Measurements of sap flow in tap roots, lateral roots and stems, as well as stable isotope labeling techniques were used to determine the occurrence and relative magnitude of hydraulic lift in several individuals of nine co-occurring Brazilian savanna (Cerrado) tree species differing in life-history traits, and to assess physical and biological determinants of this process at the tree and ecosystem level. The occurrence of reverse sap flow observed in deciduous and brevideciduous species during the dry season was consistent with hydraulic lift. The evergreen species did not exhibit reverse flow. Consistent with their ability to carry out hydraulic lift, the brevideciduous and deciduous species had both shallow and tap roots (dimorphic root systems), whereas the evergreen species had mostly deep roots (monomorphic root systems). There was an inverse relationship between rates of reverse sap flow and seasonal loss of hydraulic conductivity in lateral roots, suggesting that hydraulic lift in Cerrado woody plants may help maintain the functionality of the lateral roots in exploring dry and nutrient-rich superficial soil layers without directly enhancing the amount of water uptake.
Dendroctonus mexicanus is polyphagous within the Pinus genus and has a wide geographical distribution in Mexico and Guatemala. We examined the pattern of genetic variation across the range of this species to explore its demographic history and its phylogeographic pattern. Analysis of the mtDNA sequences of 173 individuals from 25 Mexican populations allowed to us identify 53 geographically structured haplotypes. High haplotype and low nucleotide diversities and Tajima's D indicate that D. mexicanus experienced rapid population expansion during its dispersal across mountain systems within its current range. The nested clade phylogeographic analysis indicates that the phylogeographic pattern of D. mexicanus is explained by continuous dispersion among lineages from the Sierra Madre Occidental, the Sierra Madre Oriental and the Trans-Mexican Volcanic Belt. However, we also observed isolation events among haplotypes from the Cofre de Perote/Trans-Mexican Volcanic Belt/Sierra Madre Oriental and the Trans- Mexican Volcanic Belt/Sierra Madre del Sur, which is consistent with the present conformation of mountain systems in Mexico and the emergence of geographical barriers during the Pleistocene.
We reconstructed the stand structure and composition for two western Washington old-growth forest stands harvested around 1930 (named Fresca and Rail) from field and historical data. Both old-growth stands had a codominant or dominant 250-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) overstory with a few scattered older Douglas-fir. Western hemlock (Tsuga heterophylla (Raf.) Sarg.) was codominant at Rail but was predominantly in the mid-story and understory at Fresca. The Fresca site is now dominated by Sitka spruce (Picea sitchensis (Bong.) Carr.) and western hemlock, whereas Rail has a composition similar to the previous old-growth stand. Events taking place early in succession are probably responsible for the differences between the modern and historical stands. Accelerated restoration of old-growth structural diversity may be possible at both sites through repeated creation of artificial gaps, but Fresca will remain different from its historical composition.
We reconstructed a 10,500-yr fire and vegetation history of a montane site in the North Cascade Range, Washington State based on lake sediment charcoal, macrofossil, and pollen records. High-resolution sampling and abundant macrofossils made it possible to analyze relationships between fire and vegetation. During the early Holocene (> 10,500 to ca. 8000 cal yr BP) forests were subalpine woodlands dominated by Pinus contorta. Around 8000 cal yr BP, P.contorta sharply declined in the macrofossil record. Shade tolerant, mesic species first appeared ca. 4500 cal yr BP. Cupressus nootkatensis appeared most recently at 2000 cal yr BP. Fire frequency varies throughout the record, with significantly shorter mean fire return intervals in the early Holocene than the mid and late Holocene. Charcoal peaks are significantly correlated with an initial increase in macrofossil accumulation rates followed by a decrease, likely corresponding to tree mortality following fire. Climate appears to be a key driver in vegetation and fire regimes over millennial time scales. Fire and other disturbances altered forest vegetation at shorter time scales, and vegetation may have mediated local fire regimes. For example, dominance of P. contorta in the early Holocene forests may have been reinforced by its susceptibility to frequent, stand-replacing fire events.
Bottom-up scaling of net ecosystem production (NEP) and net biome production (NBP) was used to generate a carbon budget for a large heterogeneous region (the state of Oregon, 2.5x105 km2 ) in the Western United States. Landsat resolution (30 m) remote sensing provided the basis for mapping land cover and disturbance history, thus allowing us to account for all major fire and logging events over the last 30 years. For NEP, a 23-year record (1980-2002) of distributed meteorology (1 km resolution) at the daily time step was used to drive a process-based carbon cycle model (BiomeBGC). For NBP, fire emissions were computed from remote-sensing-based estimates of area burned and our mapped biomass estimates. Our estimates for the contribution of logging and crop harvest removals to NBP were from the model simulations and were checked against public records of forest and crop harvesting. The predominately forested ecoregions within our study region had the highest NEP sinks, with ecoregion averages up to 197 gC m-2 yr-1. Agricultural ecoregions were also NEP sinks, reflecting the imbalance of NPP and decomposition of crop residues. For the period 1996-2000, mean NEP for the study area was 17.0 TgC m-2 yr-1, with strong interannual variation (SD of 10.6). The sum of forest harvest removals, crop removals, and direct fire emissions amounted to 63 percent of NEP, leaving a mean NBP of 6.1 TgC m-2 yr-1. Carbon sequestration was predominantly on public forest land, where the harvest rate has fallen dramatically in recent years. Comparison of simulation results with estimates of carbon stocks, and changes in carbon stocks, based on forest inventory data showed generally good agreement. The carbon sequestered as NBP, plus accumulation of forest products in slow turnover pools, offset 51 percent of the annual emissions of fossil fuel CO2 for the state. State-level NBP dropped below zero in 2002 because of the combination of a dry climate year and a large (200 000 ha) fire. These results highlight the strong influence of land management and interannual variation in climate on the terrestrial carbon flux in the temperate zone.
Pollen and high-resolution charcoal records from three lakes were examined to reconstruct the vegetation and fire history of the Oregon Coast Range for the last 9000 years. The sites are located along a north to- south effective precipitation gradient and changes in vegetation and fire activity provided information on the nature of this gradient in the past. The relation of vegetation to climate change was examined at millennial timescales and the relation between fire and climate was examined on centennial timescales by comparing fire interval distribution and fire synchrony between sites. The pollen data indicate more fire-adapted vegetation during the early-Holocene period (c. 9000 to 6700 cal. yr BP), followed by a shift to forests with more fire-sensitive taxa in the mid, Holocene (c. 6700 cal. yr BP to 2700 cal. yr BP) and modern forest assemblages developing over the last c. 2700 years. Comparisons of fire-interval distributions showed that the time between fires was similar between two of the three combinations of sites, suggesting that the moisture gradient has played an important role in determining long-term fire frequency. However, century-scale synchrony of fire occurrence between the two sites with the largest difference in effective precipitation suggests that centennial-scale shifts in climate may have overcome the environmental differences between these locations. Asynchrony in fire occurrence between the sites with more similar effective precipitation implies that local conditions may have played an important role in determining fire synchrony between sites with similar long-term climate histories.
This report documents the history and results of the Iron Creek installation of the cooperative Levels-of-Growing-Stock (LOGS) study in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), over the period 1966–2006 (ages 19 to 59). This is a 1949 plantation on an excellent site, and is one of nine installations in the study. Results are generally consistent with those from other LOGS installations. Volume production of thinned stands increased with increased growing stock. Current volume growth shows no sign of decreasing, and is still about twice mean annual increment. On similar public lands, rotations considerably longer than indicated by conventional economic analyses could reduce land use conflicts and increase carbon sequestration while maintaining or increasing long-term timber outputs. Small plot size prevents further thinning, which would otherwise be desirable in some treatments. The principal future value of the data is for use (in combination with other data) in development of growth models.
This paper examines the past, present, and future use of the concept of historical range and variability (HRV) in land management. The history, central concepts, benefits, and limitations of HRV are presented along with a discussion on the value of HRV in a changing world with rapid climate warming, exotic species invasions, and increased land development. This paper is meant as a reference on the strengths and limitations of applying HRV in land management. Applications of the HRV concept have specific contexts, constraints, and conditions that are relevant to any application and are influential to the extent to which the concept is applied. These conditions notwithstanding, we suggest that the HRV concept offers an objective reference for many applications, and it still offers a comprehensive reference for the short-term and possible long-term management of our nation's landscapes until advances in technology and ecological research provide more suitable and viable approaches in theory and application.
Seedling establishment is an important factor dictating the altitudinal limits of treeline species. Factors that affect seedling mortality and survival, however, have yet to be fully characterized, especially for deciduous treeline species. Here we describe microsite characteristics of successfully established Betula litwinowii seedlings at the alpine-treeline ecotone. Possible harmful effects of sky exposure on seedling physiology (i.e., photoinhibition of photosynthesis) were also examined, as well as possible facilitative effects of co-occurring Rhododendron caucasicum shrubs on northern slopes and microtopographical depressions (mainly watercourses) in ridgetop meadows. On northern slopes, seedling density was highest in newly exposed soils, with 90 percent of the youngest seedlings <2 cm) occurring in patches of rocky, bare, or moss-covered soils within the Rhododendron thicket. R. caucasicum was not a significant source of shade for B. litwinowii, as most seedlings were established 0.25 to 0.5 m away from the nearest shrub, and shade cover generated by R. caucasicum was observed in only 1 percent of seedlings at midday. On ridgetops, density of B. litwinowii was sixfold higher inside microtopographical depressions compared to outside. Sky exposure of seedlings within depressions was similar to northern slopes, ranging from 50 to 87 percent. Across all micro sites, seedlings were most abundant under 70 to 87 percent sky exposure. This preference for open microsites, combined with the observation that sustained photoinhibition of photosynthesis (Fv/Fm < 0.65) was observed only in the most open microsites (i.e., >80 percent sky exposure), suggests that sky exposure is likely not a significant factor affecting seedling mortality in B. litwinowii, in contrast to results reported for conifer and broadleaf evergreen species at treeline. A higher photosynthetic capacity and a deciduous life history may provide both tolerance and avoidance to the physiological stresses associated with high sky exposure for B. litwinowii seedlings, and other factors, such as soil moisture, more likely account for successful establishment within microtopographical depressions and R. caucasicum thickets.
The historical range of variability (HRV) in landscape structure and composition created by natural disturbance can serve as a general guide for evaluating ecological conditions of managed landscapes. HRV approaches to evaluating landscapes have been based on age classes or developmental stages, which may obscure variation in live and dead stand structure. Developing the HRV of stand structural characteristics would improve the ecological resolution of this coarse-filter approach to ecosystem assessment. We investigated HRV in live and dead wood biomass in the regional landscape of the Oregon Coast Range by integrating stand-level biomass models and a spatially explicit fire simulation model. We simulated historical landscapes of the region for 1,000 years under pre-Euro-American settlement fire regimes and calculated biomass as a function of disturbance history. The simulation showed that live and dead wood biomass historically varied widely in time and space. The majority of the forests historically contained 500 to 700 Mg/ha (50 to 70 kg/m2) of live wood and 50-200 Mg/ha (5 to 20 kg/m2) of dead wood. The current distributions are more concentrated in much smaller amounts for both biomass types. Although restoring the HRV of forest structure is not necessarily a management goal for most landowners and managing agencies, departure from the reference condition can provide relative measure to evaluate habitat conditions for managers seeking to use forest structure as a means to maintain or restore ecosystem and species diversity.