N2-fixing Tree Research Articles

Evidence of short-term belowground transfer of nitrogen from Acacia mangium to Eucalyptus grandis trees in a tropical planted forest

The short-term belowground transfer of nitrogen from nitrogen-fixing trees to companion trees has never been studied in the field. A 15N pulse-labeling study was conducted in a mixed plantation of Acacia mangium and Eucalyptus grandis at the peak of leaf area, 26 months after planting. 15N–NO3− was injected into the stem of one big Acacia tree in three plots. 15N was traced over 2 months in the labeled Acacia tree as well as in neighboring Eucalyptus trees. For both species, young leaves were sampled, as well as fine roots and the rhizosphere at a distance of 0.75 m and 2.25 m from the labeled tree. The 15N atom% was also determined in the wood, bark, branches and total foliage of the 3 labeled Acacia trees and 9 Eucalyptus trees, 60 days after labeling. Most of the leaves, fine roots and rhizosphere samples of both species were 15N enriched from 5 days after labeling. The δ15N values were higher at a distance of 0.75 m than at 2.25 m in Acacia roots, but were similar at both distances in Eucalyptus roots and the rhizospheres. The wood and bark of Eucalyptus trees sampled at a distance of 6.2 m from the labeled Acacia trees were 15N enriched. This shows belowground N transfer from Acacia to Eucalyptus trees in the field during the first few days after labeling. This facilitation process may provide a significant amount of the nitrogen requirements of trees close to N-fixing trees in mixed forests.

Alder trees enhance crop productivity and soil microbial biomass in tea plantations

Monoculture farming systems lead to soils depleted of nutrients and diminished microbial functional diversity, disrupting processes crucial to maintaining soil health. The planting of trees in these monoculture systems is one way to improve soil nutrition and biodiversity. Therefore, the objective was how planting the N-fixing tree Alnus nepalensis (7 years old), into monoculture tea (Camellia sinensis var., assamica) plantations (32 years old), influences the soil fungal and bacterial communities, and how this impacts on tea productivity. Soil samples (0–15, 15–30, 30–60cm depths) were collected from plantations of monoculture tea and tea interplanted with A. nepalensis trees. The samples were analyzed for basic soil properties and nutrients. Phospholipid fatty-acid analyses were conducted on the soil samples to determine the microbial functional groups and biomass of bacterial and fungal communities. Biomass of soil fungi and bacteria were 41% and 10% higher in the tea+A. nepalensis sites than in the tea monoculture sites, respectively. These higher levels were recorded despite there being no changes in the diversity of the soil fungi and bacteria, or the soil nutrition, between the different sites. Tea productivity increased between 52% and 72%, and is attributed to the increases in the soil community biomass. Ectomycorrhizal biomass, as well as Gram-positive, Gram-negative, and actinomycetes bacterial biomass, all increased ranging from 10% to 83%. These groups of organisms have been shown to contain plant growth promoting characteristics, contributing toward increased crop productivity. We provide clear evidence that A. nepalensis in tea plantations promotes the growth and development of the soil microbial communities and that this impacts on above ground productivity. This study highlights the benefits of introducing N-fixing tree species, such as A. nepalensis, into monoculture systems, and how this relates to soil health and harvest yield.

Soil microbial and nutrient properties in the rhizosphere of coffee under agroforestry management

The integration of shade trees in coffee production can positively contribute to increased yields in these cropping systems. However, little work has examined nutrient-microbial dynamics in the rhizosphere of coffee under shade trees. In this study, we compared nutrient concentrations and nitrifier populations in rhizosphere soil of coffee under shade and in monoculture as well as in rhizosphere versus bulk soils. Our study was performed in a low altitude, wet coffee zone of Costa Rica, at a 12-year-old agroforestry experimental farm with coffee (Coffea arabica) grown under full sun and under the commonly used dinitrogen (N2) fixing shade tree, Erythrina poeppigiana. Rhizosphere and bulk soils were collected and analyzed for inorganic N, available P, exchangeable base cations and pH status. Real-time PCR was used to determine the relative abundance of ammonia oxidizing bacteria (AOB). We show a nutrient increase (11–158%), particularly nitrates, in the rhizosphere soil compared to bulk soil of coffee. Moreover, this accumulation was greater in the rhizosphere of coffee associated with E. poeppigiana than under monoculture. AOB abundance was strongly related to soil pH (r=0.83; P=0.002) across both treatments, while AOB abundance varied with soil ammonium:nitrate ratios (r=−0.82; P=0.0420) exclusively under monoculture coffee. Our study, for the first time to our knowledge, illustrates soil nutrient differences and nutrient–microbial relationships under coffee in monoculture and in a N2-fixing shade tree system.

Acacia tree density strongly affects N and P fluxes in savanna

Studies of nutrient cycling in savanna ecosystems rarely consider how fluxes are affected by local variations in tree density and nutrient redistribution by herbivores. We studied how the density of Acacia zanzibarica trees in a humid savanna ecosystem affected the input of nitrogen (N) through N2-fixation and N and phosphorus (P) outputs through fire and also internal pathways of N and P return through herbivores. We found that N inputs and P outputs both increased with increasing density of N2-fixing trees, the N effect being due to tree density rather than to differences in the rate of N2-fixation. However, total N outputs due to fire did not vary with tree density because losses from the herb layer decreased as losses from the tree layer increased. In contrast, total P outputs did increase with tree density because P losses from the tree layer exceeded those from the herb layer. We suggest that variation in the density of N2-fixing trees coupled with the effects of fire can cause substantial differences in the local N and P balances in savanna vegetation. To some extent, these differences may be evened out by the tendency for browsing herbivores to transfer nutrients from Acacia- to grass-dominated areas. We conclude that encroachment by N2-fixing trees and shrubs has important consequences for ecosystem properties such as N and P dynamics.

Influences of N-Fixing and Non-N-Fixing Vegetation and Invasive Fish on Water Chemistry of Hawaiian Anchialine Ponds

In coastal waters, it remains unclear how terrestrial invasive species might alter nutrient availability and thus affect bottom-up control of primary production. Anchialine ponds are tidal- and groundwater-fed coastal water bodies without surface connections that provide convenient model systems in which to examine terrestrial to aquatic nutrient flow. To investigate how N-fixing and non-N-fixing terrestrial vegetation and invasive consumers affect water quality, we compared dissolved nutrient concentrations and physicochemical parameters among Hawaiian anchialine ponds surrounded by: (1) the nonnitrogen (N)-fixing tree Thespesia populnea (milo), (2) the exotic N-fixing tree Prosopis pallida (kiawe), (3) no tree cover, and (4) no tree cover but containing populations of invasive poeciliid fish. Average (±1 SE) concentration of dissolved inorganic nitrogen (DIN) across all ponds (51.15 µM ±3.1; n = 17) was high, but there was no discernible pattern among pond types. Model results suggest that leaf litter breakdown from the exotic N-fixing tree contributed <0.02% of total dissolved nitrogen (TDN) in the water column at high tide. However, additions of phosphorus (P), especially from litter of non-N-fixing T. populnea, resulted in ratios of DIN to soluble reactive phosphorus (SRP) that varied between 4.5:1 and 35:1 across pond types. Our study suggests that: (1) DIN concentrations were more dependent on watershed-scale N inputs than local tree cover or the presence of poeciliids, (2) low water residence times in these ponds reduce effects of local biota on water nutrient chemistry, and (3) P from anthropogenic inputs and/or biota cause alterations to DIN:SRP ratios in anchialine pond water that may potentially alter primary production rates in these coastal ecosystems.

Nitrogen fixation strategies can explain the latitudinal shift in nitrogen-fixing tree abundance.

The rarity of symbiotic nitrogen-fixing trees in higher-latitude compared to lower-latitude forests is paradoxical because higher-latitude soils are relatively N poor. Using national-scale forest inventories from the United States and Mexico, we show that the latitudinal abundance distribution of N-fixing trees (more than 10 times less abundant poleward of 35 degrees N) coincides with a latitudinal transition in symbiotic N-fixation type: rhizobial N-fixing trees (which are typically facultative, regulating fixation to meet nutritional demand) dominate equatorward of 35 degrees N, whereas actinorhizal N-fixing trees (typically obligate, maintaining fixation regardless of soil nutrition) dominate to the north. We then use theoretical and statistical models to show that a latitudinal shift in N-fixation strategy (facultative vs. obligate) near 35 degrees N can explain the observed change in N-fixing tree abundance, even if N availability is lower at higher latitudes, because facultative fixation leads to much higher landscape-scale N-fixing tree abundance than obligate fixation.

Impact of elevated CO2 on metal homeostasis and the actinorhizal symbiosis in early successional alder shrubs

The increasing atmospheric CO2 concentration could stimulate terrestrial ecosystem and create an important carbon (C) sink that could slow down climate change due to anthropogenic activities. The extent of this enhanced growth will strongly depends upon the availability of nitrogen (N) to plant. In alder, the predominant N2 fixing tree in boreal forest, the ability to establish actinorhizal symbiosis will thus be key. How high CO2 concentration and exogenous N impact actinorhizal symbiosis remains poorly known. In this study on speckled alder and green alder, we evaluated the effect of CO2 and exogenous N availability on (i) the efficiency and development of the actinorhizal symbiosis and (ii) on the homeostasis of eleven elements, including essential nutrients for N2 fixation such as phosphorus (P) and molybdenum (Mo). We report that Frankia sp. infection is the primary factor controlling nutrient homeostasis (allocation and concentration) in plant. P and Mo, and in a less extend magnesium (Mg) and manganese (Mn), are the elements which homeostasis was the most affected by Frankia sp. infection. Exogenous N and CO2 concentrations had limiting or no impact on the homeostasis of most elements. We also show that actinorhizal symbiosis could be critical to alder response to elevated CO2. In absence of Frankia sp. infection neither speckled alder nor green alder responded to elevated CO2. On the contrary, in presence of Frankia sp., the biomass production was significant stimulated by increasing CO2 and independent to exogenous N.

N-fixing trees in restoration plantings: Effects on nitrogen supply and soil microbial communities

Mixed-species restoration tree plantings are being established increasingly, contributing to mitigate climate change and restore ecosystems. Including nitrogen (N)-fixing tree species may increase carbon (C) sequestration in mixed-species plantings, as these species may substantially increase soil C beneath them. We need to better understand the role of N-fixers in mixed-species plantings to potentially maximize soil C sequestration in these systems. Here, we present a field-based study that asked two specific questions related to the inclusion of N-fixing trees in a mixed-species planting: 1) Do non-N-fixing trees have access to N derived from fixation of atmospheric N2 by neighbouring N-fixing trees? 2) Do soil microbial communities differ under N-fixing trees and non-N-fixing trees in a mixed-species restoration planting? We sampled leaves from the crowns, and litter and soils beneath the crowns of two N-fixing and two non-N-fixing tree species that dominated the planting. Using the 15N natural abundance method, we found indications that fixed atmospheric N was utilized by the non-N-fixing trees, most likely through tight root connections or organic forms of N from the litter layer, rather than through the decomposition of N-fixers litter. While the two N-fixing tree species that were studied appeared to fix atmospheric N, they were substantially different in terms of C and N addition to the soil, as well as microbial community composition beneath them. This shows that the effect of N-fixing tree species on soil carbon sequestration is species-specific, cannot be generalized and requires planting trails to determine if there will be benefits to carbon sequestration.

Nutrient accumulation in aboveground biomass of planted tropical trees: a meta-analysis

Efficient nutrient use is essential for biomass production by tropical trees growing in infertile soils. Accumulation of nitrogen (N) and phosphorus (P) in the aboveground biomass of four groups of tree stands [Acacia, Eucalyptus, N2-fixing trees excluding Acacia, and other non-N2-fixing broadleaved (ONNFB) trees] were investigated using meta-analyses of a range of biomass data to test the hypothesis that fast-growing Acacia and Eucalyptus trees accumulate fewer nutrients. Data for 83 tropical tree stands were selected from the literature. Standardized major axis regressions were applied between the log10-transformed biomass and N or P accumulation. Nutrient use efficiency was compared with aboveground biomass and topsoil conditions. The slope of the regression between aboveground biomass and N accumulation for Eucalyptus was significantly smaller than the slopes for the N2-fixing trees (excluding Acacia) and the ONNFB trees. N use efficiency of Eucalyptus increased with biomass more than that of N2-fixing trees (excluding Acacia) and the ONNFB trees, because their stems and twigs tended to accumulate less N than in the other groups as biomass increased. The regressions between aboveground biomass and P accumulation had a common slope, and the intercepts of Acacia and Eucalyptus were significantly lower than that of ONNFB trees. P use efficiency of Acacia was consistently higher than that of the ONNFB trees. P use efficiency is more affected by other factors like soil conditions than is N use efficiency, and the differences in the tree groups significantly affect the use efficiency of both nutrients. These results explained some aspects of the general suitability of Acacia and Eucalyptus species for tropical plantations on infertile soils.

Carbon storage landscapes of lowland Hawaii: the role of native and invasive species through space and time

Tropical forests are important storehouses of carbon and biodiversity. In isolated island ecosystems such as the Hawaiian Islands, relative dominance of native and nonnative tree species may influence patterns of forest carbon stocks and biodiversity. We determined aboveground carbon density (ACD) across a matrix of lava flows differing in age, texture, and vegetation composition (i.e., native or nonnative dominated) in wet lowland forests of Hawaii Island. To do this at the large scales necessary to accurately capture the inherent heterogeneity of these forests, we collected LiDAR data across areas of interest and developed relationships between LiDAR metrics and field-based estimates of forest ACD. This approach enabled us to inventory, rather than merely sample, the entire populations (i.e., forests) of interest. Native Hawaiian wet lowland forests exhibited ACD values similar to those of intact tropical forests elsewhere. In general, ACD of these forests increased with increasing lava flow age, but patterns differed between native and nonnative forest stands. On the youngest lavas, native-dominated forest ACD averaged < 60 Mg/ha, compared to -100 Mg C/ha for nonnative-dominated forests. This difference was due to the presence of the nonnative, N2-fixing trees F. moluccana and C. equisetifolia in the nonnative-dominated forest stands, as well as the corresponding absence of N2-fixing trees in native-dominated forest stands. Following -500 years of primary succession and thereafter, however, both forest types exhibited ACD values averaging -130 Mg C/ha, although it took nonnative forests only 75 80 years of post-establishment succession to reach those values. Given the large areas of early-successional M. polymorpha-dominated forest on young lava flows, further spread of F. moluccana and C. equisetifolia populations would likely increase ACD stocks but would constitute a significant erosion of the invaluable contribution of Hawaii's native ecosystems to global biodiversity.

Is there more soil carbon under nitrogen-fixing trees than under non-nitrogen-fixing trees in mixed-species restoration plantings?

Afforestation of agricultural land provides an important opportunity to mitigate climate change by storing carbon (C) in both plant biomass and the soil. Here we present results of a study in which we sought to determine whether soil under nitrogen(N)-fixing trees contained more C than soil under non-N-fixing trees in mixed-species plantings, and thus if inclusion of N-fixers is beneficial in terms of increasing soil C sequestration. Soils were sampled directly beneath N-fixing and non-N-fixing tree species in riparian and upland mixed-species plantings in southeastern Australia. Soil C and N contents were assessed at both the landscape and individual planting scales. At the landscape scale, there were higher levels of soil C and N under N-fixing trees compared with non-N-fixing trees. At the individual planting scale, the patterns were less clear with both large increases and decreases occurring across the range of sites. The results presented here indicate that the inclusion of N-fixers may help to increase soil C, and N, but that the response may be site- and species-specific.

N2-fixing trees and the transfer of fixed-N for sustainable agroforestry: a review

Many tropical areas lack soil nitrogen (N), an essential nutrient for plant growth and the production of food. Commercial N fertilisers are expensive, with only a fraction of this nutrient reaching the plant, which limits efficiency and potentially increases water contamination. Dinitrogen (N2)-fixing trees are a promising alternative to sustainably fertilise crops. The use of N2-fixing trees in tropical agriculture has garnered attention from researchers, development organisations, governments, and farmers in recent years as a revival of pro-environmental practices. Dinitrogen (N2)-fixing trees can establish in N-deficient soils, replace N lost in harvest and provide an as-of-yet not fully realised benefit to ecosystem services. High N2-fixation rates, upwards of 92 %, have been measured in some N2-fixing trees, using the 15N natural abundance method. The recovery of this fixed-N by associated perennial crops is of particular interest in tropical agroforestry systems. Here, we review N transfer pathways from trees to perennial crops in agroforestry. We focus on Theobroma cacao and Coffea arabica. We also draw on agroforestry systems with herbaceous alleys. We identify three pathways of N transfer from N2-fixers to non-N2-fixing crops: (1) decomposition and mineralisation of organic compounds, e.g., litter, prunings, roots, and nodules, (2) root-to-root direct transfer via exudation, and (3) common mycorrhizal networks. Both 15N natural abundance and 15N enrichment techniques have been used to study N transfer. However, various factors limit the accuracy of estimates within agroforestry systems. Under field conditions, the major limits are (1) improper reference selection and (2) unrepresentative sampling of the receiver plant and/or donor N source. We highlight key findings and provide recommendations to tackle these obstacles.

Implications of multipurpose tree leaf application on wheat productivity in dry tropics

Leaves of multipurpose tree species (those providing more than one function or product of human use) can serve as sources of fertilizer for nutrient supply, especially nitrogen (N). In this study chopped leaves of tropical tree species (5 N-fixing species, 5 non-N-fixing species and combinations of 5 N-fixing with a non-N-fixing species) were incorporated in soil to evaluate its effects on wheat biological productivity (including grain yield, GYIELD) under dryland conditions. High quality leaves of N-fixing tree species (e.g. Dalbergia sissoo, Cassia fistula and Prosopis cineraria) had lower carbon/nitrogen (C/N), lignin/nitrogen (LIG/N), polyphenol/nitrogen (PPL/N) and lignin+polyphenol /nitrogen (LIG+PPL/N) ratios than low quality leaves of non-N-fixing species. Combination treatments had intermediate values of different parameters. Application of high quality leaves caused greater increases in wheat productivity and yield than other species. By the application of leaves of N-fixing trees, on average, wheat yield increased 160% relative to the control (no addition of leaves), and when combined with non-N-fixing Terminalia chebula leaves the yield increased 108%. Mean total net productivity (TNP) with N-fixing species treatments, exceeded mean TNP with non-N-fixing species and combination treatments by 50% and 28%, respectively. Multivariate regressions indicated that nitrogen (N) concentration in leaves (main nutrient), interacting with lignin (LIG) and polyphenol (PPL) concentrations, explained 79%–86% of variability in productivity parameters and yield. Strong correlation between LIG+PPL/N ratio of leaves with GYIELD of wheat crop suggests that the ratio can be used as a reliable index for mass screening of multipurpose tree species for use as soil amendments especially in dryland agriculture. Direct application of high quality N-fixing tree species leaves (especially D. sissoo, C fistula and P. cineraria), an uncommon practice in dry tropics in India, may serve as a short-term option for rapid enhancement of wheat productivity and soil fertility.

Mining waste contaminated lands: an uphill battle for improving crop productivity

Mining drastically alters the physico-chemical and biological environment of the landscape. Low organic matter content, unfavourable pH, low water holding capacity, salinity, coarse texture, compaction, siltation of water bodies due to wash off of mineral overburden dumps, inadequate supply of plant nutrients, accelerated erosion, acid generating materials, and mobilization of contaminated sediments into the aquatic environment are the principal constraints experienced in mining contaminated sites. A variety of approaches have been considered for reclaiming mine wastes including direct revegetation of amended waste materials, top soiling, and the use of capillary barriers. The simplest technology to improve crop productivity is the addition of organic amendments. Biosolids and animal manure can support revegetation, but its rapid decomposition especially in the wet tropics, necessitates repeated applications. Recalcitrant materials such as “biochars”, which improve soil properties on a long term basis as well as promote soil carbon sequestration, hold enormous promise. An eco-friendly and cost-effective Microbe Assisted Phytoremediation system has been proposed to increase biological productivity and fertility of mine spoil dumps. Agroforestry practices may enhance the nutrient status of degraded mine spoil lands (facilitation). N-fixing trees are important in this respect. Metal tolerant ecotypes of grasses and calcium-loving plants help restore lead, zinc, and copper mine tailings and gypsum mine spoils, respectively. Overall, an integrated strategy of introduction of metal tolerant plants, genetic engineering for enhanced synthesis and exudation of natural chelators into the rhizosphere, improvement of rhizosphere, and integrated management including agroforestry will be appropriate for reclaiming mining contaminated lands.

Key role of symbiotic dinitrogen fixation in tropical forest secondary succession

Forests contribute a significant portion of the land carbon sink, but their ability to sequester CO2 may be constrained by nitrogen, a major plant-limiting nutrient. Many tropical forests possess tree species capable of fixing atmospheric dinitrogen (N2), but it is unclear whether this functional group can supply the nitrogen needed as forests recover from disturbance or previous land use, or expand in response to rising CO2 (refs 6, 8). Here we identify a powerful feedback mechanism in which N2 fixation can overcome ecosystem-scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests. Over a 300-year chronosequence in Panama, N2-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbours (greatest difference in youngest forests), and showed species-specific differences in the amount and timing of fixation. As a result of fast growth and high fixation, fixers provided a large fraction of the nitrogen needed to support net forest growth (50,000 kg carbon per hectare) in the first 12 years. A key element of ecosystem functional diversity was ensured by the presence of different N2-fixing tree species across the entire forest age sequence. These findings show that symbiotic N2 fixation can have a central role in nitrogen cycling during tropical forest stand development, with potentially important implications for the ability of tropical forests to sequester CO2.

Leaf-litter inputs from an invasive nitrogen-fixing tree influence organic-matter dynamics and nitrogen inputs in a Hawaiian river

We examined how invasion of tropical riparian forests by an exotic N-fixing tree (Falcataria moluccana) affects organic-matter dynamics in a Hawaiian river by comparing early stages of leaf-litter breakdown between the exotic F. moluccana and native Metrosideros polymorpha trees. We examined early decomposition stages because of low leaf-litter retention rates (<20 d) that result from the flashy nature of tropical Pacific Island streams. Leaf breakdown rates, fungal biomass, and invertebrate abundances were 40, 120, and 30% greater, respectively, for F. moluccana than M. polymorpha leaves. Leaf-litter breakdown was largely a result of stream flow and to a lesser extent fungal colonization. Invertebrates were not an important factor in leaf-litter breakdown. Initial tannin content, leaf C∶N, and toughness were important intrinsic factors inhibiting leaf breakdown and fungal colonization. Regression analyses between remaining N content (%) and ash-free dry mass of leaf litter revealed that the early stages of F. moluccana leaf-litter breakdown are a source of N to streams invaded by F. moluccana and contribute a conservatively estimated 2.1 to 5.7% to the available total dissolved N pool. Direct input of F. moluccana leaf litter influences early stages of leaf-litter breakdown in tropical streams with low leaf-litter retention rates. Direct input of leaf litter also contributes somewhat to N inputs, but subsurface flows through N-rich soils of F. moluccana-invaded riparian forests probably are a greater source.

Carbon and water fluxes in an arid-zone Acacia savanna woodland: An analyses of seasonal patterns and responses to rainfall events

The study of landscape gas exchange in arid and semi-arid regions is less common than those of more mesic environments, despite their large geographical extent, their importance to regional climate, their socioeconomic values and the carbon and water balances of such regions. In this study we used eddy covariance measurements to examine net ecosystem exchange and water fluxes of a landscape dominated by a N-fixing tree (Acacia aneura; Mulga) as a function of soil moisture content, vapour pressure deficit, leaf area index and pulses of rain. Seasonal budgets of carbon and water, ecosystem-scale water-use-efficiency (the ratio of net ecosystem exchange to evapotranspiration) and inherent water-use-efficiency (ecosystem water-use-efficiency×vapour pressure deficit) were also examined. Across the 12 month study, the landscape was a net sink for carbon, despite prolonged periods of zero rain.Changes in both net ecosystem exchange and evapotranspiration were tightly coupled to changes in the moisture content of the upper (10cm) soil profile, but not the deeper profile and both responded rapidly to changes in soil moisture content. As vapour pressure deficit increased over the course of several consecutive days in the wet season there was no significant response of ecosystem water-use-efficiency. In contrast, in the dry season, as vapour pressure deficit increased ecosystem water-use-efficiency declined curvilinearly. However, in both wet and dry seasons, ecosystem water-use-efficiency declined with increasing soil moisture content. Daily inherent water-use-efficiency increased gradually following each rainfall event. As daily mean vapour pressure deficit increased between rain events, inherent water-use-efficiency increased in both the wet and dry seasons but with a steeper slope in the wet season. However, inherent water-use-efficiency decreased with increasing soil moisture in both seasons, and the slope of a semi-log plot of inherent water-use-efficiency versus soil moisture content decreased faster in the dry season than in the wet season. Similarly, the marginal carbon cost of water was smaller (0.3) in the wet than dry season (0.6). Variations in ecosystem leaf area index were correlated with the under storey component, which was highest in the wet season and lowest in the dry season. We therefore conclude that changes in under storey leaf area index were significant drivers of seasonal changes in canopy gas exchange. Mulga, despite maintaining leaf area index through the dry season in a semi-arid environment, supports little dry season evapotranspiration and relies, to a very large extent, on soil moisture in the upper soil profile rather than deeper stores of water.

Forest Structure, Nutrients, and Pentaclethra macroloba Growth after Deforestation of Costa Rican Lowland Forests

Succession following deforestation in Neotropical forests has been investigated extensively, yet rarely have studies connected nutrient dynamics with vegetation. This study was conducted in lowland wet forests of Maquenque, Costa Rica. The objectives were (1) to compare carbon (C), nitrogen (N), and phosphorus (P) characteristics and understory vegetation diversity between regenerating forests and primary forests; and (2) to use these variables to evaluate P. macroloba’s successional role. Four 300 m2 plots were established in primary and secondary forests where P. macroloba was the dominant N-fixing tree. Soil and vegetation data were collected from 2008 to 2010. Values of indicators of C, N, and P cycle activity were generally greater in primary than in secondary forest soils. Efficiency of organic C use and the relative contribution of respiration and organic C to soil biomass were also greater in the primary forest. These trends corresponded with greater richness, biomass, and cover of total and leguminous plant species, greater volume of P. macroloba in primary stands, and greater density of P. macroloba in secondary stands. As cleared regions of former primary forest regenerate, P. macroloba is the important dominant N-fixing tree and a critical driver of C, N, and P recuperation and ecosystem recovery.

A meta-analysis reveals mostly neutral influence of scattered trees on pasture yield along with some contrasted effects depending on functional groups and rainfall conditions

Scattered trees occurring throughout farmland matrix are prominent features of many human-dominated landscapes around the world, especially in livestock grazing systems. They are keystone structures that may play important roles in maintaining ecosystem functions, services, and farmland biodiversity. However, scattered trees in agricultural landscapes are declining worldwide due to intensive land use. They are often perceived by farmers as having negative impacts on agricultural production. Large-scale assessments in different biomes of agricultural yield in scattered tree ecosystems remain rare. Filling this knowledge gap may help improve decision-making regarding the value of scattered trees in agricultural landscapes. Using meta-analysis, we found that, across four tree functional groups (deciduous, Eucalyptus, N2-fixing, evergreen oak), mature scattered trees do not compromise pasture yield. The sign and magnitude of scattered tree effects on pasture yield did vary, however, among tree functional groups and according to precipitation levels. Our study suggests that, as drought pressure increases abiotic stress, tree facilitation by N2-fixing trees, and competition by Eucalyptus, will become the more common interactions between scattered trees and pasture. Management options exist to conserve and restore scattered trees in agricultural landscapes, but new policies are required to support their widespread adoption by farmers.

Ecosystem legacy of the introduced N2-fixing tree Robinia pseudoacacia in a coastal forest

Habitat invasibility has been found to increase dramatically following the alteration of ecosystem properties by a nonnative species. Robinia pseudoacacia, black locust, is a nitrogen-fixing, clonal tree species that aggressively invades open habitats and expands outside of plantations worldwide. Robinia pseudoacacia stands in Cape Cod National Seashore were particularly susceptible to a hurricane in 1991 that caused widespread blowdown and a dramatic reduction in Robinia in some stands. We used this change to investigate the lasting ecological effects of this nonnative species on this upland coastal ecosystem. We established replicate clusters of 20 × 20 m field plots within 50 m of each other that contained native pitch pine (Pinus rigida) and oak (Quercus velutina, Q. alba) forest, living Robinia stands, and stands in which Robinia was eliminated or reduced to less than 5% cover by the hurricane. Net nitrification and extractable soil nitrate concentration differed significantly between stand types, in the order Robinia > former Robinia > pine-oak. Nonnative species cover differed significantly between each stand type, in the order Robinia > former Robinia > pine-oak. Invasion of Robinia pseudoacacia increased soil net nitrification and nitrogen availability and precipitated a change in forest species composition that favored nonnative species. The presence of elevated soil nitrogen and nonnative species persisted at least 14 years after the removal of the original invading tree species, suggesting that the invasion of a tree species left a legacy of altered soil biogeochemistry, a higher number of nonnative species, and greater nonnative species cover.