Morag A. McDonald

The effects of secondary forest clearance and subsequent land-use on erosion losses and soil properties in the Blue Mountains of Jamaica

As pressure on land and forest resources increases, there is a growing need to assess and improve the sustainability of slash-and-burn agriculture in tropical uplands. On steep-hillslopes (24–32 ◦ ) in the forest buffer zone of the Blue Mountains of Jamaica, following clearance of secondary forest the relative impact on surface runoff, soil erosion and soil properties of three land-use treatments: maintained weed-free without cultivation (bare); cultivated with herbaceous crops (agriculture); and cultivated with herbaceous crops and intercropped with Calliandra calothyrsuscontour hedges (agroforestry) was assessed over a 5-year period and compared with an uncleared secondary forest control (forest). The forest provided good protection against surface runoff (which was consistently less than 0.2% of rainfall) and soil erosion losses (less than 500 kg ha −1 per year); agriculture caused a seven-fold increase in surface runoff and 21-fold increase in soil erosion. However, agroforestry was effective in conservation of water (45% reduction in runoff compared with agriculture) and soil (erosion reduced by 35%). The clearance of the secondary forest led to large changes in most measured soil properties. Over 5 years concentrations of organic matter declined by 31%, total N by 38%, exchangeable K by 47%, Ca by 43% and Mg by 56%; over the same period bulk density increased by 48%. Of the soil properties more subject to year-to-year fluctuations, after 5 years the concentration of available P was 36% less in cleared plots than in forest, exchangeable Na was 50% less and moisture content 43% less. Only total P concentration and pH were unaffected. Five years after forest clearance there was no good evidence that these changes had stabilised and very little difference in soil properties was found amongst the three land-use treatments. However, within the agroforestry plots, exchangeable K, Na and sand concentrations became higher under the hedgerows than between them (by 14, 9 and 8%, respectively), whilst clay concentration became higher between them (by 9%). The results indicate that this low-input, contour-tree-hedgerow technology is effective at soil and water conservation through the sieve-barrier effect and increased water infiltration (respectively) and has the potential to enhance the sustainability of this land-use system at a plot scale.

Coppice regrowth in a disturbed tropical dry limestone forest in Jamaica

The Hellshire Hills are an extensive area of tropical dry forest over limestone, located on the south coast of Jamaica. Despite being a protected area, they are subject to encroachment and clearance by charcoal burners, leaving a relatively pristine core within more disturbed areas of forest. Future management requires a consideration of the forest’s resilience to such disturbance, and this study reports on coppice regrowth following clearance. The number of coppice shoots and the length and diameter at breast height (DBH) of the leading shoots were measured on tree stumps 14 months after they were cut in an experiment. Coppice regrowth after cutting was measured on a total of 476 stumps, representing 51 species. Coppice production and growth was vigorous as there was an extremely high incidence of coppice regrowth among and within the species in this forest after it had been cut. Initial regrowth was high with the diameter recovered by the leading shoots after 14 months approximating to 29% per year. Of the 51 species sampled three did not coppice at all and the average percent of individuals found with shoots within each species was 81%. The height and diameter of the leading shoots was significantly different among the species and the average height recorded of the leading shoots was 114 cm while the average diameter recorded was 10 mm. Larger sized stems (>10 cm DBH) on average produced more coppice shoots and larger sized shoots than smaller sized stems (<10 cm DBH) while smaller sized stems recovered more biomass than larger sized stems irrespective of species. The average diameter of the leading shoots decreased significantly with increasing shoot densities. At the species level there was a significant relationship between stem diameter and the number of shoots produced in seven of the 27 species analysed and three species returned significant relationships between stem diameter and average height of leading shoots. Coppice regrowth clearly offers considerable resilience to disturbance in this dry forest where successful regeneration by seed is highly susceptible to rainfall seasonality. However, the different abilities of species to produce coppice shoots will likely affect long-term species diversity if the present rate of clearance continues.

Plant diversity patterns in neotropical dry forests and their conservation implications

Seasonally dry tropical forests are distributed across Latin America and the Caribbean and are highly threatened, with less than 10% of their original extent remaining in many countries. Using 835 inventories covering 4660 species of woody plants, we show marked floristic turnover among inventories and regions, which may be higher than in other neotropical biomes, such as savanna. Such high floristic turnover indicates that numerous conservation areas across many countries will be needed to protect the full diversity of tropical dry forests. Our results provide a scientific framework within which national decision-makers can contextualize the floristic significance of their dry forest at a regional and continental scale.

Nutrient cycling in secondary forests in the Blue Mountains of Jamaica

Secondary forests in the Blue Mountains of Jamaica, subject to human disturbance as well as hurricanes, are coming under increasing land-use pressure with a rising population density and the remaining primary forest becoming more remote from settlements. The practice of slash-and-burn agriculture is commonly carried out by local communities. This study reports on estimates of how the secondary forests have recovered to, or close to, the functioning of undisturbed forest, in terms of key soil and nutrient cycling variables. Nutrient conservation was assessed by measuring inputs in throughfall and litterfall, which were compared to site nutrient capital and losses in surface runoff and erosion. Litterfall, litter standing crop and the growth of bioassay plants were measured in paired plots of primary and secondary forests. The results were compared with data already published for key nutrient cycling variables in primary forests of the Blue Mountains to determine the extent to which nutrient cycling and soil fertility in the secondary forest has recovered to primary forest levels. Rates of nutrient loss in runoff and eroded sediment in the secondary forest were low, basal area had recovered to 81% of primary forest levels, and rates of litterfall were high. Litterfall nutrient concentrations were high, particularly for P, and nutrient cycling was rapid as judged by the high ratio of litterfall to litter standing crop. Soil fertility had recovered well in the secondary forests as judged both by chemical analyses and the growth of the bioassay plants. The results indicate that, for forests in the middle of steep slopes, following the cessation of agriculture, tight nutrient cycling and soil condition and fertility are effectively restored during ca. 20 years of secondary succession. This results in the re-establishment of a forest with effective nutrient conservation which offers a high degree of protection of catchment soil and water resources, and the potential to sustain another cycle of agricultural production. # 2000 Elsevier Science B.V. All rights reserved.

Seedling dynamics after different intensities of human disturbance in a tropical dry limestone forest in Jamaica.

The Hellshire Hills are an extensive area of tropical dry forest over limestone, located on the south coast of Jamaica. Despite being a protected area, they are subject to encroachment and clearance by charcoal burners, leaving a relatively pristine core within more disturbed areas of forest. Future management requires a consideration of the forests resilience to such disturbance, and this study reports on seedling dynamics before and after different intensities of disturbance. The study was conducted within twelve 225-m 2 plots comprising four blocks and three treatments. The treatments were removal of all trees (≥ 2 cm dbh), removal of 50% of stems (> 5 cm) and an uncut control plot. Seedling dynamics were monitored before and 3, 8, 13 and 20 mo after the application of treatments. The study indicates that regeneration by seed was not severely affected by this small-scale disturbance. However, the density of some species was affected by the environmental conditions created by partial and clear cutting, and seasonal effects were more pronounced after disturbance. Biomass recovery by seedlings was negligible in comparison with coppice regrowth which clearly offers considerable resilience to disturbance in this dry forest where successful regeneration by seed is highly susceptible to rainfall seasonality. This will likely affect long-term species diversity if the present rate of clearance continues.

Soil and water conservation projects and rural livelihoods: options for design and research to enhance adoption and adaptation

This paper synthesizes the findings of a workshop which sought to consider the issues of poor uptake, adoption and adaptation of soil and water conservation techniques by farmers post-project by examining the experiences of projects which had research and extension elements. Critical factors contributing to the adoption and adaptation of soil and water conservation techniques by farmers are identified as a more flexible approach and which enables learning within projects; a process rather than output driven approach to soil and water conservation; demonstration of immediate and tangible benefits of soil and water conservation to farmers (production, income, risk-minimization); and avoiding a narrow focus on soil and water conservation—alternatives are ‘better land husbandry’ or ‘sustainable rural livelihoods’ approaches. A number of areas are identified as priorities for further research which would aid the successful adoption of sustainable agricultural techniques and which should guide future research, development and extension, bringing more sustained benefits to farmers, particularly in humid and subhumid hillside regions. Copyright

The repeatability of vegetation classification and mapping

The mapping of habitats as defined by plant communities is a common component of the planning and monitoring of conservation management. However, there are major concerns about the subjectivity and risk of observer bias in most commonly used plant community mapping protocols. This study provides the first test of the consistency of habitat maps based on the mapping units defined by the National Vegetation Classification (NVC), the most widely used classification of plant communities used for habitat mapping on conservation sites in the UK. Seven surveyors mapped the same upland site within five weeks in summer 2008 and the spatial correspondence of the resulting maps was assessed. The NVC is a hierarchical classification and pair-wise spatial agreement between maps decreased with lower levels of sub-classification. The average area of agreement between maps was 77.6% at the habitat level, 34.2% at the community level and 18.5% at the sub-community level. Spatial disparity in the location of mapped boundaries between vegetation types only made a small contribution to overall differences; the majority of variation between maps was due to discrepancies in classification, with vegetation types containing similar species composition most often confused. Factors relating to surveyor effort (cost, time taken and length of route) were not able to explain the substantial differences between maps. However, the methods used to assign areas to vegetation type did seem to have an effect, with surveyors who relied primarily on their own experience having the highest levels of mean agreement with other maps. The study raises serious concerns with current practice of using the NVC for site description and monitoring/surveillance. Since this is just a single case study, we recommend that further work is carried out with the aim of determining the degree and source of variation between surveyors and how consistency can be increased.

Impacts of Type of Fallow and Invasion by Chromolaena odorata on Weed Communities in Crop Fields in Cameroon

In the humid forest regions of southern Cameroon in central Africa, sectoral and macroeconomic policy reforms introduced in the late 1980s have led to intensified land use, which in turn has resulted in, among other environmental consequences, shortened fallow systems dominated by the Asteraceae shrub, Chromolaena odorata (L.) King and Robinson, rather than by secondary forest species. A trial was established to determine the effect of shortened fallow duration and invasion by C. odorata on the weed flora in subsequent mixed food cropping systems. Plots were established in cleared 5- to 7-year-old fallow fields in which the vegetation was either dominated by C. odorata or not, and in which the dominant fallow vegetation in the previous crop-fallow rotation had been either C. odorata, forest, or herbaceous (not dominated by C. odorata). Cassava (Manihot esculenta Crantz), maize (Zea mays L.), and groundnuts (Arachis hypogaea L.) were intercropped and weed species were assessed 6, 14, and 30 weeks after crop planting. Soil analyses were conducted to assess the influence of edaphic traits on the distribution and abundance of dominant weed species. The results clearly indicated an enrichment of the weed flora with time after planting, but little difference between fallow histories. Two groups of weed species corresponded with soil characteristics: C. odorata, Cyathula prostrata, Mariscus alternifolius, Mikania cordata, Musanga cecropioides, and Trema orientalis were preponderant on soils with high clay, N, and C contents, and Ageratum conyzoides, Cyperus sp., Haumania danckelmaniana, Paspalum conjugatum, Pouzolzia guineensis, Richardia brasiliensis, Sida rhombifolia, Stachytarpheta cayennensis, Talinum triangulare, and Triumfetta cordifolia were preponderant on sandier soils with high pH, P, and Mg contents.

Coppice shoot dynamics in a tropical dry forest after human disturbance.

Coppicing is an important regeneration mechanism in tropical dry forest after disturbance, but little is known about the long-term dynamics and the rate of recovery of the coppice shoots following clearance. This study reports on the growth and dynamics of coppice shoots following experimental cutting in a tropical dry forest in Jamaica. The fate of coppice shoots was tracked on a total of 481 stumps, representing 51 species over 10 y. The number of coppice shoots and the height and dbh of the leading shoots were measured on the tree stumps 14 mo and 10 y after cutting. Coppicing was vigorous for most tree species, but the average number of shoots per stump decreased significantly over the 9 y period, from 25 to 8 shoots per stump. The average height and diameter of the leading shoots after 10 y were 4.5 m and 3.8 cm, respectively, and the average percentage diameter recovered by the shoots varied between 36% and 95% among the species. Coppicing facilitates the long-term persistence of this dry forest, and the rapid growth of coppice shoots contributed to the resilience of most species after cutting.

Increased inorganic nitrogen leaching from a mountain grassland ecosystem following grazing removal: a hangover of past intensive land-use?

Heathlands and grasslands occur in montane regions, naturally or due to anthropogenic land-use. These are typically nutrient-poor but exposure to elevated nitrogen deposition and intensive livestock grazing causes large-scale ecological change. We studied the long-term implications of grazing removal on soil and drainage water biogeochemistry and the implications for nitrogen cycling in 50-year replicated grazing exclosures on a montane grassland exposed to high rates of ambient nitrogen deposition. Evidence of ‘ecosystem recovery’ represented by successional change from graminoid to shrub-dominance after cessation of grazing was not reflected in the soil biogeochemistry. Cessation of grazing had a negative impact, with increased soil extractable and soil solution nitrate concentrations; an apparent shift towards a more nitrogen-rich, bacterially dominated microbial community; and the acidification of soils and leachate. The increase in nitrate leaching appears to have been counterbalanced by a decrease in dissolved organic nitrogen leaching, approximately maintaining the overall nitrogen balance of the system, whilst apparently altering ecosystem functioning. High rates of organic matter cycling and inorganic nitrogen uptake in grazed grassland may have sustained ecosystem N limitation under elevated nitrogen deposition. Grazing removal caused long-term over-supply of nitrogen from mineralisation of enriched organic matter, exacerbated by continued high nitrogen deposition, exceeding the uptake demand of heath vegetation and resulting in nitrification and nitrate leaching. This disequilibrium between vegetation and soil following grazing removal has implications for restoration after periods of intensive grazing. Grazing may not simply leave a legacy of nutrient enrichment but its cessation may trigger nitrogen saturation and soil and freshwater eutrophication and acidification which counteract the immediate benefits of natural vegetation recovery. Long term, nitrogen saturation of abandoned grasslands is likely to reduce ecosystem resilience to invasion by nitrophilous species, pathogen attack and vulnerability to environmental pressures such as climate change. We conclude that partial and/or phased reduction in grazing levels may permit the more synchronised recovery of soils and vegetation, thereby avoiding imbalances between nitrogen supply and nitrogen demand and detrimental ecological effects.