Conducting tissues and phyletic relationships of bryophytes

Internal specialized conducting tissues, if present, are restricted to the gametophytic generation in liverworts while they may occur in both generations in mosses. Conducting tissues are unknown in the anthocerotes. Water-conducting cells (WCCs) with walls perforated by plasmodesma-derived pores occur in the Calobryales and Pallaviciniaceae (Metzgeriales among liverworts and in Takakia among mosses. Imperforate WCCs (hydroids) are present in bryoid mosses. A polarized cytoplasmic organization and a distinctive axial system of microtubules is present in the highly specialized food-conducting cells of polytrichaceous mosses (leptoids) and in less specialized parenchyma cells of the leafy stem and seta in other mosses including Sphagnumn. A similar organization, suggested to reflect specialization in long-distance symplasmic transport of nutrients, also occurs in other parts of the plant in mosses, including rhizoids and caulonemata, and may be observed in thallus parenchyma cells of liverworts. Perforate WCCs in the Calobryales, Metzgeriales and Takakia, and hydroids in bryoid mosses, probably evolved independently Because of fundamental differences in developmental design, homology of any of these cells with tracheids is highly unlikely. Likewise, putative food-conducting of bryophytes present highly distinctive characteristics and cannot be considered homologous with the sieve cells of tracheophytes.     

Warming and drought differentially influence the production and resorption of elemental and metabolic nitrogen pools in Quercus rubra

The process of nutrient retranslocation from plant leaves during senescence subsequently affects both plant growth and soil nutrient cycling; changes in either of these could potentially feed back to climate change. Although elemental nutrient resorption has been shown to respond modestly to temperature and precipitation, we know remarkably little about the influence of increasing intensities of drought and warming on the resorption of different classes of plant metabolites. We studied the effect of warming and altered precipitation on the production and resorption of metabolites in Quercus rubra. The combination of warming and drought produced a higher abundance of compounds that can help to mitigate climatic stress by functioning as osmoregulators and antioxidants, including important intermediaries of the tricarboxylic acid (TCA) cycle, amino acids including proline and citrulline, and polyamines such as putrescine. Resorption efficiencies (REs) of extractable metabolites surprisingly had opposite responses to drought and warming; drought treatments generally increased RE of metabolites compared to ambient and wet treatments, while warming decreased RE. However, RE of total N differed markedly from that of extractable metabolites such as amino acids; for instance, droughted plants resorbed a smaller fraction of elemental N from their leaves than plants exposed to the ambient control. In contrast, plants in drought treatment resorbed amino acids more efficiently (>90%) than those in ambient (65–77%) or wet (42–58%) treatments. Across the climate treatments, the RE of elemental N correlated negatively with tissue tannin concentration, indicating that polyphenols produced in leaves under climatic stress could interfere with N resorption. Thus, senesced leaves from drier conditions might have a lower nutritive value to soil heterotrophs during the initial stages of litter decomposition despite a higher elemental N content of these tissues. Our results suggest that N resorption may be controlled not only by plant demand, but also by climatic influences on the production and resorption of plant metabolites. As climate–carbon models incorporate increasingly sophisticated nutrient cycles, these results highlight the need to adequately understand plant physiological responses to climatic variables.     

Bryophyte-Cyanobacteria Associations during Primary Succession in Recently Deglaciated Areas of Tierra del Fuego (Chile)

Bryophyte establishment represents a positive feedback process that enhances soil development in newly exposed terrain. Further, biological nitrogen (N) fixation by cyanobacteria in association with mosses can be an important supply of N to terrestrial ecosystems, however the role of these associations during post-glacial primary succession is not yet fully understood. Here, we analyzed chronosequences in front of two receding glaciers with contrasting climatic conditions (wetter vs drier) at Cordillera Darwin (Tierra del Fuego) and found that most mosses had the capacity to support an epiphytic flora of cyanobacteria and exhibited high rates of N₂ fixation. Pioneer moss-cyanobacteria associations showed the highest N₂ fixation rates (4.60 and 4.96 µg N g⁻¹ bryo. d⁻¹) very early after glacier retreat (4 and 7 years) which may help accelerate soil development under wetter conditions. In drier climate, N₂ fixation on bryophyte-cyanobacteria associations was also high (0.94 and 1.42 µg N g⁻¹ bryo. d⁻¹) but peaked at intermediate-aged sites (26 and 66 years). N₂ fixation capacity on bryophytes was primarily driven by epiphytic cyanobacteria abundance rather than community composition. Most liverworts showed low colonization and N₂ fixation rates, and mosses did not exhibit consistent differences across life forms and habitat (saxicolous vs terricolous). We also found a clear relationship between cyanobacteria genera and the stages of ecological succession, but no relationship was found with host species identity. Glacier forelands in Tierra del Fuego show fast rates of soil transformation which imply large quantities of N inputs. Our results highlight the potential contribution of bryophyte-cyanobacteria associations to N accumulation during post-glacial primary succession and further describe the factors that drive N₂-fixation rates in post-glacial areas with very low N deposition.     

Winners and losers in a long-term study of vegetation change at Moor House NNR: Effects of sheep-grazing and its removal on British upland vegetation

We analysed data collected between 1954 and 2000 from nine long-term experiments designed to assess the effects of sheep-grazing versus no-sheep-grazing at Moor House NNR, an Environmental Change Network site. The experiments were set up between 1954 and 1972 across a range of vegetation types typical of much of upland Britain. Data from this type of experiment are often difficult to analyse and we describe the procedures undertaken to prepare the data for analysis. We fitted the resultant data to the British National Vegetation Classification and used ordination techniques to assess the relative positions of the experiments to each other. Finally, we used Generalized Linear Mixed-effects Modelling within a Bayesian framework to model change of species taxonomic/physiognomic groups through time in both sheep-grazed and ungrazed treatments across all nine experiments; variables included species diversity, Shannon–Weiner index and derived data on occurrence and abundance of species groups based on taxonomy and physiognomy. Hurdle analysis was used to model the species groups; this analysis separated the change through time in both probability of occurrence (binomial distribution) and abundance (Poisson distribution).In the sheep-grazed plots (the “business-as-usual” treatment hence here designated the “control”) there was a reduction in species diversity and a decrease in abundance of vascular plants, grasses, lichens, liverworts and mosses; whereas herbs, sedges and shrubs increased. When probability of occurrence was considered, there was a reduction in number of presences of both lichens and liverworts. Thus, the status quo management of continuous sheep-grazing, even though reduced since 1972, has resulted in a marked change in species composition of these plant communities, with some winners and some losers, but overall they support the concept of biotic homogenization. It is likely that some of these changes were driven by external factors such as elevated atmospheric nutrient deposition. Removal of sheep grazing had some positive benefits; with the herbs, mosses, sedges and shrubs increasing, but faster reductions in grasses and liverworts. Sedges + rushes were stable. It suggested that future monitoring schemes might use either the probability of occurrence of liverworts and lichens, or the abundance of lichens, liverworts, grasses and mosses as sensitive indicators of change in upland Britain.Moreover, during the period that Moor House has been protected as a nature reserve some key plant species groups have declined in spite of reductions in grazing pressure. To reverse this trend requires some form of interventionist management. In order to increase the diversity of vascular plants some form of disturbance will probably be needed, but for bryophytes and lichens this remains a research question. If the results from these small-scale experiments are replicated at the landscape scale a reduction of sheep grazing pressure, for example in rewilding schemes, will have little effect on species composition over a 28–44 year period.     

The influence of climatic legacies on the distribution of dryland biocrust communities

Predicting the distribution of biocrust species, mosses, lichens and liverworts associated with surface soils is difficult, but climatic legacies (changes in climate over the last 20 k years) can improve our prediction of the distribution of biocrust species. To provide empirical support for this hypothesis, we used a combination of network analyses and structural equation modelling to identify the role of climatic legacies in predicting the distribution of ecological clusters formed by species of mosses, lichens and liverworts using data from 282 large sites distributed across 0.6 million km² of eastern Australia. Two ecological clusters contained 87% of the 120 moss, lichen and liverwort species. Both clusters contained lichen, moss and liverwort species, but were dominated by different families. Sites where the air temperature increased the most over 20k years (positive temperature legacies) were associated with reductions in the relative abundance of species from the lichen (Peltulaceae and Teloschistaceae) and moss (Bryaceae) families (Cluster A species), greater groundstorey plant cover and lower soil pH. Sites where precipitation has increased over the past 20k years (positive precipitation legacy) were associated with increases in the relative abundance of lichen (Cladoniaceae, Lecideaceae and Thelotremataceae) and moss (Pottiaceae) families (Cluster B species) and lower levels of soil pH. Sites where temperatures have increased the most in the past 20k years suppressed the negative effects of plant cover on Cluster B by reducing plant cover. Increased intensity of grazing suppressed the negative effect of soil pH and the positive effect of soil carbon, on the relative abundance of Cluster B taxa. Finally, increasing temperature and precipitation legacies reduced the negative effect of soil pH on Cluster B. Understanding of the importance of climatic legacies improves our ability to predict how biocrust assemblies might respond to ongoing global environmental change associated with increasing land use intensification, increasing temperature and reduced rainfall.     

A comparison of alpha and beta diversity patterns of ferns,bryophytes and macrolichens in tropical montane forests of southern Ecuador

We present a first comparison of patterns of alpha and beta diversity of ferns, mosses, liverworts and macrolichens in neotropical montane rainforests, and explore the question whether specific taxa may be used as surrogates for others. In three localities in southern Ecuador, we surveyed terrestrial and epiphytic species assemblages in ridge and slope forests in 28 plots of 400 m2 each. The epiphytic habitat was significantly richer in ferns, liverworts, and macrolichens than the terrestrial habitat; mosses, however, were primarily terrestrial. Alpha diversity of ferns and of liverworts was congruent in both habitats. Mosses were similar to ferns and liverworts only in the epiphytic habitat. Macrolichens did not share patterns of alpha diversity with any other group. Beta diversity of ferns, mosses and liverworts (lichens excluded due to low species richness) was similar in the terrestrial habitat, but not in the epiphytic habitat. Our results demonstrate that patterns of alpha diversity of the studied taxa cannot be used to predict patterns of beta diversity. Moreover, diversity patterns observed in epiphytes are different from terrestrial plants. We noted a general coincidence in species patterns of liverworts and ferns. Diversity patterns of macrolichens, in contrast, were completely independent from any other taxonomic group studied.     

Species richness correlations among primary producers in boreal forests

Close correlations in species numbers may make it possible to indirectly assess the species richness of difficult taxonomic groups by investigating indicator groups, for which data are more easily collected. We asked if species numbers correlate among the four dominating groups of primary producers in boreal forests (liverworts, macrolichens, mosses, and vascular plants) and if substrate affiliation of species (ground vs. other substrates), sample plot size (0.01–1000 m²), and stand age (young vs. old) influence correlation strength. We used three sets of study plots from northern Sweden each including wide ranges of species richness. Although there are large differences in the ecophysiology and substrate use of vascular plants and the two bryophyte groups (mosses and liverworts), we found strong positive correlations among them not previously reported from boreal forests. In contrast, no correlation in total species richness was found between macrolichens and the two bryophyte groups, despite large overlaps in their ecology. We suggest that the positive correlations among land plants (liverworts, mosses, and vascular plants) are linked to positive relationships between site moisture and species number for all three groups. In contrast, total species number of macrolichens has not been shown to be strongly associated with moisture. However, ground‐living lichens and mosses correlated negatively in old forests. This may relate to the inability of macrolichens to exploit shaded and wet old forest ground, a habitat that is used by many moss species. Furthermore, lichens and mosses of ‘other substrates’ correlated positively in old forests, probably because the amount of boulders was positively related to species richness in both groups. Generally, correlations became stronger with increasing plot size, whereas stand age had relatively little influence. We conclude that vascular plants could be used as an indicator group for species richness of mosses and liverworts in boreal landscapes.     

N2 Fixation in Feather Mosses is a Sensitive Indicator of N Deposition in Boreal Forests

Nitrogen (N) fixation in the feather moss–cyanobacteria association represents a major N source in boreal forests which experience low levels of N deposition; however, little is known about the effects of anthropogenic N inputs on the rate of fixation of atmospheric N₂ in mosses and the succeeding effects on soil nutrient concentrations and microbial community composition. We collected soil samples and moss shoots of Pleurozium schreberi at six distances along busy and remote roads in northern Sweden to assess the influence of road-derived N inputs on N₂ fixation in moss, soil nutrient concentrations and microbial communities. Soil nutrients were similar between busy and remote roads; N₂ fixation was higher in mosses along the remote roads than along the busy roads and increased with increasing distance from busy roads up to rates of N₂ fixation similar to remote roads. Throughfall N was higher in sites adjacent to the busy roads but showed no distance effect. Soil microbial phospholipid fatty acid (PLFA) composition exhibited a weak pattern regarding road type. Concentrations of bacterial and total PLFAs decreased with increasing distance from busy roads, whereas fungal PLFAs showed no distance effect. Our results show that N₂ fixation in feather mosses is highly affected by N deposition, here derived from roads in northern Sweden. Moreover, as other measured factors showed only weak differences between the road types, atmospheric N₂ fixation in feather mosses represents a highly sensitive indicator for increased N loads to natural systems.     

Effects of warming on annual production and nutrient‐use efficiency of aquatic mosses in a high Arctic lake

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Introducing Intron Locus cox1i624 for Phylogenetic Analyses in Bryophytes: On the Issue of Takakia as Sister Genus to All Other Extant Mosses

Liverworts are well supported as the sister group to all other land plants (embryophytes) by molecular data. Observations strongly supporting this earliest dichotomy in embryophyte evolution are the strikingly different introns occurring in the mitochondrial DNAs of liverworts versus non-liverwort embryophytes (NLE), including the mosses. A final conclusion on the most basal lineages of mosses, for which genera such as Sphagnum and Takakia are the most likely candidates, is lacking. We have now investigated cox1i624, a mitochondrial group I intron conserved between the moss Physcomitrella patens and the liverwort Marchantia polymorpha. Focusing on a sampling of liverwort and moss genera, which had previously been identified as early branching taxa in their respective clades, we find that group I intron cox1i624 is universally conserved in all 33 mosses and 11 liverworts investigated. The group I intron core secondary structure is well conserved between the two ancient land plant clades. However, whereas dramatic size reductions are seen in the moss phylogeny, exactly the opposite is observed for liverworts. The cox1i624g1 locus was used for phylogenetic tree reconstruction also in combination with data sets of nad5i753g1 as well as chloroplast loci rbcL and rps4. The phylogenetic analyses revealed (i) very good support for the Treubiopsida as sister clade to all other liverworts, (ii) a sister group relationship of the nematodontous Tetraphidopsida and Polytrichopsida and (iii) two rivalling hypotheses about the basal-most moss genus with mitochondrial loci suggesting an isolated Takakia as sister to all other mosses and chloroplast loci indicating a Takakia–Sphagnum clade.     

Effects of <Emphasis Type="Italic">Penicillium bilaii</Emphasis> on maize growth are mediated by available phosphorus

Background and aims

Functional traits are promising indicators of global changes and ecosystem processes. Trait responses to environmental conditions have been examined widely in vascular plants. In contrast, few studies have focused on soil lichens and mosses composing biocrusts. We aimed to evaluate the potential of biocrust tissue traits as indicators of changes in climate and soil properties.

Methods

Isotope ratios and nutrient content in biocrust tissue were analyzed in 13 Mediterranean shrublands along an aridity gradient. Differences in tissue traits between biocrust groups (lichens and mosses), and relationships between tissue traits and climatic and soil variables were examined.

Results

Lichens and mosses differed in δ¹³C, δ¹⁵N and N content, indicating distinct physical and physiological attributes. Tissue traits correlated strongly with numerous climatic variables, likely due to a modulator effect on biocrust water relations and metabolism. We found contrasting responses of lichen and moss traits to climate, although they responded similarly to soil properties. Overall, the most responsive trait was δ¹⁵N, suggesting this trait is the best to reflect integrated processes occurring in the atmosphere and soil.

Conclusions

Biocrust tissue traits arise as cost-effective, integrative ecological indicators of global change drivers in Mediterranean ecosystems, with potential applications in response-effect trait frameworks.

     

Functional trait scaling relationships across 13 temperate mosses growing in wintertime

Mosses growing in wintertime exert important ecosystem function, but we know little about their fundamental functional trait levels and scaling relationships across species. Thus the present study chose 13 common mosses growing under a temperate deciduous forest in wintertime to measure their light-saturated assimilation rate (A_mass), dark respiration rate (Rd_mass), major element concentrations and stoichiometric ratios (C_mass, N_mass, P_mass, C:P, C:P and N:P) and the shoot mass per area (SMA). Their bivariate log–log scaling relationships were determined by standardized major axes approach. We confirmed that except C_mass, the nutrient concentrations and metabolic rates of our mosses were higher than that of mosses growing at warmer sites but the SMA was lower than for Sphagnum species. Furthermore, the functional trait levels were totally lower than those of vascular plant leaves except P_mass. Besides, we found the N_mass and P_mass were significantly positively related to A_mass and Rd_mass but negatively associated with SMA. The C:P and N:P were also closely linked with A_mass, Rd_mass and SMA. The SMA was significantly related to Rd_mass but not to A_mass. Functional trait relationships across the current species deviated from previous studies of mosses but were generally analogous to those of vascular plant leaves only with different scaling. The findings suggest that mosses allocated a greater proportion of nutrients to metabolic components rather than to the non-photosynthetic tissues. In addition, phosphorus was associated more closely with other functional traits than was nitrogen and might play a greater role to withstand the cold temperatures in wintertime for mosses.     

Inter‐specific Variation in Foliar Nutrients and Resorption of Nine Canopy‐tree Species in a Secondary Neotropical Rain Forest

The influence of environmental gradients on the foliar nutrient economy of forests has been well documented; however, we have little understanding of what drives variability among individuals within a single forest stand, especially tropical forests. We evaluated inter‐ and intra‐specific variation in nutrient resorption, foliar nutrient concentrations and physical leaf traits of nine canopy tree species within a 1‐ha secondary tropical rain forest in northeastern Costa Rica. Both nitrogen (N) and phosphorus (P) resorption efficiency (RE) and proficiency of the nine tree species varied significantly among species, but not within. Both N and P RE were significantly negatively related to leaf specific strength. Green leaf N and P concentrations were strongly negatively related to leaf mass per area, and senesced leaf nutrient concentrations were significantly positively related to green leaf nutrient concentrations. This study reveals a strong influence of physical leaf traits on foliar nutrient and resorption traits of co‐occurring species in a secondary wet tropical forest stand.     

Determinants of Lichen Diversity in a Rain Forest Understory

Change in lichen diversity is often used as a bioindicator to estimate effects of atmospheric pollution, but natural variation in lichen cover and species richness can be very high. We examined the top-down effects of spore-consuming ants and the bottom-up effects of nutrient and light availability on lichen diversity associated with the leaf surface of the rain forest understory plant, Piper cenocladum. Plots containing P. cenocladum were randomly assigned to treatments in factorial experiments that included high and low light levels, nutrient enrichment, and presence and absence of the ant mutualist, Pheidole bicornis . At the conclusion of the experiments, plants were harvested and size of leaves, secondary metabolite content (amides), epiphyll cover, and the species richness of the lichens (which comprised 85% of the epiphyll community) were quantified. Epiphyll cover (mosses, liverworts, and lichens) was greater on plants that had ant-mutualists and balanced resources. Lichen species richness was greater for plants with balanced resources, particularly for those with high light availability. Relationships between toxins and lichen cover and richness were weak and unclear. In this system, natural sources of variation were reliable determinants of lichen diversity and both biotic and abiotic influences were important.     

Ethanol synthesis, nitrogen, carbohydrates, and growth in tissues from nitrogen fertilized Pseudotsuga menziesii (Mirb.) Franco and Pinus ponderosa Dougl. ex Laws. seedlings

 Seedlings of Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, and ponderosa pine, Pinus ponderosa Dougl. ex Laws., were grown in a controlled environment and fertilized with nutrient solutions containing 150 ppm (+N), or 0 ppm nitrogen (−N). These treatments greatly altered seedling growth, and the concentrations of N and carbohydrates in their tissues. Metabolically active tissues, such as roots, incubated with a limited supply of O₂ became hypoxic faster and synthesized more ethanol than less active tissues, such as needles. All tissues that were incubated for 4 h in N₂ synthesized ethanol. Needles incubated in N₂ and light had much lower quantities of ethanol than needles in N₂ and dark, suggesting that O₂ from photosynthetsis limited internal anoxia. Most tissues from +N seedlings synthesized greater quantities of ethanol in N₂ anoxia than tissues from −N seedlings, probably because they were able to produce more enzymes with a greater availability of N. However, this increase in ethanol synthesis between N treatments was most pronounced in the phloem. Ethanol and soluble sugar concentrations were negatively related in needles and positively related in roots of N+ seedlings, but not −N seedlings. Starch concentrations had no effect on the amount of ethanol produced by any tissue. Regardless of N treatments, all tissues from ponderosa pine produced more N₂-induced ethanol than Douglas-fir, in part because its tissues contained different concentrations of soluble sugars and N as a consequence of phenological differences between the species. However, ponderosa pine tissues may also maintain greater quantities of anaerobic enzymes, or their isozymes than Douglas-fir. Received: 22 February 1998 / Accepted 23 June 1998     

Development of a diverse epiphyte community in response to phosphorus fertilization

The role of terrestrial soil nutrient supply in determining the composition and productivity of epiphyte communities has been little investigated. In a montane Hawaiian rainforest, we documented dramatic increases in the abundance and species richness of canopy epiphytes in a forest that had been fertilized annually with phosphorus (P) for 15 years; there was no response in forest that had been fertilized with nitrogen (N) or other nutrients. The response of N-fixing lichens to P fertilization was particularly strong, although mosses and non-N-fixing lichens also increased in abundance and diversity. We show that enhancement of canopy P availability is the most likely factor driving the bloom in epiphytes. These results provide strong evidence that terrestrial soil fertility may structure epiphyte communities, and in particular that the abundance of N-fixing lichens – a functionally important epiphyte group – may be particularly sensitive to ecosystem P availability.     

Tree hollows can affect epiphyte species composition

Tree hollows often harbor animals and microorganisms, thereby storing nutritive resources derived from their biological activities. The outflows from tree hollows can create unique microenvironments, which may affect communities of epiphytic organisms on trunk surfaces below the hollows. In this study, we tested whether the species richness and composition of epiphytic bryophytes (liverworts and mosses) and lichens differ above and below tree hollows of Aria japonica and Cercidiphyllum japonicum in a Japanese temperate forest. The species richness of epiphytic bryophytes and lichens did not differ above and below hollows; however, the species composition of bryophytes differed significantly above and below hollows. Indicator species analyses showed that the moss species Anomodon tristis and the liverwort species Porella vernicosa were significantly more common below than above hollows, while the liverwort species Radula japonica and four lichen species, including Leptogium cyanescens, occurred more frequently above than below hollows. Our results highlight that tree hollows can produce unique microenvironments on trunk surfaces that potentially contribute to the maintenance of epiphytic diversity on a local scale.     

Stable carbon isotopes and the metabolism of the terrestrial Devonian organism Spongiophyton

Devonian fossils of Spongiophyton have been identified as a terrestrial evolutionary intermediate between algae and vascular land plants on the basis of their dichotomously branched, tubular morphology, thick cuticles, and the scattered distribution of surface pores. Our understanding of their physiology is, however, severely limited, but may be increased through the use of stable carbon isotope measurements. One such study led to the hypothesis that Spongiophyton carried a carbon isotope (δ¹³C) signature characteristic of lichens ( Jahren et al., 2003 Geology 31 , 99–102). Here, we outline three difficulties with accepting this idea and report independent isotopic measurements of Spongiophyton fossils from Canada and Ghana. Our results show that the isotopic discrimination of analogous tissues of Spongiophyton, extant lichens, liverworts and mosses are statistically indistinguishable. We suggest therefore that claims to have definitively identified lichen metabolism are premature, and cannot be sustained.     

Correlations of stocking with the cryptogamic soil crust of a semi-arid rangeland in southwest Queensland.

Abstract The soil crust community from a sub-tropical grassland in southwest Queensland was found to include 34 taxa with cyanobacteria, other algae, lichens, fungi, mosses and liverworts represented. Cyanobacteria and liverworts were the major components of the cryptogamic cover. This is a significant component of the biodiversity of the region. Changes in the structure of this community were significantly correlated with distance from a linear water supply (bore drain) and with dung density. It was concluded that hoof impact by grazing stock had measurably affected the cryptogamic community even under a moderate stocking policy. This research suggests that management for sustainable use of low-nutrient rangelands should include consideration of soil crust condition.     

Nitrogen and phosphorus resorption in trees of a Mexican tropical dry forest

Resorption efficiency (RE) and proficiency, foliar nutrient concentrations, and relative soil nutrient availability were determined during 3 consecutive years in tree species growing under contrasting topographic positions (i.e., top vs. bottom and north vs. south aspect) in a tropical dry forest in Mexico. The sites differed in soil nutrient levels, soil water content, and potential radiation interception. Leaf mass per area (g m^–2) increased during the growing season in all species. Soil P availability and mean foliar P concentrations were generally higher at the bottom than at the top site during the 3 years of the study. Leaf N concentrations ranged from 45.4 to 31.4 mg g^–1. Leaf P varied from 2.3 to 1.8 mg g^–1. Mean N and P RE varied among species, occasionally between top and bottom sites, and were higher in the dry than in the wet years of study. Senesced-leaf nutrient concentrations (i.e., a measure of resorption proficiency) varied from 13.7 to 31.2 mg g^–1 (N) and 0.4 to 3.3 mg g^–1 (P) among the different species and were generally indicative of incomplete nutrient resorption. Phosphorus concentrations in senesced leaves were higher at the bottom than at the top site and decreased from the wettest to the the driest year. Soil N and P availability were significantly different in the north- and south-facing slopes, but neither nutrient concentrations of mature and senesced leaves nor RE differed between aspects. Our results suggest that water more than soil nutrient availability controls RE in the Chamela dry forest, while resorption proficiency may be interactively controlled by both nutrient and water availability.