Litter Nutrient Dynamics During Succession in Dry Tropical Forests of the Yucatan: Regional and Seasonal Effects

Land-use change in the tropics is creating secondary forest at an unprecedented rate. In the tropical Americas, mature dry tropical forest is rapidly being converted to secondary forest during the fallow period of shifting cultivation. We investigated litter phosphorus (P) and nitrogen (N) dynamics in forests recovering from shifting cultivation of maize (corn) in three regions of the Southern Yucatan Peninsula, Mexico. Our goal was to understand how nutrient and water availability affect forest recovery following conversion of mature forest to agricultural land. To investigate such changes at a regional scale, newly fallen litter was collected monthly along a seasonal, a successional, and a precipitation gradient. Reflecting possible P limitation, litter P concentration declined with forest age, while litter N concentration did not differ between age classes. Average litter P concentration from the southern, wettest region was 0.87 mg/g, almost twice the litter P concentration in the drier central and northern regions (0.44 and 0.45 mg/g, respectively). Average N concentrations of litter from the three regions ranged from 1.1% to 1.2%, with no regional differences. However, minima in both P and N concentration from all regions were pronouncedly timed with peak litterfall, suggesting nutrient retranslocation during periods of water stress. Additionally, successional differences in litter P were clearest during wetter months. P nutrient-use efficiency was lowest in the southern region and highest in the central and northern study regions. N nutrient-use efficiency was up to 40 times lower than P nutrient-use efficiency and showed no regional differences. Overall, our results suggest that litter nutrient dynamics in secondary dry tropical forests of the Southern Yucatan are strongly influenced by water and nutrient availability, especially P, as well as land-use history.     

Seedling Establishment of a Canopy Tree Species in Malaysian Tropical Rain Forests

Abstract Unlike many other canopy tree species in tropical rain forests, Dryobalanops aromatica (Dipterocar-paceae, local name: Kapur) establishes monospecific dominant (monodominant) forests in Peninsular Malaysia. In natural conditions, monodominance of Kapur does not necessarily mean low species diversity of the Kapur forests. While the emergent canopy layer is occupied by Kapur, many other plant species, which are common to lowland dipterocarp primary forests in the same region, are found in lower canopy layer and understory.
To understand the ecological implications of the monodominance of Kapur, we monitored post dispersal survival and seedling establishment in a pure stand of Kapur in a plantation in Kepong, near Kuala Lumpur. Immediately after seed fall, seeds and cotyledon-stage seedlings suffered high predation by vertebrates such as rodents. The predation pressure was higher in a more general fruiting year (1991) than in a sporadic fruiting year (1992). In contrast to the high mortality of seeds and newly emerged seedlings, seedlings surviving to the six-leaf stage showed low mortality, which allowed the establishment of a sapling bank.
The occurrence of saplings of Kapur with a wide range of size classes in natural forests indicates that this species is more shade tolerant than other dipterocarp species such as the Shorea group and that it could well respond to enhanced light conditions caused by canopy opening. These characteristics may partly contribute to maintaining monodominance of Kapur.     

Mating Systems of Three Tropical Dry Forest Tree Species1

Polymorphic allozyme loci were used to estimate outcrossing rates for three tree species from a disturbed dry forest in southern Costa Rica. Estimates of the multilocus outcrossing rates of Cedrela odorata and Jacaranda copaia were 0.969 and 0.982, respectively, and suggest that these species may be self-incompatible. The subcanopy tree Stemmadenia donnell-smithii also demonstrated little self-fertilization based on an estimated outcrossing rate of 0.896. Significant heterogeneity in pollen allele frequencies among maternal trees was detected for at least two enzyme loci for each species. A test of correlated mating between progeny of S. donnell-smithii revealed that all seeds within a fruit were singly sired. In addition, the low estimates of biparental inbreeding and significant differences in pollen and ovule allele frequencies for this species suggest that gene flow into the sampled forest fragment may occur. The implications of deforestation on the mating systems of these tropical tree taxa are discussed.     

Phenology of a northern hardwood forest canopy

While commonplace in other parts of the world, long‐term and ongoing observations of the phenology of native tree species are rare in North America. We use 14 years of field survey data from the Hubbard Brook Experimental Forest to fit simple models of canopy phenology for three northern hardwood species, sugar maple (Acer saccharum), American beech (Fagus grandifolia), and yellow birch (Betula alleghaniensis). These models are then run with historical meteorological data to investigate potential climate change effects on phenology. Development and senescence are quantified using an index that ranges from 0 (dormant, no leaves) to 4 (full, green canopy). Sugar maple is the first species to leaf out in the spring, whereas American beech is the last species to drop its leaves in the fall. Across an elevational range from 250 to 825 m ASL, the onset of spring is delayed by 2.7±0.4 days for every 100 m increase in elevation, which is in reasonable agreement with Hopkin's law. More than 90% of the variation in spring canopy development, and just slightly less than 90% of the variation in autumn canopy senescence, is accounted for by a logistic model based on accumulated degree‐days. However, degree‐day based models fit to Hubbard Brook data appear to overestimate the rate at which spring development occurs at the more southerly Harvard Forest. Autumn senescence at the Harvard Forest can be predicted with reasonable accuracy in sugar maple but not American beech. Retrospective modeling using five decades (1957–2004) of Hubbard Brook daily mean temperature data suggests significant trends (P≤0.05) towards an earlier spring (e.g. sugar maple, rate of change=0.18 days earlier/yr), consistent with other studies documenting measurable climate change effects on the onset of spring in both North America and Europe. Our results also suggest that green canopy duration has increased by about 10 days (e.g. sugar maple, rate of change=0.21 days longer/yr) over the period of study.     

Tree fruiting phenology in Kalinzu Forest, Uganda

The spatial and temporal variations in species composition and abundance of trees that fruited in mechanically logged, intensively pit‐sawn and essentially undisturbed forest areas in Kalinzu Forest (0°17′S, 0°30′S and 30°00′, 30°07′E) were assessed. The duration and frequency of fruiting episodes of selected pioneer, understorey and canopy tree species were also determined. These phenology aspects were monitored for 18 months (between February 1997 and July 1998) in 99 plots, each 20 m × 20 m in size. Eighty species consisting of 1489 trees fruited during the study. Most of the species were recorded in the undisturbed forest area and least in the mechanically logged areas. Monthly number of individuals and species that fruited in each forest condition were significantly related to monthly rainfall. Two pioneer species (Musanga leo‐errerae and Trema orientalis), one sub‐canopy (Funtumia africana) and canopy (Parinari excelsa) species had extended fruiting phases (>5 months). Two understorey species (Oxyancius speciosus and Tabernaemontana spp.) had an extended phase while the third (Teclea nobilis) had a short (≤5 months) fruiting phase. Of the selected species, only Strombosia scheffleri (a sub‐canopy species) and P. excelsa had more than one fruiting episode.     

Altitudinal Change in LAI and Stand Leaf Biomass in Tropical Montane Forests: a Transect Study in Ecuador and a Pan-Tropical Meta-Analysis

Abstract Leaf area index (LAI) is a key parameter controlling plant productivity and biogeochemical fluxes between vegetation and the atmosphere. Tropical forests are thought to have comparably high LAIs; however, precise data are scarce and environmental controls of leaf area in tropical forests are not understood. We studied LAI and stand leaf biomass by optical and leaf mass-related approaches in five tropical montane forests along an elevational transect (1,050–3,060 m a.s.l.) in South Ecuador, and conducted a meta-analysis of LAI and leaf biomass data from tropical montane forests around the globe. Study aims were (1) to assess the applicability of indirect and direct approaches of LAI determination in tropical montane forests, (2) to analyze elevation effects on leaf area, leaf mass, SLA, and leaf lifespan, and (3) to assess the possible consequences of leaf area change with elevation for montane forest productivity. Indirect optical methods of LAI determination appeared to be less reliable in the complex canopies than direct leaf mass-related approaches based on litter trapping and a thorough analysis of leaf lifespan. LAI decreased by 40–60% between 1,000 and 3,000 m in the Ecuador transect and also in the pan-tropical data set. This decrease indicates that canopy carbon gain, that is, carbon source strength, decreases with elevation in tropical montane forests. Average SLA decreased from 88 to 61 cm² g⁻¹ whereas leaf lifespan increased from 16 to 25 mo between 1,050 and 3,060 m in the Ecuador transect. In contrast, stand leaf biomass was much less influenced by elevation. We conclude that elevation has a large influence not only on the leaf traits of trees but also on the LAI of tropical montane forests with soil N (nitrogen) supply presumably being the main controlling factor.     

引种保护植物对西双版纳极端冷年的春季物候响应

气温被普遍认为是春季物候期最主要的控制因子之一,然而低温对植物物候的影响效应一直都存在不同的观点。西双版纳由于地处热带地区的北缘,其气温相对于赤道附近的热带地区较低。自1959年以来,西双版纳热带植物园引入了来自世界各个热带地区的4万余种植物进行保护,之前的研究证明西双版纳的低温对这些引种植物的生长有很大影响。因此,1974年西双版纳出现的极端低温势必对引种植物造成极大威胁,同时也是对这些植物低温适应能力的一个考验。通过对比43种引种植物物候期（生长抽梢期与开花期）在1974年与常年的差异情况,分析不同来源（热带亚洲、热带美洲与热带非洲）引种植物对西双版纳低温的适应性。结果表明,经历西双版纳1974年初的极端低温之后,使81%的引种植物生长抽梢期提前,同时也造成35%的引种植物在该年没有开花;而植物生长抽梢提前的主要原因则是极端低温以及低温过后气温迅速回升。引种植物均能顺利度过1974年的最冷时期,并出现生长抽梢物候,这意味着引种植物在经历极端低温之后都能够进行正常的生长活动,但极端低温对引种植物繁殖活动的不利影响大于其对生长活动的影响;引种植物对西双版纳极端低温的适应能力由大到小顺序依次为：亚洲来源植物〉美洲来源植物〉非洲来源植物。因此在迁地保护植物的选择过程中,应多选择亚洲热带植物,其次为美洲热带植物,而对非洲热带植物的引入则需谨慎考察。     

Dietary–morphological relationships in a fish assemblage of the Bolivian Amazonian floodplain

Morphological correlates of diet were examined in 48 species of freshwater fishes from floodplain lakes in the central part of the Mamoré River (Bolivian Amazon). The species were classified, according to the percentage occurrence of seven food items, into eight broad trophic categories: mud feeders, algivores, herbivores, terrestrial invertivores and omnivores, carnivores, zooplanktivores, aquatic invertivores and piscivores. There were significant relationships between the diet and morphology of the fishes even when the effect of taxonomical relatedness between species was eliminated. Relative gut length was the main morphological variable used to order species on a carnivore to mud feeder gradient. Standard length and head and mouth size were the morphological variables most closely associated with prey size. Mud feeder, algivore and piscivore species appeared as the most dietary and morphologically specialized. These results support both the hypotheses that species morphology influences the diet and that morphological similarity is conserved even in comparison with taxonomically unrelated species.     

Terrestrial wildlife responses to logging and fire in a Bolivian tropical humid forest

Logging and wildfire are significant anthropogenic disturbance agents in tropical forests. We compared the abundance and species richness of selected terrestrial wildlife taxa including small mammals, amphibians, reptiles, and terrestrial invertebrates in areas burned by wildfire and then logged and in adjacent undisturbed areas of a tropical humid forest in Bolivia. Disturbed areas had 24% less canopy cover than undisturbed areas but had 2.6 times the cover of large woody debris. Understory cover did not differ between disturbed and undisturbed areas. Small mammal abundance and species richness in disturbed areas were 43 and 70% higher, respectively, than in adjacent undisturbed areas. Herpetofaunal abundance did not differ significantly among disturbed and undisturbed areas, but trends for higher abundance were observed for both reptiles and amphibians in disturbed areas. Herpetofaunal species richness was significantly higher in disturbed compared to undisturbed areas. Total terrestrial invertebrate abundance, as estimated by pitfall traps, was significantly higher in undisturbed compared to disturbed areas mostly due to higher abundances of Formicidae and Blattidae. However, two invertebrate groups, Orthoptera and Lepidoptera (larvae) were more abundant in disturbed areas. Wildlife conservation strategies for areas where logging or wildfire occur should take into account species- or guild-specific responses to these disturbance agents.     

Productive phenology of tropical montane forests: Fertilization experiments along a moisture gradient

Phenological responses of leaves and roots were studied in the tropical montane forests of Mount Kinabalu, Borneo. Soil nutrient supply, in addition to the supply of light and water, is a potential abiotic factor influencing plant phenology in the tropics. The main objective of this study was to evaluate the contribution of soil nutrient supply to plant productive phenology. Fertilization experiments, including controls, nitrogen fertilized and nitrogen and phosphorus fertilized treatments, were conducted on three vegetation types in different moisture environments. Responses of leaves and roots were compared among treatments and among vegetation types. Leaf flushing was induced by nitrogen fertilization in the upper montane forest, where extremely wet moisture conditions are associated with cloud cover. This induction of leaf flushing by fertilization was not observed in the other forests. Root growth was suppressed by fertilization when leaf flushing was not induced by fertilization. These results indicate that soil pulsed nutrient release could be a cue for leaf flushing in a tropical wet environment, and that leaf phenology could be regulated by external abiotic factors and root phenology could be regulated by internal plant demands.     

Changed plant and animal life cycles from 1952 to 2000 in the Mediterranean region

The available data on climate over the past century indicate that the earth is warming. Important biological effects, including changes of plant and animal life cycle events, have already been reported. However, evidence of such effects is still scarce and has been mostly limited to northern latitudes. Here we provide the first long‐term (1952–2000) evidence of altered life cycles for some of the most abundant Mediterranean plants and birds, and one butterfly species. Average annual temperatures in the study area (Cardedeu, NE Spain) have increased by 1.4 °C over the observation period while precipitation remained unchanged. A conservative linear treatment of the data shows that leaves unfold on average 16 days earlier, leaves fall on average 13 days later, and plants flower on average 6 days earlier than in 1952. Fruiting occurs on average 9 days earlier than in 1974. Butterflies appear 11 days earlier, but spring migratory birds arrive 15 days later than in 1952. The stronger changes both in temperature and in phenophases timing occurred in the last 25 years. There are no significant relationships among changes in phenophases and the average date for each phenophase and species. There are not either significant differences among species with different Raunkiaer life‐forms or different origin (native, exotic or agricultural). However, there is a wide range of phenological alterations among the different species, which may alter their competitive ability, and thus, their ecology and conservation, and the structure and functioning of ecosystems. Moreover, the lengthening of plant growing season in this and other northern hemisphere regions may contribute to a global increase in biospheric activity.