Necromass Production Research Articles

Necromass responses to warming: A faster microbial turnover in favor of soil carbon stabilisation

Microbial byproducts and residues (hereafter ‘necromass’) potentially play the most critical role in soil organic carbon (SOC) sequestration. However, little is known about the influence of climate warming on necromass accumulation in the agroecosystem and the underlying mechanisms associated with microbial life strategies. In order to address these knowledge gaps, we used amino sugars as biomarkers of microbial necromass, and investigated their variation through an 8-year trial in an agroecosystem with two warming levels (+1.6 and + 3.2 °C) compared to ambient temperature. The results showed that the lower warming level had no impact on total microbial necromass carbon. Conversely, warming the soil 3.2 °C above ambient increased total microbial necromass by 17 % and its contribution to SOC by 21.3 %, mainly by increasing fungal necromass (+19.8 %), whereas +3.2 °C warming had no impact on bacterial necromass. At the phylum level, compared with the ambient control, +3.2 °C warming induced an increase in the abundance of Proteobacteria and a decrease in both Acidobacteria and Actinobacteria, whereas in the fungal community, Ascomycota increased and Mortierellomycota decreased. This indicates that r-strategists outcompete K-strategists in warmer climates, which led to increased microbial necromass production and accumulation, as supported by the positive correlation between r-strategists and microbial necromass. Stronger microbial competition for resources also resulted in a higher biomass turnover rate, greater cell death, and greater production of microbial necromass. This was supported by the lower bacterial and fungal network complexity and trophic links under warming conditions. In addition, the necromass generated from accelerated microbial turnover further offsets warming-induced deceases in microbial biomass. Consequently, bulk SOC did not change, despite microbial necromass having a much greater response to warming than the soil C pool. Therefore, future climate warming may influence the composition and persistence of SOC during microbial degradation.

Microbial metabolic traits drive the differential contribution of microbial necromass to soil organic carbon between the rhizosphere of absorptive roots and transport roots

The rhizosphere is a typical soil microbial hotspot, however, not a homogeneous entity. Due to root functional differentiation, different root functional modules (i.e., absorptive roots and transport roots) can play distinct roles in microbial necromass formation and subsequent soil organic carbon (SOC) sequestration by influencing microbial metabolic activity in the surrounding soil. Yet, how microbial metabolic traits mediated by different root functional modules regulate the accumulation of microbial necromass C (MNC) in the rhizosphere remains poorly understood. Herein, we quantified and compared the differences in the contribution of MNC to SOC between the rhizosphere of two root functional modules, and explored the role of microbial metabolic traits in influencing the contribution of MNC to rhizosphere SOC in different root functional modules in two spruce (Picea asperata Mast.) plantations. Our findings revealed that absorptive roots exhibited a significantly higher contribution of MNC to SOC (32.9–37.5%) compared to transport roots (27.7–30.5%) in the rhizosphere. This suggests that absorptive roots possess a greater ability to promote MNC accumulation in the rhizosphere than transport roots. This observation was mainly attributed to the difference in the trade-offs between microbial growth and investment traits between the two root functional modules. Specifically, the rhizosphere of absorptive roots had greater microbial C use efficiency (CUE), faster growth and turnover rates, lower respiratory quotients and biomass-specific enzyme activity than did those of transport roots, suggesting that absorptive roots support greater microbial growth yields and subsequently greater necromass production. Collectively, our findings demonstrate that the contribution of MNC to SOC in the rhizosphere largely depends on the trade-offs of microbial metabolic traits mediated by root functional differentiation. Our study also provides novel and direct empirical evidence supporting the need to integrate function-based fine root classifications with the different contributions of MNC to SOC sequestration in the rhizosphere into land surface models of C cycling.

Macroecology Differentiation Between Bacteria and Fungi in Topsoil Across the United States

AbstractBacteria and fungi possess distinct physiological traits. Their macroecology is vital for ecosystem functioning such as carbon cycling. However, bacterial and fungal biogeography and underlying mechanisms remain elusive. In this study, we investigated bacterial versus fungal macroecology by integrating a microbial‐explicit model—CLM‐Microbe—with measured fungal (FBC) and bacterial biomass carbon (BBC) from 34 NEON sites. The distribution of FBC, BBC, and FBC: BBC (F:B) ratio was well simulated across sites, with variations in 99% (P < 0.001), 97% (P < 0.001), and 99% (P < 0.001) being explained by the CLM‐Microbe model, respectively. We found stronger biogeographic patterns of FBC relative to BBC across the United States. Fungal and bacterial turnover rates showed similar trends along latitude. However, latitudinal trends of their component fluxes (carbon assimilation, respiration, and necromass production) were distinct between bacteria and fungi, with those latitudinal trends following inverse unimodal patterns for fungi and showing exponential declining responses for bacteria. Carbon assimilation was dominated by vegetation productivity, and respiration was dominated by mean annual temperature for bacteria and fungi. The dominant factor for their necromass production differs, with edaphic factors controlling fungal and mean annual temperature controlling bacterial processes. The understanding of fungal and bacterial macroecology is an important step toward linking microbial metabolism and soil biogeochemical processes. Distinct fungal and bacterial macroecology contributes to the microbial ecology, particularly on microbial community structure and its association with ecosystem carbon cycling across space.

Earthworms as catalysts in the formation and stabilization of soil microbial necromass.

Microbial necromass is a central component of soil organic matter (SOM), whose management may be essential in mitigating atmospheric CO2 concentrations and climate change. Current consensus regards the magnitude of microbial necromass production to be heavily dependent on the carbon use efficiency of microorganisms, which is strongly influenced by the quality of the organic matter inputs these organisms feed on. However, recent concepts neglect agents relevant in many soils: earthworms. We argue that the activity of earthworms accelerates the formation of microbial necromass stabilized in aggregates and organo‐mineral associations and reduces the relevance of the quality of pre‐existing organic matter in this process. Earthworms achieve this through the creation of transient hotspots (casts) characterized by elevated contents of bioavailable substrate and the efficient build‐up and quick turnover of microbial biomass, thus converting SOM not mineralized in this process into a state more resistant against external disturbances, such as climate change. Promoting the abundance of earthworms may, therefore, be considered a central component of management strategies that aim to accelerate the formation of stabilized microbial necromass in wide locations of the soil commonly not considered hotspots of microbial SOM formation.

Fast-decaying plant litter enhances soil carbon in temperate forests but not through microbial physiological traits

Conceptual and empirical advances in soil biogeochemistry have challenged long-held assumptions about the role of soil micro-organisms in soil organic carbon (SOC) dynamics; yet, rigorous tests of emerging concepts remain sparse. Recent hypotheses suggest that microbial necromass production links plant inputs to SOC accumulation, with high-quality (i.e., rapidly decomposing) plant litter promoting microbial carbon use efficiency, growth, and turnover leading to more mineral stabilization of necromass. We test this hypothesis experimentally and with observations across six eastern US forests, using stable isotopes to measure microbial traits and SOC dynamics. Here we show, in both studies, that microbial growth, efficiency, and turnover are negatively (not positively) related to mineral-associated SOC. In the experiment, stimulation of microbial growth by high-quality litter enhances SOC decomposition, offsetting the positive effect of litter quality on SOC stabilization. We suggest that microbial necromass production is not the primary driver of SOC persistence in temperate forests. Factors such as microbial necromass origin, alternative SOC formation pathways, priming effects, and soil abiotic properties can strongly decouple microbial growth, efficiency, and turnover from mineral-associated SOC.

T4-like Phages Reveal the Potential Role of Viruses in Soil Organic Matter Mineralization.

Viruses are the most abundant biological entities in the world, but their ecological functions in soil are virtually unknown. We hypothesized that greater abundance of T4-like phages will increase bacterial death and thereby suppress soil organic carbon (SOC) mineralization. A range of phage and bacterial abundances were established in sterilized soil by reinoculation with 10-3 and 10-6 dilutions of suspensions of unsterilized soil. The total and viable 16S rRNA gene abundance (a universal marker for bacteria) was measured by qPCR to determine bacterial abundance, with propidium monoazide (PMA) preapplication to eliminate DNA from non-viable cells. Abundance of the g23 marker gene was used to quantify T4-like phages. A close negative correlation between g23 abundance and viable 16S rRNA gene abundance was observed. High abundance of g23 led to lower viable ratios for bacteria, which suggested that phages drove microbial necromass production. The CO2 efflux from soil increased with bacterial abundance but decreased with higher abundance of T4-like phages. Elimination of extracellular DNA by PMA strengthened the relationship between CO2 efflux and bacterial abundance, suggesting that SOC mineralization by bacteria is strongly reduced by the T4-like phages. A random forest model revealed that abundance of T4-like phages and the abundance ratio of T4-like phages to bacteria are better predictors of SOC mineralization (measured as CO2 efflux) than bacterial abundance. Our study provides experimental evidence of phages' role in organic matter turnover in soil: they can retard SOC decomposition but accelerate bacterial turnover.

Sticky dead microbes: Rapid abiotic retention of microbial necromass in soil

Microbial necromass dominates soil organic matter. Recent research on necromass and soil carbon storage has focused on necromass production and stabilization mechanisms but not on the mechanisms of necromass retention. We present evidence from soil incubations with stable-isotope labeled necromass that abiotic adsorption may be more important than biotic immobilization for short-term necromass retention. We demonstrate that necromass adsorbs not only to mineral surfaces, but may also interact with other necromass. Furthermore, necromass cell chemistry alters necromass-necromass interaction, with more bacterial tracer retained when there is yeast necromass present. These findings suggest that the adsorption and abiotic interaction of microbial necromass and its functional properties, beyond chemical stability, deserve further investigation in the context of soil carbon sequestration.

Zonal distribution of coarse woody debris and its contribution to net primary production in a secondary mangrove forest

AbstractCoarse woody debris (CWD) plays an important role in long‐term carbon storage in forest ecosystems. However, few studies have examined CWD in mangrove forests. A secondary mangrove forest on an estuary of the Trat River showed different structures along vegetation zones ranging from the river's edge to inland parts of the forest (the Sonneratia–Avicennia, Avicennia, Rhizophora, and Xylocarpus zones, respectively). The mass distribution of CWD stock in downed wood and standing dead trees along these vegetation zones was evaluated. Most of the CWD stock in the Sonneratia–Avicennia and Avicennia zones was found in downed wood, while it mainly accumulated in standing dead trees in the Rhizophora and Xylocarpus zones. The total mass of CWD stock that accumulated in each zone ranged from 1.56–8.39 t ha−1, depending on the forest structure and inundation regimes. The annual woody debris flux in each zone was calculated by summing the necromass (excluding foliage) of dead trees and coarse litter from 2010 to 2013. The average woody debris flux was 5.4 t ha−1 year−1, and its zonal variation principally depended on the necromass production that resulted from forest succession, high tree‐density, and lightning. Over all the zones, the above‐ and below‐ground net primary production (ANPP and BNPP, respectively) was estimated at 18.0 and 3.6 t ha−1 year−1, respectively. The magnitude of BNPP and its contribution to the NPP was markedly increased when fine root production was taken into consideration. The contribution of the woody debris flux without root necromass to the ANPP ranged from 12 to 28%.

Romul_Hum—A model of soil organic matter formation coupling with soil biota activity. II. Parameterisation of the soil food web biota activity

A soil food web (FW) based approach to modelling the contribution of soil micro- and mesofauna to the formation of soil organic matter (SOM) was developed as an improvement to the soil organic matter model ROMUL, which is the predecessor of Romul_Hum. The main improvement is that the by-products (excrement and necromass) of soil micro- and mesofauna, and their important role as precursors to formation of stable SOM, that are not represented in any soil organic matter model, are now included in the Romul_Hum. First, parameters were compiled to explicitly and mechanistically model the foundational role of microbial communities in the soil FW by using a substrate C:N ratio based function that allows (a) the fungal to bacterial biomass ratio to vary from 3 to 15, (b) fungal C:N to vary from 8 to 30, and (c) bacterial C:N to vary from 5 to 8. These equations were used in model algorithms instead of fixed values that are used in the SOM models. Second, ordination was used on 15 published FWs biomass C datasets to define six FW patterns as combinations of two energy channels (fungal and bacterial) and three types of soil horizons (L, F+H, Ah or Ahe) in temperate and boreal forests. Third, FW analysis methods were used to quantify excrement and necromass C produced by soil micro- and mesofauna at different trophic levels. Soil heterotrophic respiration (expressed as the rate of SOM mineralisation), a core rate variable in all SOM models, was associated with the formation of the faunal by-products (excrement and necromass) using stoichiometric relationships between the production of faunal excrement, necromass and total FW biotic respiration. The ratios of “excrement mass/necromass C to soil heterotrophic respiration C” were key parameters incorporated into the Romul_Hum. The values of these ratios vary from 0.03 to 0.41 for the bacterial energy channels (with maximum in the F+H soil horizon) and from 0.02 to 0.17 in fungal energy channels (wih maximum in the Ah/Ahe soil horizon). The well established mechanistic algorithm for available nitrogen (N) of the FW methodology was used to develop functions for the “ratio of available N to heterotrophic respiration C” depending on substrate C:N ranging from 5 to 120. The resulting algorithms, functions and new parameters for excrement and necromass production, as well as parameterisation of available N production, are described for all six FW patterns. The newly developed FW module was incorporated in the simulation model of SOM formation, Romul_Hum, allowing for the calculation of faunal organic matter contributions to SOM formation. This novel approach to including soil faunal activity in modelling of soil C dynamics takes advanatage of the high degree of organisation within soil biotic communities in FWs and integrates the effects of microorganisms and soil fauna that govern the processes of organic residues transformation and mineralisation in the soil system.

Romul_Hum model of soil organic matter formation coupled with soil biota activity. I. Problem formulation, model description, and testing

For many decades in the late 20th century, the processes involved in the formation of soil organic matter (SOM) (also known as humification) constituted one of the most important areas of scientific inquiry in soil science. However, these processes have not been included in modern models of SOM dynamics, despite their relevance to quantifying carbon stabilisation (i.e., sequestration) in soil. Furthermore, modern models have focussed on mineralisation processes (e.g., mainly microbial heterotrophic respiration) and have not included the important effects of soil fauna that are known as key agents of SOM formation. To address these issues, we developed a modelling approach predicated on the existence of definable stoichiometric relations among the processes leading to SOM formation that are mediated by soil biota (e.g., correlations among biotic respiration, production of faunal excrement, and necromass as sources of SOM formation). Soil respiration, a core rate variable in all SOM models, was associated with the production of faunal by-products in soil food webs as precursors of stable SOM, specifically micro- and meso-faunal excrement, necromass, and earthworm casts. We developed a food-web based module, using a synthesis of decades of published data, to describe micro- and meso-faunal excrement and necromass production. We developed a separate module for anecic earthworms, with explicit representation of processes related to fresh casts. The contributions of these two modules were compiled and integrated with the ROMUL model of SOM dynamics, without changing the structure of the original model. These modules enabled calculation of the proportional contribution of faunal by-products to humification (i.e., recalcitrant SOM formation) and carbon sequestration. Testing of the new version of the model, known as Romul_Hum, showed consistent accumulation of faunal by-products in the “final” SOM fractions: the well-decomposed SOM of the organic soil horizons and the stable SOM of the mineral soil.

NECROMASS PRODUCTION: STUDIES IN UNDISTURBED AND LOGGED AMAZON FORESTS

Necromass stocks account for up to 20% of carbon stored in tropical forests and have been estimated to be 14-19% of the annual aboveground carbon flux. Both stocks and fluxes of necromass are infrequently measured. In this study, we directly measured the production of fallen coarse necromass (> or = 2 cm diameter) during 4.5 years using repeated surveys in undisturbed forest areas and in forests subjected to reduced-impact logging at the Tapajos National Forest, Belterra, Brazil (3.08 degrees S, 54.94 degrees W). We also measured fallen coarse necromass and standing dead stocks at two times during our study. The mean (SE) annual flux into the fallen coarse necromass pool in undisturbed forest of 6.7 (0.8) Mg x ha(-1) x yr(-1) was not significantly different from the flux under a reduced-impact logging of 8.5 (1.3) Mg x ha(-1) x yr(-1) With the assumption of steady state, the instantaneous decomposition constants for fallen necromass in undisturbed forests were 0.12 yr(-1) for large, 0.33 yr(-1) for medium, and 0.47 yr(-1) for small size classes. The mass weighted decomposition constant was 0.15 yr(-1) for all fallen coarse necromass. Standing dead wood had a residence time of 4.2 years, and approximately 0.9 Mg x ha(-1) x yr(-1) of this pool was respired annually to the atmosphere through decomposition. Coarse necromass decomposition at our study site accounted for 12% of total carbon remineralization, and total aboveground coarse necromass was 14% of the aboveground biomass. Use of mortality rates to calculate production of coarse necromass leads to an underestimation of coarse necromass production by 45%, suggesting that nonlethal disturbance such as branch fall contributes significantly to this flux. Coarse necromass production is an important component of the tropical forest carbon cycle that has been neglected in most previous studies or erroneously estimated.

BIOMASS DYNAMICS IN AMAZONIAN FOREST FRAGMENTS

Habitat fragmentation affects aboveground biomass in Amazonian forests, with potentially important implications for carbon storage and greenhouse gas emissions. We assessed the dynamics of aboveground‐biomass stocks by combining long‐term (10– 19 yr) data on mortality, damage, growth, and recruitment of large (≥10 cm diameter at breast height [dbh]) trees with measurements of nearly all other live and dead plant material (seedlings, saplings, small trees, palms, lianas, downed wood debris, snags, litter) in 50 1‐ha plots in fragmented and continuous Amazonian forests.The key process altering biomass dynamics in fragmented forests is the chronically elevated mortality of large trees, which apparently results from microclimatic changes and increased wind turbulence near forest edges. This, in turn, accelerates the production of necromass and leads to significantly increased wood debris and litter on the forest floor. Near forest edges, frequent canopy disturbance increases the amount of light in the understory, resulting in accelerated tree recruitment, significantly higher biomass of small (5– 10 cm dbh) trees, and higher liana densities. Surprisingly, the estimated annual turnover of wood debris increases significantly near forest edges, suggesting that decomposition is occurring more rapidly in fragmented than continuous forests.These results reveal that habitat fragmentation fundamentally alters the distribution and dynamics of aboveground biomass in Amazonian forests. The rate of carbon cycling probably increases sharply, both because long‐lived canopy and emergent trees decline in favor of shorter‐lived successional trees and lianas, and because necromass production and turnover both appear to increase. Carbon storage in live vegetation also declines because small successional trees and lianas (which typically have low wood density) store substantially less carbon than do large, old‐growth trees. Finally, the decline and rapid decay of live biomass in forest fragments may produce substantial atmospheric carbon emissions, above and beyond that resulting from deforestation per se.

Effects of abandonment on the energy balance and evapotranspiration of wet subalpine grassland

Abandonment of grasslands is an important form of land-use change in subalpine regions. It leads to marked changes in the vegetation cover, which, in turn, may affect the radiative properties of the canopy and exchange processes between the ecosystem and the atmosphere. The aim of this study was to quantify the differences in net radiation ( R n), albedo ( α) and energy balance components between a managed and an abandoned wet grassland field in relationship to biomass and necromass production. The study in the Rotenbach watershed on the northern slope of the Swiss Alps (1400 m above sea level) was based on continuous measurements of the energy and radiation balances during two consecutive seasons. The managed site, which was mown once during the autumn, was characterized by a larger number of species, but in the abandoned field, the dominating species ( Ranunculus aconitifolius) was much more abundant. Annual production of aboveground plant biomass was similar in both fields, but litter accumulation and the amount of standing necromass was larger in the abandoned field. Early in the season, the litter layer at the abandoned site had only a minor influence on R n, but the larger amount of standing necromass in the course of the season decreased α and increased R n. This effect was most pronounced during mid-season and compensated for the higher ratio of sensible to latent heat fluxes, i.e., the Bowen ratio of the abandoned field. The resulting difference in total water vapour flux (evapotranspiration, ET) between the fields was small. Largest reduction in ET occurred early and late in the season, and this was partly due to decreased soil evaporation. Although the effect of abandonment on the cumulative ET during the two measuring periods was small, it was sufficient to increase the soil water content in the abandoned field, particularly during the later part of the season. It is concluded that abandonment causes a change in canopy structure which leads to reduced ET and temporarily increased soil water content.

Book review

For many decades in the late 20th century, the processes involved in the formation of soil organic matter (SOM) (also known as humification) constituted one of the most important areas of scientific inquiry in soil science. However, these processes have not been included in modern models of SOM dynamics, despite their relevance to quantifying carbon stabilisation (i.e., sequestration) in soil. Furthermore, modern models have focussed on mineralisation processes (e.g., mainly microbial heterotrophic respiration) and have not included the important effects of soil fauna that are known as key agents of SOM formation. To address these issues, we developed a modelling approach predicated on the existence of definable stoichiometric relations among the processes leading to SOM formation that are mediated by soil biota (e.g., correlations among biotic respiration, production of faunal excrement, and necromass as sources of SOM formation). Soil respiration, a core rate variable in all SOM models, was associated with the production of faunal by-products in soil food webs as precursors of stable SOM, specifically micro- and meso-faunal excrement, necromass, and earthworm casts. We developed a food-web based module, using a synthesis of decades of published data, to describe micro- and meso-faunal excrement and necromass production. We developed a separate module for anecic earthworms, with explicit representation of processes related to fresh casts. The contributions of these two modules were compiled and integrated with the ROMUL model of SOM dynamics, without changing the structure of the original model. These modules enabled calculation of the proportional contribution of faunal by-products to humification (i.e., recalcitrant SOM formation) and carbon sequestration. Testing of the new version of the model, known as Romul_Hum, showed consistent accumulation of faunal by-products in the “final” SOM fractions: the well-decomposed SOM of the organic soil horizons and the stable SOM of the mineral soil.

Effects of elevated CO 2, N fertilization, and cutting regime on the production and quality of Lolium perenne L. shoot necromass

In the Swiss grassland FACE experiment, we measured the effect of elevated CO 2 on the shoot necromass production and quality of Lolium perenne in 1995. Dead stubble of reproductive tillers and dead leaf sheaths were the main components of necromass. Elevated CO 2 did not significantly change the amount and the nitrogen concentration of necromass. Significantly more necromass was produced and the N concentration was lower in the low N supply treatments. Total necromass amounted to 250–500 g m −2. Necromass N content was in the order of 5–6 g m −2. This underscores the importance of the carbon and nitrogen fluxes included in necromass and their importance for soil biology and fertility.

A quantitative study of microbial decomposition of biopolymers in Recent sediments from the Peru Margin

The relationship between depth changes in total organic matter (TOC), biopolymers (“carbohydrates”, “proteins” and lipids) and bacterial populations and their activity, were investigated on two cruises to an upwelling site on the Peru Margin. Samples were obtained from the sediment surface to 8020 cm below sea floor (cbsf). Bacterial concentrations increased from near surface to a maximum at 12.5 cbsf, then rapidly decreased to 20 cbsf and a slower decrease to 8020 cbsf. The bacterial population at 8020 cbsf was 9.5% of the near surface value, but at 3.3 × 10 8 cells/cm 3 it was still very significant. The presence of dividing bacterial cells indicated that a portion of the population was active and this was confirmed by radiotracer measurements. Biopolymers were overestimated in the surface 4 cm (138% of the TOC). By 5.5 cbsf, however, organic matter uncharacterized as biopolymers was present. This increased rapidly to 17% of the TOC at 10 cbsf and then more slowly to 79% at 2229 cbsf. Bacterial populations and activity were significantly correlated with depth changes in both TOC and biopolymers ( p < 0.002). Calculation of rates of anaerobic carbon metabolism from the measured rates of sulphate reduction and methanogenesis demonstrated that these processes could account for all (101%) of the decrease in “carbohydrate” and “protein” in the 2.5 to 22.5 cbsf interval, where bacterial activity was intense. These data confirm the importance of anaerobic bacterial processes in these high organic matter sediments. At deeper intervals, 22.5–480 and 480–8020 cbsf, the decrease in “carbohydrate” and “protein” only accounted for 40% and 15% respectively of the anaerobic bacterial metabolism and hence the significant bacterial population present must have been utilizing a portion of the increasing uncharacterized organic matter. There was a negative relationship between the total bacterial population and the percentage of uncharacterized organic matter, suggesting that a portion of the uncharacterized organic matter may have come from dead bacterial cells. Estimates of bacterial necromass production from rates of thymidine incorporation were significantly correlated with increases in uncharacterized organic matter ( p < 0.05, 2.5–197.5 cbsf) and could account for 16% of the increase within the top 25 cm. However, as the thymidine incorporation technique underestimates bacterial productivity in anoxic sediments it is likely that a much greater proportion of the increase in uncharacterized organic matter should be attributed to “bacterial necromass” production. It is suggested that bacterial necromass production may be an important mechanism for the production of recalcitrant, and hence preserved, organic matter in sediments in high productivity regions.

Hurricane Damage to a Flood Plain Forest in the Luquillo Mountains of Puerto Rico

Hurricane Hugo caused low to moderate damage to a flood plain forest that was partially protected by its topographic position. Treefalls and the location of damage suggested N to NW wind direction during the storm. Thirty percent of the trees, or 693 trees/ha, had some damage and 84 percent of the damage was to the canopy. Most of the damage to trees was caused by direct wind impact (83%) as opposed to secondary effects (16%). Over 80 percent of the snapped, leaning, and uprooted trees were dicotyledonous. Tree mortality was only 1 percent, and most of the damage to the sierra palm Prestoea montana (R. Grah.) Nichols was loss of leaves. Rapid refoliation, epicormic branching, adventitious root production, resprouting, and regeneration from seed in open areas were observed nine months after the event. Ten percent of the aboveground biomass and 12-16 percent of the nutrient stocks (N, P, K, Ca, Mg) were transferred to the forest floor, mostly in the form of woody biomass and nutrient-rich leaves. Palm leaves were the dominant leaf component of necromass. Instantaneous in situ fine and coarse necromass production was 10 and 9.2 Mg/ha, respectively. Net changes in aboveground mass, N, P, K, Ca, and Mg (in percent of prehurricane value) were 8, 3, 0, 3, 12, and 1, respectively, in spite of a high rate of loss by export. The source of additional mass and nutrients were boles from upland forests that fell into and remained inside the flood plain.

Ecological Significance of Necromass Production In Mat-Forming Lichens

AbstractAvailable evidence indicates that basal regions of well-developed lichen mats are composed largely of dead or senescent thallus material. Marked vertical gradients in concentrations of N, P and K in thalli of mat lichens, together with results of N budget studies in Cladonia stellaris and Stereocaulon paschale, suggest that these key elements may be recycled internally to fund new growth at the apices. It is argued that nutrient recycling will increase rates of both apical growth and basal necromass production and that under steady-state conditions this will result in deeper mats and facilitate dominance sensu Grime; thus accumulation of necromass may be an ecological advantage in mat-forming lichens. Basal necromass of the N2-fixing S. paschale contains more N than necromass of the non-N2-fixing C. stellaris; this may be due in part to a greater allocation of N to irrecoverable fungal cell wall components in S. paschale and may underlie a greater requirement for N in this mycobiont.