Daniel J. Vogt

Production, Turnover, and Nutrient Dynamics of Above- and Belowground Detritus of World Forests

Publisher Summary Evergreen forests accumulate higher forest floor masses than deciduous in similar climatic zones. In needle-leaved forests, none of the climatic factors or latitude explains the variation in amount of aboveground litterfall mass or litterfall nitrogen (N) input. This is in contrast to the broad-leaved forests in which half of the variation in aboveground litterfall mass or litterfall N input is explained by latitude, mean annual temperature, minimum monthly mean temperature, or logarithmic transformations of precipitation. Lower total root masses were measured in cold temperate needle-leaved deciduous, Mediterranean needle-leaved evergreen, and tropical broad-leaved semi-deciduous forests while higher total mean root masses occurred in tropical broad-leaved evergreen forests. In the warm temperate zones, evergreen forests had similar mean total root masses whether broad-leaved or needle-leaved. However, deciduous forests averaged 5000 kg ha -1 less total root mass than evergreen forests. In cold temperate forests, the proportion of total detrital input to forest floor occurring as aboveground litterfall varied from 23 to 80%. Inclusion of root turnover as part of total litter input changed the calculated mean residence time of organic matter in the forest floor from 7.8 to 6.3 years in a red pine plantation and 68.6 to 15.7 years in a Pacific silver fir stand.

Review of root dynamics in forest ecosystems grouped by climate, climatic forest type and species

Patterns of both above- and belowground biomass and production were evaluated using published information from 200 individual data-sets. Data sets were comprised of the following types of information: organic matter storage in living and dead biomass (e.g. surface organic horizons and soil organic matter accumulations), above- and belowground net primary production (NPP) and biomass, litter transfers, climatic data (i.e. precipitation and temperature), and nutrient storage (N, P, Ca, K) in above- and belowground biomass, soil organic matter and litter transfers. Forests were grouped by climate, foliage life-span, species and soil order. Several climatic and nutrient variables were regressed against fine root biomass or net primary production to determine what variables were most useful in predicting their dynamics. There were no significant or consistent patterns for above- and belowground biomass accumulation or NPP change across the different climatic forest types and by soil order. Similarly, there were no consistent patterns of soil organic matter (SOM) accumulation by climatic forest type but SOM varied significantly by soil order—the chemistry of the soil was more important in determining the amount of organic matter accumulation than climate. Soil orders which were high in aluminum, iron, and clay (e.g. Ultisols, Oxisols) had high total living and dead organic matter accumulations-especially in the cold temperate zone and in the tropics. Climatic variables and nutrient storage pools (i.e. in the forest floor) successfully predicted fine root NPP but not fine root biomass which was better predicted by nutrients in litterfall. The importance of grouping information by species based on their adaptive strategies for water and nutrient-use is suggested by the data. Some species groups did not appear to be sensitive to large changes in either climatic or nutrient variables while for others these variables explained a large proportion of the variation in fine root biomass and/or NPP.

Analysis of some direct and indirect methods for estimating root biomass and production of forests at an ecosystem level

The relationship of global climate change to plant growth and the role of forests as sites of carbon sequestration have encouraged the refinement of the estimates of root biomass and production. However, tremendous controversy exists in the literature as to which is the best method to determine fine root biomass and production. This lack of consensus makes it difficult for researchers to determine which methods are most appropriate for their system. The sequential root coring method was the most commonly used method to collect root biomass data in the past and is still commonly used. But within the last decade the use of minirhizotrons has become a favorite method of many researchers. In addition, due to the high labor-intensive requirements of many of the direct approaches to determine root biomass, there has been a shift to develop indirect methods that would allow fine root biomass and production to be predicted using data on easily monitored variables that are highly correlated to root dynamics. Discussions occur as to which method should be used but without gathering data from the same site using different methods, these discussions can be futile. This paper discusses and compares the results of the most commonly used direct and indirect methods of determining root biomass and production: sequential root coring, ingrowth cores, minirhizotrons, carbon fluxes approach, nitrogen budget approach and correlations with abiotic resources. No consistent relationships were apparent when comparing several sites where at least one of the indirect and direct methods were used on the same site. Until the different root methods can be compared to some independently derived root biomass value obtained from total carbon budgets for systems, one root method cannot be stated to be the best and the method of choice will be determined from researchers personal preference, experiences, equipment, and/or finances.

Biodiversity and the productivity and stability of ecosystems

Attempts to unveil the relationships between the taxonomic diversity, productivity and stability of ecosystems continue to generate inconclusive, contradictory and controversial conclusions. New insights from recent studies support the hypothesis that species diversity enhances productivity and stability in some ecosystems, but not in others. Appreciation is growing for the ways that particular ecosystem features, such as environmental variability and nutrient stress, can influence biotic interactions. Alternatives to the diversity-stability hypothesis have been proposed, and experimental approaches are starting to evolve to test these hypotheses and to elucidate the mechanisms underlying the functional role of species diversity.

Decay rate and substrate quality of fine roots and foliage of two tropical tree species in the Luquillo Experimental Forest, Puerto Rico

Decomposition rates, initial chemical composition, and the relationship between initial chemistry and mass loss of fine roots and foliage were determined for two woody tropical species, Prestoea montana and Dacryodes excelsa, over a gradient of sites in two watersheds in the Luquillo Experimental Forest, Puerto Rico. At all locations, fine roots decayed significantly more slowly than foliage during the initial 6 months.Substrate quality of the initial tissue showed marked differences between roots and foliage when using cell wall chemistry, secondary chemistry and total elemental analysis as indices. Quantity of acid detergent fiber (ADF) (non-digestible cell wall fiber) and lignin content were higher for roots than leaves: D. excelsa roots had 55.3% ADF and 28.7% lignin while leaves had 36.2% ADF and 11.8% lignin; P. montana roots had 68.0% ADF and 26.8% lignin while leaves had 48.5% ADF and 16.1% lignin. Aluminum concentrations were higher in fine roots (843 mg kg−1 in D. excelsa, 1500 mg kg−1 in P. montana) than leaves (244 mg kg−1 in D. excelsa, 422 mg kg−1 in P. montana), while calcium concentrations were higher in foliage (5.5 mg g−1 in D. excelsa, 7.8 mg g−1 in P. montana) than roots (3.4 mg g−1 in D. excelsa, 3.1 mg g−1 in P. montana). Nitrogen did not show any trend with tissue or species type. A linear model between mass remaining after 6 months and initial tissue chemistry could be developed only for calcium (r2=0.64).

Belowground responses as indicators of environmental change

Abstract This paper discusses when and under what circumstances belowground parameters become advantageous to monitor as sensitive, early indicators of an environment being exposed to anthropogenic stress. The following specific belowground attributes are presented as useful parameters to monitor because they are sensitive to stress and should be influenced prior to the occurrence of visible aboveground symptoms: fine root biomass and turnover, mycorrhizal biomass and species of fungi colonizing the roots, secondary defensive chemical production in roots, carbohydrate reserves in roots, and nutrient and/or trace element concentrations and ratios in roots.

STRUCTURAL AND FUNCTIONAL RESPONSES OF A SUBTROPICAL FOREST TO 10 YEARS OF HURRICANES AND DROUGHTS

Little is known about ecosystem-level responses to multiple, climatic disturbance events. In the subtropical forests of Puerto Rico, the major natural disturbances are hurricanes and droughts. We tested the ecosystem-level effects of these disturbances in sites with different land use histories. From 1989 to 1992, data were collected to determine the effects of Hurricane Hugo and two droughts on litterfall inputs, fine-root biomass, and decomposition rates in three topographic locations (stream, riparian, upslope) within two watersheds. From 1994 to 1998, we added a third watershed and an experiment in which coarse-wood levels were manipulated to simulate hurricane inputs. Data were collected on tree and palm growth rates, litterfall inputs, fine-root biomass, and decomposition rates. From 1994 to 1998, four hurricanes and three droughts were recorded. Measured parameters had unique responses and recovery rates to hurricanes and droughts. Litterfall inputs returned to long-term mean rates within one month...

Roots, nutrients and their relationship to spatial patterns

Ecosystem sustainability and resilience after a disturbance may be regulated by processes occurring at smaller spatial scales. The matrix of different spatial environments are created by (1) individual plants that accumulate higher concentrations of specific nutrients, trace elements or defensive plant secondary chemicals and thereby modify the chemistry of their ecological space and/or rates of processes, (2) the presence of structures (e.g., coarse woody debris) that may buffer some micro-environments from disturbances by functioning as a hospitable environment or as a reservoir for mycorrhizal fungi to sustain them into the next phase of stand development, and (3) chemical changes in soils during soil development which may result in distinct soil chemical environments. The response of the plants or change in the sustainability of carbon and nutrient cycles may be expressed more strongly at this smaller ecological space of an individual plant and furthermore must be frequently examined separately by the above- and belowground space of that individual.This paper will present three case studies from temperate and tropical forest ecosystems which suggest the importance of studying plant growth and nutrient and trace element cycling by stratifying sampling to encompass the mosaic patterns of existing spatial variability within the ecosystem. The examples show how individual plant species are able to create ecologically distinct spatial environments because of their distribution patterns within the landscape, how nutrient transfers in roots respond to the chemical variations in the soil, and how roots and mycorrhizal fungi are able to maintain themselves in the mosaic of coarse woody debris remaining on a site after the elimination of aboveground tree biomass.

The effects of the frog Eleutherodactylus coqui on invertebrates and ecosystem processes at two scales in the Luquillo Experimental Forest, Puerto Rico

Determining the ubiquity of top-down control effects of predators on their prey and ecosystem processes is important for understanding community and ecosystem-level consequences that may result from predator loss. We conducted experiments at two spatial scales to investigate the effects of terrestrial frogs (Eleutherodactylus coqui )o n aerial and litter invertebrates, plant growth and herbivory, and litter decomposition. At both scales, frogs reduced aerial invertebrates and leaf herbivory, but had no effect on litter invertebrates. At the smaller scale, frogs increased foliage production rates, measured as the number of new leaves and new leaf area produced, by 80% and decomposition rates by 20%. The influence of E. coqui on increasing primary productivity and decomposition rates at the smaller scale appeared to be a result of elimination and excretion rather than of controlling prey. While the results provide evidence for frogs controlling herbivorous prey at both scales, species effects on ecosystem processes were only detectable at the smaller scale. The results highlight the difficulties in conducting experiments at large spatial scales. The findings from this study imply that the loss of amphibians and other species of higher trophic levels may affect nutrient cycling rates in tropical forests.

A critique of the role of ectomycorrhizas in forest ecology

Abstract This paper identifies areas of mycorrhizal ecology that the authors believe are crucial for advancing our understanding of the role of mycorrhizas in forest ecosystems. Increased research is needed into the relationships between plant carbon fixation and allocation and nutrient acquisition by mycorrhizas in many forest ecosystems and for different plant physiological stages. These comparisons need to consider not only how symbiotic associations modify plant uptake of nutrients but the interactions between mycorrhizas and decomposer microbes and fauna, and how these interactions influence the conservation of, or competition for, available nutrients. The authors identify four areas of future study that should be accorded priority: (1) the cost-benefit ratio (to the host) of maintaining mycorrhizal fungi on roots and the optimal mycorrhizal biomass that can be supported before the cost-benefit ratio becomes unfavorable; (2) the significance of mycorrhizas in storing nutrients in ecosystems; (3) the significance of interconnecting (naturally grafted) roots relative to the links between different roots attributable to mycorrhizal hyphae; (4) the ability of mycorrhizal fungi to use complex organic substrates as sources of nutrients (such as N and P) in forests.