Soil pH accounts for differences in species distribution and leaf nutrient concentrations of Brazilian woodland savannah and seasonally dry forest species

Understanding the environmental factors shaping savannah and tropical forest boundaries is important to predict tropical vegetation responses to climate change and other human-mediated disturbances. To better understand the soil characteristics affecting the distribution of Cerradão (Brazilian woodland savannah) and seasonally dry forest (SDF), two vegetation types occurring next to each other in a similar seasonal climate in south-eastern Brazil, we compared several leaf chemical and morphological traits associated with soil pH and resource availability of Cerradão and SDF woody species. Leaf functional traits were measured for 25 Cerradão and 27 SDF species. We performed between-site comparisons with either all species pooled using phylogenetically independent contrasts or species shared between Cerradão and SDF, as well as congeneric pairs. We found higher specific leaf area and leaf nitrogen, potassium, calcium and sulfur concentrations for SDF species. We did not find higher concentrations for leaf phosphorus and manganese (Mn) for SDF species, despite a higher concentration of these nutrients in SDF soil. Cerradão plants had higher leaf iron (Fe), Mn and aluminium (Al) concentrations. For most of the traits assessed, variance was higher among species and genera than between sites. Nutrients with greater availability in the SDF soil did not invariably exhibit higher concentrations in the leaves of SDF species, indicating that these were not limiting for plant productivity. Higher concentrations in the leaves of Cerradão species for Al, Fe and Mn are probably a consequence of lower soil pH, which increases the availability of these elements. In spite of the differences in belowground resources and the divergence for some traits between Cerradão and SDF, our results show high diversity in leaf functional traits within communities and a similarity of leaf functional traits in closely related species in the contrasting habitats. Besides, we surmise that soil pH is an important factor affecting Cerradão and SDF species distribution, excluding SDF species from more acidic soils, because of the toxic effects of Al, and possibly also Mn and Fe.     

Water uptake and transport in lianas and co-occurring trees of a seasonally dry tropical forest

Water uptake and transport were studied in eight liana species in a seasonally dry tropical forest on Barro Colorado Island, Panama. Stable hydrogen isotope composition (D) of xylem and soil water, soil volumetric water content (_v), and basal sap flow were measured during the 1997 and 1998 dry seasons. Sap flow of several neighboring trees was measured to assess differences between lianas and trees in magnitudes and patterns of daily sap flow. Little seasonal change in _v was observed at 90–120 cm depth in both years. Mean soil water D during the dry season was –19 at 0–30 cm, –34 at 30–60 cm, and –50 at 90–120 cm. Average values of xylem D among the liana species ranged from –28 to –44 during the middle of the dry season, suggesting that water uptake was restricted to intermediate soil layers (30–60 cm). By the end of the dry season, all species exhibited more negative xylem D values (–41 to –62), suggesting that they shifted to deeper water sources. Maximum sap flux density in co-occurring lianas and trees were comparable at similar stem diameter (DBH). Furthermore, lianas and trees conformed to the same linear relationship between daily sap flow and DBH. Our observations that lianas tap shallow sources of soil water at the beginning of the dry season and that sap flow is similar in lianas and trees of equivalent stem diameter do not support the common assumptions that lianas rely primarily on deep soil water and that they have higher rates of sap flow than co-occurring trees of similar stem size.     

Patterns of restoration of soil physciochemical properties and microbial biomass in different landslide sites in the sal forest ecosystem of Nepal Himalaya

The pattern of natural restoration in soil components and processes was documented in five landslide-damaged (1–58-year-old) sites in the moist tropical sal (Shorea robusta) forest ecosystem of Nepal Himalaya. Comparisons were made with an undisturbed forest site in the same region. Concentrations of soil organic C, total N, total P and extractable nutrients (Ca, Mg and K) increased with the age of sites. The 58-year-old site showed concentrations of soil organic C, total N and total P that were 75–89% of concentrations in the undisturbed sal forest. The soil microbial biomass, the active fraction of soil organic matter, showed similar seasonal variations at all sites. The amount of mean microbial biomass (expressed as C, N and P contents) increased 4–5 times at the 58-year-old site relative to the 1-year-old site, and the bulk increase occurred within the initial 15 year. The increase in the C/N ratio of soil microbial biomass with age (9.4–11.6 years) reflected change in its composition. Although the net N-mineralization rate increased consistently until 58 years of age, the proportion of nitrification rate relative to ammonification rate distinctly decreased beyond 40 years. On the other hand, the soil available-N (both NO₃⁻ and NH₄⁺) concentrations increased from 1 to 40 year and then declined; with age the proportion of NH₄⁺ increased, however. Rates of restoration in soil properties were faster in the early successional stages (1–15 year) than late stages. Among different soil properties the restoration of soil microbial biomass (C and N) was faster than soil organic C and total N. Best fit power function models showed that the estimated times for the 58-year-old site to reach the level of the undisturbed, mature sal forest would be about 30–35 year for microbial biomass (C and N) and about 100–150 year for organic C and total N. Higher accumulation of soil microbial biomass and high N-mineralization rate at late successional stages indicated the re-establishment of enriched soil and restitution of nutrient cycling during the course of ecosystem restoration.     

Carbon stocks in above‐ground biomass of monoculture plantations,mixed species plantations and environmental restoration plantings in north‐east Australia

Summary The emergence of carbon markets has provided a potential source of funding for reforestation projects. However, there is concern amongst ecologists that these markets will promote the establishment of monoculture plantations rather than more diverse restoration plantings, on the assumption that fast‐growing monocultures are likely to store more carbon than restoration plantings. We examined the validity of this assumption for three predominantly rainforest plantation types established in the moist tropical uplands of north‐east Australia: monoculture plantations of native rainforest conifers (n = 5, mean age 13 years); mixed species plantations of rainforest cabinet timber species, rainforest conifers and eucalypts (n = 5, mean age 13 years); and, environmental restoration plantings comprised mostly of a diverse range of rainforest trees (n = 10, mean age 14 years). We found that restoration plantings stored significantly more carbon in above‐ground biomass than monoculture plantations of native conifers (on average, 106 t vs 62 t carbon per ha); and tended to store more carbon than mixed species timber plantations which were intermediate in value (86 t carbon per ha). Carbon stocks were higher in restoration plantings than in monoculture and mixed species plantations for three reasons. First, and most importantly, restoration plantings were more densely stocked than monoculture and mixed species plantations. Second, there were more large diameter trees in restoration plantings than monoculture plantations. Third, the trees used in restoration plantings had a higher average wood density than the conifers used in monoculture plantations. While, on average, wood density was higher in mixed species plantations than restoration plantings, the much higher stocking rate of restoration plantings meant they stored more carbon than mixed species plantations. We conclude that restoration plantings in the moist tropics of north‐east Australia can accumulate relatively high amounts of carbon within two decades of establishment. Comparison with reference rainforest sites suggests that restoration plantings could maintain their high stocking rates (and therefore high biomass) as they develop in future decades. However, because restoration plantings are currently much more expensive to establish than monoculture plantations, restoration plantings are unlikely to be favoured by carbon markets. Novel reforestation techniques and designs are required if restoration plantings are to both provide habitat for rainforest biota and store carbon in biomass at a cost comparable to monoculture plantations.     

Nitrogen and phosphorus resorption in trees of a Mexican tropical dry forest

Resorption efficiency (RE) and proficiency, foliar nutrient concentrations, and relative soil nutrient availability were determined during 3 consecutive years in tree species growing under contrasting topographic positions (i.e., top vs. bottom and north vs. south aspect) in a tropical dry forest in Mexico. The sites differed in soil nutrient levels, soil water content, and potential radiation interception. Leaf mass per area (g m^–2) increased during the growing season in all species. Soil P availability and mean foliar P concentrations were generally higher at the bottom than at the top site during the 3 years of the study. Leaf N concentrations ranged from 45.4 to 31.4 mg g^–1. Leaf P varied from 2.3 to 1.8 mg g^–1. Mean N and P RE varied among species, occasionally between top and bottom sites, and were higher in the dry than in the wet years of study. Senesced-leaf nutrient concentrations (i.e., a measure of resorption proficiency) varied from 13.7 to 31.2 mg g^–1 (N) and 0.4 to 3.3 mg g^–1 (P) among the different species and were generally indicative of incomplete nutrient resorption. Phosphorus concentrations in senesced leaves were higher at the bottom than at the top site and decreased from the wettest to the the driest year. Soil N and P availability were significantly different in the north- and south-facing slopes, but neither nutrient concentrations of mature and senesced leaves nor RE differed between aspects. Our results suggest that water more than soil nutrient availability controls RE in the Chamela dry forest, while resorption proficiency may be interactively controlled by both nutrient and water availability.     

Non‐continuous reproductive phenology of animal‐dispersed species in young forest restoration plantings

Tree species that produce resources for fauna are recommended for forest restoration plantings to attract pollinators and seed dispersers; however, information regarding the flowering and fruiting of these species during early growth stages is scarce. We evaluated the reproductive phenology of animal‐dispersed tree species widely used in Atlantic Forest restoration. We marked 16 animal‐dispersed tree species in 3‐ to 8‐year‐old forest restoration plantings in Itu‐São Paulo, southeast Brazil. We noted the age of the first reproductive event, flowering and fruiting seasonality, percentage of trees that reached reproductive stages, and intensity of bud, flower, and fruit production for each species. Flowering and fruiting are seasonal for most species; only two, Cecropia pachystachya and Ficus guaranitica, exhibited continuous flowering and fruiting throughout the year; we also identified Schinus terebinthifolia and Dendropanax cuneatus fruiting in the dry season during resource scarcity. Therefore, we recommend all as framework species, that is, species that are animal‐dispersed with early flowering and fruiting potential, for forest restoration. Further, we recommend identifying and planting similar animal‐dispersed tree species that produce fruits constantly or in the dry season to maximize fauna resource availability throughout the year in tropical forest restoration plantings. Abstract in Portuguese is available with online material     

Water relations of indigenous versus exotic tree species,growing at the same site in a tropical montane forest in southern Ethiopia

The objective of the study was to compare the water relations of two indigenous [Podocarpus falcatus (Thunb.) Endl., Croton macrostachys Hochst. ex. Del.] and two exotic tree species (Eucalyptus globulus Labille., Cupressus lusitanica Miller) growing in the same location in the montane Munessa State Forest, southern Ethiopia. Stem flow was measured with Granier type thermal dissipation probes. Sap flux, normalized per unit sapwood area, and the total sapwood areas of the particular trees were used to estimate daily transpiration. Maximum daily transpiration values (60 kg water) were recorded for Croton when at full foliage. After shedding most of its leaves in the dry season transpiration was reduced to 8 kg per day. Eucalyptus had the next highest transpiration (55 kg), in this case at the peak of the dry season. It transpired 4–5 times more than Podocarpus and Cupressus trees of similar size. Maximum stem flux density was tree-size dependent only in Croton. Diurnal patterns of stem flux indicated that Croton, Eucalyptus and Podocarpus, in contrast to Cupressus, responded more directly to light than to atmospheric water pressure deficit. At high VPD (>1.0 kPa) stem flux reached a plateau in Croton and Podocarpus indicating stomatal limitation. Per unit leaf area Croton had the highest and Podocarpus and Cupressus the lowest daily transpiration rates. In summary, the pioneer tree Croton had the lowest and Podocarpus the highest water use efficiency. The contribution of the study to the understanding of the role of each tree species in the hydrology of the natural forest and the plantations is discussed.