Biomass,Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pasture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of México, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha^–1, whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha^–1. The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha^–1. Dead wood (standing and downed combined) comprised 27%–29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass averaged 32.0 Mg ha^–1 in floodplain forest, 17.1 Mg ha^–1 in dry forest, and 5.8 Mg ha^–1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha^–1 in pasture and reached 180 and 12.9 Mg ha^–1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha^–1 in pasture and reached 2623 and 264 kg ha^–1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha^–1 in floodplain forest, 141 Mg ha^–1 in dry forest, and 124 Mg ha^–1 in pasture. Soil C comprised 37%–90% of the total ecosystem C, whereas soil N comprised 85%–98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in México, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmosphere from the burning of biomass in the dry tropical landscapes of México may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.     

The nutrition of grazed pastures in New Zealand

Models of nutrient cycles (PKS) in grazed pastures are used to estimate fertiliser requirements for New Zealand farmers. Work relevant to the development of these models is reviewed. Fertiliser recommendations should be based on the long-term economic maintenance rate. Short-term modifications based on current soil nutrient status (PS) estimated by either soil tests or dynamic cycling models are probably not justified unless current fertiliser use differs greatly from the estimated maintenance rate. The effect of maintenance recommendations should be monitored, and if necessary modified, by soil testing and plant analysis.     

Response of the dung beetle community to different climatic zones: Does the land use system matter?

The conversion of Brazilian savanna into exotic pastures leads to the loss of dung beetles and a decrease in their contribution to ecological functions. We hypothesized that the dung beetle communities from exotic pastures would show greater significant differences between climatic zones, when contrasted to communities from Brazilian savanna in the same region, since dung beetle assemblages in pastures are more simplified. We assessed which variables (purpose of production, type of management, percentage the habitat per buffer, soil penetration resistance, pasture area and herd size) affect more the dung beetle community in exotic pastures. We carried out this study in 48 areas of native Brazilian savannas and exotic pastures distributed across four bioclimatic zones: BZ1, hot with three dry months; BZ2, hot with 4–5 dry months; BZ3, sub-hot with 4–5 dry months and BZ4, meso-thermal with 4–5 dry months of Minas Gerais State, Brazil. In each BZ, six areas of Brazilian savannas and six areas of exotic pasture were selected. In the Brazilian savanna areas, the species richness, abundance and biomass of dung beetles did not differ between the bioclimatic zones, unlike the exotic pastures. The composition of the dung beetle community was different between land use systems and between bioclimatic zones; the interaction between the two factors was also significant. Our results provide evidence that dung beetle communities active in exotic pastures are more susceptible to climatic environmental variations than communities from more complex and stable habitats, such as savannas. Finally, the best model suggested that all the six variables combined explained about 91% of the total variability in species composition observed between sampling sites.     

Transpiration efficiency of three Mediterranean annual pasture species and wheat

Attempts to improve water use efficiency in regions with Mediterranean climates generally focus on increasing plant transpiration relative to evaporation from the soil and increasing transpiration efficiency. Our aim was to determine if transpiration efficiency differs among key species occurring in annual pastures in southern Australia. Two glasshouse experiments were conducted with three key pasture species, subterranean clover (Trifolium subterraneum L.), capeweed [Arctotheca calendula (L.) Levyns] and annual ryegrass (Lolium rigidum Gaudin), and wheat (Triticum aestivum L.). Transpiration efficiency was assessed at the levels of␣whole-plant biomass and water use (W), leaf gas exchange measurements of the ratio of CO₂ assimilation to leaf conductance to water vapour (A/g), and carbon isotope discrimination (Δ) in leaf tissue. In addition, Δ was measured on shoots of the three pasture species growing together in the field. In the glasshouse studies, annual ryegrass had a consistently higher transpiration efficiency than subterranean clover or capeweed by all methods of measurement. Subterranean clover and capeweed had similar transpiration efficiencies by all three methods of measurement. Wheat had W values similar to ryegrass but A/g and Δ values similar to subterranean clover or capeweed. The high W of annual ryegrass seems to be related to a conservative leaf gas exchange behaviour, with lower assimilation and conductance but higher A/g than for the other species. In contrast to the glasshouse results, the three pasture species had similar Δ values when growing together in mixed-species swards in the field. Reasons for these differing responses between glasshouse and field-grown plants are discussed in terms of the implications for improving the transpiration efficiency of mixed-species annual pasture communities in the field. Received: 6 March 1997 / Accepted: 23 December 1997     

Biology of slugs (Agriolimacidae and Arionidae: Mollusca) in New Zealand hill country pastures

Summary Life cycles of the slugs Deroceras reticulatum and Arion intermedius were studied over a 2.5-year period in a sheep-grazed pasture in the Kaimai Range, New Zealand. D. reticulatum approximated a bivoltine phenology, with intervals between consecutive generations ranging from 4 to 7 months and maximum life span from 8 to 12 months. Egg laying occurred in autumn and spring-early summer. Periods of egg hatching were followed by a phase of juvenile growth characterised by linear increase in the logarithm of population mean body weights. This was followed by a phase of reproductive activity. A. intermedius had an annual life cycle. Eggs laid during late summer and autumn hatched during autumn-winter. Immature slugs were characterised by a period of low growth rate during winter followed by a period of rapid growth culminating in reproductive maturity. In both species, the phase of rapid growth rate was associated with enlargement of the hermaphrodite gland. The size of the hermaphrodite gland relative to body weight reached a maximum during the spermatozoon stage, generally coincident with maximum body weight. During the reproductive period of D. reticulatum and A. intermedius the body weight remained relatively stable but the hermaphrodite gland became progressively smaller as the slugs approached the post-reproductive stage. The albumen gland attained maximum weight at the oocyte stage of the hermaphrodite gland, at the onset of reproductive activity. The pasture, strongly dominated by grasses, was heterogenous in slope, vegetation and treading/grazing influences from sheep due to stratification of the habitat into contour tracks, associated with sheep movement around the slope contours, and intertrack areas. High population densities of D. reticulatum were associated with the track edges (kerbs) and the upper intertrack slopes, while densities of A. intermedius were highest for the intertrack slopes. Low densities of both species occurred on the tracks and at sheep campsites. Analysis of cohort life tables indicated that these dispersion patterns were important in the dynamics of the populations. The stage mortality contributing most to the variance in reproductive populations of both species was that between hatching and onset of reproduction; most of the mortality occurred shortly after hatching on south-facing slopes, but tended to occur later in the life cycle on north slopes. For D. reticulatum this mortality was inversely related to initial density and associated, at least in part, with predation by carabids and birds and with treading by sheep. Density relationships, both at whole plot and plot stratum level, indicated that variations in natality had a stabilizing influence on D. reticulatum populations.     

Vapor–pressure deficit and extreme climatic variables limit tree growth

Assessing the effect of global warming on forest growth requires a better understanding of species‐specific responses to climate change conditions. Norway spruce and European beech are among the dominant tree species in Europe and are largely used by the timber industry. Their sensitivity to changes in climate and extreme climatic events, however, endangers their future sustainability. Identifying the key climatic factors limiting their growth and survival is therefore crucial for assessing the responses of these two species to ongoing climate change. We studied the vulnerability of beech and spruce to warmer and drier conditions by transplanting saplings from the top to the bottom of an elevational gradient in the Jura Mountains in Switzerland. We (1) demonstrated that a longer growing season due to warming could not fully account for the positive growth responses, and the positive effect on sapling productivity was species‐dependent, (2) demonstrated that the contrasting growth responses of beech and spruce were mainly due to different sensitivities to elevated vapor–pressure deficits (VPD), (3) determined the species‐specific limits to VPD above which growth rate began to decline, and (4) demonstrated that models incorporating extreme climatic events could account for the response of growth to warming better than models using only average values. These results support that the sustainability of forest trees in the coming decades will depend on how extreme climatic events will change, irrespective of the overall warming trend.     

<Emphasis Type="Italic">Camponotus punctulatus</Emphasis> ant’s demography: a temporal study across land-use types and spatial scales

Agricultural activities promote the explosion of diverse pest populations. In Argentina, the ant Camponotus punctulatus invades agricultural fields after production ceases. The temporal demography and spatial distribution of colonies of C. punctulatus were studied over a five year period using replicated plots of different land use types representing a gradient of increasing agricultural disturbance. We experimentally tested the hypothesis that the increase in C. punctulatus colony density was related to increasing levels of agricultural disturbance. Abandoned rice fields represented the situation with greatest disturbance. Sown pastures were intermediate. Natural grasslands represented no agricultural disturbance. The predictions were (1) the greater the soil disturbance produced by agriculture, the greater the susceptibility for invasion by C. punctulatus, (2) rice fields offers greater opportunities for establishment of colonizing species than sown pastures, and (3) disturbed land use areas that were more recently colonized as well as land use areas with greater soil disturbance will exhibit patterns of colony aggregation at a small scale but with time the patterns will become uniform. Initially, colonies in the abandoned rice fields had a higher annual mortality and larger turnover than in sown pastures. Over five years, abandoned rice fields sustained higher densities of colonies than sown pastures. The colonies were the largest and had the longest lifespans in abandoned ricefields. Natural grasslands had the lowest colony density, survivorship, and size but had variable levels of colonization. More than one type of spatial distribution was found in field replicates. At small spatial scales across disturbed land use types, replicates exhibited regular distributions. At greater spatial scales, spatial distributions were mostly random in sown pastures, there were many cases of aggregation in rice fields, although some cases of uniform distributions were also found in all disturbed land uses. These results highlight significant intraspecific variation in ant demography across types of land use, space, and time, and show a clear predisposition of C. punctulatus to invade and successfully establish in the most disturbed land use types. Hypotheses that can account for the changes in demography across land use types are discussed. Received 20 January 2006; revised 3 August 2006; accepted 26 October 2006.