Complex mountain terrain and disturbance history drive variation in forest aboveground live carbon density in the western Oregon Cascades, USA

Abstract

Forest carbon (C) density varies tremendously across space due to the inherent heterogeneity of forest ecosystems. Variation of forest C density is especially pronounced in mountainous terrain, where environmental gradients are compressed and vary at multiple spatial scales. Additionally, the influence of environmental gradients may vary with forest age and developmental stage, an important consideration as forest landscapes often have a diversity of stand ages from past management and other disturbance agents. Quantifying forest C density and its underlying environmental determinants in mountain terrain has remained challenging because many available data sources lack the spatial grain and ecological resolution needed at both stand and landscape scales. The objective of this study was to determine if environmental factors influencing aboveground live carbon (ALC) density differed between young versus old forests. We integrated aerial light detection and ranging (lidar) data with 702 field plots to map forest ALC density at a grain of 25 m across the H.J. Andrews Experimental Forest, a 6369 ha watershed in the Cascade Mountains of Oregon, USA. We used linear regressions, random forest ensemble learning (RF) and sequential autoregressive modeling (SAR) to reveal how mapped forest ALC density was related to climate, topography, soils, and past disturbance history (timber harvesting and wildfires). ALC increased with stand age in young managed forests, with much greater variation of ALC in relation to years since wildfire in old unmanaged forests. Timber harvesting was the most important driver of ALC across the entire watershed, despite occurring on only 23% of the landscape. More variation in forest ALC density was explained in models of young managed forests than in models of old unmanaged forests. Besides stand age, ALC density in young managed forests was driven by factors influencing site productivity, whereas variation in ALC density in old unmanaged forests was also affected by finer scale topographic conditions associated with sheltered sites. Past wildfires only had a small influence on current ALC density, which may be a result of long times since fire and/or prevalence of non-stand replacing fire. Our results indicate that forest ALC density depends on a suite of multi-scale environmental drivers mediated by complex mountain topography, and that these relationships are dependent on stand age. The high and context-dependent spatial variability of forest ALC density has implications for quantifying forest carbon stores, establishing upper bounds of potential carbon sequestration, and scaling field data to landscape and regional scales.

Keywords: Forest carbon; Forest management; Landscape heterogeneity; Lidar; Topography; Wildfire.

Fig. 1

Observed versus predicted aboveground live carbon (ALC). Gray points represent the mean observed and predicted ALC for each of 42 spatially independent groups of plots. 1:1 line in black. Predicted ALC values are back transformed from cubed root ALC in the regression model.

Fig. 2

Study area map of back transformed predicted aboveground live carbon (ALC). Streams depicted in blue. Note: cell size is 25 m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Aboveground live carbon (ALC) in relation to years since harvest and wildfire. Model fit (black solid line) and 95% confidence interval (gray dashed lines) are for best models of ALC in relation to years since harvest (YR_HARVEST) and years since wildfire (YR_FIRE) in 135 harvest units and 114 unmanaged sample polygons (see methods section for description of harvest units and polygon sampling).

Fig. 4

Variable importance plots for environmental variables from random forests models of ALC for 150 sample plots across the entire HJA landscape (upper left), 35 plots in managed forests (upper right), and 115 plots in unmanaged forests (lower right). See Table 1 for descriptions of environmental variables.

Fig. 5

Maps of aboveground live carbon (ALC) from lidar and sequential autoregressive (SAR) model predictions for the HJA study area. Dashed rectangles in maps on the left correspond to enlarged areas on the right. Map cell size is 25 m. Black areas denote partial harvests with variable tree retention that were excluded from SAR analysis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Boxplots of aboveground live carbon (ALC) in relation to TEMP, TPI30, TPI300, and CLAY for young managed and old unmanaged forests. Boxplots of aboveground live carbon (ALC) in relation to YR_HARV, YR_FIRE, and SLOPE for young managed forests; and YR_FIRE, RAD, TOPEX, and SILT in old unmanaged forests.

Fig. 6

Boxplots of aboveground live carbon (ALC) in relation to TEMP, TPI30, TPI300, and CLAY for young managed and old unmanaged forests. Boxplots of aboveground live carbon (ALC) in relation to YR_HARV, YR_FIRE, and SLOPE for young managed forests; and YR_FIRE, RAD, TOPEX, and SILT in old unmanaged forests.