Ecological integrity of boreal streams

Running waters provide a number of services for humans, such as drinking water and food resources and many freshwater animals are confined and specialised to this environment. However, this natural resource has become increasingly impacted by humans resulting in a substantial loss of biodiversity and services. To assess ecological integrity of streams a number of bioassessment schemes have been developed and most of these are based on community structure and composition. Although many of the biological metrics developed have been used successfully in bioassessment, it has been suggested that ecosystem functions, such as leaf-litter decomposition, should be incorporated in modern bioassessment schemes. In this thesis I compare a number of structural metrics with functional metrics along a nutrient gradient in nine boreal streams in south-central Sweden to assess the potential of ecosystem function as a biomonitoring tool. Leaf-litter breakdown (Alnus glutinosa (L.) Gaertner) was studied during four seasons and stable isotope (δ13C and δ15N) and stoichiometric ratios (C:N) of phytobenthos, CPOM, FPOM, invertebrates and fish were also analysed. My results indicate that leaf-litter breakdown is a relatively insensitive tool to assess ecosystem impairment compared to invertebrate metrics. However, δ15N in organic matter has been suggested as a potential tool to assess ecological integrity of streams and my results support this conjecture. A strong response in δ15N in organic matter with nutrient enrichment was revealed, suggesting that δ15N could serve as a simple tool to assess nutrient enrichment effects in boreal streams. I also found that leaf-litter associated fungi and invertebrates were positively correlated with leaf-litter breakdown rates and a nutrient gradient. Moreover, I found that certain dominating species, e.g. waterlouse (Asellus aquaticus (L.)), can have a strong influence on ecosystem processes. In this thesis I show that leaf-litter breakdown is not a simple low-cost biomonitoring tool as several field trips were necessary to assure adequate litter-bag recovery. Also natural factors, such as fluctuating water levels and heavy snow fall, resulted in substantial loss of litter bags thereby confounding data interpretation

Shifts in leaf litter breakdown along a forest–pasture–urban gradient in Andean streams

Tropical montane ecosystems of the Andes are critically threatened by a rapid land-use change which can potentially affect stream variables, aquatic communities, and ecosystem processes such as leaf litter breakdown. However, these effects have not been sufficiently investigated in the Andean region and at high altitude locations in general. Here, we studied the influence of land use (forest–pasture–urban) on stream physico-chemical variables (e.g., water temperature, nutrient concentration, and pH), aquatic communities (macroinvertebrates and aquatic fungi) and leaf litter breakdown rates in Andean streams (southern Ecuador), and how variation in those stream physico-chemical variables affect macroinvertebrates and fungi related to leaf litter breakdown. We found that pH, water temperature, and nutrient concentration increased along the land-use gradient. Macroinvertebrate communities were significantly different between land uses. Shredder richness and abundance were lower in pasture than forest sites and totally absent in urban sites, and fungal richness and biomass were higher in forest sites than in pasture and urban sites. Leaf litter breakdown rates became slower as riparian land use changed from natural to anthropogenically disturbed conditions and were largely determined by pH, water temperature, phosphate concentration, fungal activity, and single species of leaf-shredding invertebrates. Our findings provide evidence that leaf litter breakdown in Andean streams is sensitive to riparian land-use change, with urban streams being the most affected. In addition, this study highlights the role of fungal biomass and shredder species (Phylloicus; Trichoptera and Anchytarsus; Coleoptera) on leaf litter breakdown in Andean streams and the contribution of aquatic fungi in supporting this ecosystem process when shredders are absent or present low abundance in streams affected by urbanization. Finally, we summarize important implications in terms of managing of native vegetation and riparian buffers to promote ecological integrity and functioning of tropical Andean stream ecosystems

COMPARING TWO TRANSECT METHODS FOR THE DETECTION OF RED-BACKED SALAMANDERS

Typical monitoring methods for terrestrial salamanders are subject to extensive variation, driven by the environmental conditions in effect during sampling. As rigorous salamander sampling methods are needed to monitor populations, there is a need to assess commonly used methods under a variety of environmental conditions. We hypothesized that of two methods used for capturing red-backed salamanders (Plethodon cinereus), leaf litter searches and natural cover searches, the latter would perform best for adults and in situations where moisture was limiting. We compared captures for paired transect surveys: one a leaf litter search and the other a natural cover search, relative to age, proximity to streams, rainfall events, vapor pressure deficit and season. We found that natural cover searches outperformed leaf litter searches when conducted away from streams and in the absence of rainfall. Natural cover searches performed better regardless of vapor pressure deficit and season (spring or fall). Natural cover searches detected more adults than juveniles. We recommend natural cover searches as more efficient than leaf litter searches

Effects of burial on leaf litter quality, microbial conditioning and palatability to three shredder taxa

1. Heterotrophic microorganisms are crucial for mineralising leaf litter and rendering it more palatable to leaf-shredding invertebrates. A substantial part of leaf litter entering running waters may be buried in the streambed and thus be exposed to the constraining conditions prevailing in the hyporheic zone. The fate of this buried organic matter and particularly the role of microbial conditioning in this habitat remain largely unexplored. 2. The aim of this study was to determine how the location of leaf litter within the streambed (i.e. at the surface or buried), as well as the leaf litter burial history, may affect the leaf-associated aquatic hyphomycete communities and therefore leaf consumption by invertebrate detritivores. We tested the hypotheses that (i) burial of leaf litter would result in lower decomposition rates associated with changes in microbial assemblages compared with leaf litter at the surface and (ii) altered microbial conditioning of buried leaf litter would lead to decreased quality and palatability to their consumers, translating into lower growth rates of detritivores. 3. These hypotheses were tested experimentally in a second-order stream where leaf-associated microbial communities, as well as leaf litter decomposition rates, elemental composition and toughness, were compared across controlled treatments differing by their location within the streambed. We examined the effects of the diverse conditioning treatments on decaying leaf palatability to consumers through feeding trials on three shredder taxa including a freshwater amphipod, of which we also determined the growth rate. 4. Microbial leaf litter decomposition, fungal biomass and sporulation rates were reduced when leaf litter was buried in the hyporheic zone. While the total species richness of fungal assemblages was similar among treatments, the composition of fungal assemblages was affected by leaf litter burial in sediment. 5. Leaf litter burial markedly affected the food quality (especially P content) of leaf material, probably due to the changes in microbial conditioning. Leaf litter palatability to shredders was highest for leaves exposed at the sediment surface and tended to be negatively related to leaf litter toughness and C⁄P ratio. In addition, burial of leaf litter led to lower amphipod growth rates, which were positively correlated with leaf litter P content. 6. These results emphasise the importance of leaf colonisation by aquatic fungi in the hyporheic zone of headwater streams, where fungal conditioning of leaf litter appears particularly critical for nutrient and energy transfer to higher trophic levels

Linking spatial patterns of leaf litterfall and soil nutrients in a tropical forest: a neighborhood approach

Leaf litter represents an important link between tree community composition, forest productivity and biomass, and ecosystem processes. In forests, the spatial distribution of trees and species-specific differences in leaf litter production and quality are likely to cause spatial heterogeneity in nutrient returns to the forest floor and, therefore, in the redistribution of soil nutrients. Using mapped trees and leaf litter data for 12 tree species in a subtropical forest with a well-documented history of land use, we: (1) parameterized spatially explicit models of leaf litter biomass and nutrient deposition; (2) assessed variation in leaf litter inputs across forest areas with different land use legacies; and (3) determined the degree to which the quantity and quality of leaf litter inputs and soil physical characteristics are associated with spatial heterogeneity in soil nutrient ratios (C:N and N:P). The models captured the effects of tree size and location on spatial variation in leaf litterfall (R2 = 0.31–0.79). For all 12 focal species, most of the leaf litter fell less than 5 m away from the source trees, generating fine-scale spatial heterogeneity in leaf litter inputs. Secondary forest species, which dominate areas in earlier successional stages, had lower leaf litter C:N ratios and produced less litter biomass than old-growth specialists. In contrast, P content and N:P ratios did not vary consistently among successional groups. Interspecific variation in leaf litter quality translated into differences in the quantity and quality (C:N) of total leaf litter biomass inputs and among areas with different land use histories. Spatial variation in leaf litter C:N inputs was the major factor associated with heterogeneity in soil C:N ratios relative to soil physical characteristics. In contrast, spatial variation soil N:P was more strongly associated with spatial variation in topography than heterogeneity in leaf litter inputs. The modeling approach presented here can be used to generate prediction surfaces for leaf litter deposition and quality onto the forest floor, a useful tool for understanding soil–vegetation feedbacks. A better understanding of the role of leaf litter inputs from secondary vegetation in restoring soil nutrient stocks will also assist in managing expanding secondary forests in tropical regions

Closely Related Tree Species Differentially Influence the Transfer of Carbon and Nitrogen from Leaf Litter Up the Aquatic Food Web

Decomposing leaf litter in streams provides habitat and nutrition for aquatic insects. Despite large differences in the nutritional qualities of litter among different plant species, their effects on aquatic insects are often difficult to detect. We evaluated how leaf litter of two dominant riparian species (Populus fremontii and P. angustifolia) influenced carbon and nitrogen assimilation by aquatic insect communities, quantifying assimilation rates using stable isotope tracers (13C, 15N). We tested the hypothesis that element fluxes from litter of different plant species better define aquatic insect community structure than insect relative abundances, which often fail. We found that (1) functional communities (defined by fluxes of carbon and nitrogen from leaf litter to insects) were different between leaf litter species, whereas more traditional insect communities (defined by relativized taxa abundances) were not different between leaf litter species, (2) insects assimilated N, but not C, at a higher rate from P. angustifolia litter compared to P. fremontii, even though P. angustifolia decomposes more slowly, and (3) the C:N ratio of material assimilated by aquatic insects was lower for P. angustifolia compared to P. fremontii, indicating higher nutritional quality, despite similar initial litter C:N ratios. These findings provide new evidence for the effects of terrestrial plant species on aquatic ecosystems via their direct influence on the transfer of elements up the food web. We demonstrate how isotopically labeled leaf litter can be used to assess the functioning of insect communities, uncovering patterns undetected by traditional approaches and improving our understanding of the association between food web structure and element cycling

Leaf litter decomposition rates increase with rising mean annual temperature in Hawaiian tropical montane wet forests

Decomposing litter in forest ecosystems supplies nutrients to plants, carbon to heterotrophic soil microorganisms and is a large source of CO2 to the atmosphere. Despite its essential role in carbon and nutrient cycling, the temperature sensitivity of leaf litter decay in tropical forest ecosystems remains poorly resolved, especially in tropical montane wet forests where the warming trend may be amplified compared to tropical wet forests at lower elevations. We quantified leaf litter decomposition rates along a highly constrained 5.2 ◦C mean annual temperature (MAT) gradient in tropical montane wet forests on the Island ofHawaii. Dominant vegetation, substrate type and age, soil moisture, and disturbance history are all nearly constant across this gradient, allowing us to isolate the effect of rising MAT on leaf litter decomposition and nutrient release. Leaf litter decomposition rates were a positive linear function of MAT, causing the residence time of leaf litter on the forest floor to decline by ∼31 days for each 1 ◦C increase in MAT. Our estimate of the Q10 temperature coefficient for leaf litter decomposition was 2.17, within the commonly reported range for heterotrophic organic matter decomposition (1.5–2.5) across a broad range of ecosystems. The percentage of leaf litter nitrogen (N) remaining after six months declined linearly with increasing MAT from ∼88% of initial N at the coolest site to ∼74% at the warmest site. The lack of net N immobilization during all three litter collection periods at all MAT plots indicates that N was not limiting to leaf litter decomposition, regardless of temperature. These results suggest that leaf litter decay in tropical montane wet forests may be more sensitive to rising MAT than in tropical lowland wet forests, and that increased rates of N release from decomposing litter could delay or prevent progressive N limitation to net primary productivity with climate warming

Nutritional quality and calorific value of Amazonian forest litter

A study on the nutritional quality of litter from an Amazon terra firme forest was carried out to supplement quantitative data on litter production previously published by KLINGE and RODRIGUES (1968). Analyses for the following constituents were carried out: cell-wall and non cell-wall fractions, crude protein, total mineral ash, polyphenols, and caloric values. Reasons are given for choosing these variables. Mineral ash and protein values were very low, whilst cell-wall fractions, which are a measure of the amount of undigestible material, were high, as were caloric values. Polyphenols were also relatively high. These factors together indicate that the litter is a very low grade forage. Amazon leaf litter has high caloric values compared with published figures from other tropical forests. The following hypothesis was offered to explain these high values: as mineral nutrients are severely limiting in this ecosystem, not all the products of photosynthesis can be channeled into plant growth. Large proportions of these photosynthetic products are therefore probably accumulated in the leaves as reduced high energy compounds such as waxes, resins etc. Available data do in fact indicate that primary production is relatively low. The low quality forage which the leaf litter offers may be a contributing factor to the low animal biomass of the Amazon forests