Creighton M. Litton

EFFECTS OF TREE DENSITY AND STAND AGE ON CARBON ALLOCATION PATTERNS IN POSTFIRE LODGEPOLE PINE

Validating the components of the carbon (C) budget in forest ecosystems is essential for developing allocation rules that allow accurate predictions of C pools and fluxes. In addition, a better understanding of the effects of natural disturbances on C cycling is critical, particularly in light of alterations to disturbance regimes that may occur with global climate change. However, quantitative data about how postfire differences in eco- system structure affect C allocation patterns are lacking. For this study, we examined how above- and belowground C pools, fluxes, and allocation patterns varied with fire-initiated differences in tree density and stand age in lodgepole pine stands in Yellowstone National Park of four forest types: low ( ,1000 trees/ha), moderate (7000-40 000 trees/ha), and high tree densities (.50 000 trees/ha) in 13-year-old stands, and in ;110-year-old mature stands. C pools in live biomass and detritus were estimated with allometric equations and direct sampling. Aboveground net primary productivity (ANPP) was estimated as aboveground biomass increment plus fine litterfall, and total belowground carbon allocation (TBCA) was estimated using a C balance approach. Our results indicate that the magnitude of C pools and fluxes varies greatly with fire-initiated differences in tree density and stand age. Coarse woody debris and mineral soil carbon accounted for the majority of total ecosystem C in young stands (91-99%), in contrast to mature stands where the largest amount of C was found in live biomass (64%). ANPP and TBCA increased with tree density (mean ANPP was 59, 122, and 156 g C·m 22 ·yr 21 , and TBCA was 68, 237, and 306 g C·m 22 ·yr 21 for low-, moderate-, and high-density young stands, respectively), and with stand age (ANPP was 218 g C·m 22 ·yr 21 and TBCA was 382 g C·m 22 ·yr 21 for 110-year-old stands). ANPP and TBCA were positively correlated, and both variables were well correlated with leaf area index. Notably, the ratio of TBCA to (TBCA 1 ANPP) remained remarkably constant (0.63-0.66) across extreme gradients of tree density and stand age, differing only slightly for the low-density young stands (0.54). These results suggest that C allocation patterns in a postfire lodgepole pine ecosystem are independent of tree density and stand age.

Comparison of modeling approaches for carbon partitioning: Impact on estimates of global net primary production and equilibrium biomass of woody vegetation from MODIS GPP

[1] Partitioning of gross primary production (GPP) to aboveground versus belowground, to growth versus respiration, and to short versus long‐lived tissues exerts a strong influence on ecosystem structure and function, with potentially large implications for the global carbon budget. A recent meta‐analysis of forest ecosystems suggests that carbon partitioning to leaves, stems, and roots varies consistently with GPP and that the ratio of net primary production (NPP) to GPP is conservative across environmental gradients. To examine influences of carbon partitioning schemes employed by global ecosystem models, we used this meta‐analysis‐based model and a satellite‐based (MODIS) terrestrial GPP data set to estimate global woody NPP and equilibrium biomass, and then compared it to two process‐based ecosystem models (Biome‐BGC and VISIT) using the same GPP data set. We hypothesized that different carbon partitioning schemes would result in large differences in global estimates of woody NPP and equilibrium biomass. Woody NPP estimated by Biome‐BGC and VISIT was 25% and 29% higher than the meta‐analysis‐based model for boreal forests, with smaller differences in temperate and tropics. Global equilibrium woody biomass, calculated from model‐specific NPP estimates and a single set of tissue turnover rates, was 48 and 226 Pg C higher for Biome‐BGC and VISIT compared to the meta‐analysis‐based model, reflecting differences in carbon partitioning to structural versus metabolically active tissues. In summary, we found that different carbon partitioning schemes resulted in large variations in estimates of global woody carbon flux and storage, indicating that stand‐level controls on carbon partitioning are not yet accurately represented in ecosystem models.

Leaf litter decomposition rates increase with rising mean annual temperature in Hawaiian tropical montane wet forests

Decomposing litter in forest ecosystems supplies nutrients to plants, carbon to heterotrophic soil microorganisms and is a large source of CO2 to the atmosphere. Despite its essential role in carbon and nutrient cycling, the temperature sensitivity of leaf litter decay in tropical forest ecosystems remains poorly resolved, especially in tropical montane wet forests where the warming trend may be amplified compared to tropical wet forests at lower elevations. We quantified leaf litter decomposition rates along a highly constrained 5.2 °C mean annual temperature (MAT) gradient in tropical montane wet forests on the Island of Hawaii. Dominant vegetation, substrate type and age, soil moisture, and disturbance history are all nearly constant across this gradient, allowing us to isolate the effect of rising MAT on leaf litter decomposition and nutrient release. Leaf litter decomposition rates were a positive linear function of MAT, causing the residence time of leaf litter on the forest floor to decline by ∼31 days for each 1 °C increase in MAT. Our estimate of the Q10 temperature coefficient for leaf litter decomposition was 2.17, within the commonly reported range for heterotrophic organic matter decomposition (1.5–2.5) across a broad range of ecosystems. The percentage of leaf litter nitrogen (N) remaining after six months declined linearly with increasing MAT from ∼88% of initial N at the coolest site to ∼74% at the warmest site. The lack of net N immobilization during all three litter collection periods at all MAT plots indicates that N was not limiting to leaf litter decomposition, regardless of temperature. These results suggest that leaf litter decay in tropical montane wet forests may be more sensitive to rising MAT than in tropical lowland wet forests, and that increased rates of N release from decomposing litter could delay or prevent progressive N limitation to net primary productivity with climate warming.

Ecosystem carbon storage does not vary with mean annual temperature in Hawaiian tropical montane wet forests

Theory and experiment agree that climate warming will increase carbon fluxes between terrestrial ecosystems and the atmosphere. The effect of this increased exchange on terrestrial carbon storage is less predictable, with important implications for potential feedbacks to the climate system. We quantified how increased mean annual temperature (MAT) affects ecosystem carbon storage in above- and belowground live biomass and detritus across a well-constrained 5.2 °C MAT gradient in tropical montane wet forests on the Island of Hawaii. This gradient does not systematically vary in biotic or abiotic factors other than MAT (i.e. dominant vegetation, substrate type and age, soil water balance, and disturbance history), allowing us to isolate the impact of MAT on ecosystem carbon storage. Live biomass carbon did not vary predictably as a function of MAT, while detrital carbon declined by ~14 Mg of carbon ha(-1) for each 1 °C rise in temperature - a trend driven entirely by coarse woody debris and litter. The largest detrital pool, soil organic carbon, was the most stable with MAT and averaged 48% of total ecosystem carbon across the MAT gradient. Total ecosystem carbon did not vary significantly with MAT, and the distribution of ecosystem carbon between live biomass and detritus remained relatively constant across the MAT gradient at ~44% and ~56%, respectively. These findings suggest that in the absence of alterations to precipitation or disturbance regimes, the size and distribution of carbon pools in tropical montane wet forests will be less sensitive to rising MAT than predicted by ecosystem models. This article also provides needed detail on how individual carbon pools and ecosystem-level carbon storage will respond to future warming.

Biology and impacts of Pacific island invasive species. 9. Capra hircus, the feral goat (Mammalia: Bovidae).

Abstract: Domestic goats, Capra hircus, were intentionally introduced to numerous oceanic islands beginning in the sixteenth century. The remarkable ability of C. hircus to survive in a variety of conditions has enabled this animal to become feral and impact native ecosystems on islands throughout the world. Direct ecological impacts include consumption and trampling of native plants, leading to plant community modification and transformation of ecosystem structure. Although the negative impacts of feral goats are well known and effective management strategies have been developed to control this invasive species, large populations persist on many islands. This review summarizes impacts of feral goats on Pacific island ecosystems and management strategies available to control this invasive species.

The Contemporary Scale and Context of Wildfire in Hawai'i

Abstract: Wildfire is a major threat to natural resources and native species in Hawai‘i, but the frequency and extent of wildfires across the archipelago has not been well quantified. Our objective was to summarize the available wildfire data for Hawai‘i and synthesize the social and ecological dimensions of wildfire drivers, impacts, and management responses. We constructed a 110-yr span of wildfire records for the state of Hawai‘i to examine historical trends (1904–2011) and summarized relationships between contemporary wildfire occurrence (2005–2011) and land use/land cover types and human population. Total area burned statewide increased more than fourfold from 1904 to 1959 to peaks in the 1960s–1970s and mid-1990s to present. From 2005 to 2011, on average, 1,007 wildfires were reported across the state per year (±77 SE), burning an average of 8,427 ha yr-1 (±2,394 SE). Most fires (95%) were <4 ha, while most area burned (93%) was attributed to fires ≥40 ha. Ignition frequency was positively correlated with human population across islands. Wildfires were most frequent in developed areas, but most areas burned occurred in dry nonnative grasslands and shrublands that currently compose 24% of Hawai‘i’s total land cover. These grass-dominated landscapes allow wildfires to propagate rapidly from areas of high ignition frequencies into the forested margins of the states watersheds, placing native habitat, watershed integrity, and human safety at risk. There is an urgent need to better assess fire risk and impacts at landscape scales and increase the integration of prefire planning and prevention into existing land management goals.

Future directions for forest restoration in Hawai‘i

Hawai‘i has served as a model system for studies of nutrient cycling and conservation biology. The islands may also become a laboratory for exploring new approaches to forest restoration because of a common history of degradation and the growing number of restoration projects undertaken. Approximately half of the native ecosystems of Hawai‘i have been converted to non-native conditions. Many restoration projects have focused on intensively managed out plantings of native plants with emphasis on threatened and endangered species. While these projects have been effective in stabilizing plant populations, this model is often prohibitively expensive for restoration at the scale needed to protect watersheds and provide habitat for rare bird species. Here we suggest ways of rethinking ecological restoration that are applicable across the tropics, particularly on islands and fire-prone grasslands. First, we suggest making use of non-native, non-invasive species to help reclaim degraded or invaded sites or as long-term components of planned restoration outcomes. Second, we suggest incorporating remote sensing techniques to refine where restoration is carried out. Finally, we suggest borrowing technologies in plant production, weed control, and site preparation from industrial forestry to lower restoration costs. These suggestions would result in ecosystems that differ from native reference systems in some cases but which could be applied to much larger areas than most current restoration efforts while providing important ecosystem services. We also stress that community involvement is key to successful restoration, as a major goal of almost all restoration projects is to re-connect the community with the forest.

Spatial and temporal variability of guinea grass (Megathyrsus maximus) fuel loads and moisture on Oahu, Hawaii

Frequent wildfires in tropical landscapes dominated by non-native invasive grasses threaten surrounding ecosystems and developed areas. To better manage fire, accurate estimates of the spatial and temporal variability in fuels areurgentlyneeded.Wequantifiedthespatialvariabilityinliveanddeadfinefuelloadsandmoisturesatfourguineagrass (Megathyrsus maximus) dominated sites. To assess temporal variability, we sampled these four sites each summer for 3 years(2008-2010)andalsosampledfuelloads,moisturesandweathervariablesbiweeklyatthreesitesfor1year.Liveand dead fine fuel loads ranged spatially from 0.85 to 8.66 and 1.50 to 25.74Mgha � 1 respectively, and did not vary by site or year. Biweekly live and dead fuel moistures varied by 250 and 54% respectively, and were closely correlated (P,0.05) with soil moisture, relative humidity, air temperature and precipitation. Overall, fine fuels and moistures exhibited tremendous variability, highlighting the importance of real-time, site-specific data for fire prevention and management. However, tight correlations with commonly quantified weather variables demonstrates the capacity to accurately predict fuel variables across large landscapes to better inform management and research on fire potential in guinea grass ecosystems in Hawaii and throughout the tropics.

Early post-fire succession in a Nothofagus glauca forest in the Coastal Cordillera of south-central Chile

The temperate deciduous species Nothofagus glauca (Phil.) Krasser exhibits characteristics commonly found in fire-adapted vegetation, yet the role of fire in the evolutionary history of the vegetation in south-central Chile has not been well investigated. We examined the effects of a wildfire on early succession in a Nothofagus glauca forest in the Coastal Cordillera of south-central Chile by comparing data from a burned forest to the vegetation in an adjacent, unburned stand.

Belowground impacts of perennial grass cultivation for sustainable biofuel feedstock production in the tropics

Perennial grasses can sequester soil organic carbon (SOC) in sustainably managed biofuel systems, directly mitigating atmospheric CO2 concentrations while simultaneously generating biomass for renewable energy. The objective of this study was to quantify SOC accumulation and identify the primary drivers of belowground C dynamics in a zero‐tillage production system of tropical perennial C4 grasses grown for biofuel feedstock in Hawaii. Specifically, the quantity, quality, and fate of soil C inputs were determined for eight grass accessions – four varieties each of napier grass and guinea grass. Carbon fluxes (soil CO2 efflux, aboveground net primary productivity, litterfall, total belowground carbon flux, root decay constant), C pools (SOC pool and root biomass), and C quality (root chemistry, C and nitrogen concentrations, and ratios) were measured through three harvest cycles following conversion of a fallow field to cultivated perennial grasses. A wide range of SOC accumulation occurred, with both significant species and accession effects. Aboveground biomass yield was greater, and root lignin concentration was lower for napier grass than guinea grass. Structural equation modeling revealed that root lignin concentration was the most important driver of SOC pool: varieties with low root lignin concentration, which was significantly related to rapid root decomposition, accumulated the greatest amount of SOC. Roots with low lignin concentration decomposed rapidly, but the residue and associated microbial biomass/by‐products accumulated as SOC. In general, napier grass was better suited for promoting soil C sequestration in this system. Further, high‐yielding varieties with low root lignin concentration provided the greatest climate change mitigation potential in a ratoon system. Understanding the factors affecting SOC accumulation and the net greenhouse gas trade‐offs within a biofuel production system will aid in crop selection to meet multiple goals toward environmental and economic sustainability.