Ecosystems最新文献

Ecosystem dynamics are shaped by plant adaptation to environmental stress, yet the conditions under which this occurs remain poorly understood. We developed a theoretical framework to predict how strategies used by tropical trees to cope with low-phosphorus (P) availability (that is, traits related to P uptake, and use) influence growth under P limitation. We then tested this framework against data on tree species in Borneo and a meta-analysis of results from pantropical nutrient addition experiments. Our theoretical framework predicts that plant traits associated with low-P environments, including enhanced P allocation to leaves, efficient P resorption, and root phosphatase activity, alleviate the negative effects of P scarcity more strongly for “inefficiently” growing plants, represented by large trees and old-growth forests, compared with saplings or secondary forests. In agreement with this prediction, changes in traits related to low-P environments increased the potential relative growth rate of large trees more than small trees in Borneo. Finally, theoretical expectation was supported by a meta-analysis which revealed stronger P limitation in saplings and secondary forests than in old-growth forests. Together, these findings provide a novel framework to interpret the relationship between resource constraints and plant performance and reinforce the importance of accounting for plant adaption to predict ecosystem responses to P limitation in tropical forests.

Abstract

Fishing-down-marine-food-webs has resulted in alarming declines of various species worldwide. Benthic rays are one examples of such overexploited species. On tidal flats, these rays are highly abundant and play an ecologically important role. They use tidal flats as refuge, feeding and resting grounds, during which they bury into the sediment, which results in sediment bioturbation. Changes in bioturbation intensity, following ray removal, may affect the biogeomorphology of tidal flats with possible cascading effects on the macrozoobenthic community. However, it is poorly understood how these indirect effects could influence ecosystem function. We therefore studied the geomorphic impact of benthic rays (specifically the pearl whipray/stingray Fontitrygon margaritella) on the tropical tidal flats of the Bijagós Archipelago, Guinea-Bissau, on a landscape scale. We investigated 1) bioturbation rates by rays using drone and ground surveys, 2) the spatial distribution of ray pits on multiple tidal flats, 3) the impact of rays on sediment properties and macrozoobenthos by experimental exclusion (15 months). Benthic rays bioturbated 3.7 ± 0.35% of the tidal flat’s sediment surface per day over one single 24-h period, which equals a complete top-sediment-surface turnover every 27 days. The spatial distribution of ray pits was affected by tidal flat geomorphology since pits decayed faster at areas exposed to strong hydrodynamic forces. Predator exclusion altered sediment properties, leading to changes in sedimentation (− 17%) and erosion (− 43%) rates. In addition, macrozoobenthic species composition changed, marked by an increase in Capitellidae worms and a greater biomass of Malacostraca over time. These changes indicated substantial effects of ray bioturbation on the biotic and geomorphic landscape of tidal flats. Overall, we conclude that changing abundances of benthic rays can have clear landscape-wide geomorphological effects on intertidal ecosystems. These indirect consequences of fisheries should be incorporated in integrative management plans to preserve tidal flats and connected ecosystems.

High-latitude warming is increasing soil temperatures and driving permafrost thaw, potentially altering soil nutrient conditions by enhancing microbial mineralization and making nutrients from previously frozen soils accessible for plant and microbial use. Increases in soil nutrient concentrations may alter plant community processes and, consequently, carbon (C) fluxes. We conducted an experiment in a boreal permafrost peatland, monitoring forest floor C flux and functional traits of the entire moss and vascular plant communities to the addition of nutrients at 20 and 40 cm soil depths and under closed and open canopy conditions. Plant functional trait responses were investigated at both community level (using community-weighted means) and intraspecific scales. Using fertilizer additions, we emulated nutrient increases at different depths in the soil profile, replicated at high and low canopy cover sites to assess the influence of light availability. Our results demonstrate rapid responses of vascular plant community-level traits as well as ecosystem respiration and gross primary productivity to fertilization treatments under low canopy cover, suggesting an influence of local environmental variation. We found that moss community-level traits played a more important role in mediating C flux response to nutrient fertilization than vascular plants but led to little change in C sink–source dynamics. This provides insight into existing ambiguities of the response of boreal C fluxes to increased nutrient availability following soil warming and permafrost thaw: Local environmental conditions and moss community can strongly mediate the response, whereas vascular plant communities may play a more minor role. However, our results suggest that these changes may not alter overall C sink–source dynamics of peatlands in the near term.

More than half of the forest area of the North German Lowlands is stocked with Scots pine-dominated forests, mostly plantations. Climate change suggests a declining suitability of Europe’s temperate zone for conifer plantations, but only a few studies have examined the long-term growth trends of Scots pine in relation to environmental and site factors in this region. We studied the radial growth patterns of Scots pine over the last 60 years at ten sites along a precipitation gradient (830–530 mm mean annual precipitation) from an oceanic to a subcontinental climate, analyzing the spatial and temporal variability of the climate sensitivity of growth to identify the main climatic factors influencing pine growth across this gradient, which covers a large part of the species’ tolerated precipitation range. Annual radial increment was sensitive to late-winter temperatures (February, March) and summer drought and heat (June–August), with sensitivity increasing from the oceanic to the drier continental sites. Warmer late-winter periods apparently have stimulated growth during the last decades, while the sensitivity to summer-drought has remained fairly stable. Until recently, the negative impact of warming summers on growth has been compensated by the positive effect of late-winter warming, resulting in stable (or increasing) growth trends. However, our comparison of the climate sensitivity across sites suggests that the drought effect compensation through winter warming will in future be limited by increasing drought exposure. Thus, future productivity declines are likely in the northern German lowlands despite warming winters, discouraging large-scale pine plantations in the face of climate warming.

While vascular plants in drylands can spatially self-organize and persist under climatic stress through gradual changes in patch attributes, dryland patch dynamics largely assumes bare soil between plants. Biological soil crusts (BSCs) are communities living in the soil surface of drylands and mediate water redistribution in space. BSCs often occur in patches of light cyanobacteria and dark-mixed aggregates; however, little is known about their spatial patterns and dynamics. Here, we investigate spatial attributes of BSC patches, their spatial interactions with vascular plants, and factors that drive variation in these attributes using ultra-high-resolution (1 cm) maps from UAV imagery across three ecoregions of the southwest United States. Our analysis showed that light cyanobacteria BSCs varied most in patch shape complexity with aridity, while dark-mixed BSCs varied most in abundance. The distribution of dark-mixed BSCs was strongly affected by the soil template (texture and calcareousness) and vascular plants. Light cyanobacteria BSCs and woody plants spatially aggregated with aridity, while slope enhanced the spatial association between BSC functional groups. We conclude that light cyanobacteria BSCs can likely persist under stress through patch shape alterations, while dark-mixed BSC patches may have a lower capacity to do so—corroborating that dark-mixed BSC abundance may decline under altered climatic regimes. Light cyanobacteria BSCs may also buffer the effects of aridity for other biota by promoting runoff. BSCs and vascular plants coordinate in space in response to resource availability, suggesting the need to consider self-organization of multiple unique assemblages to better predict dryland response to climate change.

Tropical forests are the world’s most structurally complex ecosystems, providing key functions like biomass accumulation, which is linked to this complexity. Tropical forests are also exposed to chronic, non-severe winds, yet their effect on forest structural complexity is understudied. Here we examine drivers of forest structural complexity in Puerto Rico with a particular focus on chronic wind exposure. We used airborne light detection and ranging data collected in 2016 to quantify canopy height and rugosity (variation in height) in ~ 20,000, 0.28 ha forested sites stratified by forest age. We used random forest models to analyze variation in canopy height and rugosity as a function of chronic wind exposure, forest age, mean annual precipitation, elevation, slope (in degrees), soil type, soil available water storage, and exposure to a previous hurricane. Canopy height was driven by precipitation, forest age, and chronic wind exposure, decreasing by 2.12 m (16%) on average in wind-exposed forests across all forest ages. Canopy height increased by 4.0 m (41%) on average in forests aged 25–66 years, and by 4.0 m between sites with 1000 and 2000 mm y⁻¹ precipitation. Canopy rugosity was driven by canopy height, precipitation, forest age, and elevation, increasing log-linearly with canopy height and precipitation, decreasing with elevation, and was highest in younger forests. Chronic wind exposure did not drive variation in canopy rugosity. Our results suggest that chronic wind exposure plays an integral role in limiting canopy height, potentially reducing aboveground carbon accumulation in older tropical forests.

Abstract

Quantifying uncertainty is important to establishing the significance of comparisons, to making predictions with known confidence, and to identifying priorities for investment. However, uncertainty can be difficult to quantify correctly. While sampling error is commonly reported based on replicate measurements, the uncertainty in regression models used to estimate forest biomass from tree dimensions is commonly ignored and has sometimes been reported incorrectly, due either to lack of clarity in recommended procedures or to incentives to underestimate uncertainties. Even more rarely are the uncertainty in predicting individuals and the uncertainty in the mean both recognized for their contributions to overall uncertainty. In this paper, we demonstrate the effect of propagating these two sources of uncertainty using a simple example of calcium concentration of sugar maple foliage, which does not require regression, then the mass of foliage and calcium content of foliage, and finally an entire forest with multiple species and tissue types. The uncertainty due to predicting individuals is greater than the uncertainty in the mean for studies with few trees—up to 30 trees for foliar calcium concentration and 50 trees for foliar mass and calcium content in the data set we analyzed from the Hubbard Brook Experimental Forest. The most correct analysis will take both sources of uncertainty into account, but for practical purposes, country-level reports of uncertainty in carbon stocks can safely ignore the uncertainty in individuals, which becomes negligible with large enough numbers of trees. Ignoring the uncertainty in the mean will result in exaggerated confidence in estimates of forest biomass and carbon and nutrient contents.

Ecosystem processes in rivers are thought to be controlled more by extrinsic than intrinsic factors, that is, the result of processes that occur upstream or within their watersheds. However, large floodplain rivers have a diverse assemblage of aquatic areas spanning gradients of connectivity with the main channel and internal controls may at times regulate long-term dynamics. When and where internal controls are important has not been widely explored in rivers. The Upper Mississippi River System (UMRS) provides a unique opportunity to assess regulation of ecosystem processes in a large floodplain river as water clarity has increased in several reaches over the last two decades. To better understand when and where intrinsic variables (for example, aquatic vegetation and common carp) and extrinsic variables (for example, upstream main channel total suspended solids (TSS) concentration and discharge) regulate water clarity, we describe 24-year trends of TSS in six study reaches of the UMRS. We evaluated the degree to which trends were shared across aquatic areas within each study reach and identified potential drivers of long-term TSS dynamics. Results varied across and within UMRS reaches, but common carp abundance was the strongest predictor in nearly all study reaches. Several models indicated associations with both intrinsic and extrinsic factors, and the marginal model r² values (0.26–0.61) suggest that additional environmental factors may have influenced water clarity. Knowledge of the degree to which intrinsic and extrinsic processes regulate water clarity is important for understanding and managing large, floodplain rivers worldwide.

Understanding how cool-season C₃ and warm-season C₄ grasses will respond to climate change is critical for predicting future ecosystem functioning in many grasslands. With warming, C₄ grasses are expected to increase relative to C₃ grasses, but alterations in the seasonal availability of water may also influence C₃/C₄ dynamics because of their distinct seasons of growth. To better understand how shifts in the seasonal availability of water can affect ecosystem function in a northern mixed-grass prairie in southeastern Wyoming, we reduced early season rainfall (April–June) using rainout shelters and added the amount of excluded precipitation later in the growing season (July–September), effectively shifting spring rainfall to summer rainfall. As expected, this shift in precipitation seasonality altered patterns of soil water availability, leading to a 29% increase in soil respiration and sustained canopy greenness throughout the growing season. Despite these responses, there were no significant differences in C₃ aboveground net primary production (ANPP) between the seasonally shifted treatment and the plots that received ambient precipitation, likely due to the high levels of spring soil moisture present before rainout shelters were deployed that sustained C₃ grass growth. However, in plots with high C₄ grass cover, C₄ ANPP increased significantly in response to increased summer rainfall. Overall, we provide the first experimental evidence that shifts in the seasonality of precipitation, with no change in temperature, will differentially impact C₃ versus C₄ species, altering the dynamics of carbon cycling in this geographically extensive semi-arid grassland.

Coral reefs are facing a constant barrage of human impacts, including eutrophication, overharvesting and climate change. While the local effects of overharvesting have been well-studied, regional nutrient loading from anthropogenic activities on land and global climate change-induced disturbances are increasing in magnitude and necessitating cross-scale multi-stressor approaches for coral reef ecology. Here, we expand on longstanding theory to develop an integrated multi-stressor framework for coral reefs. We show that: (i) The geometry of a simple, empirically motivated model suggests nutrients and harvesting can operate similarly, and synergistically, in driving shifts from coral- to algae-dominated reefs, resulting in clear context-dependent management implications; and (ii) this same geometry suggests climate-driven coral mortality can drive the presence of long transients and climate-driven alternate states, even in moderately impacted ecosystems. Reefs seemingly in a “safe space” based on individual stressors may in fact be much more susceptible to increasingly frequent storms and bleaching events in multi-stressor conditions. By integrating these findings with general ecological and theoretical concepts, we suggest that responses in benthic composition may act as “signatures of change” to multi-stressors, allowing us to develop a predictive and generalizable multi-stressor framework for coral reefs under global change. In line with this theory, we detail empirical evidence from Barbados of historical changes in reef composition and multi-stressor impacts within our framework. By bridging coral reef ecology and general ecological concepts, we can better understand ecosystem functioning and resilience in these important yet highly threatened systems.