@article{ ISI:000263570800016, Author = {Fornara, D. A. and Tilman, D.}, Title = {{Ecological mechanisms associated with the positive diversity-productivity relationship in an N-limited grassland}}, Journal = {{ECOLOGY}}, Year = {{2009}}, Volume = {{90}}, Number = {{2}}, Pages = {{408-418}}, Month = {{FEB}}, Abstract = {{In a 13-year grassland biodiversity experiment in Minnesota, USA, we addressed two main questions: What set of ecological mechanisms caused aboveground productivity to become; 340\% greater in highly diverse plant mixtures than in the average monoculture? Why did the effect of diversity on productivity become so much stronger through time? Because our grassland system is N limited, we simultaneously measured critical variables associated with the storage and cycling of this element, such as plant and soil N pools, soil N availability, soil N mineralization rates, and plant N-use efficiency, as well as the initial soil N concentration of each diversity plot when the experiment was established in 1994. We used linear and multiple regression analyses to test for potential effects of these variables on aboveground productivity and to address whether and how such variables were in turn affected by plant species diversity and functional composition across years and also at different time intervals within the same year. We found that seven variables simultaneously controlled productivity: (1) initial total soil nitrogen (N) of each plot, (2) diversity-dependent increases in total soil N through time, ( 3) soil N mineralization rates, (4) soil nitrate (NO3-) utilization, (5) increases in plant N-use efficiency at greater plant diversity, (6) legume presence, and (7) higher species numbers. The surprising continued significance of higher plant diversity may occur because of its effects on seasonal capture of soil NO3- and moisture and on the accumulation of root-N pools, all of which may have also increased productivity through time at higher species numbers.}}, ISSN = {{0012-9658}}, Unique-ID = {{ISI:000263570800016}}, } @article{ ISI:000261684400008, Author = {Fornara, Dario A. and Tilman, David and Hobbie, Sarah E.}, Title = {{Linkages between plant functional composition, fine root processes and potential soil N mineralization rates}}, Journal = {{JOURNAL OF ECOLOGY}}, Year = {{2009}}, Volume = {{97}}, Number = {{1}}, Pages = {{48-56}}, Month = {{JAN}}, Abstract = {{1. Plant functional composition may indirectly affect fine root processes both qualitatively (e.g. by influencing root chemistry) and quantitatively (e.g. by influencing root biomass and thus soil carbon (C) inputs and the soil environment). Despite the potential implications for ecosystem nitrogen (N) cycling, few studies have addressed the linkages between plant functional composition, root decay, root detritus N dynamics and soil N mineralization rates. 2. Here, using data from a large grassland biodiversity experiment, we first show that plant functional composition affected fine root mass loss, root detritus N dynamics and net soil N mineralization rates through its effects on root chemistry rather than on the environment of decomposition. In particular, the presence of legumes and non-leguminous forbs contributed to greater fine root decomposition which in turn enhanced root N release and net soil N mineralization rates compared with C3 and C4 grasses. 3. Second, we show that all fine roots released N immediately during decomposition and showed very little N immobilization regardless of plant composition. As a consequence, there was no evidence of increased root or soil N immobilization rates with increased below-ground plant biomass (i.e. increased soil C inputs) even though root biomass negatively affected root decay. 4. Our results suggest that fine roots represent an active soil N pool that may sustain plant uptake while other soil N forms are being immobilized in microbial biomass and/or sequestered into soil organic matter. However, fine roots may also represent a source of recalcitrant plant detritus that is returned to the soil (i.e. fine roots of C4 and C3 grasses) and that can contribute to an increase in the soil organic matter pool. 5. Synthesis. An important implication of our study is that the simultaneous presence of different plant functional groups (in plant mixtures) with opposite effects on root mass loss, root N release and soil N mineralization rates may be crucial for sustaining multiple ecosystem services such as productivity and soil C and N sequestration in many N-limited grassland systems.}}, DOI = {{10.1111/j.1365-2745.2008.01453.x}}, ISSN = {{0022-0477}}, Unique-ID = {{ISI:000261684400008}}, } @article{ ISI:000259819800009, Author = {Dybzinski, Ray and Fargione, Joseph E. and Zak, Donald R. and Fornara, Dario and Tilman, David}, Title = {{Soil fertility increases with plant species diversity in a long-term biodiversity experiment}}, Journal = {{OECOLOGIA}}, Year = {{2008}}, Volume = {{158}}, Number = {{1}}, Pages = {{85-93}}, Month = {{NOV}}, Abstract = {{Most explanations for the positive effect of plant species diversity on productivity have focused on the efficiency of resource use, implicitly assuming that resource supply is constant. To test this assumption, we grew seedlings of Echinacea purpurea in soil collected beneath 10-year-old, experimental plant communities containing one, two, four, eight, or 16 native grassland species. The results of this greenhouse bioassay challenge the assumption of constant resource supply; we found that bioassay seedlings grown in soil collected from experimental communities containing 16 plant species produced 70\% more biomass than seedlings grown in soil collected beneath monocultures. This increase was likely attributable to greater soil N availability, which had increased in higher diversity communities over the 10-year-duration of the experiment. In a distinction akin to the selection/complementarity partition commonly made in studies of diversity and productivity, we further determined whether the additive effects of functional groups or the interactive effects of functional groups explained the increase in fertility with diversity. The increase in bioassay seedling biomass with diversity was largely explained by a concomitant increase in N-fixer, C4 grass, forb, and C3 grass biomass with diversity, suggesting that the additive effects of these four functional groups at higher diversity contributed to enhance N availability and retention. Nevertheless, diversity still explained a significant amount of the residual variation in bioassay seedling biomass after functional group biomass was included in a multiple regression, suggesting that interactions also increased fertility in diverse communities. Our results suggest a mechanism, the fertility effect, by which increased plant species diversity may increase community productivity over time by increasing the supply of nutrients via both greater inputs and greater retention.}}, DOI = {{10.1007/s00442-008-1123-x}}, ISSN = {{0029-8549}}, Unique-ID = {{ISI:000259819800009}}, } @article{ ISI:000254728300016, Author = {Fornara, Dario. A. and Du Toit, Johan T.}, Title = {{Responses of woody saplings mammalian herbivory in an exposed to chronic African savanna}}, Journal = {{ECOSCIENCE}}, Year = {{2008}}, Volume = {{15}}, Number = {{1}}, Pages = {{129-135}}, Note = {{Symposium on Restoring and Designing Ecosystems for a Crowded Planet - Provision of Ecosystem Services or Mere Window Dressing, Montreal, CANADA, AUG, 2005}}, Abstract = {{Suppressed growth forms of woody species are common where fire and herbivory are major ecosystem drivers, such as in African savannas. Nevertheless, despite their importance in maintaining plant population viability, woody plants in suppressed growth forms have received little attention in ecological studies. We measured a set of morpho-functional traits and investigated plant density variation in suppressed growth forms (hereafter we refer to them as saplings) of two common species, Acacia nigrescens and Acacia tortilis, across sites that had undergone very different histories of attack from large herbivores while fire had been absent for at least 13 y. We show that heavily browsed saplings have higher regrowth abilities, twice the number of stems produced by the main root crown and twice the root diameter at 5 cm soil depth, than lightly browsed saplings. This suggests that Acacia saplings are resilient to chronic herbivory and show high morphological plasticity. However, we show that mammalian herbivores can strongly limit sapling recruitment to mature size classes and possibly affect variation in sapling density between heavily and lightly browsed sites. Further studies should investigate whether the persistence of the sapling bank can be ascribed to the ``storage effect{''} by which a plant's reproductive potential is ``stored{''} in a suppressed growth form until a window of opportunity allows rapid maturity.}}, ISSN = {{1195-6860}}, Unique-ID = {{ISI:000254728300016}}, } @article{ ISI:000254517200023, Author = {Fornara, D. A. and du Toit, J. T.}, Title = {{Community-level interactions between ungulate browsers and woody plants in an African savanna dominated by palatable-spinescent Acacia trees}}, Journal = {{JOURNAL OF ARID ENVIRONMENTS}}, Year = {{2008}}, Volume = {{72}}, Number = {{4}}, Pages = {{534-545}}, Month = {{APR}}, Abstract = {{We studied the composition of a savanna woody plant community across a natural herbivory gradient maintained by both browsing and grazing ungulates in an and part of the Kruger National Park, South Africa. We focused on (1) short-term browsing effects on reproductive and morphological traits of a dominant-palatable woody species, Acacia nigrescens, Miller, (2) the relationship between browsing-grazing intensity and soil parameters, (3) the effects of herbivore-soil interactions on woody species richness and composition, and (4) browser-induced effects on the representation of woody plant functional traits. We show that the number of pods carried by A. nigrescens trees as well as the internode length of external tree branches both decreased significantly at high browsing intensity. Moreover, we found that total soil nitrogen (N) and soil cation (Ca, Na, Mg, and K) concentrations varied significantly according to grazing rather than browsing intensity with soil nutrients decreasing at heavily grazed sites. Although herbivory and soil properties together explained similar to 79\% of the total variability in woody species composition, neither herbivory intensity nor soil properties taken separately could explain variation in woody species richness across sites. Browsing intensity could also not account for variation in the mix of evergreen, spinescent and preferred browse plants across sites. Ordination analysis showed that the palatable but spinescent A. nigrescens had the lowest fit on the first two ordination axes and was distributed across study sites irrespective of herbivory intensity and soil properties. We suggest that community-level interactions between browsers and woody plants are different in and eutrophic African savannas where the dominance by spinescent trees might prevent shifts in species composition under high browsing intensity, which is contrary to what has been observed in woodlands at higher latitudes. (c) 2007 Elsevier Ltd. All rights reserved.}}, DOI = {{10.1016/j.jaridenv.2007.07.010}}, ISSN = {{0140-1963}}, Unique-ID = {{ISI:000254517200023}}, } @article{ ISI:000254235500005, Author = {Fornara, D. A. and Du Toit, J. T.}, Title = {{Browsing-induced effects on leaf litter quality and decomposition in a southern african savanna}}, Journal = {{ECOSYSTEMS}}, Year = {{2008}}, Volume = {{11}}, Number = {{2}}, Pages = {{238-249}}, Month = {{MAR}}, Abstract = {{We investigated the linkages between leaf litter quality and decomposability in a savanna plant community dominated by palatable-spinescent tree species. We measured: (1) leaf litter decomposability across five woody species that differ in leaf chemistry; (2) mass decomposition, nitrogen (N); and carbon (C) dynamics in leaf litter of a staple browse species (Acacia nigrescens) as well as (3) variation in litter composition across six sites that experienced very different histories of attack from large herbivores. All decomposition trials included litter bags filled with chopped straw to control for variation in site effects. We found a positive relationship between litter quality and decomposability, but we also found that Acacia and straw litter mass remaining did not significantly vary between heavily and lightly browsed sites. This is despite the fact that both the quality and composition of litter returned to the soil were significantly different across sites. We observed greater N resorption from senescing Acacia leaves at heavily browsed sites, which in turn contributed to increase the C:N ratio of leaf litter and caused greater litter N immobilization over time. This, together with the significantly lower tree- and herb-leaf litter mass beneath heavily browsed trees, should negatively affect decomposition rates. However, estimated dung and urine N deposition from both browsers and grazers was significantly greater at high- than at low-herbivory sites. We hypothesize that N inputs from dung and urine boost litter N mineralization and decomposition (especially following seasonal rainfall events), and thereby offset the effects of poor leaf litter quality at chronically browsed sites.}}, DOI = {{10.1007/s10021-007-9119-7}}, ISSN = {{1432-9840}}, Unique-ID = {{ISI:000254235500005}}, } @article{ ISI:000252899300008, Author = {Fornara, D. A. and Tilman, D.}, Title = {{Plant functional composition influences rates of soil carbon and nitrogen accumulation}}, Journal = {{JOURNAL OF ECOLOGY}}, Year = {{2008}}, Volume = {{96}}, Number = {{2}}, Pages = {{314-322}}, Month = {{MAR}}, Abstract = {{1. The mechanisms controlling soil carbon (C) and nitrogen (N) accumulation are crucial for explaining why soils are major terrestrial C sinks. Such mechanisms have been mainly addressed by imposing short-term, step-changes in CO2, temperature and N fertilization rates on either monocultures or low-diversity plant assemblages. No studies have addressed the long-term effects of plant functional diversity (i.e. plant functional composition) on rates of soil C accumulation in N-limited grasslands where fixation is the main source of N for plants. 2. Here we measure net soil C and N accumulation to 1 m soil-depth during a 12-year-long grassland biodiversity experiment established on agriculturally degraded soils at Cedar Creek, Minnesota, USA. 3. We show that high-diversity mixtures of perennial grassland plant species stored 500\% and 600\% more soil C and N than, on average, did monoculture plots of the same species. Moreover, the presence of C4 grasses and legumes increased soil C accumulation by 193\% and 522\%, respectively. Higher soil C and N accrual resulted both from increased C and N inputs via (i) higher root biomass, and (ii) from greater root biomass accumulation to 60 cm soil depth resulting from the presence of highly complementary functional groups (i.e. C4 grasses and legumes). 4. Our results suggest that the joint presence of C4 grass and legume species is a key cause of greater soil C and N accumulation in both higher and lower diversity plant assemblages. This is because legumes have unique access to N, and C4 grasses take up and use N efficiently, increasing below-ground biomass and thus soil C and N inputs. 5. Synthesis. We demonstrate that plant functional complementarity is a key reason why higher plant diversity leads to greater soil C and N accumulation on agriculturally degraded soils. We suggest the combination of key C4 grass-legume species may greatly increase ecosystem services such as soil C accumulation and biomass (biofuel) production in both high- and low-diversity N-limited grassland systems.}}, DOI = {{10.1111/j.1365-2745.2007.01345.x}}, ISSN = {{0022-0477}}, Unique-ID = {{ISI:000252899300008}}, } @article{ ISI:000245668300023, Author = {Fornara, D. A. and Du Toit, J. T.}, Title = {{Browsing lawns? Responses of Acacia nigrescens to ungulate browsing in an African savanna}}, Journal = {{ECOLOGY}}, Year = {{2007}}, Volume = {{88}}, Number = {{1}}, Pages = {{200-209}}, Month = {{JAN}}, Abstract = {{We measured browsing-induced responses of Acacia trees to investigate ``browsing lawns'' as an analogy to grazing lawns in a semiarid eutrophic African savanna. During the two-year field study, we measured plant tolerance, resistance, and phenological traits, while comparing variation in leaf nitrogen and specific leaf area ( SLA) across stands of Acacia nigrescens, Miller, that had experienced markedly different histories of attack from large herbivores. Trees in heavily browsed stands developed ( 1) tolerance traits such as high regrowth abilities in shoots and leaves, high annual branch growth rates, extensive tree branching and evidence of internal N translocation, and ( 2) resistance traits such as close thorn spacing. However, phenological ``escape'' responses were weak even in heavily browsed stands. Overall, browsing strongly affected plant morpho-functional traits and decreased both the number of trees carrying pods and the number of pods per tree in heavily browsed stands. Hence, there is experimental evidence that tolerance and resistance traits may occur simultaneously at heavily browsed sites, but this comes at the expense of reproductive success. Such tolerance and resistance traits may coexist if browsers trigger and maintain a positive feedback loop in which trees are continually investing in regrowth ( tolerance), and if the plant's physical defenses ( resistance) are not nutritionally costly and are long-lived. Our results confirm that chronic browsing by ungulates can maintain A. nigrescens trees in a hedged state that is analogous to a grazing lawn. Further research is needed to fully understand the long-term effects of chronic browsing on reproduction within such tree populations, as well as the overall effects on nutrient cycling at the ecosystem level.}}, ISSN = {{0012-9658}}, Unique-ID = {{ISI:000245668300023}}, } @article{ ISI:000228028100012, Author = {Fornara, DA and Dalling, JW}, Title = {{Seed bank dynamics in five Panamanian forests}}, Journal = {{JOURNAL OF TROPICAL ECOLOGY}}, Year = {{2005}}, Volume = {{21}}, Number = {{Part 2}}, Pages = {{223-226}}, Month = {{MAR}}, DOI = {{10.1017/S002467404002184}}, ISSN = {{0266-4674}}, Unique-ID = {{ISI:000228028100012}}, } @article{ ISI:000227083400008, Author = {Fornara, DA and Dalling, JW}, Title = {{Post-dispersal removal of seeds of pioneer species from five Panamanian forests}}, Journal = {{JOURNAL OF TROPICAL ECOLOGY}}, Year = {{2005}}, Volume = {{21}}, Number = {{Part 1}}, Pages = {{79-84}}, Month = {{JAN}}, Abstract = {{Variation among forests in environmental and biotic conditions may strongly influence seed fate with important consequences for the abundance and distribution of plant species. Here we examine the post-dispersal seed removal rates of six pioneer species (Cecropia peltata, Miconia argentea, Luchea seemannii. Trenia microntha. Apeiba aspera and jacaranda copaia) from the soil surface at five sites in Panama varying in elevation (0-1100 m) and seasonality (0-4 mo dry season). We compared removal rates of washed seeds placed in vertebrate exclosures, invertebrate exclosures, and unprotected controls in January and June. Overall, removal rates of unprotected seeds were similar among sites. Almost all seed removal could be attributed to litter ants in two subfamilies (Myrmicinae and Ponerinae). Little or no removal was recorded from invertebrate exclosures while vertebrate exclosures had no effect on removal either in lowland and montane forests. Seed removal rates were high for four animal-dispersed species (mean 45\%, removed over 2 d), whereas two wind-dispersed species were largely untouched (mean 2\%, removed). These results indicate that seed dispersal characteristics, rather than site characteristics, may be the strongest determinant of the post-dispersal seed fate of pioneers.}}, DOI = {{10.1017/S026646740400197X}}, ISSN = {{0266-4674}}, Unique-ID = {{ISI:000227083400008}}, }