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Forest composition, structure and productivity of browning and greening forests in Interior Alaska
Claudia
I
Czimczik, University Of California, Irvine, czimczik@uci.edu
(Presenter)
Nicole
Fiore, UC Irvine, nmfiore@uci.edu
Shawn
A
Pedron, UC Irvine, spedron@uci.edu
Clayton
D
Elder, Jet Propulsion Laboratory, cdelder@uci.edu
Michael
Goulden, University Of California, Irvine, mgoulden@uci.edu
Previous remote sensing analyses of boreal forest structure have yielded several key findings, including a widespread trend toward declining Normalized Difference Vegetation Indices (NDVIs), which suggests decreasing forest productivity, and large changes in surface reflectance and brightness temperature with and following wildfire, which implies large shifts in biophysical properties during fire recovery. These observations provide a powerful tool for assessing boreal forest structure and function at large spatial scales, but also underscore the challenge of linking remotely-sensed observations to the actual conditions on the ground. Direct field observations are needed to link remote sensing observations to biophysical properties, and to further explore recent possible declines in boreal productivity, as well as the biophysical changes during succession.
We surveyed the composition and structure of 20 forest stands in interior Alaska (63.8004 to 65.5720 N and 149.3965 to 149.9545 W) that over the past 30 years showed increasing, decreasing or no change in Landsat TIR, NDII, NDVI, Brightness, Greenness and, or Wetness. Some of these sites had recently burned, while others were considered old growth. We surveyed the forest at each site along 3-5 100-m transects, and assessed the forest structure, including tree height, diameter per canopy, vegetative cover (stratified by canopies and ground), fraction and type of standing and downed dead trees, and leaf area index (LAI). Data were recorded by tree species and ground cover type (moss, lichen, forb, grass). Fire history and productivity of each stand were assessed with increment cores or disks for 2-5 trees per species.
Key findings include:
1) Various remote sensing proxies from Landsat are tied to distinct biophysical properties. The local TIR relative to the scene average was correlated with canopy height. Wetness and NDII were correlated with canopy cover and LAI. The ratio of greenness or NDVI to wetness was correlated to the relative abundance of deciduous vs. evergreen foliage. These correlations proved useful for quantifying the biophysical changes with fire recovery.
2) Fire recovery can be distinguished into three stages: i.) A rapid increase in NDVI and greenness with an expansion of deciduous foliage, ii.) A gradual increase in wetness and decrease in brightness and TIR with an expansion of total LAI and a co-dominant evergreen and conifer canopy, and iii.) A gradual decrease in NDVI and greenness despite a high wetness and LAI as deciduous species senesce and evergreen dominate.
3) Some of the recent NDVI decline in the area appears to reflect this mid to late succession transition from high NDVI, deciduous stands to low NDVI, evergreen stands. Several of the "browning" stands we visited had a healthy evergreen overstory that had recently overtopped a senescent deciduous stratum. However, “browning” was also observed in mature forests transitioning to sphagnum moss-dominated peat plateaus or recently burnt forests, where permafrost thaw and collapse promoted shrubs, and “greening” in a forest stand that had been recently drained by regional flood-control projects.
Together, our data show that fire history and changes in hydrology strongly modify the reflective properties of boreal forests. Thus, quantifying effects of climate change on these forests requires knowledge of typical succession trajectories in relation to aspect, and drainage.
Presentation:
ASTM4_Poster_Czimczik_63_64.pdf (1272k)
Associated Project(s):
Poster Location ID: 63
Session Assigned: Vegetation Dynamics and Distribution
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