Live Tree Carbon Research Articles

Estimates of live-tree carbon stores in the Pacific Northwest are sensitive to model selection

BackgroundEstimates of live-tree carbon stores are influenced by numerous uncertainties. One of them is model-selection uncertainty: one has to choose among multiple empirical equations and conversion factors that can be plausibly justified as locally applicable to calculate the carbon store from inventory measurements such as tree height and diameter at breast height (DBH). Here we quantify the model-selection uncertainty for the five most numerous tree species in six counties of northwest Oregon, USA.ResultsThe results of our study demonstrate that model-selection error may introduce 20 to 40% uncertainty into a live-tree carbon estimate, possibly making this form of error the largest source of uncertainty in estimation of live-tree carbon stores. The effect of model selection could be even greater if models are applied beyond the height and DBH ranges for which they were developed.ConclusionsModel-selection uncertainty is potentially large enough that it could limit the ability to track forest carbon with the precision and accuracy required by carbon accounting protocols. Without local validation based on detailed measurements of usually destructively sampled trees, it is very difficult to choose the best model when there are several available. Our analysis suggests that considering tree form in equation selection may better match trees to existing equations and that substantial gaps exist, in terms of both species and diameter ranges, that are ripe for new model-building effort.

Impacts of fire exclusion and recent managed fire on forest structure in old growth Sierra Nevada mixed-conifer forests

We re-sampled areas included in an unbiased 1911 timber inventory conducted by the U.S. Forest Service over a 4000 ha study area. Over half of the re-sampled area burned in relatively recent management- and lightning-ignited fires. This allowed for comparisons of both areas that have experienced recent fire and areas with no recent fire, to the same areas historically based on early forest inventories. Our results indicate substantially altered present forest conditions, relative to the 1911 data, and can largely be attributed to the disruption of the key ecosystem process for these forests, fire. For areas that burned recently there was a noticeable difference in forest structure based on fire severity. Current tree density and canopy cover in areas burned recently with moderate severity did not differ from 1911 estimates, while areas that burned recently with low severity or were unburned had higher tree density and canopy cover relative to the 1911 estimates. This emphasizes an important distinction with regard to using fire to restore forests, resting primarily on whether fires kill trees in the lower and intermediate canopy strata. Our results also demonstrate nearly a doubling of live tree carbon stocks in the present forest compared to the historical forest. The findings presented here can be used by managers and ecologists interested in restoring Sierra Nevada mixed conifer systems.

The carbon costs of mitigating high-severity wildfire in southwestern ponderosa pine

Forests provide climate change mitigation benefit by sequestering carbon during growth. This benefit can be reversed by both human and natural disturbances. While some disturbances such as hurricanes are beyond the control of humans, extensive research in dry, temperate forests indicates that wildfire severity can be altered as a function of forest fuels and stand structural manipulations. The purpose of this study was to determine if current aboveground forest carbon stocks in fire-excluded southwestern ponderosa pine forest are higher than prefire exclusion carbon stocks reconstructed from 1876, quantify the carbon costs of thinning treatments to reduce high-severity wildfire risk, and compare posttreatment (thinning and burning) carbon stocks with reconstructed 1876 carbon stocks. Our findings indicate that prefire exclusion forest carbon stocks ranged from 27.9 to 36.6 Mg C ha 1 and that the current fire-excluded forest structure contained on average 2.3 times as much live tree carbon. Posttreatment carbon stocks ranged from 37.9 to 50.6 Mg C ha 1 as a function of thinning intensity. Previous work found that these thinning and burning treatments substantially increased the 6.1 m wind speed necessary for fire to move from the forest floor to the canopy (torching index) and the wind speed necessary for sustained crown fire (crowning index), thereby reducing potential fire severity. Given the projected drying and increase in fire prevalence in this region as a function of changing climatic conditions, the higher carbon stock in the fire-excluded forest is unlikely to be sustainable. Treatments to reduce high-severity wildfire risk require trade-offs between carbon stock size and carbon stock stability.

High-severity wildfire effects on carbon stocks and emissions in fuels treated and untreated forest

Forests contain the world's largest terrestrial carbon stocks, but in seasonally dry environments stock stability can be compromised if burned by wildfire, emitting carbon back to the atmosphere. Treatments to reduce wildfire severity can reduce emissions, but with an immediate cost of reducing carbon stocks. In this study we examine the tradeoffs in carbon stock reduction and wildfire emissions in 19 fuels-treated and -untreated forests burned in twelve wildfires. The fuels treatment, a commonly used thinning ‘from below’ and removal of activity fuels, removed an average of 50.3 Mg C ha −1 or 34% of live tree carbon stocks. Wildfire emissions averaged 29.7 and 67.8 Mg C ha −1 in fuels treated and untreated forests, respectively. The total carbon (fuels treatment plus wildfire emission) removed from treated sites was 119% of the carbon emitted from the untreated/burned sites. However, with only 3% tree survival following wildfire, untreated forests averaged only 7.8 Mg C ha −1 in live trees with an average quadratic mean tree diameter of 21 cm. In contrast, treated forest averaged 100.5 Mg C ha −1 with a live tree quadratic mean diameter of 44 cm. In untreated forests 70% of the remaining total ecosystem carbon shifted to decomposing stocks after the wildfire, compared to 19% in the fuels-treated forest. In wildfire burned forest, fuels treatments have a higher immediate carbon ‘cost’, but in the long-term may benefit from lower decomposition emissions and higher carbon storage.

Forest carbon densities and uncertainties from Lidar, QuickBird, and field measurements in California

Greenhouse gas inventories and emissions reduction programs require robust methods to quantify carbon sequestration in forests. We compare forest carbon estimates from Light Detection and Ranging (Lidar) data and QuickBird high-resolution satellite images, calibrated and validated by field measurements of individual trees. We conducted the tests at two sites in California: (1) 59 km2 of secondary and old-growth coast redwood (Sequoia sempervirens) forest (Garcia–Mailliard area) and (2) 58 km2 of old-growth Sierra Nevada forest (North Yuba area). Regression of aboveground live tree carbon density, calculated from field measurements, against Lidar height metrics and against QuickBird-derived tree crown diameter generated equations of carbon density as a function of the remote sensing parameters. Employing Monte Carlo methods, we quantified uncertainties of forest carbon estimates from uncertainties in field measurements, remote sensing accuracy, biomass regression equations, and spatial autocorrelation. Validation of QuickBird crown diameters against field measurements of the same trees showed significant correlation (r = 0.82, P < 0.05). Comparison of stand-level Lidar height metrics with field-derived Lorey's mean height showed significant correlation (Garcia–Mailliard r = 0.94, P < 0.0001; North Yuba R = 0.89, P < 0.0001). Field measurements of five aboveground carbon pools (live trees, dead trees, shrubs, coarse woody debris, and litter) yielded aboveground carbon densities (mean ± standard error without Monte Carlo) as high as 320 ± 35 Mg ha− 1 (old-growth coast redwood) and 510 ± 120 Mg ha− 1 (red fir [Abies magnifica] forest), as great or greater than tropical rainforest. Lidar and QuickBird detected aboveground carbon in live trees, 70–97% of the total. Large sample sizes in the Monte Carlo analyses of remote sensing data generated low estimates of uncertainty. Lidar showed lower uncertainty and higher accuracy than QuickBird, due to high correlation of biomass to height and undercounting of trees by the crown detection algorithm. Lidar achieved uncertainties of < 1%, providing estimates of aboveground live tree carbon density (mean ± 95% confidence interval with Monte Carlo) of 82 ± 0.7 Mg ha− 1 in Garcia–Mailliard and 140 ± 0.9 Mg ha− 1 in North Yuba. The method that we tested, combining field measurements, Lidar, and Monte Carlo, can produce robust wall-to-wall spatial data on forest carbon.

Fire suppression and fuels treatment effects on mixed‐conifer carbon stocks and emissions

Depending on management, forests can be an important sink or source of carbon that if released as CO2 could contribute to global warming. Many forests in the western United States are being treated to reduce fuels, yet the effects of these treatments on forest carbon are not well understood. We compared the immediate effects of fuels treatments on carbon stocks and releases in replicated plots before and after treatment, and against a reconstruction of active-fire stand conditions for the same forest in 1865. Total live-tree carbon was substantially lower in modern fire-suppressed conditions (and all of the treatments) than the same forest under an active-fire regime. Although fire suppression has increased stem density, current forests have fewer very large trees, reducing total live-tree carbon stocks and shifting a higher proportion of those stocks into small-diameter, fire-sensitive trees. Prescribed burning released 14.8 Mg C/ha, with pre-burn thinning increasing the average release by 70% and contributing 21.9-37.5 Mg C/ha in milling waste. Fire suppression may have incurred a double carbon penalty by reducing stocks and contributing to emissions with fuels-treatment activities or inevitable wildfire combustion. All treatments reduced fuels and increased fire resistance, but most of the gains were achieved with understory thinning, with only modest increases in the much heavier overstory thinning. We suggest modifying current treatments to focus on reducing surface fuels, actively thinning the majority of small trees, and removing only fire-sensitive species in the merchantable, intermediate size class. These changes would retain most of the current carbon-pool levels, reduce prescribed burn and potential future wildfire emissions, and favor stand development of large, fire-resistant trees that can better stabilize carbon stocks.

An Exploratory, Post-Harvest Comparison of Ecological and Economic Characteristics of Forest Stewardship Council Certified and Uncertified Northern Hardwood Stands*

ABSTRACT As more forest entities worldwide consider pursuing Forest Stewardship Council (FSC) certification, a critical question remains on whether stand-level management impacts differ between certified and uncertified forests. To begin to answer this question, we measured forest structure on three FSC-certified stands, three uncertified stands, and six adjacent unharvested reference stands (12 stands total) composed primarily of sugar maple (Acer saccharum) on non-industrial private properties in central Vermont, USA. The certified and uncertified partial harvests reduced total tree biomass and live tree carbon storage by one-third compared to reconstructed pre-harvest conditions. Both treatments also contained significantly lower densities of saplings and some mid-size trees compared to non-harvested references due to similar impacts from harvesting. The net present value of merchantable sugar maple over 10 year projections was consistently lower on certified than uncertified stands, but this difference was insignificant at discount rates from 4–8%. The certified stands contained significantly greater total residual volumes of coarse woody debris (standing and downed) than uncertified stands, although the debris was smaller than that found in unmanaged mature forests. Overall, our data suggest that FSC-certified harvested stands in northern hardwood forests have similar sugar maple timber value, aboveground live tree carbon storage value, similar live tree structure, and greater residual coarse woody debris than uncertified harvested stands.

Strategies to estimate national forest carbon stocks from inventory data: the 1990 New Zealand baseline

SummaryAn estimate of live tree carbon stored in New Zealand forests at 1990 was made to partially satisfy New Zealand's international obligations under the Framework Convention for Climate Change. A national database was compiled of 4956 forest inventory plots measured as recently as possible to 1990. Plot biomass estimates were obtained by applying species allometric relationships derived from harvested stands. Forest areas and classes were taken from a 1987 national map of vegetation cover. Regularly spaced grids, based on an initial 1 km × 1 km grid, were overlaid on the total forest area and plots were tested for bias against site characteristics at the grid points. As grid point density and sample size increased, bias was minimal in regional sampling intensity and in total annual precipitation. Differences in mean elevation and annual temperature remained stable as grid point density increased, and showed little correlation with stem biomass. This sampling method gave a measure of precision not available from previous estimates.An efficient sample size to estimate the mean within a 5% level of precision (at 95% probability) required a sample of 574 plots selected from a 4‐km grid. This strategy generated a mean estimate for the 1990 New Zealand forest carbon biomass of 179.3 ± 4.9 Mg ha−1 (± SE), totalling 919.1 ± 25.1 Mt for the 5.1 million ha mapped forest area. The mean was 6–10% lower than previous estimates, and was within the range reported for other countries. Within forest classes, mean carbon biomass ranged from 105 Mg ha−1 in pure podocarp forest to 215 Mg ha−1 in mixed lowland podocarp–broadleaved–beech forest. Of the major taxa groups throughout the forest estate, beech (Nothofagus) contributed 60% of the national forest carbon biomass reservoir, 26.7% was in other hardwoods, 13.2% in conifers, and 0.1% in other taxa (e.g. tree ferns).