Scale-dependent climate-disturbance interactions as drivers of ecosystem and landscape change
A central focus of our research has been to develop a mechanistic understanding of the long-term ecological consequences of climate-driven increases in fire severity and its impacts on ecosystem structure and function. We are continuing to develop this work through ongoing ecosystem-scale studies of post-fire permafrost dynamics, tree seedling establishment, and plant-soil-microbial feedbacks that control carbon sequestration over succession. As we established the BNZ Regional Site Network, however, we discovered that our mechanistic model, largely ecosystem-scale in perspective, did not account for the surprising variability in ecosystem properties that we see across the landscape. Catastrophic events such as wildfires share characteristic non-linear behaviors that are often generated by cross-scale interactions and feedbacks among system elements. We suspect that these types of processes are creating surprises across the Regional Site Network: patterns that cannot easily be predicted based on information obtained at a single spatial or temporal scale. Our current research expands beyond the ecosystem-scale to focus on when and where cross-scale phenomena shape post-fire ecosystem structure, function, and spatial distributions.
Our research points towards four key cross-scale interactions or feedbacks we hypothesize to be important controls over ecological impacts of an intensifying fire regime at the landscape scale.
- First is the negative feedback between fire severity and flammability of dominant tree species, an effect that could dampen or even decouple the relationship between climate warming and fire.
- Second is the cross-scale interaction between patterns of fire severity and plant seed or mycorrhizal spore sources; increasing fire severity could decrease local seed and spore sources, making post-fire recruitment dependent upon long-distance dispersal, as well as seedbed characteristics. We expect that patterns of long-distance dispersal are highly sensitive to the spatial configuration of unburned stands on the landscape.
- Third is the population dynamics and movements of browsing animals; at a decadal time- scale, browsing of deciduous tree species could deflect severely burned ecosystems from deciduous successional trajectories, even if deciduous seedlings dominate in the first decade after fire.
- Fourth is the post-fire degradation of permafrost; in sites where permafrost temperature is a legacy of past climate, loss of the organic layer could lead to loss of permafrost, altering site drainage and coupling current carbon and nutrient dynamics to the material legacy of past ecosystems.
Diagram showing how tasks (notations in red) link components pertaining to climate-permafrost-hydrology interactions directly (red arrows) and via feedbacks (blue arrows), and how hypothesis 2b is tied to the other research themes.
Hypothesis 2a: Climate-driven changes in fire regime affect ecosystem structure and function through interactions between fire characteristics and vegetation or permafrost that alter the spatial connectivity of biophysical processes and the influence of past material legacies on present ecosystems.
Task D1: Examine the effects of plant and microbial propagule dispersion on post-fire establishment of key plant species and successional trajectories, and determine how propagule availability will respond to an altered fire regime.
We are exploring the relationship between fire severity and the relative importance of local regeneration vs. long-distance dispersal for the assembly of plant communities and their symbiotic microbiomes. We are using information obtained from Task C3 to identify sites with varying distance to unburned and lightly burned seed/spore sources, including unburned islands within a burn scar. We are focusing exploratory analysis on BNZ Regional Site Network sites that have burned within the past 15 years. In particular, we are building on more than a decade of work on our network of 2004 burned sites to explore this relationship by identifying distance to unburned seed/spore source for each site using maps of fire disturbance derived from remote sensing. In these sites, we are focusing our efforts on the establishment dynamics of four plant species or physiognomic groups that have key impacts on biogeochemical processes:
- Dominant boreal tree species and their mycorrhizal symbionts
- Understory vascular species and their mycorrhizal symbionts
- Mosses and their nitrogen fixing microbiomes
- Alders and their N-fixing actinorhyzal symbionts
Task D2: Examine the spatial patterning and strength of plant-herbivore interactions across the post-fire landscape in relation to plant growth, species dominance, successional pathway, and biogeochemical cycling.
With the recent establishment of the BNZ Regional Site Network, we are poised to extend our more-than-25 years of research on plant-herbivore interactions along the Tanana River to understand how vertebrate herbivory influences successional trajectories after fire. This research complements ongoing studies on wildlife population ecology initiated during the previous funding cycle, and fits well with current efforts to examine linkages between food webs and biogeochemical cycles in relation to climate-disturbance interactions across the region.
In particular, we move to examine the coupling of biogeochemical cycling and plant-herbivore interactions in the context of fluctuating browsing pressures (herbivore densities), which has strong feedbacks to plant demography, successional dynamics, and the sustainability of herbivore populations and their predators. We are measuring plant responses to herbivory in terms of regrowth, stem/species turnover, and plant primary and secondary chemistry. Biogeochemical effects will be examined by way of nitrogen supply, soil nitrogen composition, and carbon turnover (soil enzyme activities and litter decomposition).
These studies are being conducted across a browsing gradient based on moose and hare densities, and by expanding our long-term network of vertebrate herbivore exclosures within 10 burns scars spanning in age of 15-50 years. This design will help address questions regarding ecological services (sustainability of both plant and animal communities) as well as how plant communities recover from the combined effects of physical (fire) and biotic (herbivory) disturbances.
Task D3. Determine the consequences of a changing fire regime and fire-driven permafrost thaw for biogeochemical connectivity between past and present ecosystems.
Low severity burning of boreal forest and tundra ecosystems leaves a residual soil organic layer that is a biogeochemical legacy of one or more past fire cycles. In upland black spruce ecosystems characteristic of our Regional Site Network, this legacy can contribute >50% of ecosystem carbon and nutrient pools.
High severity burning, by contrast, erases the surficial legacy of past ecosystems, and exposes permafrost organic matter to decomposition and contemporary biogeochemical cycles. Loss of the organic layer also offers an opportunity for plant community reorganization; plant and microbial species that sequester propagules in the organic layer are killed and novel seedbeds are exposed, shifting the dominant recruitment mode towards long-distance dispersal. Increasing severity of burning of the soil organic layer thus catalyzes opposing shifts in the biogeochemical connectivity of past and present ecosystems.
Our over-arching goal is to understand the consequences of these shifts for net ecosystem carbon balance and carbon cycling feedbacks to climate. Because nutrients such as nitrogen strongly limit plant productivity in these ecosystems, we are particularly interested in how N released from thawing permafrost may couple past ecosystems to contemporary patterns of net primary productivity.
We are using upland and lowland chronosequences together with mechanistic biogeochemical studies in a subset of sites to address the following questions:
- How does fire severity, successional trajectory, time after fire, and landscape position affect the relative contribution organic layer vs. newly thawed permafrost C and N to contemporary ecosystem pools?
- How does decomposition of permafrost soil organic matter contribute to contemporary C cycling, and how does this vary across sites that differ in landscape position?
- In sites that thaw deeply after fire, do plants acquire N released by thawing permafrost and, if so, how do differences in plant functional traits or landscape position affect their contribution to net primary productivity?
To address question 1, we are continuing our work characterizing ecosystem carbon and nitrogen pools across sites in the BNZ Regional Site Network that represent chronosequences of ecosystem recovery after fire. By combining these results with our study of recent (2004) fire impacts on carbon and nitrogen pools, we will determine the net ecosystem balance of carbon and nitrogen across conifer and deciduous successional trajectories and the contribution of legacy carbon or nitrogen to those balances. We are also developing similar characterizations of carbon and nitrogen pools in the thermokarst chronosequence sites, where succession was similarly initiated by fire.
In a subset of upland and lowland sites that are intermediate- and mature-aged, we will use radiocarbon dating of the soil organic layer to confirm or modify mass-balance based estimates of legacy organic matter contributions. These data will be key for validation of model performance for post-fire succession. To address question 2, we will measure seasonal variation in the radiocarbon age of soil respired carbon dioxide (CO 2) and methane (CH4) across paired burned and unburned sites that vary in landscape position (ice-rich floodplain, valley bottom toe slope, and well-drained hillslope), where we continuously monitored soil temperature in the top 1.5 m since 2012.
By comparing seasonal change in age of C respired at the surface to depth of thaw in paired burned and unburned sites, we will determine the contribution of permafrost soil C to respiration dynamics. These data will be used to validate modeling of post-fire soil respiration dynamics. Plant acquisition of N from deep permafrost will be determined by the reach of both plant roots and their mycorrhizae, so to address question 3, we will survey plant root density, functional traits, and mycorrhizal symbiont identity by depth across a subset of young, intermediate, and old sites in both the Regional Site Network and thermokarst chronosequence. In replicate plots that are part of the C4 watershed of the Caribou-Poker Creeks Research Watershed, where stream nitrogen exports are measured (below) as well as deep soil temperature dynamics (above), we will employ an experimental addition of a 15N tracer to determine which plant species are capable of acquiring nitrogen from deep in the soil profile, at the face of thawing permafrost. We will also examine partitioning and retention of the tracer in the plant-soil system, and determine the allocation and C:N stoichiometry of plant-acquired tracer. These data will be used for validation of the new depth-stratified rooting and N dynamics in the combined Dynamic Organic Soil - Dynamic Vegetation Model - Terrestrial Ecosystem Model (DOS-DVM-TEM).
Alaska’s boreal forest is experiencing broad-scale hydrologic changes in the timing and forms of precipitation, frequency of drought, and river discharge. Over the past century, summer drought conditions have become more frequent with the climate shifting towards warmer conditions and reduced precipitation. The effects of changes in the timing and amount of precipitation on ecosystem functioning are modulated by the distribution of permafrost, landscape topography, infiltration rate, soil water holding capacity, and vegetation structure.
Permafrost has a dominant control on hydrology by forming an impermeable barrier that restricts subsurface flows to the shallow active layer of soil -- the shallow soil layer above permafrost that freezes and thaws each year. In upland catchments, degradation of permafrost may lead to drying of surface soil and reduced stream flow as the impermeable barrier recedes deeper in the soil profile.
In contrast, in lowland ecosystems, the combination of low gradient and subsidence caused by local permafrost degradation, serves to maintain saturated soils and may result in ponding of surface waters. Interactions between changing climate, vegetation, permafrost, and water availability will control ecosystem carbon and nitrogen cycling.
In upland landscapes, loss of permafrost and soil drying will likely lead to deepening flow paths through catchments, drying of soil, and increased aeration leading to loss of organic matter stored within soil. In contrast, thawing of lowland permafrost and impounding of water can reduce soil decomposition rate and stimulate production of methane. Lastly, while deepening of the active layer typically occurs gradually, thermokarst in areas of ice rich permafrost can cause abrupt thaw and collapse of the soil profile, which accelerate losses of soil carbon and nitrogen. How will vegetation and permafrost thaw interact with changing climate to alter water availability and surface hydrology within and across different landscapes throughout interior Alaska, and what are the feedbacks to landscape connectedness, ecosystem structure and function, and disturbance regimes?
Diagram showing how tasks (notations in red) link components pertaining to climate-trophic interactions directly (red arrows) and via feedbacks (blue arrows), and how hypothesis 2c is tied to the other research themes.
Hypothesis 2b: Vegetation, landscape setting, and soil drainage characteristics modulate the response of permafrost to climate warming, changes in hydrological connectivity resulting from permafrost thaw, and post-thaw changes in ecosystem function.
Task D4: Examine the interactions between changes in climate, permafrost, and vegetation on soil water retention, hydrologic partitioning, and stream export of C and N across upland boreal forest catchments.
Research examining the coupling among climate, permafrost, soils and vegetation with watershed hydrology and stream solute exports is focused in the Caribou-Poker Creeks Research Watershed (CPCRW) and builds on ~35+ years of stream flow and climate measurements. The CPCRW is the core site for the taiga domain of National Ecological Observatory Network (NEON) and will host a tower and aquatic array, which will provide infrastructure to examine solute fluxes at higher temporal resolution than currently possible.
We are coupling ongoing measurements of stream flow with patterns of summer precipitation using collectors located in sub-catchments of the CPCRW to characterize hydrologic partitioning and develop a model describing change in actual evapotranspiration and stream discharge with timing of precipitation and extent of permafrost. Sub-catchment measures of hydrologic partitioning will be coupled to plot scale measures using wells to collect soil and near-stream water to examining how changes in the ecohydrology of boreal forest will affect stream exports of C and N. We are initiating data collection across the Regional Site Network to examine carbon and nitrate (NO3) production and transport in soil, ground water, and streams. Data collection will focus on concentrations and isotopes of solutes in streams and waters contributing to streamflow, as well as measures of the production of DOC and NO3- in soil. Sampling will target locations of contrasting disturbance history, vegetative characteristics, drainages, and permafrost conditions, to provide a broad context for examining effects of vegetation shift from black spruce to deciduous stands. We are applying an existing process and mixing-based model to quantify relative contributions of NO3- in precipitation, and rates of nitrification and denitrification.
Task D5: Determine influences of vegetation and permafrost thaw on soil C storage and soil water retention and hydraulic properties.
Relationships between carbon storage and hydraulic properties in soils vary with soil organic content, texture, and disturbance history, and have the potential to change as permafrost thaw leads to the gradual deepening of the seasonally thawed active layer. We are evaluating properties of soil hydrology in relation to carbon stocks across a range of plant-water and permafrost-water relationships associated with the Regional Site Network.
We are quantifying depth dependent soil water infiltration and water retention curves in sites spanning the range of forest cover type, stand age, organic layer thickness, and soil texture offered by the RSN. Subsamples used in our soil hydrology assessments will be analyzed for C and organic matter concentrations. These data also support Task D6, as they will allow us to make predictions regarding how soil saturation and runoff response will vary as a function of disturbance and changing active layer conditions.
Task D6: Use global change experiments situated in contrasting upland and lowland ecosystems to determine ecosystem responses to changes in permafrost extent and surface hydrology.
We are using two long-term manipulative experiments that have been established in contrasting ecosystem types of the boreal domain:
- Upland tundra at the altitudinal treeline (CiPEHR/DryPEHR )
- Lowland black spruce forest (APEX). These experiments were established independently, and both manipulate aspects of permafrost and hydrology to understand controls over ecosystem function.
Though located in different settings, both represent trajectories of thaw in ice-rich permafrost, and both experiments are focused on similar key questions:
- Does warming and permafrost degradation cause a net release of C from the ecosystem to the atmosphere, and how does the magnitude change over years, decades, and centuries?
- What proportion of this C release is derived from old C that comprises the bulk of the soil C pool?
- How does water table position interact with warming to control old C losses and release of C to the atmosphere?
In this round of LTER research, we will integrate datasets to determine whether the partitioning of total C release into CO2 and CH4 responds similar to variation in water table position across the two experimental sites. Increasing CH4:CO2 ratios should occur as a result of anaerobic decomposition in saturated soils, and can have a significant climate effect because of the much higher greenhouse warming potential of CH4. Because permafrost thaw can have very different impacts of hydrology dependent on ice content, geomorphology, and soil characteristics, integrating data across all of our experimental treatments will provide information on C flux responses across a wide range of soil moisture conditions.
High densities of vertebrate herbivores in Alaska’s boreal forest and the associated evolution of chemical defense against browsing are believe to be linked with global variation in climate-driven fire at the end of the Pleistocene, resulting in high forage availability and intensity of selective browsing during winter across the Alaskan landscape.
The BNZ LTER has been studying mechanisms and consequences of plant-herbivore interactions for over three decades. We are now focusing on how changing disturbance regimes are influencing the population dynamics and movement patterns of vertebrate herbivores and their predators across the region, and how plant-browser interactions are shaping regional vegetation responses to environmental change. Changing disturbance regimes are also affecting the complex interactions among vertebrate and invertebrate herbivores and plant pathogens.
For example, browsing promotes the dominance of chemically-defended alders (N-fixing species), which influence N cycling, but the outbreak of a fungal stem canker on thin-leaf alder ( Valsa melanodiscus) has led to near-complete alder mortality in some riparian habitats. Quaking aspen, a preferred forage species for moose, is predicted to increase in abundance on drying slopes following high severity fires, but the fate of the aspen leaf miner (Phyllocnistis populiella, hereafter ALM), which has been at outbreak densities for over a decade in interior Alaska, is uncertain.
Other invertebrate herbivores, such as the willow leaf blotch miner ( Micurapteryx salicifolliela), can severely impact vertebrate forage species, but we are just beginning to study how moose and plant pests/pathogens interact, including feedbacks that may influence outbreak dynamics, such as whether bark stripping of aspen by moose fosters the spread of aspen cankers.
Hypothesis 2c: Climate- and disturbance-driven changes in trophic dynamics affect the population dynamics of plants and animals and ecosystem function by altering the abundance of key plant species.
Task D7: Characterize patterns and drivers of recent changes in regional distributions of key plant pathogens, assess pathogen effects on plant growth, community composition, and successional dynamics, and predict future impacts on ecosystem function at regional scales.
We collaborate with USDA State and Private Forestry and utilize state-wide aerial (USFS) and ground-based (USFS & LTER) monitoring efforts to assess how recent warming and associated plant drought stress are affecting key plant pathogens, and coordinate pathogen abundances with measurements of plant growth, community composition and stand structure across LTER and State permanent plots.
We are focusing on fungal cankers specific to trembling aspen and alder, because of the importance of these hosts in ecosystem function, their critical role in vegetation regime shifts following fire, and the apparent rapid spread of their fungal cankers. Related stem cankers are now rapidly spreading to infect Siberian, Sitka and red alder throughout the state. We will initiate new monitoring programs for these hosts in collaboration with the USFS, and quantify the effects of canker on N fixation by Siberian alder and associated rates of plant growth and successional patterns of C and N storage. We are helping coordinate a new effort to study and monitor the identities, distributions, and impacts of the suite of fungal cankers that are now increasing on aspen, and incorporate results into models that forecast regional impacts on herbivores, stand production dynamics, and climate feedbacks.
Task D8: Examine the direct and interactive effects of insect herbivores and vertebrate browsers on plant growth, biogeochemical cycling, and vegetation development in early successional stands.
This task is motivated by the desire to better understand the impact of insect herbivory within natural communities on the foraging of mammalian species highly valued as a subsistence resource. While vertebrate browsers clearly have strong direct effects on plant community structure and biogeochemical cycling in the boreal forest, the long-term impacts of insect herbivores are less well understood. In addition to direct effects on plant performance, which can be substantial during outbreaks, insect herbivores may indirectly affect patterns of browsing by mammals by reducing plant quality and slowing growth, leading to underappreciated interactive effects. Conversely, winter browsing by mammals appears to improve leaf quality, which could in turn have positive effects on insect herbivores. We are to continuing a manipulative experiment begun in 2012, in which mammal exclusion (via fencing) is crossed with insect suppression (via annual insecticide application). Dependent variables are plant community composition, cover, shrub growth, litter production, decomposition, and soil chemical and physical variables. Measurements of decomposition rates are planned for the future. The work builds on a history of LTER-related research that seeks to understand the influence of herbivores on boreal communities and ecosystems.
Task D9: Determine how post-fire stand age and area influence aspen’s susceptibility to insect herbivory and impact the population dynamics of an outbreak insect herbivore.
The aspen leaf miner feeds on epidermal leaf tissues, disrupting photosynthesis, reducing growth, and causing dieback of it primary host, quaking aspen. Although it is not unusual for aspen leaf miner populations to remain at high density for decades, the mechanisms sustaining such outbreaks are poorly understood. Between 2002 and 2011, the aspen leaf miner maintained high densities throughout interior Alaska. Damage declined in 2011, then rebounded in many areas in 2014, creating a patchwork of high and low density populations across interior Alaska. The current pattern of spatial variation in aspen leaf miner density is ideal for the investigation of environmental factors fostering outbreaks. We plan to expand an ongoing, long-term field study of aspen leaf miner populations near Fairbanks to a regional scale in order to relate variation in leaf miner oviposition, survivorship, fecundity, and leaf damage to environmental and host characteristics across stands ranging in age from <5 to 80 years. A central hypothesis is that aspen leaf miner survivorship and fitness increase with aspen stand age. The expression of both direct (chemical) and indirect (predator-mediated) defenses declines as aspen trees mature, and preliminary data indicate that the leaves of mature aspen trees produce more aspen leaf miner survivors of higher mass than the leaves of young trees. Larger-scale temporal and spatial patterns of outbreak timing and distribution will be investigated using data from annual aerial forest damage surveys conducted by the state of Alaska. A better understanding of the relationship between stand age and aspen leaf miner outbreak will assist in predicting future outbreaks. As the climate warms and wildfires increase in extent and intensity, pre-fire black spruce stands are predicted to convert to post-fire deciduous stands with increasing frequency. Large, aging stands of poorly defended trees could produce disproportionate numbers of aspen leaf miner individuals, fueling additional widespread outbreaks.
Task D10: Examine population dynamics of snowshoe hares and their spatial synchrony across a latitudinal boreal transect in relation to the abundance and space use of their primary mammalian predators.
Recently we have demonstrated the importance of predation and habitat structure as controls over snowshoe hare survival, and that their primary predator (Canada lynx) is capable of traversing vast distances and significant dispersal barriers. This observation corroborates the idea that a “travelling wave” of predator abundance could affect the spatial synchrony of hare population dynamics. Populations of snowshoe hares are being actively monitored via annual pellet counts or biannually by capture-mark-recapture techniques in four areas: Tetlin National Wildlife Refuge (Tok), BCEF (Fairbanks), Koyukuk/Nowitna NWR (Galena), and Gates of the Arctic NP (Wiseman), representing a ∼700km latitudinal gradient from the Canadian border to the central Brooks Range of Alaska. Across this gradient we also live-capture lynx using cage traps and modified foot snares. We collect morphometric and genetic data on each animal and outfit the animals with 350 g GPS2110L Iridium transmitters. The transmitter fix schedule can be controlled remotely which allows us to obtain detailed information on habitat use, travel rates, and activity patterns. We are currently expanding this effort to include additional wildlife refuges (Yukon Flats and Kanuti NWR), as well coordinated capture efforts with researchers in Yukon Territory, Canada. These efforts also involve cooperation with residents of rural communities across interior Alaska.