Spatial separation of litter decomposition and mycorrhizal nitrogen uptake in a boreal forest

Our understanding of how saprotrophic and mycorrhizal fungi interact to re-circulate carbon and nutrients from plant litter and soil organic matter is limited by poor understanding of their spatiotemporal dynamics. In order to investigate how different functional groups of fungi contribute to carbon and nitrogen cycling at different stages of decomposition, we studied changes in fungal community composition along vertical profiles through a Pinus sylvestris forest soil. We combined molecular identification methods with 14C dating of the organic matter, analyses of carbon:nitrogen (C:N) ratios and 15N natural abundance measurements. Saprotrophic fungi were primarily confined to relatively recently (< 4 yr) shed litter components on the surface of the forest floor, where organic carbon was mineralized while nitrogen was retained. Mycorrhizal fungi dominated in the underlying, more decomposed litter and humus, where they apparently mobilized N and made it available to their host plants. Our observations show that the degrading and nutrient-mobilizing components of the fungal community are spatially separated. This has important implications for biogeochemical studies of boreal forest ecosystems.     

Diversity and species distribution of ectomycorrhizal fungi along productivity gradients of a southern boreal forest

Coniferous forests with diverse ectomycorrhizal fungus (EMF) communities are associated with nutrient-poor, acidic soils but there is some debate whether EMF can be equally adapted to more productive, nitrogen-rich sites. We compared EMF species distribution and diversity along a replicated productivity gradient in a southern boreal forest of British Columbia (Canada). Roots from subalpine fir (Abies lasiocarpa) saplings of the understory were sampled and EMF species were identified by morphotypes supplemented with ITS rDNA analysis. There were significant changes in the distribution and abundance of 74 EMF species along the productivity gradient, with as little as 24% community similarity among contrasting sites. Species richness per plot increased asymptotically with foliar nitrogen concentrations of subalpine fir, demonstrating that many EMF species were well suited to soils with high rates of nitrogen mineralization. EMF species abundance in relation to site productivity included parabolic, negative linear, and positive exponential curves. Both multi-site and more narrowly distributed EMF were documented, and a diverse mix of mantle exploration types was present across the entire productivity gradient. The results demonstrate strong associations of EMF fungal species with edaphic characteristics, especially nitrogen availability, and a specialization in EMF communities that may contribute to the successful exploitation of such contrasting extremes in soil fertility by a single tree host.     

Beetle species richness along the forest productivity gradient in northern Finland

The occurrence and habitat associations of the majority of invertebrate groups in boreal forests are poorly known, even though these groups represent perhaps over 99% of the animal species diversity in the forests. We studied the beetle (Coleoptera) fauna of four forest site types in northern Finland: in spruce mires, herb rich, mesic and sub-xeric forests. We sampled beetles in 32 study sites with five window and five pitfall traps in each. We describe the species abundance and diversity patterns within and among forest types and relate these patterns to structural components of the forests. The volume of decaying wood varied from 14 to 93 m³ ha⁻¹ among sampling sites. The total beetle catch consisted of 100 333 individuals and 435 species. The beetle species richness did not vary according to site fertility but the number of specimens increased with increasing fertility in heath forest sites. The richness of beetle species correlated only weakly with any of the stand structure characteristics at the stand level. Nevertheless, the detrended correspondence analysis (DCA) indicated that different beetle assemblages are characteristic of different forest types. The high level of beta-diversity in beetles among forest types indicates that focusing exclusively on, for example, key-biotopes (presumed biodiversity hotspots) when selecting areas to be set aside would result in a situation where a large proportion of species, even of the rare and threatened ones, is not included in this network of protected areas. This suggests that the complementary set of different forest types may be the best general strategy to maintain the overall beetle species diversity in boreal forests.     

Within-species correlations in leaf traits of three boreal plant species along a latitudinal gradient

Evergreen boreal plant species express high variability in their leaf traits. It remains controversial whether this within-species variability is constrained to the same leaf trait relationships as has been observed across species. We sampled leaves of three boreal evergreen woody species along a latitudinal gradient (from 57o56′N to 69o55′N). Leaf longevity (LL) of Pinus sylvestris L. and Vaccinium vitis-idaea L. correlated negatively with mean annual air temperature (MAT), whereas the LL of Ledum palustre L. was not affected by MAT. V. vitis-idaea and L. palustre had a negative relationship between leaf mass per area (LMA) and MAT. In P. sylvestris, the LMA–MAT relationship was positive. A negative correlation between LL and LMA was significant only for P. sylvestris. Leaf nitrogen concentration was positively related to leaf phosphorus concentration in all three species. Leaf potassium concentration was related to nitrogen concentration only in L. palustre, and to phosphorus concentration in P. sylvestris and L. palustre. Our results demonstrate that although within the studied species the variation in some of the leaf traits may have the same degree as interspecific variation, there is no such intercorrelation of leaf traits within the studied species as has been observed across species.     

Soil N chemistry in oak forests along a nitrogen deposition gradient

Anthropogenic N deposition may change soil conditions in forest ecosystems as demonstrated in many studies of coniferous forests, whereas results from deciduous forests are relatively scarce. Therefore the influence of N deposition on several variables was studied in situ in 45 oak-dominated deciduous forests along a N deposition gradient in southern Sweden, where the deposition ranged from 10 to 20 kg N ha⁻¹ year⁻¹. Locally estimated NO ₋ ³ deposition, as measured with ion-exchange resins (IER) on the soil surface, and grass N concentration (%) were positively correlated with earlier modelled regional N deposition. Furthermore, the δ¹⁵N values of grass and uppermost soil layers were negatively correlated with earlier modelled N deposition. The data on soil NO ₋ ³ , measured with IER in the soil, and grass N concentration suggest increased soil N availability as a result of N deposition. The δ¹⁵N values of grass and uppermost soil layers indicate increased nitrification rates in high N deposition sites, but no large downward movements of NO ₋ ³ in these soils. Only a few sites had NO ₋ ³ concentrations exceeding 1 mg N l⁻¹ in soil solution at 50 cm depth, which showed that N deposition to these acid oak-dominated forests has not yet resulted in extensive leaching of N. The δ¹⁵N enrichment factor was the variable best correlated with NO ₋ ³ concentrations at 50 cm and is thus a variable that potentially may be used to predict leaching of NO ₋ ³ from forest soils.     

Species composition of an ectomycorrhizal fungal community along a local nutrient gradient in a boreal forest

Soil abiotic factors are considered to be important in determining the distribution of ectomycorrhizal (ECM) fungal species; however, there are few field data to support this. Here, we relate ECM species distributions to changes in soil chemistry along a short (90-m), natural nutrient gradient. The ECM community was characterized, using morphological and molecular techniques, in soil samples collected at 10-m intervals. There were pronounced changes in ECM fungal community structure along the transect, with many taxa showing discrete distributions. Although there was a change of host from Pinus to Picea along the gradient, host-specific fungi did not account for the observed change in community structure. Ordination analyses showed that community structure was strongly correlated with soil characteristics, in particular extractable ammonium and base saturation. However, autocorrelation among soil parameters makes it difficult to isolate the effects of individual parameters. The distinctive changes in soil and vegetation along the transect used in this study provided an exceptional opportunity to examine the local-scale impact of natural spatial heterogeneity on an ECM fungal community.     

Effects of enhanced nitrogen inputs and climate warming on a forest understorey plant assessed by transplant experiments along a latitudinal gradient

Global warming and enhanced nitrogen (N) inputs are two key global-change drivers affecting temperate forest ecosystems simultaneously. Interactive effects of multiple drivers might cause species responses to differ from those in single-factor experiments; therefore, there is an urgent need for more multi-factor studies. Here, we assessed the growth and reproductive performance of multiple populations of a widespread grass of deciduous forests (Milium effusum) sampled along a latitudinal gradient and subjected to experimental manipulations of temperature and nitrogen availability. Common garden transplant experiments along the latitudinal gradient were used to manipulate temperatures and combined with experimental N addition to assess intraspecific responses of the study species to global-change drivers as well as to determine local adaptation. The total biomass, number of seeds and seedling emergence time of M. effusum increased when transplanted in the southern common garden. Apart from effects on the seed mass, the species did not respond to N addition alone. Yet, interactive effects between warming and N addition were found: N addition led to increased biomass growth but only in the northern common garden. Significant home-site advantages were apparent, most likely because of increased mycorrhizal colonization of roots of local transplants. We show that multiple global-change drivers may alter dynamics in understorey communities of temperate forests. Our study reinforces the need to increase our understanding of plant responses to future environmental changes by expanding the multi-factor research framework.     

Radial growth response of four dominant boreal tree species to climate along a latitudinal gradient in the eastern Canadian boreal forest

To address the central question of how climate change influences tree growth within the context of global warming, we used dendroclimatological analysis to understand the reactions of four major boreal tree species –Populus tremuloides, Betula papyrifera, Picea mariana, and Pinus banksiana– to climatic variations along a broad latitudinal gradient from 46 to 54°N in the eastern Canadian boreal forest. Tree‐ring chronologies from 34 forested stands distributed at a 1° interval were built, transformed into principal components (PCs), and analyzed through bootstrapped correlation analysis over the period 1950–2003 to identify climate factors limiting the radial growth and the detailed radial growth–climate association along the gradient. All species taken together, previous summer temperature (negative influences), and current January and March–April temperatures (positive influences) showed the most consistent relationships with radial growth across the gradient. Combined with the identified species/site‐specific climate factors, our study suggested that moisture conditions during the year before radial growth played a dominant role in positively regulating P. tremuloides growth, whereas January temperature and growing season moisture conditions positively impacted growth of B. papyrifera. Both P. mariana and P. banksiana were positively affected by the current‐year winter and spring or whole growing season temperatures over the entire range of our corridor. Owing to the impacts of different climate factors on growth, these boreal species showed inconsistent responsiveness to recent warming at the transition zone, where B. papyrifera, P. mariana, and P. banksiana would be the most responsive species, whereas P. tremuloides might be the least. Under continued warming, B. papyrifera stands located north of 49°N, P. tremuloides at northern latitudes, and P. mariana and P. banksiana stands located north of 47°N might benefit from warming winter and spring temperatures to enhance their radial growth in the coming decades, whereas other southern stands might be decreasing in radial growth.     

Strong and weak trophic cascades along a productivity gradient

Variation in the strengths of predator effects, although commonly observed in natural communities, and predicted from theoretical models, remains poorly understood in the study of food web interactions and community structure. In this study, I first showed that prey species in benthic pond food webs were highly variable in their susceptibility to predators. Some were vulnerable throughout their lives, whereas others were vulnerable as juveniles, but able to grow to a size-refuge. Next, I showed that herbivore and predator abundance increased along a natural productivity gradient among 29 ponds, and herbivore species composition shifted from dominance by vulnerable to dominance by invulnerable herbivore species along this gradient. However, there was a considerable amount of variation both in herbivore biomass and composition at intermediate productivity; some were dominated by small species and others by larger species. Finally, in in situ exclosure experiments, I found that predator effects were strong and cascaded to plants in a low productivity pond and in an intermediate productivity pond dominated by small herbivore species. Alternatively, in a high productivity pond and in an intermediate productivity pond dominated by larger herbivores, I found that predator effects on prey biomass were weak, and did not cascade to plants.     

Nitrogen uptake and nitrogen use efficiency above and below ground along a topographic gradient of soil nitrogen availability

Nitrogen (N) uptake and nitrogen use efficiency (NUE) are closely related through feedback mechanisms to soil N availability and N cycling in forested ecosystems. We investigated N uptake and NUE not only at the leaf, litterfall, and aboveground levels but also belowground and whole stand levels along a topographic gradient of soil N availability in a cool temperate deciduous forest in Japan. In this study, we addressed how whole stand level N uptake and NUE affect C and N cycling in forested ecosystems. At the leaf, litterfall, and aboveground levels, N uptake decreased and NUE increased with decreasing soil N availability. This pattern resulted from decreasing leaf N concentrations and increasing N resorption efficiencies as soil N availability declined. Low N concentrations in litterfall may have resulted in little soil N being available to plants, due to microbial immobilization. In contrast, when belowground components were included, N uptake and NUE were not correlated with soil N availability. This was mainly due to higher levels of fine root production when soil N availability was low. Higher fine root allocation can result in a high input of detritus to decomposer systems and, thus, contribute to accumulation of soil organic matter and immobilization by microbes, which may result in further soil N availability decline. Our results suggest that allocation to the fine root rather than whole stand level NUE is important for C and N cycling in forested ecosystems, as is the feedback mechanism in which litterfall level NUE shifts with changes in the N concentration of litterfall.     

The response of forest plant regeneration to temperature variation along a latitudinal gradient

Background and Aims

The response of forest herb regeneration from seed to temperature variations across latitudes was experimentally assessed in order to forecast the likely response of understorey community dynamics to climate warming.

Methods

Seeds of two characteristic forest plants (Anemone nemorosa and Milium effusum) were collected in natural populations along a latitudinal gradient from northern France to northern Sweden and exposed to three temperature regimes in growth chambers (first experiment). To test the importance of local adaptation, reciprocal transplants were also made of adult individuals that originated from the same populations in three common gardens located in southern, central and northern sites along the same gradient, and the resulting seeds were germinated (second experiment). Seedling establishment was quantified by measuring the timing and percentage of seedling emergence, and seedling biomass in both experiments.

Key Results

Spring warming increased emergence rates and seedling growth in the early-flowering forb A. nemorosa. Seedlings of the summer-flowering grass M. effusum originating from northern populations responded more strongly in terms of biomass growth to temperature than southern populations. The above-ground biomass of the seedlings of both species decreased with increasing latitude of origin, irrespective of whether seeds were collected from natural populations or from the common gardens. The emergence percentage decreased with increasing home-away distance in seeds from the transplant experiment, suggesting that the maternal plants were locally adapted.

Conclusions

Decreasing seedling emergence and growth were found from the centre to the northern edge of the distribution range for both species. Stronger responses to temperature variation in seedling growth of the grass M. effusum in the north may offer a way to cope with environmental change. The results further suggest that climate warming might differentially affect seedling establishment of understorey plants across their distribution range and thus alter future understorey plant dynamics.     

Variation of carbon and nitrogen cycling processes along a topographic gradient in a central Amazonian forest

It is well recognized in the literature that topography can influence soil nutrient stocks and dynamics in temperate regions, but for tropical forests, this source of variation has sometimes been ignored. The nature of such variations may depend upon the soil type, which in turn, is closely linked to local or regional topography. This study characterizes the soil and describes the status of carbon and nitrogen in vegetation, litterfall, litter‐layer and soil upper layers along the main positions of a topographic gradient (plateau, slope and valley), 60 km north of Manaus, on Cuieiras Reserve watershed. Nitrogen concentrations in living leaves, fresh litterfall, litter‐layer and soil upper layers were lower in the valley than in both slope and plateau plots. Carbon concentrations in plant material were not significantly different among the three topographic positions, resulting in higher C : N ratios in valley plots. Local topography (plateau, slope and valley) clearly was an influential factor in the nutrient distribution along the study locations. Lower rates of N cycling processes in the valley are probably related to its sandy soil texture and seasonal flooding.     

Patterns of diversity in plant and soil microbial communities along a productivity gradient in a Michigan old-field

The relationship between plant diversity and productivity has received much attention in ecology, but the relationship of these factors to soil microbial communities has been little explored. The carbon resources that support soil microbial communities are primarily derived from plants, so it is likely that the soil microbial community should respond to changes in plant diversity or productivity, particularly if the plant community affects the quality or quantity of available carbon. We investigated the relationship of plant diversity and productivity to the composition of the soil microbial community along a topographic gradient in a mid-successional old-field in southwestern Michigan. Soil moisture, soil inorganic N, and plant biomass increased from the top to the base of the slope, while light at ground level decreased along this same gradient. We characterized the changes in resource levels along this gradient using an index of productivity that incorporated light levels, soil N, soil moisture, and plant biomass. Average plant species richness declined with this productivity index and there were associated compositional changes in the plant community along the gradient. The plant community shifted from predominantly low-growing perennial forbs at low productivities to perennial grasses at higher productivities. Although there was variation in the structure of the soil microbial community [as indicated by fatty acid methyl ester (FAME) profiles], changes in the composition of the soil microbial community were not correlated with plant productivity or diversity. However, microbial activity [as indicated by Biolog average well color development and substrate-induced respiration (SIR)] was positively correlated with plant productivity. The similarity between patterns of plant biomass and soil microbial activity suggests that either plant productivity is driving microbial productivity or that limiting resources for each of these two communities co-vary.     

Nutrient limitation along a productivity gradient in wet meadows

Conservation management in meadows often focuses on reducing soil fertility and consequently community productivity as to promote and sustain species-rich vegetations. The productivity level to which nutrients are limiting growth is, however, unclear, as well as the relationship between productivity and the type of nutrient limitation. We carried out a fertilization experiment with N, P and K in six annually mown meadows along an aerial phytomass gradient (200–650 g m^–2). All meadows were found to be growth-limited by nutrients. Low-productive meadows were N-limited, or N+P co-limited, whereas our higher productive meadows were co-limited by a combination of N, P and/or K. The results from our experiments were compared with the results from 45 other fertilization experiments with N, P and K in grasslands and wetlands (aerial phytomass range 50–1500 g m^–2). Our results were consistent in nitrogen being the most frequent (co)-limiting nutrient, and regarding the equal frequence of occurrence of P (co)-limitation and K (co)-limitation (both in ca. 25–30% of all sites). Co-limitation occurred more often in our sites than in the other experiments. There was no clear relationship between aerial phytomass and type of nutrient limitation, except that K (co)-limitation only occurred at sites with phytomass above 200 g m^–2, and P (co)-limitation below 600 g m^–2. A comparison of productivity and nutrient concentrations in aerial phytomass among two years indicated that the type of nutrient limitation is not a static site characteristic but may vary with dynamic environmental conditions such as soil wetness; drought seems to enhance N-availability which may induce P- and K-limitation.     

Local plant species richness increases with regional habitat commonness across a gradient of forest productivity

We tested the prediction that forest habitat types with relatively high productivity are not only relatively low in species richness but are also regionally uncommon. This relationship was supported by an analysis of data from 146 forest communities in southern Ontario, Canada. Potential forest habitat productivity was determined based on a classification scheme developed for the Canadian Land Inventory (CLI) project. Vascular plant species richness approximated a unimodal distribution across forest productivity classes with the lowest mean species richness recorded for the two most productive classes. The contemporary regional commonness of forest habitat productivity classes were also displayed as a unimodal frequency distribution. Hence, mean species richness per CLI class was positively correlated with the regional area of land encompassing each of these productivity classes and this relationship was increasingly significant at increasingly larger spatial scales of regional CLI class land areas. These results are consistent with the species pool hypothesis, which postulates that species richness is relatively low in highly productive habitats because such habitats have been relatively uncommon in both space and time and hence, have had relatively little historical opportunity for the origination of adapted species.     

Competition intensity along a productivity gradient in a low-diversity grassland

Competition among plants often shifts from roots to shoots as productivity increases and species composition changes. We examined competition in an old field with low diversity to test whether this shift occurred along a productivity gradient without species turnover. Forty plots received one of four nitrogen treatments (0, 5, or 15 g added N m-2 yr-1 or 400 g m-2 yr-1 of sawdust added to immobilize N) annually for 5 yr. All N levels were dominated by the perennial grasses Agropyron cristatum and Bromus inermis. Transplants of Agropyron were grown with all neighbors, roots of neighbors, or no neighbors present to measure total, root, and shoot competition. Transplant growth was 22%-165% higher in subplots without neighbors present, which indicates that competition occurred. Competition from neighbor roots was primarily responsible for suppression of transplant growth over the entire productivity gradient. In contrast to previous field experiments that found either an increase in total competition intensity or a shift from root to shoot competition with increasing productivity, we found neither. Increases in total competition intensity or shifts from root to shoot competition found along other gradients may be caused by changes in species composition and not by increased resources or neighbor biomass. These results suggest that different competitive mechanisms may operate in low-diversity vegetation than in more diverse natural vegetation.