributed from India through Myanmar, Laos and Thailand. The extents of teak forests are shrinking due to deforestation and the local impacts of global climate change. However, the direct impacts of climate changes on the continental-scale distributions of native and non-native teak have not been examined. In this study, we developed a species distribution model for teak across its entire native distribution in tropical Asia, and its non-native distribution in Bangladesh. We used presence-only records of trees and twelve environmental variables that were most representative for current teak distributions in South and Southeast Asia. MaxEnt (maximum entropy) models were used to model the distributions of teak under current and future climate scenarios. We found that land use/land cover change and elevation were the two most important variables explaining the current and future distributions of native and non-native teak in tropical Asia. Changes in annual precipitation, precipitation seasonality and annual mean actual evapotranspiration may result in shifts in the distributions of teak across tropical Asia. We discuss the implications for the conservation of critical teak habitats, forest management planning, and risks of biological invasion that may occur due to its cultivation in non-native ranges.

Density, diversity, composition and distribution of street trees in Sylhet Metropolitan City of Bangladesh

Both for environmental and aesthetic purposes, urban street trees have significant importance. In most of the developed cities, this type of plantation is an integral part of landscape planning. Nevertheless, this issue is poorly considered in the cities of developing countries. Consequently, we observe a scarcity of urban street trees. Considering this scenario, we explored the state of urban street tree planting in Sylhet City, which is a growing urban centre in Bangladesh. We assessed the density, diversity, composition and distribution of street trees in the city using different diversity computation indices (e.g. relative abundances, dominance, importance and richness, evenness and diversity) in main and link roads. We also measured the growth pattern of the trees in the study area. It was found that out of 486 studied transects in both main and link roads, 404 had no trees. Sweitenia mahagoni was the dominant species in both road categories. The Mann Whitney U test results suggest that there was no significant difference in tree density and size attributes between the road categories. From these assessments, we concluded that the diversity and distribution of street trees in the study area was very limited. Moreover, there was no scientific arboricultural consideration in species selection to increase street tree planting for the city. We conclude that only proper management and objective-specific endeavor can change the prevailing scenario of the study area.

Comparing normal, lognormal and Weibull distributions for fitting diameter data from Akashmoni plantations in the north-eastern region of Bangladesh

Statistical probability density functions are widely used to model tree diameter distributions and to describe stand structure. The objective of this study was to compare the performance of normal, logarithmic-normal and three-parameter Weibull distributions for fitting diameter data from Akashmoni (Acacia auriculiformis A. Cunn. ex Benth.) plantations grown in the north-eastern region of Bangladesh. Data from 96 plots, established in 24 plantations in north-eastern Bangladesh and ranging in age from 1 to 6 years, were used for this study. The parameters of the Weibull distribution were calculated using maximum likelihood estimation (MLE) and moment estimation (ME) methods. The goodness of fit of normal, lognormal, Weibull MLE and Weibull ME were tested using one-sample Kolmogorov-Smirnov (KS) tests. The KS test results showed that both lognormal and Weibull distributions were equally effective for describing the diameter distributions of these Akashmoni plantations grown in the north-eastern region of Bangladesh.

Freshwater Swamp Forest Trees of Bangladesh Face Extinction Risk from Climate Change

Global climate change is impacting the distribution and abundance of species acting as a major cause of species extinction. It is rapid in freshwater swamp forest ecosystems, since they support disproportionate levels of biodiversity compared to their spatial coverage. The natural swamp forests of Bangladesh have been especially susceptible to climate change as they are limited in range to a few scattered patches in the north-eastern region. We sought to understand how climate change may impact the swamp forests of Bangladesh by modelling distributional changes in Pongamia pinnata and Barringtonia acutangula species, which dominate or co-dominate most swamp forest ecosystems across Bangladesh. We used the maximum entropy (MaxEnt) modelling tool, combined presence-only data of species and bioclimatic variables for two climate scenarios (RCP6.0 and RCP8.5). We compared current, 2050 and 2070 distributions. Results suggest that plant extractable water holding capacity of soil, annual precipitation, precipitation of warmest quarter and mean annual actual evapotranspiration are the key bioclimatic variables for the distribution of both trees. The MaxEnt models indicate that Pongamia pinnata and Barringtonia acutangula trees of Bangladesh face increasing climate stress and may become extinction under both mid-range and extreme climate scenarios.

Structure and composition of understory treelets and overstory trees in a protected area of Bangladesh

Species diversity, composition, and structure of the understory treelet and overstory tree layers were investigated at Lawachara National Park in Bangladesh. Plots were taken following a systematic sampling technique from the whole forest during January to May 2012. In this study, all the overstory trees (diameter at breast height [DBH] ≥ 10 cm) and understory treelets (2 cm ≤ DBH < 10 cm) were investigated and the structure and diversity between them were compared. A total number of 347 individual trees belonging to 69 species and 29 families, and 311 individual treelets belonging to 61 species and 27 families were found in this study. Artocarpus chaplasha and Terminalia catappa were the most important species of tree and treelet respectively. The five most important families both for trees and treelets were Meliaceae, Elaeocarpaceae, Lythraceae, Combretaceae, and Moraceae. The different diversity indices and true diversity value indicate that the overstory tree layer was much more diverse than the understory treelet layer and thus trees may be an appropriate representation of the diversity status of a particular tropical forest type. However, treelet diversity at Lawachara National Park is also considerable and more attention needs to be paid to include treelets in any biodiversity inventory of tropical forest regardless of the size limit of 10 cm DBH.

Characterising the diameter distribution of Sal plantations by comparing normal, lognormal and Weibull distributions at Tilagarh Eco-park, Bangladesh

For many years foresters have been using statistical probability density functions to describe and characterise stand structure. Predicting the current and future yields of a stand is essential for successful stand and timber management. Implicit prediction of current yield is accomplished by using diameter distribution methods. All diameter distribution yield systems predict the number of trees per unit area by diameter class. In this study, the normal, lognormal and the three-parameter Weibull probability density function were compared to characterise the diameter distributions of Sal (Shorea robusta) plantations grown at Tilagarh Eco-park, Bangladesh. Data from 70 plots, established in three plantations, were used for this study. The Weibull parameters were estimated by the maximum likelihood and moments estimator methods. A one-sample Kolmogorov–Smirnov test was used for the goodness of fit for all models. The Kolmogorov–Smirnov test results showed that both lognormal and Weibull distributions were suitable to characterise the diameter distributions of Sal plantations in the study area and may be applicable for other Sal forests in Bangladesh.

Contrasting the financial efficiency of agroforestry practices in buffer zone management of Madhupur National Park, Bangladesh

This research contrasts the financial profitability of different agroforestry practices in community-based buffer zone management of Madhupur National Park (MNP), Bangladesh. This park is the second largest in Bangladesh and subjected to enormous anthropogenic pressure and land rights-related conflicts. This paper reports the different agroforestry practices of villagers in different modules of the buffer zone of MNP, identified by a participatory research approach. It then assesses the profitability of each module on the basis of cost–benefit analysis, net present value, internal rate of return, and annualized income. Results indicate three agroforestry systems were being practised in the buffer zone by the participants: agro-silviculture, agro-silvo-horticulture, and combined woodlot and agroforestry. Among these modules, agro-silvo-horticulture was the most profitable, followed by agro-silviculture then combined woodlot and agroforestry. We recognize that not all of the participants can practise intensive agroforestry; our results suggest that the community-based buffer zone management strategy for MNP would benefit from prioritizing agroforestry practices where possible. This will lead to more successful buffer zone management for the conservation of natural forests while supporting the development of local resource-dependent communities.

An allometric equation for estimating stem biomass of Acacia auriculiformis in the north-eastern region of Bangladesh

Tree biomass plays an important role in sustainable management and in estimating forest carbon stocks. The objective of this study was to select the best model for measuring stem biomass of Acacia auriculiformis in the study area. Data from five hillocks and 120 individual trees from each hillock were used in this study. Twelve different forms of linear, power and exponential equations were compared in this study to select the best model. Two models (VI and XI) were selected based on R 2, adjusted R 2, the Akaike information criterion, F-statistics and the five assumptions of linear regression. Model VI was discarded based on the Durbin-Watson value of autocorrelation of the residuals, then the ARIMA (2, 0, 1) model was used to remove the autocorrelation from the model and the final bias-corrected model XI was derived. The model was validated with a test data set having the same range of DBH and stem height of the training data set on the basis of linear regression, Morisitas similarity index, and t-test for mean difference between predicted and expected biomass. A comparison between the best logarithmic and non-linear allometric model shows that the non-linear model produces systematic biases and overestimates stem biomass for larger trees. The overall results showed that the bias-corrected logarithmic model XI can be used efficiently for estimating stem biomass of A. auriculiformis in the northeastern region of Bangladesh.

Adaptive management and planning for the conservation of four threatened large Asian mammals in a changing climate

Mammals can serve as an indicator of global climate change impacts on species’ distributions due to the wide range of ecological niches they utilize. Tropical Asia encompasses several biodiversity hotspots, is the largest reservoir of mammalian diversity on earth, and has already experienced the extinction of several mammal species either regionally or locally. Global climate change could become a significant driver of species extinction, either directly or synergistically with other factors, such as habitat loss, agricultural expansion, overexploitation, and land use change. Despite the variability of climatic regimes across tropical Asia, the potential impacts of climate change on continental-scale distributions of mammals have not been examined. To address this issue, we developed habitat suitability models for four threatened large mammals (Ursus thibetanus, Elephas maximus, Hoolock hoolock, and Panthera tigris tigris), across their entire distributions in Asia. We used presence-only distribution records and nine bioclimatic and environmental variables and built species-specific habitat suitability models using a maximum entropy algorithm (MaxEnt). We used a moderate and an extreme climate scenario (RCP6.0 and RCP8.5) and three time steps: current, 2050, and 2070. Our results suggest that changes in annual precipitation, annual mean temperature, precipitation, and temperature seasonality could reduce suitable habitat for these mammals and therefore increase their extinction risks. However, several patches of stable habitat are projected to persist through the late twenty-first century, and these climate change refugia areas can be managed as an important strategy for conservation of the mammal species and the maintenance of biodiversity in the face of ongoing climate change. In this context, we recommend the following steps for the conservation of global mammal populations: (i) define the spatial extent (local, regional, or continental scale) of the target mammals, (ii) identify and prioritize climate change refugial areas following ecological niche models or other methods based on biological data, and (iii) implement management actions by analyzing current management tools and the strategies required (e.g., habitat restoration or assisted migration for prioritized species) to achieve long-term conservation goals.