Climate Disturbance

Section II. 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.

This research centers around three hypotheses:

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.

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.

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.

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 1: 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:

  1. Dominant boreal tree species and their mycorrhizal symbionts
  2. Understory vascular species and their mycorrhizal symbionts
  3. Mosses and their nitrogen fixing microbiomes
  4. Alders and their N-fixing actinorhizal symbionts

Task 2: 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.

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 have 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 3: 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 overarching 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.

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 4: 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.

We are coupling ongoing measurements of streamflow with patterns of summer precipitation using collectors located in sub-catchments of the Caribou-Poker Creek Research Watershed (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 examine how changes in the ecohydrology of boreal forest will affect stream exports of carbon (C) and nitrogen (N). We are initiating data collection across the Regional Site Network to examine C 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 dissolved organic carbon 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 5: 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.

Task 6: 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:

  1. Upland tundra at the altitudinal treeline (CiPEHR/DryPEHR )
  2. Lowland black spruce forest (APEX). These experiments were established independently, and both manipulate aspects of permafrost and hydrology to understand controls over ecosystem function.

In this round of LTER research, we will integrate datasets to determine whether the partitioning of total C release into CO2 and CH4 responds similarly 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.

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 7: 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 8: 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 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 9: 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 its 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 10: 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.

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