Effects of Climate Change

Direct effects of climate change on ecosystems and disturbance regimes

BNZ LTER research indicates that responses of ecosystems and disturbance regimes to climate variability and change in interior Alaska are controlled more by indirect and interactive effects than by the direct effects of climate warming alone. 

These responses are modulated by strong feedback controls of ecosystem structure on disturbance regimes, and the interaction of multiple disturbances across the landscape. For example, sensitivity of net primary production to warming appears to be more a function of soil moisture and soil nutrient availability, than to temperature per se

Vegetation has strong influences on soil moisture and thermal properties that regulate permafrost stability, interactions that affect and are affected by fire frequency and severity. Inventories across the BNZ Regional Site Network have quantified large, and somewhat surprising, variability in biophysical parameters and disturbance regimes both within and among ecosystem of varying ages across the landscape, while reinforcing our understanding of the driving role of landscape position and biological legacies on both. 

To expand our current understanding, we are focusing on quantifying patterns in the response of both ecosystems and disturbance regimes to climate change that are apparent across spatial and temporal scales. 

  • First, we are examining the spatial variability in species diversity, functional and life history traits and productivity across the Regional Site Network, as these community/ecosystem characteristics link across spatial scales. 
  • Second, we will examine temporal and spatial evidence for the widespread browning that has been documented via remote-sensing. 
  • Third, we will assess landscape vulnerability to change by quantifying the current spatial heterogeneity of disturbance regimes (fire and permafrost), and the temporal legacies, current severity, and distribution of these disturbances.

Credit: Jeremy Jones

Our research on direct climate change effects is organized around two hypotheses and four research tasks.

Hypothesis 1a: Climate change influences ecosystem structure and function at multiple temporal scales through effects on site conditions, key species and vegetation community types, transforming landscape structure and heterogeneity.

Task C1: Quantify the climate sensitivities of vegetation communities across multiple temporal and spatial scales, with an emphasis on relationships among plant species diversity, life history and functional traits, and productivity.

Alaska’s boreal forest constitutes the northern extent of the range for many species, and ongoing and future climate change across Alaska’s boreal forest is expected to influence the growth and interactions among species. Directional changes in mean climate or increases in climatic variation will likely have major implications for vegetation communities and ecosystem processes, resulting in loss or addition of species, shifts in phenology, and/or changes in community dynamics and productivity. 

The BNZ Regional Site Network provides a hierarchical framework for long-term monitoring of community processes, their sensitivity to changing climate, and their effects on ecosystem function across spatial and temporal scales. 

  • First, we are using the Regional Site Network to test the hypothesis that diversity increases resilience to perturbation at some spatial scales but may decrease resilience at other scales, and explore the role of non-vascular plants in driving these relationships. Non-vascular plants comprise large components of species diversity and productivity in boreal ecosystems but they are difficult to identify and are seldom included in community- or trait-based studies. We have complete vascular, and non-vascular diversity data or all Regional Site Network sites. Net primary production is being measured at a variety of scales using both field and remote sensing techniques. 
  • Second, we are monitoring the phenology of approximately 70 plant species across multiple sites, including recently introduced species, and combine these measurements with historical records based on herbarium specimens to evaluate how individual species will respond to changes in climate such as changes in season length, mean temperature, and amount and timing of precipitation. 
  • Third, we are quantifying a suite of plant functional traits to provide a mechanistic framework for explaining shifts in phenology and community processes across environmental conditions captured by the Regional Site Network.

Task C2: Determine the direct and interactive effects of climate sensitivity vs. intrinsic factors on wide-spread patterns of browning in the boreal forest.

Widespread declines in satellite indicators of productivity have fueled concerns about the health and carbon sink strength of boreal forests in Alaska and the circumpolar North. These signals of changing forest productivity are consistent with BNZ observations of declining radial growth of tree species in interior Alaska. 

Radioisotopes in tree wood link declines in Alaskan tree growth to recent climate warming across a surprisingly broad spectrum of the forest landscape. Forest inventory records in Canada suggest a wide range of boreal forests are vulnerable to increasing rates of tree mortality caused by drought stress, an emerging pattern in forests worldwide. However, patterns of tree mortality are also shaped by the autogenic processes of stand development through succession.

Confounding intrinsic processes of succession with impacts of external drivers is likely to cause biased estimates of climate effects on mortality, including both overestimates and underestimates. Here we are combining long-term records of tree growth and mortality from BNZ plots and other statewide data sets in productive, floodplain forests with our recently initiated Regional Site Network in upland forests to disentangle intrinsic vs. extrinsic drivers of forest productivity. Our approach integrates monitoring data with tree-ring records and demographic reconstruction of stand mortality to provide a basis for interpreting satellite records of changing forest productivity.

Credit: BNZ Photo Archive

Hypothesis 1b: Disturbance regimes that have shaped the boreal forest for millennia are changing due to increases in mean annual temperature, the length of the snow-free period, and the incidence of drought.

Task C3: Examine the relationship between climate and the spatial heterogeneity of fires (variation in burn area and severity, fragmentation of burn scars, composition of unburned islands) and determine what parts of the landscape are most vulnerable to reburn and vegetation change.

Plot-level data, satellite-based analyses and modeling studies suggest that recent changes in Alaska’s fire regime are promoting a shift towards a more deciduous forest. However, we have little information concerning how the conifer:hardwood ratio has shifted across the landscape due to changes in fire regime, which areas are most and least vulnerable to change, and the extent to which recent patterns of vegetation change are due to changes in fire regime characteristics other than burn severity. 

Our goal is to better understand where and when Alaska conifer forests are vulnerable to conversion to deciduous forest following wildfire by examining the role of a suite of fire regime characteristics, including past and current forest fragmentation, topographic position, fire seasonality, and fire-return interval. 

Landscape fragmentation is a legacy of past disturbance (both wildfire and human-caused), but also influences current fire behavior and the spatial heterogeneity of vegetation trajectories we see on the landscape today. LANDSAT imagery from the 1980s to present will be used to examine how post-fire vegetation has been driven by various components of the fire regime. 

We use daily MODIS hot spot locations to examine how seasonality within a burn perimeter influences post-fire vegetation on similar landscape types and are using plot-level data from two very different large fire years (2004, that burned late in the growing season and 2015, that burned early in the growing season) along with archived post-fire plot-level data dating back to 1950, to ground-truth pre- to post-fire conifer:hardwood ratios and assess whether there was significant change in post-fire vegetation that cannot be explained by topography and relay succession, indicating a true state-change as a result of climate-induced changes to the fire regime. Based on information from field and remote-sensing data, we will create maps of landscape vulnerability and determine the landscape components most susceptible to change.

Task C4: Examine how interactions between climate, fire severity, and landscape characteristics govern patterns of permafrost thaw and subsequent recovery.

Discontinuous permafrost in interior Alaska is heterogeneous at multiple scales (i.e., broad variations in permafrost thickness, finer scale distributions of ice content and closed and open taliks), and our ability to predict changes in permafrost regimes requires more information on this heterogeneity and how it interacts with ecological and hydrological systems. 

We are using a combination of vertical ground temperature monitoring, electrical resistivity tomography (ERT), seasonal thaw depth measurements, and permafrost coring to investigate spatial variation in permafrost thickness, spatial extent, and soil properties and ice content across a subset of RSN sites encompassing a range in vegetation cover, fire history, soil type, and geomorphology. 

Of primary interest is how fire severity influences both short- and long-term rates of thaw. We include sites that burned during the extreme fire seasons of 2015 and 2004, using a space-for-time substitution to examine the spatial variation in thaw depth and subsidence following fire. These data will be analyzed using understanding gained from Task C3. Within a single location, repeat ERT measurements can provide insight into the sustained impact of climatic variation, fire disturbance, and post-fire vegetation recovery on permafrost stability and dynamics (108). We are using these approaches to monitor changes in permafrost structure that occurred following a record flood in 2014 that led to large increases in active layer thickness. 

Together, the proposed research will allow us to understand where and when permafrost is most vulnerable to degradation, what landscape characteristics (vegetation, substrate composition and properties, geomorphology, aspects of ground or stream hydrology) have the potential to alter the sensitivity of permafrost to climate, as well as what controls the spatial nature of permafrost recovery following disturbance. We will then use the thermal and geophysical data in combination with GIS techniques to analyze permafrost distributions and identify which areas are most vulnerable to thaw and thermokarst development. Overlays with fire perimeter maps will allow us to make inferences about relationships between fire history and contemporary permafrost regimes.

Credit: Kevin Petrone

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