Ecosystems最新文献

Quantifying nitrogen uptake rates across different forest types is critical for a range of ecological questions, including the parameterization of global climate change models. However, few measurements of forest nitrogen uptake rates are available due to the intensive labor required to collect in situ data. Here, we seek to optimize data collection efforts by identifying measurements that must be made in situ and those that can be omitted or approximated from databases. We estimated nitrogen uptake rates in 18 mature monodominant forest stands comprising 13 species of diverse taxonomy at the Morton Arboretum in Lisle, IL, USA. We measured all nitrogen concentrations, foliage allocation, and fine root biomass in situ. We estimated wood biomass increments by in situ stem diameter and stem core measurements combined with allometric equations. We estimated fine root turnover rates from database values. We analyzed similar published data from monodominant forest FACE sites. At least in monodominant forests, accurate estimates of forest nitrogen uptake rates appear to require in situ measurements of fine root productivity and are appreciably better paired with in situ measurements of foliage productivity. Generally, wood productivity and tissue nitrogen concentrations may be taken from trait databases at higher taxonomic levels. Careful sorting of foliage or fine roots to species is time consuming but has little effect on estimates of nitrogen uptake rate. By directing research efforts to critical in situ measurements only, future studies can maximize research effort to identify the drivers of varied nitrogen uptake patterns across gradients.

In a warming world, the input of glacier meltwater to inland water ecosystems is predicted to change, potentially affecting their productivity. Meta-ecosystem theory, which posits that the nutrient availability in the recipient ecosystem can determine the extent of cross-ecosystem boundary utilization, can be useful for studying landscape-scale influences of glacier meltwater on inland waters. Here, we investigate how the input of glacier meltwater in a river system in Southern Greenland influences the utilization of marine subsidies in freshwater fish. Our study system comprised four sites, with controls for glacial meltwater and marine subsidies, harboring a partially migrating population of arctic char, meaning that some individuals migrate to the ocean and others remain in freshwaters, and two fully resident populations as a freshwater reference. We assessed the incorporation of marine carbon in freshwater resident char using both bulk and amino acid stable isotope analysis of muscle tissue. In the population with partial migration, marine subsidies were a significant resource for resident char individuals, and estimates of trophic position suggest that egg cannibalism is an important mechanism underlying the assimilation of these marine subsidies. In proglacial streams, namely those with high glacial meltwater, the total dependence on marine subsidies increased and reached 83% because char become cannibals at smaller sizes. In the configuration of our focal meta-ecosystem, our results suggest that the importance of marine subsidies to freshwater fish strengthens within increasing meltwater flux from upstream glaciers.

With climate warming and drying, fire activity is increasing in Cajander larch (Larix cajanderi Mayr.) forests underlain by continuous permafrost in northeastern Siberia, and initial post-fire tree demographic processes could unfold to determine long-term forest carbon (C) dynamics through impacts on tree density. Here, we evaluated above- and belowground C pools across 25 even-aged larch stands of varying tree densities that established following a wildfire in ~ 1940 near Cherskiy, Russia. Total C pools increased with increased larch tree density, from ~ 9,000 g C m⁻² in low-density stands to ~ 11,000 g C m⁻² in high and very high-density stands, with increases most pronounced at tree densities < 1 stem m⁻² and driven by increased above- and belowground (that is, coarse roots) and live and dead (that is, woody debris and snags) larch biomass. Total understory vegetation and non-larch coarse root C pools declined with increased tree density due to decreased shrub C pools, but these pools were relatively small compared to larch biomass. Fine root, soil organic matter (OM), and near surface (0–30 cm) mineral soil (MS) C pools varied little with tree density, although soil C pools held most (18–28% in OM and 44–51% in MS) C stored in these stands. Thus, if changing fire regimes promote denser stands, C storage will likely increase, but whether this increase offsets C lost during fires remains unknown. Our findings highlight how post-fire tree demographic processes impact C pool distribution and stability in larch forests of Siberian permafrost regions.

Discerning ecosystem change and food web dynamics underlying anthropogenic eutrophication and the introduction of non-native species is necessary for ensuring the long-term sustainability of fisheries and lake biodiversity. Previous studies of eutrophication in Lake Victoria, eastern Africa, have focused on the loss of endemic fish biodiversity over the past several decades, but changes in the plankton communities over this same time remain unclear. To fill this gap, we examined sediment cores from a eutrophic embayment, Mwanza Gulf, to determine the timing and magnitude of changes in the phytoplankton and zooplankton assemblages over the past century. Biogeochemical proxies indicate nutrient enrichment began around ~ 1920 CE and led to rapid increases in primary production, and our analysis of photosynthetic pigments revealed three zones: pre-eutrophication (prior to 1920 CE), onset of eutrophication with increases in all pigments (1920–1990 CE), and sustained eutrophication with cyanobacterial dominance (1990 CE–present). Cladoceran remains indicate an abrupt decline in biomass in ~ 1960 CE, in response to the cumulative effects of eutrophication and lake-level rise, preceding the collapse of haplochromine cichlids in the 1980s. Alona and Chydorus, typically benthic littoral taxa, have remained at relatively low abundances since the 1960s, whereas the abundance of Bosmina, typically a planktonic taxon, increased in the 1990s concurrently with the biomass recovery of haplochromine cichlid fishes. Overall, our results demonstrate substantial changes over the past century in the biomass structure and taxonomic composition of Mwanza Gulf phytoplankton and zooplankton communities, providing a historical food web perspective that can help understand the recent changes and inform future resource management decisions in the Lake Victoria ecosystem.

Interactions such as mutualism and facilitation are common in ecosystems established by foundation species; however, their outcomes vary and show conditionality. In a Mexican Caribbean Bay, a seagrass-coralline algae (rhodoliths) mutualism protects the seagrass Thalassia testudinum from green turtle overgrazing. We postulate that the state of the seagrass meadow in this bay depends on the strengths of the interactions among seagrasses, green turtles, and coralline algae. Spatio-temporal changes through satellite imagery showed rhodolith bed developed rapidly from 2009 (undetected) to 2016 (bed of 6934 m²). Typically, such rapid expansion of the rhodoliths does not occur in seagrass meadows. An in situ growth experiment of coralline algae showed that a combination of reduction in light and wave movement (usual in dense seagrass meadows) significantly reduced their growth rates. In the rhodolith beds, the growth rates of the coralline algae Neogoniolithon sp. and Amphiroa sp. were high at 9.5 mm and 15.5 mm per growth tip y⁻¹, respectively. In a second experiment, we found lower mortality in coralline algae within a rhodolith bed compared to algae placed outside the bed, likely explained by the reduced resuspension that we found in a third experiment, and this positive feedback may explain the high population increase in the rhodoliths, once established when the turtles grazed down the seagrass canopy. Therefore, the grazing-protection mutualism between seagrasses and coralline algae is thus conditional and came into existence under a co-occurrence of intensive grazing pressure and rapid population growth of coralline algae facilitated by positive feedback from increased growth and reduced sediment resuspension by the dense rhodolith bed.

Interactions between tropical cyclones and wildfires occur widely and can tip closed forests into open-canopy structures that initiate a ‘grass–fire’ cycle. We examined cyclone–fire interactions in a monodominant tropical montane pine forest in the Dominican Republic using remotely-sensed imagery to quantify damage from fires between 1986 and 2004, a category 1 cyclone in 1998, and an extensive wildfire in 2005. We also measured forest structure and composition 14.7 years after the 2005 fire. The area inside the 2005 burn scars (fire perimeters) totaled 25,206 ha, of which 81% burned and 14% was cyclone damaged. Cyclone damage made the fire markedly more extensive and severe—high-severity fires were > 3 times more frequent with high-severity cyclone damage than no cyclone damage—but these markedly synergistic effects were restricted to areas that had not burned for at least 19 years before the 2005 fire. Though earlier fires from 1986 to 2004 were small and low-severity, they were sufficient, when present, to prevent high-severity fire in 2005 irrespective of cyclone severity. In areas with strong cyclone–fire interactions, there was a complete loss of pine canopies, yet these stands had abundant pine canopy recruitment by 2019 and showed no evidence of compositional shifts toward open-canopy structures with pyrogenic herbaceous understories, illustrating the resilience of this ecosystem to a range of cyclone–fire synergies. However, the future resilience of tropical montane pine forests to cyclone–fire synergies is uncertain as climate change increases the intensity of cyclones and frequency of drought-triggered fires in these ecosystems.

Global environmental change has redistributed earth’s biomass and the inputs and dynamics of basal detrital resources in ecosystems, contributing to the decline of biodiversity. Yet efforts to manage detrital necromass for biodiversity conservation are often overlooked or consider only singular resource types for focal species groups. We argue there is a significant opportunity to broaden our perspective of the spatiotemporal complexity among multiple necromass types for innovative biodiversity conservation. Here, we introduce an ecosystem-scale perspective to disentangling the spatial and temporal characteristics of multiple and distinct forms of necromass and their associated biota. We show that terrestrial and aquatic ecosystems contain a diversity of necromass types, each with contrasting temporal frequencies and magnitudes, and spatial density and configurations. By conceptualising an ecosystem in this way, we demonstrate that specific necromass dynamics can be identified and targeted for management that benefits the unique spatiotemporal requirements of dependent decomposer organisms and their critical role in ecosystem biomass conversion and nutrient recycling. We encourage conservation practitioners to think about necromass quantity, timing of inputs, spatial dynamics, and to engage with researchers to deepen our knowledge of how necromass might be manipulated to exploit the distinct attributes of different necromass types to help meet biodiversity conservation goals.

Hurricanes are extreme climatic events frequently affecting tropical regions such as the tropical dry forests (TDFs) in Mexico, where its frequency/intensity is expected to increase toward the year 2100. To answer how resistant is a Mexican tropical dry forest to a high-intensity hurricane, and if its degree of resistance was mediated by its conservation degree, we evaluated the effect of a category 4 hurricane over the tree community, soil nutrients, and soil enzymatic activity in two contrasting TDF ecosystems: Old-Growth Forest (OGF) and Secondary Forest (SF). In general, vegetation richness and diversity showed very high resistance one year after the hurricane, but several structural attributes did not, especially in the OGF where the tree mortality related to vegetation structure and spatial distribution of individuals was higher. Then, in the short term, SF vegetation appeared to be more resistant, whereas the OGF, with more biomass to lose, appeared to be more vulnerable. Conversely, most soil attributes showed low resistance in both stages, but especially in SF which could face more severe nutrient limitations. The response of TDF to high-intensity hurricanes, in terms of above- and belowground processes, was in part dependent on its disturbance level. Moreover, an increase in the intensity/frequency of hurricanes could lead this TDF toward a high nutrient limitation (especially by phosphorus) for the plants and consequently toward a loss of soil functioning, especially in the SF. This eventually could produce a severe degradation in fundamental attributes and functions of the ecosystem.

The function of tropical peatland as a carbon sink is a balance between peat accumulation and peat loss; however, various interacting factors are involved affecting this process. In this study, we collected and intensively radiocarbon dated peat cores from two peat domes, visualized their cross-sectional profiles of geochemical properties, and developed three macrocharcoal records from each peat dome. We find that the young (4500 y calBP) and shallow (6 m) coastal peat has a simple and linear age–depth relationship, showing stable accumulation of carbon during the late Holocene. In contrast, the older (ca. 40,000 y cal BP) and deeper (15 m) inland peat shows a more complex history, where we observed age reversals and hiatuses, likely caused by climate variability from the Last Glacial Maximum (LGM) to the Holocene. The charcoal record reveals a continuous presence of low-severity fire as indicated by charcoal morphotypes, though fire frequency increased after agriculture was established. An age reversal during the LGM was likely caused by a flood. Two periods of hiatuses occurred, each several millennia in length, at the end of the LGM and during the early Holocene. One cause of the hiatuses may have been a climatically halted peat formation from low precipitation and cooler climate during the LGM. Another cause may have been that severe fires consumed thousands of years of accumulated peat. If the hiatuses were entirely due to fire, the carbon released from these paleo-fire events (600 t C ha⁻¹) suggests several times the impact of the most intense modern peat fires.

Abstract

Decomposition is a key determinant of forest functioning, controlling nutrient and carbon cycling. Although litter-mixing effects on decomposition (that is, using mixtures of litter of different species) have been studied extensively, less is known about the indirect effects of modified microenvironments via overstory tree species mixing. To investigate the effects of tree species diversity on decomposition, we installed 384 standardized litterbags, filled with leaf litter of four broadleaved tree species with contrasting litter quality, in a large, 10-year-old tree diversity experiment. To quantify microenvironments, we used microclimate sensors, below-canopy rain gauges and measured soil characteristics. We then analysed indirect tree species diversity effects, that is, tree species richness effects on mass loss rates via tree species-induced alterations in the microclimate, throughfall and soil characteristics. We found that understory microenvironmental conditions indeed affect mass loss rates, with the main drivers differing among incubation stages. Predominantly soil phosphorus, but also vapour pressure deficit and throughfall amounts, was negatively associated with mass loss rates across litter types during the first 2 months of the decomposition process. After 6 months of the decomposition, soil moisture was found to be the key determinant positively affecting mass loss rates. In sum, our research contributes to a better understanding of the determinants of decomposition and shows an important pathway in which tree species diversity affects decomposition, via modified microenvironmental conditions acting via the soil, microclimate and throughfall.