半干旱地区不同森林类型土壤水分动态模拟

采用暖温带落叶阔叶次生林、油松人工林和华北落叶松人工林样地土壤水分的生长季内观测数据和其他辅助观测数据,检验了Georgakakos等提出的土壤水分模型在半干旱林地的适用性。结果表明,该模型用于模拟半干旱林地日尺度土壤水分动态具有一定的可信度,且能够较好的反映不同森林类型的水文效应。模型参数的敏感性分析表明,不同目标函数的参数敏感度信息反映了该模型"异参同效"现象不显著,模型结构不确定性也较小。各参数的敏感度结果揭示了各参数在降雨入渗、深层渗漏和蒸散部分中的控制作用。从模型模拟的土壤水分变化通量来看,油松人工林地实际年蒸散发量大于其他林地,落叶阔叶林地年入渗量大于其他林地,而3种森林类型林地深层渗漏所占生长季降雨量的比例都较小。研究半干旱地区多年生人工林土壤水分的情况,不仅有助于从根本上认清半干旱地区土壤-大气-植被连续体的复杂作用关系,也为半干旱地区树种选择及造林后的生态水文效应研究提供理论依据。     

Whole tree xylem sap flow responses to multiple environmental variables in a wet tropical forest

In order to quantify and characterize the variance in rainforest tree physiology, whole tree sap flow responses to local environmental conditions were investigated in 10 species of trees with diverse traits at La Selva Biological Station, Costa Rica. A simple model was developed to predict tree sap flow responses to a synthetic environmental variable generated by a principle components analysis. The best fit was obtained with a sigmoid function which explained between 74 and 93% of the variation in sap flux of individual trees. Sap flow reached an asymptote where higher light and evaporative demand did not cause sap flux to increase further. Soil moisture had little influence on sap flux. The morphological characteristics of the trees significantly affected sap flow; taller trees responded to changes in environmental variables sooner than shorter trees and high liana cover buffered tree sap flow responses to weather. The effect of species‐specific differences on the model was small; the mean effectiveness of the model was reduced by 6% when parameters were estimated from a single pool of measurements taken from all individuals. The results indicate that sap flow response could be effectively estimated using a simple general model and composite environmental index for these 10 diverse tree species.     

Sensitivity of canopy transpiration to altered precipitation in an upland oak forest: evidence from a long-term field manipulation study

Climate‐induced changes in regional precipitation could have important implications for the carbon, water, and nutrient cycles of forest ecosystems. However, few studies have examined the response of deciduous forests to increases or decreases in precipitation. Therefore, the throughfall displacement experiment (TDE) was established in 1993 near Oak Ridge, Tennessee to examine the sensitivity of an upland oak (Quercus spp.) forest to ambient, wet (+33%), and dry (?33%) precipitation regimes. Sap flux measurements on co‐occurring tree species were scaled using species‐specific estimates of stand sapwood area to derive daily and seasonal rates of canopy transpiration (E_C) from 2000 to 2003. With the exception of 2003, which was an extremely wet year, daily E_C in the dry plot, and occasionally during extended droughts in the ambient and wet plots, declined as water potential in the upper 0.35 m soil profile approached ‐3.0 MPa. Seasonal patterns of soil water potential and treatment‐specific differences in E_C were dependent on precipitation frequency and intensity. Supplemental precipitation added to the wet plot increased seasonal E_C on average by 9% (range ?1% to 19%), whereas extended periods of drought on the dry plot in 2000, 2001, and 2002 were sufficient to reduce seasonal E_C by 26–30% compared with the ambient plot. There was a strong correlation between seasonal E_C and the water stress integral, a cumulative index of drought severity and duration. A polynomial fitted to these data indicated that reductions in seasonal E_C on the order of 40% were possible given TDE‐imposed reductions in soil water potential. Application of this equation to all years of the TDE (1994–2003) revealed considerable interannual and treatment‐specific variation in canopy transpiration. In general, a 33% removal of throughfall on the dry plot during 1995, 1998, and 2002 resulted in a 23–32% reduction in seasonal E_C compared with the ambient plot. While droughts in deciduous forests are often limited in duration and tend to occur late in the growing season, soil water deficits of the magnitude observed in this study have the potential to impact local and regional forest water budgets.     

Frequent fires reduce tree growth in northern Australian savannas: implications for tree demography and carbon sequestration

Tropical savannas are typically highly productive yet fire‐prone ecosystems, and it has been suggested that reducing fire frequency in savannas could substantially increase the size of the global carbon sink. However, the long‐term demographic consequences of modifying fire regimes in savannas are difficult to predict, with the effects of fire on many parameters, such as tree growth rates, poorly understood. Over 10 years, we examined the effects of fire frequency on the growth rates (annual increment of diameter at breast height) of 3075 tagged trees, at 137 locations throughout the mesic savannas of Kakadu, Nitmiluk and Litchfield National Parks, in northern Australia. Frequent fires substantially reduced tree growth rates, with the magnitude of the effect markedly increasing with fire severity. The highest observed frequencies of mild, moderate and severe fires (1.0, 0.8 and 0.4 fires yr^?1, respectively) reduced tree growth by 24%, 40% and 66% respectively, relative to unburnt areas. These reductions in tree growth imply reductions in the net primary productivity of trees by between 0.19 t C ha^?1 yr^?1, in the case of mild fires, and 0.51 t C ha^?1 yr^?1, in the case of severe fires. Such reductions are relatively large, given that net biome productivity (carbon sequestration potential) of these savannas is estimated to be just 1–2 t C ha^?1 yr^?1. Our results suggest that current models of savanna tree demography, that do not account for a relationship between severe fire frequency and tree growth rate, are likely to underestimate the long‐term negative effects of frequent severe fires on tree populations. Additionally, the negative impact of frequent severe fires on carbon sequestration rates may have been underestimated; reducing fire frequencies in savannas may increase carbon sequestration to a greater extent than previously thought.     

A seasonal evergreen forest in Belize: unusually high tree species richness for northern Central America

Most of the undisturbed forests of northern Central America remain undescribed. Some studies predict that tree species richness in lowland forests of northern Central America should be much lower than in similar forest types in southern Central America. This paper describes the physical and biological structure of two permanent, one-hectare plots on a valley floor in the Bladen Nature Reserve, Belize and compares these plots with other forests in the Neotropics. The plots have 91 and 89 species of trees ≥ 10 cm diameter (327 and 358 individuals, respectively) and comparable or higher measures of alpha-diversity than other such forests in Central America. The recorded tree species richness from other forests in northern Central America may be lower than those in southern Belize. The plots are dominated by Mortoniella pittieri Woodson (Apocynaceae), a disjunct from Costa Rica and Nicaragua. More than 50% of the tree species have distributions ranging into Bolivia or Peru, and >75% are wide-ranging species. The plots have a lower tree density and a higher proportion of large trees (>70 cm dbh) than other wet Neotropical forests. Like other Neotropical forests, tree species in the plots are generally aggregated and have a high proportion of vertebrate-dispersed fruits. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 138 , 275–296.     

民勤绿洲-荒漠过渡区水量平衡规律研究

以甘肃省民勤县刘家地村外缘的绿洲 -荒漠过渡地带为定位研究区域 ,利用长期调查和水文学研究方法从生态系统的角度对绿洲 -荒漠过渡区的水量平衡特别是土壤水分和蒸散量的时空格局与动态特征进行了研究。结果显示 :1沿绿洲至流动沙丘这一过渡带 ,随着与绿洲距离的逐渐拉大 ,土壤含水量减小 ,0～ 80 cm土层的月平均土壤含水量的大小顺序为红柳沙包(5 .6 1% ) >白刺沙包 (5 .4 7% ) >流动沙丘 (5 .2 2 % ) ;土壤水分垂直变化规律是由表层到深层依次递增。 2在蒸散日进程中 ,红柳沙包和白刺沙包的蒸散率峰值到来时间比流动沙丘早 1～ 2 h,且日最大蒸散速率大小顺序为红柳沙包 (0 .2 4 mm / h) >白刺沙包 (0 .2 0 m m/ h) >流动沙丘 (0 .18mm/ h) ;影响绿洲荒漠 -过渡区蒸散的主要气象因子为日照百分率、气温、空气饱和差以及风速等 ,它们与日蒸散量的斜率关联度分别为 0 .717、0 .6 4 3、0 .6 4 9和 0 .70 5。3生态系统总的水量平衡特点是 ,水分输入主要靠降水和土壤水分的水平运动补给 ,而土壤和植被蒸发散是系统的主要输出项。     

Direct response of tree growth to soil water and its implications for terrestrial carbon cycle modelling

Wood growth constitutes the main process for long‐term atmospheric carbon sequestration in vegetation. However, our understanding of the process of wood growth and its response to environmental drivers is limited. Current dynamic global vegetation models (DGVMs) are mainly photosynthesis‐driven and thus do not explicitly include a direct environmental effect on tree growth. However, physiological evidence suggests that, to realistically model vegetation carbon allocation under increased climatic stressors, it is crucial to treat growth responses independently from photosynthesis. A plausible growth response function suitable for global simulations in DGVMs has been lacking. Here, we present the first soil water‐growth response function and parameter range for deciduous and evergreen conifers. The response curve was calibrated against European larch and Norway spruce in a dry temperate forest in the Swiss Alps. We present a new data‐driven approach based on a combination of tree ring width (TRW) records, growing season length and simulated subdaily soil hydrology to parameterize ring width increment simulations. We found that a simple linear response function, with an intercept at zero moisture stress, used in growth simulations reproduced 62.3% and 59.4% of observed TRW variability for larch and spruce respectively and, importantly, the response function slope was much steeper than literature values for soil moisture effects on photosynthesis and stomatal conductance. Specifically, we found stem growth stops at soil moisture potentials of ?0.47 MPa for larch and ?0.66 MPa for spruce, whereas photosynthesis in trees continues down to ?1.2 MPa or lower, depending on species and measurement method. These results are strong evidence that the response functions of source and sink processes are indeed very different in trees, and need to be considered separately to correctly assess vegetation responses to environmental change. The results provide a parameterization for the explicit representation of growth responses to soil water in vegetation models.     

Development of microsatellite markers for Carapa guianensis (Aublet), a tree species from the Amazon forest

Carapa guianensis is a timber species found in Central America and the north of South America. We have developed microsatellite primers which will allow analysis of gene flow and population genetic structure in natural populations of this tree species. Polymorphism of five microsatellite loci was evaluated using a total of 12 adult trees from a natural population. An average of 4.2 alleles per locus was detected, and expected heterozygosity ranging from 0.397 to 0.806. These loci are being used for genetic population analysis in a managed forest in the state of Pará, in Amazonian Brazil, as part of the Dendrogene project.     

Interannual and seasonal variations of energy and water vapor fluxes above a tropical seasonal rain forest in Xishuangbanna, SW China

The values and variation characteristics of energy components, their relationship with net radiation and the characteristics of water balance in the forest were analyzed, based on the observation data of energy fluxes, meteorological parameters and biomass in a tropical seasonal rain forest in Xishuangbanna from January 2003 to December 2004. The results show that annual net radiation was 3516.4 MJ/(m² · a) and 3516.6 MJ/(m² · a) in 2003 and 2004, respectively, of which 46% and 44% were used in latent heat flux, and 12% and 11% were lost as sensible heat flux. Annual mean canopy surface conductance was 10.3 mm/s and 10.0 mm/s in 2003 and 2004, respectively. Moreover, canopy surface conductance was lower in dry-hot seasons than in fog-cool and rainy seasons. Canopy surface conductance correlated significantly and positively with leaf area index, but negatively with water vapor pressure deficit. In general, canopy surface conductance was not affected directly by soil water content, but highly depended on soil moisture status when soil water content was below 0.15 m³/m³. Annual total evapotranspiration of this forest ecosystem in dry seasons was lower than that in rainy seasons, which was considered as one of the most important reasons that tropical seasonal rain forest could survive and flourish in Xishuangbanna at limit of water and heat.     

西双版纳热带季节雨林水热通量

利用西双版纳热带季节雨林2003和2004年常规气象、生物量以及水热通量观测资料,对该林地两年内各能量分量的数值大小和变化规律、能量分配以及水量平衡特征等进行了分析研究。结果表明,2003和2004年净辐射总量分别为3516.4MJ/(m.2a)和3516.6MJ/(m.2a)。在能量分配过程中潜热通量占优势,2003年和2004年的总量分别是相应年份净辐射总量的46%和44%,显热通量则分别只有12%和11%。2003年和2004年林冠传导率均值分别为10.3mm/s和10.0mm/s,其中干热季期间的林冠传导率明显低于雾凉季和雨季。林冠传导率与叶面积指数和空气饱和水汽压差值之间分别呈极显著的正、负线性相关关系;它基本上不受土壤含水量的影响,只是当长期无雨或雨量很小导致土壤含水量低于0.15m3/m3时,林冠传导率才与土壤含水量间存在极显著的相关关系。西双版纳热带雨林2003和2004年的蒸散量分别是663mm和634mm,受浓雾和林冠传导率的综合影响,该森林生态系统干季蒸散量低于雨季,这是西双版纳热带季节雨林能够在水热极限条件下生存并良好发育的重要原因。     

Interannual and seasonal variations of energy and water vapor fluxes above a tropical seasonal rain forest in Xishuangbanna, SW China

The values and variation characteristics of energy components, their relationship with net radiation and the characteristics of water balance in the forest were analyzed, based on the observation data of energy fluxes, meteorological parameters and biomass in a tropical seasonal rain forest in Xishuangbanna from January 2003 to December 2004. The results show that annual net radiation was 3516.4 MJ/(m² · a) and 3516.6 MJ/(m² · a) in 2003 and 2004, respectively, of which 46% and 44% were used in latent heat flux, and 12% and 11% were lost as sensible heat flux. Annual mean canopy surface conductance was 10.3 mm/s and 10.0 mm/s in 2003 and 2004, respectively. Moreover, canopy surface conductance was lower in dry-hot seasons than in fog-cool and rainy seasons. Canopy surface conductance correlated significantly and positively with leaf area index, but negatively with water vapor pressure deficit. In general, canopy surface conductance was not affected directly by soil water content, but highly depended on soil moisture status when soil water content was below 0.15 m³/m³. Annual total evapotranspiration of this forest ecosystem in dry seasons was lower than that in rainy seasons, which was considered as one of the most important reasons that tropical seasonal rain forest could survive and flourish in Xishuangbanna at limit of water and heat.     

Impacts of fire on sources of soil CO2 efflux in a dry Amazon rain forest

Fire at the dry southern margin of the Amazon rainforest could have major consequences for regional soil carbon (C) storage and ecosystem carbon dioxide (CO₂) emissions, but relatively little information exists about impacts of fire on soil C cycling within this sensitive ecotone. We measured CO₂ effluxes from different soil components (ground surface litter, roots, mycorrhizae, soil organic matter) at a large‐scale burn experiment designed to simulate a severe but realistic potential future scenario for the region (Fire plot) in Mato Grosso, Brazil, over 1 year, and compared these measurements to replicated data from a nearby, unmodified Control plot. After four burns over 5 years, soil CO₂ efflux (R_s) was ~5.5 t C ha^?1 year^?1 lower on the Fire plot compared to the Control. Most of the Fire plot R_s reduction was specifically due to lower ground surface litter and root respiration. Mycorrhizal respiration on both plots was around ~20% of R_s. Soil surface temperature appeared to be more important than moisture as a driver of seasonal patterns in R_s at the site. Regular fire events decreased the seasonality of R_s at the study site, due to apparent differences in environmental sensitivities among biotic and abiotic soil components. These findings may contribute toward improved predictions of the amount and temporal pattern of C emissions across the large areas of tropical forest facing increasing fire disturbances associated with climate change and human activities.     

Amazon drought and forest response: Largely reduced forest photosynthesis but slightly increased canopy greenness during the extreme drought of 2015/2016

Amazon droughts have impacted regional ecosystem functioning as well as global carbon cycling. The severe dry‐season droughts in 2005 and 2010, driven by Atlantic sea surface temperature (SST) anomaly, have been widely investigated in terms of drought severity and impacts on ecosystems. Although the influence of Pacific SST anomaly on wet‐season precipitation has been well recognized, it remains uncertain to what extent the droughts driven by Pacific SST anomaly could affect forest greenness and photosynthesis in the Amazon. Here, we examined the monthly and annual dynamics of forest greenness and photosynthetic capacity when Amazon ecosystems experienced an extreme drought in 2015/2016 driven by a strong El Niño event. We found that the drought during August 2015–July 2016 was one of the two most severe meteorological droughts since 1901. Due to the enhanced solar radiation during this drought, overall forest greenness showed a small increase, and 21.6% of forests even greened up (greenness index anomaly ≥1 standard deviation). In contrast, solar‐induced chlorophyll fluorescence (SIF), an indicator of vegetation photosynthetic capacity, showed a significant decrease. Responses of forest greenness and photosynthesis decoupled during this drought, indicating that forest photosynthesis could still be suppressed regardless of the variation in canopy greenness. If future El Niño frequency increases as projected by earth system models, droughts would result in persistent reduction in Amazon forest productivity, substantial changes in tree composition, and considerable carbon emissions from Amazon.     

Robust ecological drought projections for drylands in the 21st century

Dryland ecosystems may be especially vulnerable to expected 21st century increases in temperature and aridity because they are tightly controlled by moisture availability. However, climate impact assessments in drylands are difficult because ecological dynamics are dictated by drought conditions that are difficult to define and complex to estimate from climate conditions alone. In addition, precipitation projections vary substantially among climate models, enhancing variation in overall trajectories for aridity. Here, we constrain this uncertainty by utilizing an ecosystem water balance model to quantify drought conditions with recognized ecological importance, and by identifying changes in ecological drought conditions that are robust among climate models, defined here as when >90% of models agree in the direction of change. Despite limited evidence for robust changes in precipitation, changes in ecological drought are robust over large portions of drylands in the United States and Canada. Our results suggest strong regional differences in long‐term drought trajectories, epitomized by chronic drought increases in southern areas, notably the Upper Gila Mountains and South‐Central Semi‐arid Prairies, and decreases in the north, particularly portions of the Temperate and West‐Central Semi‐arid Prairies. However, we also found that exposure to hot‐dry stress is increasing faster than mean annual temperature over most of these drylands, and those increases are greatest in northern areas. Robust shifts in seasonal drought are most apparent during the cool season; when soil water availability is projected to increase in northern regions and decrease in southern regions. The implications of these robust drought trajectories for ecosystems will vary geographically, and these results provide useful insights about the impact of climate change on these dryland ecosystems. More broadly, this approach of identifying robust changes in ecological drought may be useful for other assessments of climate impacts in drylands and provide a more rigorous foundation for making long‐term strategic resource management decisions.     

A review of the relationships between drought and forest fire in the United States

The historical and presettlement relationships between drought and wildfire are well documented in North America, with forest fire occurrence and area clearly increasing in response to drought. There is also evidence that drought interacts with other controls (forest productivity, topography, fire weather, management activities) to affect fire intensity, severity, extent, and frequency. Fire regime characteristics arise across many individual fires at a variety of spatial and temporal scales, so both weather and climate – including short‐ and long‐term droughts – are important and influence several, but not all, aspects of fire regimes. We review relationships between drought and fire regimes in United States forests, fire‐related drought metrics and expected changes in fire risk, and implications for fire management under climate change. Collectively, this points to a conceptual model of fire on real landscapes: fire regimes, and how they change through time, are products of fuels and how other factors affect their availability (abundance, arrangement, continuity) and flammability (moisture, chemical composition). Climate, management, and land use all affect availability, flammability, and probability of ignition differently in different parts of North America. From a fire ecology perspective, the concept of drought varies with scale, application, scientific or management objective, and ecosystem.     

Influence of large termitaria on soil characteristics, soil water regime, and tree leaf shedding pattern in a West African savanna

Termitaria are major sites of functional heterogeneity in tropical ecosystems, through their strong influence on soil characteristics, in particular soil physico-chemical properties and water status. These factors have important consequences on nutrient availability for plants, plant spatial distribution, and vegetation dynamics. However, comprehensive information about the influence of termite-rehandled soil on soil water regime is lacking. In a humid shrubby savanna, we characterized the spatial variations in soil texture, soil structure and maximum soil water content available for plants (AWC max) induced by a large termite mound, at three deepths (0–0.10, 0.20–0.30 and 0.50–0.60 m). In addition, during a three month period at the end of the rainy season, soil water potential was surveyed by matrix sensors located on the termite mound and in the surrounding soil at the same depths and for the 80–90 cm layer. Concurrently, the leaf shedding patterns of two coexisting deciduous shrub species exhibiting contrasted soil water uptake patterns were compared for individuals located on termite mounds and in undisturbed control areas. For all the soil layers studied, clay and silt contents were higher for the mound soil. Total soil clods porosity was higher on the mound than in control areas, particularly in the 0.20-0.60 m layer, and mound soil exhibited a high shrinking/swelling capacity. AWC_max of the 0-0.60 m soil layer was substantially higher on the termite mound (112 mm) than in the surroundings (84 mm). Furthermore, during the beginning of the dry season, soil water potential measured in situ for the 0.20-0.90 m soil layer was higher on the mound than in the control soil. In contrast, soil water potential of the 0-0.10 m soil layer was similar on the mound and in the control soil. In the middle of the dry season, the leaf shedding pattern of Crossopteryx febrifuga shrubs (which have limited access to soil layers below 0.60 m) located on mounds was less pronounced than that of individuals located on control soil. In contrast, the leaf shedding pattern of the shrub Cussonia barteri (which has a good access to deep soil layers) was not influenced by the termite mound. We conclude that in this savanna ecosystem, termite mounds appear as peculiar sites which exhibit improved soil water availability for plants in upper soil layers, and significantly influence aspects of plant function. Implications of these results for understanding and modelling savanna function and dynamics, and particularly competitive interactions between plant species, are discussed.     

Evapotranspiration and soil water content in a scrub-oak woodland under carbon dioxide enrichment

Leaf conductance often decreases in response to elevated atmospheric CO₂ concentration (C_a) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated C_a during an eight‐month period two years after C_a manipulation began. Whole‐chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated C_a, despite an increase in leaf area index (LAI). Elevated C_a also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5–3 m), suggesting that the water savings in elevated C_a in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated C_a.     

Leaf attributes and tree growth in a tropical dry forest

Questions: How are leaf attributes and relative growth rate (RGR) of the dominant tree species of tropical deciduous forest (TDF) affected by seasonal changes in soil moisture content (SMC)? What is the relationship of functional attributes with each other? Can leaf attributes singly or in combination predict the growth rate of tree species of TDF? Location: Sonebhadra district of Uttar Pradesh, India. Methods: Eight leaf attributes, specific leaf area (SLA); leaf carbon concentration (LCC); leaf nitrogen concentration (LNC); leaf phosphorus concentration (LPC); chlorophyll concentration (Chl), mass‐based stomatal conductance (Gs_mass); mass based photosynthetic rate (A_mass); intrinsic water use efficiency (WUEi); and relative growth rate (RGR), of six dominant tree species of a dry tropical forest on four sites were analysed for species, site and season effects over a 2‐year period. Step‐wise multiple regression was performed for predicting RGR from mean values of SMC and leaf attributes. Path analysis was used to determine which leaf attributes influence RGR directly and which indirectly. Results: Species differed significantly in terms of all leaf attributes and RGR. The response of species varied across sites and seasons. The attributes were positively interrelated, except for WUEi, which was negatively related to all other attributes. The positive correlation was strongest between Gs_mass and A_mass and the negative correlation was strongest between Gs_mass and WUEi. Differences in RGR due to site were not significant when soil moisture was controlled, but differences due to season remained significant. The attributes showed plasticity across moisture gradients, which differed among attributes and species. Gs_mass was the most plastic attribute. Among the six species, Terminalia tomentosa exhibited the greatest plasticity in six functional attributes. In the step‐wise multiple regression, A_mass, SLA and Chl among leaf attributes and SMC among environmental factors influenced the RGR of tree species. Path analysis indicated the importance of SLA, LNC, Chl and A_mass in determining RGR. Conclusion: A _mass, SMC, SLA and Chl in combination can be used to predict RGR but could explain only three‐quarters of the variability in RGR, indicating that other traits/factors, not studied here, are also important in modulating growth of tropical trees. RGR of tree species in the dry tropical environment is determined by soil moisture, whereas the response of mature trees of different species is modulated by alterations in key functional attributes such as SLA, LNC and Chl.     

Effects of high- and low-intensity fires on soil properties and plant growth in a Bolivian dry forest

We compared soil nutrient availability and soil physical properties among four treatments (high-intensity fire, low-intensity fire, plant removal, and harvesting gap) and a control (intact forest understory) over a period of 18 months in a tropical dry forest in Bolivia. The effect of treatments on plant growth was tested using a shade intolerant tree species (Anadenanthera colubrina Vell. Conc.) as a bioassay. Surface soils in high-intensity fire treatments had significantly greater pH values, concentrations of extractable calcium (Ca), potassium (K), magnesium (Mg), and phosphorus (P), and amounts of resin-available P and nitrogen (N) than other treatments; however, a loss of soil organic matter during high-intensity fires likely resulted in increased bulk density and strength, and decreased water infiltration rates. Low intensity fires also significantly increased soil pH, concentrations of extractable Ca, K, Mg, and P, and amounts of resin-available P and N, although to a lesser degree than high-intensity fires. Low-intensity fires did not lower soil organic matter contents or alter soil physical properties. Plant removal and harvesting gap treatments had little effect on soil chemical and physical properties. Despite the potentially negative effects of degraded soil structure on plant growth, growth of A. colubrina seedlings were greater following high-intensity fires. Evidently, the increase in nutrient availability caused by high-intensity fires was not offset by degraded soil structure in its effects on seedling growth. Long-term effects of high intensity fires require further research.