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Species richness of tropical wood-inhabiting macrofungi provides support for species-energy theory

     Department of Plant Biology, University of Illinois at Urbana–Champaign, 265 Morrill Hall, 505 S. Goodwin Ave., Urbana, Illinois 61801

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

A study was undertaken at the El Verde Field Station in Puerto Rico to determine the effect of energy available from newly dead trees on the species richness of macrofungal communities that inhabit them. It is hypothesized that there is a positive relationship between available energy and species richness. Energy was measured using the volume of the dead trees and the wood density of living trees of the same species. One hundred ninety-four logs of known tree species were surveyed 1 y for fruiting bodies of macrofungi at monthly intervals. For individual logs, log volume had a significant positive effect on macrofungal species richness. Younger logs had significantly higher species richness than older logs, and those with less apparent decay had more species than those with more decay. When logs were grouped by tree species, total wood volume and density of live wood had a significant positive effect and average log diameter had a negative effect on total species richness and abundance of the wood-inhabiting macrofungi. Macrofungal richness and abundance constantly increased with initial wood density; there was no evidence for a unimodal relationship. These results support the proposed relationship between species richness and energy.

Key words: biodiversity, competition, host-specificity, Puerto Rico

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Understanding the processes that underlie patterns of species richness continues to be a major challenge for ecology. There is increasing evidence indicating that the amount of energy available to a community is positively correlated with its species richness, at least at some spatial scales and even might be responsible for patterns of latitudinal diversity (see review in Wright et al 1993). In this hypothesis "energy" takes into account both the amount of energy available per unit area and the total area occupied by the community (Wright 1983). Support for this theory comes from a variety of taxa, including Lepidoptera in Britain (Turner et al 1987), coral reefs worldwide (Fraser and Currie 1996), ants in the western hemisphere (Kaspari et al 2000) and in New England (Gotelli and Ellison 2002), birds worldwide (Hawkins et al 2003) and vertebrates and trees in North America (Currie 1991). So far no studies have examined the species-energy relationship for fungi.

Most studies of the energy-richness take place over large scales, across nations or continents, which lets researchers compare areas with large differences in both available energy and species richness. However, at these scales factors other than those related to energy may influence species richness and thus bias results. For example, areas that are widely separated generally have different sets of species living in them (different "species pools") and may have very different habitat structures (e.g. forests vs. grasslands), both of which will have an impact on species richness (Gotelli and Ellison 2002).

Studies at smaller scales could avoid these problems but present different challenges. First, a study of the relationship between species and available energy must encompass variation in available energy, but the climatic variables that are typically used to measure energy (e.g. net primary production, annual potential evapotranspiration, etc.) may not show sufficient variation at small scales. Second, at small spatial scales, competition theory predicts that for both primary produces and consumers, interactions between species may result in a unimodal (hump-shaped) relationship between energy and richness, which would not be present at regional or global scales (Rosenzweig and Abramsky 1993).

To overcome these challenges, I studied the relationship between species richness and energy in communities of unit-restricted macrofungi that inhabit dead wood in a 16 ha plot in a tropical forest in Puerto Rico. "Macrofungi" are any fungi that produce large, easily visible fruiting structures such as mushrooms, puffballs and bracket fungi. A unit-restricted species is a type of decomposer that is limited during at least one life history stage to using resources present on only a single patch of a discontinuous resource. This group of decomposers includes many fungal species that decompose wood, leaves, dung, etc. (Cooke and Rayner 1984). These can be contrasted with fungi that decompose resources with a continuous distribution or fungi that can forage by growing from one resource patch to another through the use of rhizomorphs or mycelial cords.

Studying unit-restricted macrofungi on dead wood has many of the advantages of both large- and small-scale energy-richness studies, without many of the drawbacks. Studies at large spatial scales can take advantage of sizable differences in species richness and energy availability. Fortunately, even at small spatial scales, macrofungi on dead wood in the tropics are noted for being taxonomically diverse and showing few signs of host specificity (Lodge 1997, Lindblad 2000, Gilbert et al 2002; cf. Gilbert and Sousa 2002 for an exception in mangrove forests). The high richness of these fungi makes it easier to detect differences among communities. Furthermore dead wood of different tree species provides different amounts of energy to the macrofungal community decomposing it. These differences can be measured and tested as a predictor of fungal species richness. By focusing on a small area in a tropical forest, the study can be carried out in an area with a common species pool and climatic regime, thus avoiding some of the problems of studies on larger scales.

In addition, competition among unit-restricted species differs from competition among other types of consumers (Schmit 1999). As a result unit-restricted species may not have the unimodal relationship between richness and energy at small spatial scales that has been found for other types of consumers. For unit-restricted species, a successful competitor cannot be defined as a species that can survive on a patch indefinitely, because no population of a unit-restricted species will survive the decomposition of its patch. Instead a successful competitor is one that survives long enough to produce propagules that can then disperse and colonize new patches. Often individual patches are decomposed by a single generation of decomposers, so reproduction of multiple species consuming the same limiting resource is common, provided each species captures sufficient resources to reproduce. This contrasts with other types of consumers, where a successful competitor often is defined as a species that can persist indefinitely on a single patch.

Interspecific competition of fungi has been studied with two unit-restricted, dung-inhabiting species of the mushroom genus Coprinus competing for agar in Petri plates (Schmit 1999). This experiment demonstrated the effect of initial resource density of a patch (equivalent to the energy available on the patch) on competition. Both species reproduced when they competed for resources on a patch (i.e. a single Petri plate), and both produced more spores when they were on patches with a higher initial resource density (= more energy). One species, Coprinus congregatus, grew more slowly and captured a smaller territory than its competitor. C. congregatus was affected more by competition, in that it suffered a greater reduction in spore production as compared to being grown alone. However C. congregatus produced more spores when competing on plates with a high initial resource density than on low initial resource density plates.

One conclusion of this experiment is that it is easier for inferior competitors to persist on patches with high resource densities, because they are more likely to produce propagules than inferior competitors on patches with a low resource density. On patches with a high initial resource density, slow growing individuals will have more resources at their disposal and will be more likely to capture sufficient resources to reproduce. Furthermore some species may not use resources efficiently enough to survive on patches with a low resource density but may be able to survive on high resource density patches. Therefore more species should reproduce successfully on patches that have a high initial resource density than on patches with a low initial resource density. This conclusion is in contrast with models of consumers competing for a single resource (Tilman 1982), which predict that the species that is able to maintain itself at the lowest resource density will be the only species to survive, regardless of initial resource densities or resource supply rates.

This paper presents a test of the species-energy theory using unit-restricted macrofungal communities found on logs in proximity in a tropical forest. Based on this theory I predicted that there would be a higher species richness of macrofungi growing on logs with higher energy and that the relationship between energy and species richness would be constantly increasing rather than unimodal. Furthermore I predicted that the mechanism for this increase in species richness would be that individual fungi which capture small territories would be more likely to reproduce on logs with higher energy than on logs with lower energy. Therefore there should be a greater abundance of fungal fruiting bodies on high-energy logs.

The basic strategy of the study was to examine the species richness of macrofungal communities on logs from different tree species. The volume of the logs and the initial density of the wood (the density of wood from live trees of the same species) were used to measure the total energy available to the macrofungal community. Following Wright et al (1993), individual logs were considered to be patches, and logs from different species represented "regions" with different amounts of energy available to the macrofungal community. The relationship between species richness and energy was examined on the regional level (i.e. looking at differences among communities on wood of different tree species).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Study site.— – Fieldwork was carried out at the University of Puerto Rico’s El Verde Field Station in the Luquillo Long Term Ecological Research Site, Puerto Rico, (18°19'N, 65°45'W; 300–500 meters altitude) described in Reagan and Wade (1996). The research took advantage of the long-term vegetation study in progress at the field station. A 16 ha plot, called the Hurricane Recovery Plot, was established in 1989 to study regeneration of the forest after hurricane Hugo. Trees with >10 cm dbh that were felled by Hugo were identified, tagged and mapped for future work on decay fungi. Afterward standing trees >5 cm dbh were tagged permanently and identified to the species level.

Field methods.— – In Mar 1997, 194 fallen, dead trees with intact tags were found in the Hurricane Recovery Plot. Based on the tag, the tree species of each of these logs was determined. In total 37 tree species with 1–23 replicate logs were encountered (TABLE I). Once a month from Mar 1997 to Feb 1998 all logs were surveyed for fruiting bodies of macrofungi. Fruiting bodies from all species found on each log were collected, dried and identified to the species or morphospecies level.

The initial and current wood densities were estimated. The current wood density was quantified by removing five wood cores of 0.9 cm diam and 5 cm deep from each log with an increment borer. The cores were dried and weighed for an estimate of the current density (mass/volume) of each log. Many logs had decayed to such an extent that it no longer was possible to core them, so no estimate was made for those logs. For each tree species represented in the study, cores were taken from five live trees and used to determine the initial density of the logs.

Estimating species richness and abundance.— – To determine the relationship between the initial resource density of a log and its species richness, one ideally would collect macrofungi from the log from the time it fell until the time it had decayed. Unfortunately it can take more than a decade for many tree species to decay. To overcome this obstacle, many logs of different ages were surveyed for each tree species whenever possible. Species richness of an individual log was estimated as number of macrofungal species found on the log over the study period. Macrofungal species richness for a tree species was estimated as the number of fungal species that were found on all the surveyed logs of that species. It was assumed that all fungal species found were unit-restricted. Despite frequent searches, no rhizomorphs or mycelial cords were found in association with the study logs. In addition, due to the rocky nature of the terrain, many logs were balanced on large rocks and had limited contact with the forest floor; one-quarter had less than 20% of their length in contact with the earth, and an additional one-quarter had 20–80% contact. While there may have been a small number of species that are not unit-restricted included in the dataset, there is no indication that they had an impact on the outcome (see results below).

Due to the unpredictable nature of fungal reproduction, it is likely some macrofungal species inhabiting individual logs did not fruit during the fieldwork. Surveying macrofungi over many logs of individual tree species should reduce any bias resulting from these gaps in data. However species richness numbers should be considered minimum estimates. It would be possible to identify additional species using culturing or molecular based techniques, but these are not practical when surveying hundreds of logs.

Abundance was measured by estimating the number of meters along each log that contained fruiting bodies of each macrofungal species. This technique is similar to that used in plot-based studies where abundance is determined by the number of small subplots in which a species occurs (e.g. Bills et al 1986).

Estimating energy.— – "Energy" refers to the energy that is available for use by the macrofungal decomposer community, which comes from the biomass in the trees at the time that they die. Therefore variables that determine the initial biomass of a tree when it dies are correlated to the energy available to the macrofungal community. In practice two variables were used to determine the energy available: the volume of the log and initial wood density. To determine the total wood volume available to the macrofungal community of a given tree species, the volume of wood was added up over all replicate logs of that species. Initial resource density was chosen over current resource density because it more accurately reflected the total amount of energy available to the community over its history.

Statistical analysis.— – I first analyzed the data from the individual logs to determine whether any of the factors measured, other than those related to energy, could bias the results. All statistical analyses were performed with the Minitab 14.1 statistical package. Following Fraser and Currie (1996), the relationship between the volume of a log and its species richness was determined with least squares regression, and the other factors were tested to see if they could explain the residuals. One-way ANOVAs were used to determine whether the residuals differed with presence of bark, ants or termites, canopy cover, broken vs. tipped trees, contact with ground, decay class and age class. Regression analysis was used to evaluate whether current wood density was a significant predictor of the residuals. In those cases where residuals could be explained by a confounding factor, a general linear model was used to study the interactions of the factors.

The relationship between energy, specifically volume and initial wood density, and species richness and abundance then was examined. While both factors influence the energy available to the community, volume may be correlated with many factors not related to energy, such as surface area, tree age, intralog variability, etc. Therefore the analysis was designed to carefully measure the relationship between species richness and volume and density both independently and jointly as well as to look at potential confounding factors.

Regression analyses were used to determine the relationship between macrofungal species richness and volume and macrofungal abundance and volume. Initial resource density was used to explain the residuals of the species richness-volume and abundance-volume relationships. The regression of density on the residuals provides a measure of the effect of density that is independent of the effects of wood volume. Multiple regression analysis was used to construct a model of species richness and abundance using both volume and initial resource density. Significant positive relationships between species richness and abundance and wood volume and initial density provide support for the species-energy hypothesis.

To confirm that little host specificity was found in this community, I tested for host specificity using the method of Lindblad (2000). A regression analysis determined the relationship between the number of logs and the number of tree species on which a macrofungal species occurs. Only macrofungal species that were found on more than two logs are included in this analysis because little can be concluded about the host specificity of species found on only one or two logs. If there is little host specificity, the log-host relationship should be linear when both measures are logarithmically transformed. Outliers, species with studentized residuals >±3, are removed from the dataset, starting with the largest outlier. After each outlier is removed the regression is recalculated, and the process is iterated until there are no more outliers. Outliers with negative residuals were found on fewer tree species than would be expected based on the number of logs they inhabit and are considered to show some degree of host specificity.

Investigators have suggested that the number of spores that land on a piece of wood will be positively correlated with the surface area of the wood (Bader et al 1995, Høiland and Bendiksen 1996). This in turn would lead to a positive correlation between wood surface area and species richness. This proposed mechanism does not rely on energy to explain species richness. However it is possible that surface area could be positively correlated with both volume and/or initial density of wood, which could limit the ability of this study to test the species-energy hypothesis as related to wood volume. To account for this possibility, the Pearson’s correlation was determined among surface area, wood density and volume. Based on this correlation, multiple regression analyses were carried out to determine if initial wood density is a significant predictor of species richness and abundance once the influence of wood surface area is accounted for.

An additional potential source of error lies in the fact that some tree species are represented by more logs than others are. Previous studies have shown that fine woody debris (<10 cm diam) has community that is distinct from coarse woody debris (>10 cm diam) (Kruys and Jonsson 1999, Nordén et al 2004). Furthermore, for a given volume of wood, coarse woody debris harbors fewer species than fine woody debris (Nordén et al 2004). Although all but 10 of the logs surveyed in this study had diameters >10 cm, it is possible that smaller tree species will have greater species richness per unit volume than larger tree species. To test for this the average diameter of each tree, measured at the base of the logs, was used as a variable in the regressions relating macrofungal species richness and abundance to wood volume, surface area and initial wood density.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Individual logs.— – The 194 individual logs had an average of 3.6 macrofungal species, with a range of 0–18 species. In total 207 macrofungal species were collected. Log volume was a significant predictor of the species richness of its macrofungal community (FIG. 1) (Log₁₀ [species richness +1] = 0.65 + 0.31 Log₁₀ [wood volume m³], F[1,192] = 111.6, P < 0.001, R²adj = 36.4%). One-way ANOVAs indicate that the residuals of the species-volume regression differed significantly with decay class and age class (FIGS. 2–3) but not with other factors (TABLE II).

View larger version (14K): [in this window] [in a new window]   FIG. 1. Regression of fungal richness on log volume for individual logs.

View larger version (12K): [in this window] [in a new window]   FIGS. 2–3. ANOVA tests of the effects of physical factors on the residuals of the species-volume regression for individual logs. 2. Decay class. 3. Year discovered dead.

Based on the ANOVA results, a general linear model was constructed for species richness. When both age class and decay class were used in the model, decay class was not significant. This is because older logs tend to fall into higher decay classes and therefore the two measures are explaining the same portion of the variance in the species-volume relationship. The GLM model is Log₁₀ (species richness + 1) = 0.71+0.31 Log₁₀ volume (m³) + age class, where age class 1 = –0.061, age class 2 = –0.085, age class 3 = +0.146, P < 0.001, R²adj = 44.5%. Based on this analysis four tree species represented only by logs in age class 3 (Andira inermis, Buchenavia capitata, Ocotea leucoxylon and Prestoea montana) were removed from the dataset. P. montana was represented by three replicate logs, whereas the other species were represented by one log each.

Logs grouped by tree species.— – The remaining 33 tree species each were represented by 1–23 logs, with the average being 5.7. Each tree species had 0–38 fungal species with an average of 14. Macrofungal species were found on 1–56 logs, with the average being 4.7, which represented 1–25 tree species, with an average of 3.2 tree species per macrofungal species. The average ratio of logs:host trees across all fungal species was 1.2:1.

Host specificity.— – Analysis shows evidence of a low degree of host specificity (FIG. 4). The regression equation is linear and significant (Log₁₀ [hosts] = 0.07 + 0.76Log₁₀ [Logs], F [1,69] = 575.9, P < 0.001). Of the 75 macrofungal species found on three or more logs, only four species (5.4%) showed clear signs of host specificity. A Stereum species was present on five logs, all Miconia tetrandra; a pleurotoid species was present on four logs, all M. tetrandra; Camillea verruculospora was present on eight logs, seven of them M. tetrandra, and one log of them Dacryodes excelsa and a marasmioid species was present on three logs, all Chionanthus domingensis, the most common tree species in the study.

View larger version (10K): [in this window] [in a new window]   FIG. 4. Relationship between the number of logs a macrofungal species occurs on and the number of tree species those logs represent. Circles are species that are not outliers and are used to determine the regression line. Triangles are outliers not used in determining the regression.

View larger version (20K): [in this window] [in a new window]   FIGS. 5–8. Effect of energy on species richness. 5. Regression of total number of macrofungal species found inhabiting a tree species on total volume of logs of that tree species. 6. Regression of the residuals from Fig. 5 on the live wood density of the tree species 7. Regression of total macrofungal abundance from a tree species on total volume of logs of that tree species. 8. Regression of the residuals from FIG. 7 on the live wood density of the tree species. In all panels, arrow indicates the data point corresponding to Miconia tetrandra.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The results of this study provide support for the species-energy theory. Two variables, which are clearly related to energy, volume of the logs and the density of live wood, explained significant amounts of variation in macrofungal species richness among logs from different tree species. This is the first study to examine the effect of initial wood density on species richness of macrofungal decomposers of wood. It is also the first study to examine the relationship between species richness and resource density at the scale of individual patches of unit-restricted species.

Individual logs.— – Looking at individual logs, the only factors in addition to wood volume that were shown to have an impact on species richness were the age and state of decay of the logs. In at least one other study older logs have been shown to harbor fewer wood-decay macrofungi (Heilmann-Clausen 2001). Some studies have shown cases where highly decayed wood harbored more macrofungal species (Bader et al 1995, Lindblad 2001, Nordén and Paltto 2001), but in other cases logs at intermediate stages of decay harbored the most species (Renvall 1995, Høiland and Bendiksen 1997, Edmonds and Lebo 1998, Lindblad 1998, Kruys et al 1999, Sippola and Renvall 1999, Lindblad 2001). Lower species richness on older, more decayed wood could be a response to energy in the form of wood density. The density of a log is reduced as it is broken down, so any macrofungal species requiring a high wood density most likely will not be present late in the decomposition process. In addition it could be that in young logs replacement interactions (where one macrofungus invades another’s territory and replaces it) have not yet had a chance to come to completion and that the drop in species is due to these interactions.

On individual logs, current wood density had no effect on species richness. Given the effect of age/decay class on species diversity, however, it is somewhat surprising that there was no relationship between current wood density and species richness. This discrepancy may be due partially to the fact that density of extremely decayed logs was not measured. In addition, because the logs were not decayed evenly, the technique of taking five samples may not have adequately reflected the variability in the current density of the dead logs. Age and state of decay were overall measurements and therefore may have been more representative of each log as a whole.

I similarly found no effect of initial wood density on species richness. This is not surprising because the logs were not studied during the entire decay process. As a result some species already might have already been eliminated from individual logs before the study began and others might have been present but not reproducing during the study.

Species-energy theory.— – When logs were grouped by tree species, initial wood density explained one-fifth of the variance in species richness and abundance once volume of wood sampled was accounted for, providing support for the species-energy theory. Wood densities found in this study span almost the entire range of wood densities found in tropical American forests (Reyes et al 1992). This indicates that the study was not biased as a result of failure to take into account the entire range of resource density (Rosenzweig and Abramsky 1993).

Like other tests of species-energy theory, this study did not take into account differences among species in their abilities to use different resources. Wood from different tree species vary in lignin, cellulose and hemicellulose content. Some trees may produce chemicals that retard the growth of some saprotrophic fungal species. As a result wood volume and density may not be the only factors that determine energy available for each fungal species. It unfortunately was not practical to determine the decomposition abilities of each species in relation to the surveyed logs.

Taken together, log volume, average log diameter and initial wood density can explain more than 80 percent of the variation in species richness and abundance. All three of these factors can be measured at the time that the tree initially falls. This leaves relatively little variance to be explained by the stochastic nature of the colonization process, differences in wood chemistry and the variety of microenvironments found under the forest canopy. However these factors may determine the distribution of individual species, even if they do not determine the richness of the community as a whole.

Host specificity.— – Fewer than 6% of the species found on three or more logs in this study showed signs of host specificity, slightly less than was found in a tropical forest in Costa Rica (Lindblad 2000). Camillea verruculospora previously had been shown to be largely restricted to wood of Miconia species (Lodge and Laessøe 1995), a finding confirmed by this study. While three of the four species showing host specificity occur on M. tetrandra, it is not an outlier in any of the regressions involving the relationship among species richness, abundance, wood volume, wood surface area and initial wood density (FIGS. 5–8). This implies some degree of independence between the factors that influence species richness and those that influence the distribution of individual species.

Wood volume, area and diameter.— – At both the scale of individual logs and of logs grouped by tree species, a significant relationship between wood volume and species richness was discovered. This is not surprising because the relationship between species richness and area sampled is well known (Rosenzweig 1995) and researchers frequently have found that larger pieces of wood harbor more macrofungal species (Bader et al 1995, Bendiksen 1997, Lindblad 1998, Allen et al 2000, Rubino and McCarthy 2003). When the logs are grouped by tree species, both wood volume and wood surface area explain a significant amount of the variation of species richness and abundance. Because these two measures are correlated tightly in this study, the two are largely explaining the same variation in species richness and abundance. Regardless of the use of volume or surface area, wood density, a measure of available energy, is still a significant predictor of species richness and abundance.

Surface area of a log could influence species richness and abundance in a variety of ways. The probability of spores of a particular species landing on a piece of wood might be proportional to the surface area of the wood. Larger pieces of wood could receive spores from more species of fungi and thus have a more diverse fungal community (Bader et al 1995). Furthermore surface area of a log could influence the environmental conditions within the log through such means as amount of rainwater captured by the log, gas exchange between the log and the atmosphere and presence of plants growing on the log, which in turn can influence fungal activity (Rayner and Boddy 1988, Bader et al 1995, Kruys et al 1999) and potentially the composition of the fungal community. Finally, larger logs also might have a greater diversity of microhabitats, which in turn could lead to a greater number of species inhabiting them. (Allen et al 2000, Rubino and McCarthy 2003).

Trees species with a large average diameter had a lower macrofungal species richness and abundance when the effects of initial wood density and wood volume or surface area also were taken into account, a result also shown by Nordén et al (2004) in oak-dominated forests in Sweden. Logs with large diameters presumably would be more heterogeneous and have more microhabitats than those with smaller diameters, so diversity of microhabitats within a log does not seem to be a factor in the current study. Nordén et al (2004) hypothesized that the higher species richness of fine woody debris could be a due to the fact that for a given volume of wood fine woody debris has more individual pieces than intact wood and would be scattered over a larger area with more microhabitats and have a greater surface area to catch spores than intact wood. However average log diameter and number of replicate logs are not correlated in the current study, which implies that the smaller logs were not present in a greater number of microhabitats. The effect of average log diameter on macrofungal species richness and abundance was still significant or nearly so when wood surface area was taken into account, which indicates that surface area alone is not a factor in enhanced diversity on smaller logs. It has been shown that different species are present on logs of different sizes (Kruys and Jonsson 1999, Nordén et al 2004). It might be that those species on larger diameter logs have access to more resources and are more likely to invade their neighbors, an effect demonstrated in the laboratory (Holmer and Stenlid 1993). On the other hand, it might be that fungi on small diameter logs make use of resources more efficiently, a fact also demonstrated in the laboratory (Schmit 2002). A fungus on a small diameter log may not need to move resources as far (from the log core to the surface where fruiting bodies are formed), leading to greater efficiency in resource use. If fungi exist more efficiently on small diameter logs, some fungi might be unable to reproduce on large logs. It also is possible that large logs, which generally are from older trees, differ in ways not measured in this study that affect the quality of wood for the macrofungal community.

Macrofungi species richness on dead wood clearly is influenced simultaneously by energy and other factors. This study unfortunately was not designed to elucidate the contrasting, highly correlated effects of wood volume, surface area and diameter. Studies with an experimental approach, using wood blocks whose volume, surface area and diameter varied independently would help to clarify the effects of these factors on species diversity.

Implications.— – As predicted, the relationship among initial wood density and species richness and abundance was not unimodal but instead constantly increased. This has important implications for the role of individual tree species in maintaining species richness. Given the low host specificity of wood-inhabiting macrofungi, it would be logical to conclude that the role of particular tree species is minor at best. However this study demonstrates that individual trees with high-density wood can support a richer community of macrofungi than can trees with low-density wood. In maintaining a rich macrofungal community on individual logs, trees with high density wood might be crucial for the survival of species that are poor competitors. These species occasionally may reproduce on trees with low density wood but that frequency would be insufficient to ensure the survival of the colony.

A wide variety of other factors also have been shown to influence the species richness of wood decay macrofungi in plots. These include forestry practices (Bader et al 1995, Høiland and Bendiksen 1997, Lindblad 1998, Sippola and Renvall 1999), amount of dead wood on the forest floor (Bader et al 1995, Sippola and Renvall 1999, Ferris et al 2000, Humphrey et al 2000, Nordén and Paltto 2001, Rubino and McCarthy 2003), rainfall and forest type (Renvall 1995, Fryar et al 1999, Lindblad 2001). Trees that are more common in forest communities and are more widely distributed have been shown to be hosts to more polypore species (Gilbert et al 2002, Ortega and Navarro 2004). One of the challenges for fungal ecology is to determine how these factors interact at a variety of scales to produce the patterns of species richness and distribution found in nature.

	ACKNOWLEDGMENTS

 
I thank Jon Bithorn, Sharon Cantrell, Zhigang Liu, D. Jean Lodge, Elvia Melendez, Jill Thompson and Jess Zimmerman for help and advice in Puerto Rico. Melinda Brady Schmit, Greg Mueller, Mathew Leibold, Mike Miller, Carol Shearer and Ellen Simms and two anonymous reviewers provided helpful comments on the manuscript. This study was financed by NSF Doctoral dissertation Improvement Grant DEB-9623523 and by grants BSR-8811902 and DEB9411973 from NSF to the Institute for Tropical Ecosystem Studies (ITES), University of Puerto Rico, and to the International Institute of Tropical Forestry, USDA Forest Service, as part of the Long-Term Ecological Research Program in the Luquillo Experimental Forest, and grant BSR-9015961 from NSF to ITES for establishment of the hurricane recovery plot.

	FOOTNOTES

 
Accepted for publication March 23, 2005.

¹ Current address: National Park Service, Center for Urban Ecology, 4598 MacArthur Blvd., NW, Washington DC 20007. E-mail: john_schmit{at}nps.gov

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