Patterns of species richness and biogeographic regionalization of the avifaunas of the seasonally dry tropical forest in Mesoamerica

Biogeographic patterns of avifaunas associated with seasonally dry tropical forests in Mesoamerica are poorly understood despite their high levels of species richness and endemism. Through the parsimony analysis of endemicity, we analyzed biogeographic relationships of 650 resident species of birds associated with seasonally dry tropical forests from Mexico to Panama, based on potential distributions obtained through ecological niche modeling. Results show two general avifaunal groups, east and west of the Isthmus of Tehuantepec. Patterns of biogeographic distribution and species richness also helped illuminate the importance of key areas for birds associated to this habitat in the region.

Los patrones biogeográficos de las avifaunas asociadas a los bosques tropicales estacionalmente secos en Mesoamérica están pobremente entendidos, a pesar de que estas áreas poseen una gran riqueza de especies y endemismo. Analizamos las relaciones biogeográficas con base en distribuciones potenciales, hechas a partir de modelos del nicho ecológico usando el análisis de parsimonia de endemismos, de 650 especies de aves residentes asociadas a los bosques tropicales estacionalmente secos desde México hasta Panamá. Los resultados muestran dos grupos generales de la avifauna, al este y oeste del Istmo de Tehuantepec. El contexto biogeográfico y la riqueza de especies resalta también la importancia de áreas clave para las aves asociadas a este tipo de hábitat en la región.     

Carbon distribution of a well- and poorly-drained black spruce fire chronosequence

The objective of this study was to quantify carbon (C) distribution for boreal black spruce (Picea mariana (Mill.) BSP) stands comprising a fire chronosequence in northern Manitoba, Canada. The experimental design included seven well‐drained (dry) and seven poorly‐drained (wet) stands that burned between 1998 and 1850. Vegetation C pools (above‐ground + below‐ground) steadily increased from 1.3 to 83.3 t C ha^?1 for the dry chronosequence, and from 0.6 to 37.4 t C ha^?1 for the wet chronosequence. The detritus C pools (woody debris + forest floor) varied from 10.3 to 96.0 t C ha^?1 and from 12.6 to 77.4 t C ha^?1 for the dry and wet chronosequence, respectively. Overstorey biomass, mean annual biomass increment (MAI), woody debris mass, and litterfall were significantly greater (α = 0.05) for the dry stands than for the wet stands, but the bryophyte, understorey, and forest floor C pools were significantly less for the dry than for the wet stands. The root mass ratio decreased with stand age until 37 years after fire, was fairly constant thereafter, and was not significantly affected by soil drainage. The C pools of the overstorey and bryophyte tended to increase with stand age. Foliage biomass, litterfall, and MAI (for the dry stands) peaked at 71 years after fire and declined in the oldest stands. The results from this study illustrate that the effects of disturbance and edaphic conditions must be accounted for in boreal forest C inventories and C models. The appropriateness of using chronosequences to examine effects of wildfire on ecosystem C distribution is discussed.     

Soil carbon fluxes and stocks in a Great Lakes forest chronosequence

We measured soil respiration and soil carbon stocks, as well as micrometeorological variables in a chronosequence of deciduous forests in Wisconsin and Michigan. The chronosequence consisted of (1) four recently disturbed stands, including a clearcut and repeatedly burned stand (burn), a blowdown and partial salvage stand (blowdown), a clearcut with sparse residual overstory (residual), and a regenerated stand from a complete clearcut (regenerated); (2) four young aspen ( Populus tremuloides ) stands in average age of 10 years; (3) four intermediate aspen stands in average age of 26 years; (4) four mature northern hardwood stands in average age of 73 years; and (5) an old-growth stand approximately 350-years old. We fitted site-based models and used continuous measurements of soil temperature to estimate cumulative soil respiration for the growing season of 2005 (days 133–295). Cumulative soil respiration in the growing season was estimated to be 513, 680, 747, 747, 794, 802, 690, and 571 g C m⁻² in the burn, blowdown, residual, regenerated, young, intermediate, mature, and old-growth stands, respectively. The measured apparent temperature sensitivity of soil respiration was the highest in the regenerated stand, and declined from the young stands to the old-growth. Both, cumulative soil respiration and basal soil respiration at 10 °C, increased during stand establishment, peaked at intermediate age, and then decreased with age. Total soil carbon at 0–60 cm initially decreased after harvest, and increased after stands established. The old-growth stand accumulated carbon in deep layers of soils, but not in the surface soils. Our study suggests a complexity of long-term soil carbon dynamics, both in vertical depth and temporal scale.     

Ecology and diversity of leaf litter fungi during early-stage decomposition in a seasonally dry tropical forest

Leaf litter samples of 12 dicotyledonous tree species (belonging to eight families) growing in a dry tropical forest and in early stages of decomposition were studied for the presence of litter fungi. Equal-sized segments of the leaves incubated in moist chambers were observed every day for 30 d for the presence of fungi. Invariably, the fungal assemblage on the litter of each tree species was dominated by a given fungal species. The diversity of fungi present in the litter varied with the tree species although many species of fungi occurred in the litter of all 12 species. A Pestalotiopsis species dominated the litter fungal assemblage of five trees and was common in the litter of all tree species. The present study and earlier studies from our lab indicate that fungi have evolved traits such as thermotolerant spores, ability to utilize toxic furaldehydes, ability to produce cell wall destructuring enzymes and an endophyte-litter fungus life style to survive and establish themselves in fire-prone forests such as the one studied here. This study shows that in the dry tropical forest, the leaf litter fungal assemblage is governed more by the environment than by the plant species.     

Hydraulic and photosynthetic co-ordination in seasonally dry tropical forest trees

In the present study the linkage between hydraulic, photosynthetic and phenological properties of tropical dry forest trees were investigated. Seasonal patterns of stem‐specific conductivity (K_SP) described from 12 species, including deciduous, brevi‐deciduous and evergreen species, indicated that only evergreen species were consistent in their response to a dry‐to‐wet season transition. In contrast, K_SP in deciduous and brevi‐deciduous species encompassed a range of responses, from an insignificant increase in K_SP following rains in some species, to a nine‐fold increase in others. Amongst deciduous species, the minimum K_SP during the dry season ranged from 6 to 56% of wet season K_SP, indicating in the latter case that a significant portion of the xylem remained functional during the dry season. In all species and all seasons, leaf‐specific stem conductivity (K_L) was strongly related to the photosynthetic capacity of the supported foliage, although leaf photosynthesis became saturated in species with high K_L. The strength of this correlation was surprising given that much of the whole‐plant resistance appears to be in the leaves. Hydraulic capacity, defined as the product of K_L and the soil–leaf water potential difference, was strongly correlated with the photosynthetic rate of foliage in the dry season, but only weakly correlated in the wet season.     

Biomass and carbon dynamics of a tropical mountain rain forest in China

Biometric inventories for 25 years, from 1983 to 2005, indicated that the Jianfengling tropical mountain rain forest in Hainan, China, was either a source or a modest sink of carbon. Overall, this forest was a small carbon sink with an accumulation rate of (0.56±0.22) Mg C ha⁻¹yr⁻¹, integrated from the long-term measurement data of two plots (P9201 and P8302). These findings were similar to those for African and American rain forests ((0.62±0.23) Mg C ha⁻¹yr⁻¹). The carbon density varied between (201.43±29.38) Mg C ha⁻¹ and (229.16±39.2) Mg C ha⁻¹, and averaged (214.17±32.42) Mg C ha⁻¹ for plot P9201. Plot P8302, however, varied between (223.95±45.92) Mg C ha⁻¹ and (254.85±48.86) Mg C ha⁻¹, and averaged (243.35±47.64) Mg C ha⁻¹. Quadratic relationships were found between the strength of carbon sequestration and heavy rainstorms and dry months. Precipitation and evapotranspiration are two major factors controlling carbon sequestration in the tropical mountain rain forest.     

Identification of microsatellite markers in a neotropical seasonally dry forest tree,Astronium urundeuva (Anacardiaceae)

Nine dinucleotide microsatellites were developed in Astronium urundeuva (Anacardiaceae), a typical tree of the seasonally‐dry tropical forests of South America and characterized on three populations from Paraguay and Argentina. Seven microsatellites were found polymorphic in within population gene diversities ranging from 0.32 to 0.91, and an observed number of alleles varying between four and 20. Despite their relatively low number of alleles, these markers proved valuable tools in detecting genetic structure between three populations in Paraguay and North Argentina.     

Leaf-litter and changing nutrient levels in a seasonally dry tropical hardwood forest,Belize, C.A.

Summary Total above ground plant biomass in a 45 year old seasonally dry tropical hardwood forest was estimated to be approximately 56,000 kg/ha oven dry weight. Nutrients immobilized in the standing vegetation were: N, 203 kg/ha; P, 24 kg/ha; K, 234 kg/ha; Ca, 195 kg/ha; Mg, 47 kg/ha; Na, 9 kg/ha; Mn, 1 kg/ha; Cu, 0.5 kg/ha; Zn, 3 kg/ha; Fe, 4 kg/ha. Total nutrients returned each year through the litter were: N, 156 kg/ha; P, 9 kg/ha; K, 59 kg/ha; Ca, 373 kg/ha; Mg, 32 kg/ha; Na, 5 kg/ha; Mn, 1 kg/ha; Al, 21 kg/ha; Zn, 0.3 kg/ha; Fe, 9 kg/ha. Half of the nutrients immobilized in the standing vegetation were found in the leaves and are returned annually to the soil. Although litter fall is interrupted during the year, the mean nutrient content of the litter was high –5.2%.A decomposition rate of 0.48 percent per day was considered high for a seasonally dry tropical hardwood forest. Fluctuations in soil nutrient levels showed a sharp increase at the start of the rainy season. Later during the dry season nutrient levels decreased to concentrations similar to what they were just prior to the rainy season. Soil organic matter levels were very high –20% in the top 12 cm.     

Development and characterization of 11 microsatellite markers in a widespread Neotropical seasonally dry forest tree species,Geoffroea spinosa Jacq. (Leguminosae)

Eleven dinucleotide microsatellites were developed in Geoffroea spinosa (Leguminosae), a widespread tree of the seasonally dry Neotropical forests, and characterized on six populations from Peru, Argentina and Paraguay. Four of them amplified on the Peruvian populations only, probably because of mutations in the microsatellite flanking regions in the other populations. Ten microsatellites were found polymorphic, with within population gene diversities ranging from 0.17 to 0.95, and a number of alleles varying from seven to 19. A significant overall genetic differentiation was also found (θ = 0.212; P < 0.01).     

Differences in leaf phenology between juvenile and adult individuals of two tree species in a seasonally dry tropical woodland

In temperate forests, juvenile trees anticipate leaf phenology compared to adults, thus avoiding shading and herbivory. This is also expected to occur in seasonal tropical forests due to intense herbivory and shading during the rainy season; however, the anticipation of leaf phenology by juveniles in seasonal tropical forests has yet to be demonstrated. Stem‐succulent species are expected to be prone to juvenile phenological anticipation because these species are able to use water stored in their stems for leaf flushing in the dry season. We investigated this hypothesis by comparing leaf phenology (bud break, leaf expansion) of juveniles and adults of two species with contrasting wood densities in the transition between dry and rainy seasons in a tropical dry woodland. We also investigated the level of light limitation that juveniles experience in the rainy season. Both species exhibited bud break during the dry season, but only expanded their leaves with the occurrence of the first rains. In general, the stem‐succulent species had a more precocious bud break; however, anticipation by juveniles occurred only in the species with more dense wood. Canopy openness was lower than in temperate deciduous forests, but the fact that the full expansion of leaves occurred only with rainfall indicates that bud break in anticipation of canopy closure contributes only to keeping leaf photosynthetic balance from going negative, and not to higher carbon gain. The importance of anticipated budding for escaping herbivory remains an alternative explanation in need of investigation.     

Functional diversity of macromycete communities along an environmental gradient in a Mexican seasonally dry tropical forest

Macromycetes are important for ecosystem functioning due to their role in the nutrient cycling, and their function as pathogens and mutualists. Diversity metrics based on functional traits are robust predictors of ecosystem functionality since they incorporate an evolutionary and ecologic background. We examined diversity patterns of macrofungi using functional trait-based metrics of diversity along an altitudinal gradient in a seasonally dry tropical forest in southern Mexico. Our findings show that: (1) functional diversity varies with elevation, relating more to climatic variables than to vegetation structure; (2) functional diversity indexes exhibited contrasting patterns, so measures reflecting heterogeneity on trait abundance and niche complementarity tend to increase with elevation, whereas the measure of trait evenness decreases; and (3) functional diversity patterns depend on the type of functional trait considered and how they respond to environmental conditions. Our results indicate that functional diversity analyses help understanding of how macrofungal communities respond to environmental variation.     

Drivers of postfire soil organic carbon accumulation in the boreal forest

The accumulation of soil carbon (C) is regulated by a complex interplay between abiotic and biotic factors. Our study aimed to identify the main drivers of soil C accumulation in the boreal forest of eastern North America. Ecosystem C pools were measured in 72 sites of fire origin that burned 2–314 years ago over a vast region with a range of ? mean annual temperature of 3°C and one of ? 500 mm total precipitation. We used a set of multivariate a priori causal hypotheses to test the influence of time since fire (TSF), climate, soil physico‐chemistry and bryophyte dominance on forest soil organic C accumulation. Integrating the direct and indirect effects among abiotic and biotic variables explained as much as 50% of the full model variability. The main direct drivers of soil C stocks were: TSF >bryophyte dominance of the FH layer and metal oxide content >pH of the mineral soil. Only climate parameters related to water availability contributed significantly to explaining soil C stock variation. Importantly, climate was found to affect FH layer and mineral soil C stocks indirectly through its effects on bryophyte dominance and organo‐metal complexation, respectively. Soil texture had no influence on soil C stocks. Soil C stocks increased both in the FH layer and mineral soil with TSF and this effect was linked to a decrease in pH with TSF in mineral soil. TSF thus appears to be an important factor of soil development and of C sequestration in mineral soil through its influence on soil chemistry. Overall, this work highlights that integrating the complex interplay between the main drivers of soil C stocks into mechanistic models of C dynamics could improve our ability to assess C stocks and better anticipate the response of the boreal forest to global change.     

Fire history influences on the spatial heterogeneity of soil nitrogen transformations in three adjacent stands in a dry tropical forest in Thailand

Spatial patterns of soil nitrogen (N) transformations were examined using geostatistical analysis in three adjacent stands with different fire history (0, 10 and 35 years since the latest fire, respectively) in a dry tropical forest in Thailand. A larger pool of total inorganic N and a faster rate of N mineralization were recorded in the stand with longer fire prevention. At the spatial scale analyzed, the proportion of spatially dependent variance to the total variance of N mineralization and nitrification increased from 0.39 to 0.73, and from 0.40 to 0.77, respectively, with the time since the latest fire. The spatial autocorrelation ranges of N mineralization and nitrification decreased from 9.0 to 3.28 m, and 9.0 to 2.77 m, respectively, with the time since the latest fire. These results suggested that fire history affected not only the level of available soil N, but also the spatial heterogeneity of soil N transformations, presumably due to the difference in plant influences on soil.     

Phylogenetic structure of angiosperm communities during tropical forest succession

The phylogenetic structure of ecological communities can shed light on assembly processes, but the focus of phylogenetic structure research thus far has been on mature ecosystems. Here, I present the first investigation of phylogenetic community structure during succession. In a replicated chronosequence of 30 sites in northeastern Costa Rica, I found strong phylogenetic overdispersion at multiple scales: species present at local sites were a non-random assemblage, more distantly related than chance would predict. Phylogenetic overdispersion was evident when comparing the species present at each site with the regional species pool, the species pool found in each age category to the regional pool or the species present at each site to the pool of species found in sites of that age category. Comparing stem size classes within each age category, I found that during early succession, phylogenetic overdispersion is strongest in small stems. Overdispersion strengthens and spreads into larger size classes as succession proceeds, corroborating an existing model of forest succession. This study is the first evidence that succession leaves a distinct signature in the phylogenetic structure of communities.     

Survival and growth of dominant tree seedlings in seasonally tropical dry forests of Yucatan: site and fertilization effects

Aims Seasonally tropical dry forests of the Yucatan Peninsula are typically found in sites with nutrient-poor soils because of the recent geological origin of the region. The landscape is dominated by extensive karstic plates that shape environments where vegetation regeneration through seed germination may be limited by the availability of suitable microsites. In this study, we documented the survival and growth of seedlings from three dominant tree species (Bursera simaruba, Piscidia piscipula and Lysiloma latisiliquum) in seasonally tropical dry forests in Yucatan. Specifically, we evaluated the effect of nutrient addition (N and P, separately and in combination) on seedling survival and growth across three sites with differing levels of precipitation.Methods We conducted a nutrient addition experiment, whereby we established 12 plots of dimensions 10×10 m (100 m 2) at each site, from which three plots were randomly selected to receive one of four treatments: N addition, P addition, N and P addition and no nutrient addition (controls). Prior to treatment application, in each plot, we planted 10 seedlings of each species in October 2010 and subsequently conducted surveys of plant growth and survival every 20 days from November 2010 to April 2011.Important findings Overall, nutrient addition increased seedling survival and the magnitude of this effect was similar among sites. We did not observe an additive effect of the N + P treatment on survival. Similarly, we observed a positive effect of nutrient addition on seedling growth, but this effect was contingent upon site; regarding survival, the effects of N and P on seedling growth were not additive. These results suggest that seedling recruitment and growth in the three dominant species of trees in Yucatan are limited by nutrient availability but that the magnitude of this effect, particularly on seedling growth, is specific for species and site.     

Large contribution of arbuscular mycorrhizal fungi to soil carbon pools in tropical forest soils

The origins and composition of soil organic matter (SOM) are still largely uncertain. Arbuscular mycorrhizal fungi (AMF) are recognized as indirect contributors through their influence on soil aggregation, plant physiology, and plant community composition. Here we present evidence that AMF can also make large, direct contributions to SOM. Glomalin, a recently discovered glycoprotein produced by AMF hyphae, was detected in tropical soils in concentrations of over 60 mg cm^–3. Along a chronosequence of soils spanning ages from 300 to 4.1 Mio years, a pattern of glomalin concentrations is consistent with the hypothesis that this protein accumulates in soil. Carbon dating of glomalin indicated turnover at time scales of several years to decades, much longer than the turnover of AMF hyphae (which is assumed to be on the order of days to weeks). This suggests that contributions of mycorrhizae to soil carbon storage based on hyphal biomass in soil and roots may be an underestimate. The amount of C and N in glomalin represented a sizeable amount (ca. 4–5%) of total soil C and N in the oldest soils. Our results thus indicate that microbial (fungal) carbon that is not derived from above- or below-ground litter can make a significant contribution to soil carbon and nitrogen pools and can far exceed the contributions of soil microbial biomass (ranging from 0.08 to 0.2% of total C for the oldest soils).     

西双版纳热带山地雨林生物量研究

观测了西双版纳山地气候,建立了山地雨林生物量回归方程,调查了海拔1 100~1 820 m范围5块样地(面积0.16~0.25 hm2)的热带山地雨林生物量。结果表明,海拔1 105和1 610 m的年平均温度分别为20.1和16.6℃,年降雨量分别为1 659和2 011 mm,旱季(11~4月)降雨量分别为295和283mm,年平均相对湿度分别为81%和84%;5块样地生物量变化为256.4~368.6 t.hm-2,平均为312.6t.hm-2,其中乔木占97.1%、木质藤本占1.2%、幼树和灌木占1.3%、草本和幼苗占0.4%;采用热带季节雨林生物量回归方程估计山地雨林生物量,会使得总生物量以及树干和树根生物量高估38.3%~61.5%,树枝生物量低估7.6%~30.8%。可见,西双版纳山地海拔增加导致雨季降雨量增加,山地雨林生物量较热带季节雨林降低32.6%,季节雨林生物量方程不适用于山地雨林。     

Organic matter accumulation following fires in a moorland soil chronosequence

Severe fires in 1957 and 1976 removed the vegetation and soil organic matter from the litter layers and organic horizons of soils at two adjacent moorland sites leaving exposed the uppermost mineral horizon of the soil. In the period since, plant recolonization and soil organic matter reaccumulation have occurred to give a chronosequence. Assuming no major changes in the carbon and nitrogen content of the unburned soil since 1957, the rates of accumulation of soil C and N were estimated to be 0.035 kg C m^–2 y^–1 and 0.001 kg N m^–2 y^–1 over the first 19 years, and 0.50 kg C m^–2 y^–1 and 0.023 kg N m^–2 y^–1 over the period from 19 to 38 years after burning. Solid-state ¹³C NMR (cross-polarization, magic angle spinning ¹³C nuclear magnetic resonance spectroscopy) showed that the ratio of alkyl- and methyl-C-to-O-alkyl-C increased with stage of decomposition and in the unburned soil with decreasing particle-size. For the organic matter that had reaccumulated in the 1957-burned soil, the alkyl-C-to-O-alkyl-C ratio of the > 2000 μm and 2000–250 μm particle-size fractions were greater than those of the corresponding size fractions from the unburned soil, indicating that the reaccumulated soil organic matter was subject to decomposition but limited fragmentation or comminution.