Margot W. Kaye

Nutrient and carbon dynamics in a replacement series of Eucalyptus and Albizia trees.

Tree plantations are an important component of tropical landscapes, providing wood, fuel, and perhaps carbon (C) sequestration. Primary production in wet tropical plantations is typically nutrient limited. In some Hawaiian Eucalyptus plantations, nitrogen (N) limitations to production are alleviated by intercropping N-fixing Albizia trees that may decrease available phosphorus (P). Thus, sustainable productivity and C sequestration may depend on species composition. We measured soil N and P availability and ecosystem N and C sequestration in a 17-yr-old replicated replacement series of Eucalyptus and Albizia in Hawaii. Species composition included pure plots of each species and four proportions of mixtures. Soil N availability increased with the proportion of Albizia in the plot, but soil P availability declined. Aboveground tree C accumulation showed a synergistic response to increasing percentage of Albizia, with the mixed stands having more tree C than pure stands of Eucalyptus or Albizia. In the top 50 cm of soil, total N and C increased linearly with percentage of Albizia. Stands with the highest percentage of Albizia had 230 g/m2 more soil N and 2000 g/m2 more soil C than stands without Albizia. Stable C isotope analyses showed that increased soil C resulted from differences in both tree-derived C and “old” sugarcane-derived C. Deeper soil C (50–100 cm) was a substantial fraction (0.36) of total soil C but did not vary among treatments. Our results demonstrate that tree species effects on nutrient and C dynamics are not as simple as monocultures suggest. Mixed-species afforestation increased tree and soil C accrual over 17 years, and N inputs may increase soil C storage by decreasing decomposition.

Expansion of forest stands into tundra in the Noatak National Preserve, northwest Alaska

AbstractTemperatures across the northern regions of North America have been increasing for 150 years, and forests have responded to this increase. In the Noatak National Preserve in Alaska, white spruce (Picea glauca [Moench] Voss) forests reach their northern limit, occurring primarily on well-drained sites and as gallery forests along streams. Rolling plateaus of tundra separate the white spruce forests into disjunct stands. We examined patterns of tree age, tree growth, and tree encroachment into tundra ecosystems in six stands along the Agashashok River. Warming over the past 150 years appears to have increased tree growth and resulted in forest expansion into adjacent tundra ecosystems. The forest/tundra ecotone shifted by about 80 to 100 m into the tundra in the past 200 years, as evidenced by declining maximum tree age with distance towards the tundra. The decadal-scale pattern of tree establishment at the farthest extent of trees into the tundra (the tundra-forest ecotone) correlated with the detr...

Fire history along environmental gradients in the Sacramento Mountains, New Mexico: Influences of local patterns and regional processes

Abstract Patterning in fire regimes occurs at multiple spatiotemporal scales owing to differences in scaling of local and regional influences. Local fire occurrence and behavior may be controlled largely by site factors, while regional climate and changes in human land use can synchronize fire timing across large areas. We examined historical patterns in fires during the past five centuries across gradients in forest types and physiography and in relation to regional climate variability and land use change in the Sacramento Mountains in southern New Mexico. Forest stand-level chronologies of fires were reconstructed for 19 pinyon-juniper, ponderosa pine, and mixed-conifer stands using fire-scar records in crossdated tree-ring series. The fire history documents both local and regional factors effected fire occurrences in stands. Lower-elevation stands recorded more frequent fire than higher-elevation stands, although there were not significant differences between means of fire frequencies from clusters of ponderosa pine and mixed-conifer stands. Mean fire intervals ranged from approximately 3 to 11 years in ponderosa pine sites to 4 to 14 years in mixed-conifer sites. Sites on the steeper west side of the range, where fire spread more readily between forest types, recorded significantly more frequent fire than sites on the more physiographically heterogeneous east side. Fires were also synchronized by regional factors. Fire occurrences and fire-free years are related to variability in both annual Palmer Drought Severity Indices and El Niño-Southern Oscillation events. Fire regimes in the stands were also profoundly effected by changes in human land use patterns, with fire cessation in all sites following intensive Euro-American settlement beginning in the 1880s.

INITIAL CARBON, NITROGEN, AND PHOSPHORUS FLUXES FOLLOWING PONDEROSA PINE RESTORATION TREATMENTS

Southwestern ponderosa pine forests were dramatically altered by fire regime disruption that accompanied Euro-American settlement in the 1800s. Major changes include increased tree density, diminished herbaceous cover, and a shift from a frequent low- intensity fire regime to a stand-replacing fire regime. Ecological restoration via thinning and prescribed burning is being widely applied to return forests to the pre-settlement condition, but the effects of restoration on ecosystem function are unknown. We measured carbon (C), nitrogen (N), and phosphorus (P) fluxes during the first two years after the implementation of a replicated field experiment comparing thinning and composite (thin- ning, forest floor fuel reduction, and prescribed burning) restoration treatments to untreated controls in a ponderosa pine forest in northern Arizona, USA. Total net primary productivity (260 g C·m 22 ·yr 21 ) was similar among treatments because a 30-50% decrease in pine foliage and fine-root production in restored ecosystems was balanced by greater wood, coarse root, and herbaceous production. Herbaceous plants accounted for ,20% of total plant C, N, and P uptake in the controls but from 25% to 70% in restored plots. Total plant N uptake was ;3 g N·m 22 ·yr 21 in all treatments, but net N mineralization was just one-half and two- thirds of this value in the control and composite restoration, respectively. Element flux rates in controls generally declined more in a drought year than rates in restoration treat- ments. In this ponderosa pine forest, ecological restoration that emulated pre-settlement stand structure and fire characteristics had a small effect on plant C, N, and P fluxes at the whole ecosystem level because lower pine foliage and fine-root fluxes in treated plots (compared to controls) were approximately balanced by higher fluxes in wood and her-

EFFECTS OF CONIFERS AND ELK BROWSING ON QUAKING ASPEN FORESTS IN THE CENTRAL ROCKY MOUNTAINS, USA

Elk browsing and conifer species mixing with aspen (Populus tremuloides Michx.) present current challenges to aspen forest management in the western United States. We evaluated the effects of conifers and elk browsing on quaking aspen stands in and near Rocky Mountain National Park using tree rings to reconstruct patterns of aspen establish- ment, growth, and mortality over the past 120 years. High conifer encroachment and elk browse were both associated with decreased aspen recruitment, with mean recruitment dropping over 30% from pure aspen to mixed stands and over 50% from low-browse to high-browse stands. Maximum aspen recruitment was lower in mixed stands than in pure stands with the same tree basal area. High levels of elk browsing were also associated with a 30% decrease in stand-level growth of aspen. Neither high conifer abundance nor elk browse affected the growth of individual trees or aspen mortality. Aspen establishment was negatively influenced by conifers and elk browsing; however, aspen growth and mortality appeared to be resilient to these two external influences. Overall, these results suggest that long-term preservation of aspen forests could be achieved by enhancing aspen recruitment.

An assessment of fire, climate, and apache history in the Sacramento mountains, New Mexico

Fire historians typically attribute the causes of temporal change in past fire regimes to climatic variation, human land use, or some combination of the two. Most long-term historical reconstructio...

Influence of elk grazing on soil properties in Rocky Mountain National Park

We used three 35-year exclosures to examine the effects of high elk populations on a variety of soil properties in three vegetation types: upland sagebrush, aspen, and meadow. Grazing and hoof action by elk significantly increased bulk density (from 0.87 kg/l ungrazed to 0.94 kg/l grazed), with greater effects on soils with fewer rocks. Grazing substantially reduced extractable calcium, magnesium, potassium and phosphorus in the sagebrush type, but not in the aspen or meadow types. The only grazing effect on pH came in aspen types, where grazing prevented aspen establishment, and kept soil pH about 0.7 units higher than under aspen inside the exclosures. Grazing had no overall effect on total soil C and N across all exclosures and vegetation types. The availability of soil nitrogen, indexed by in-field resin bags and net mineralization in soil cores, showed little overall effect of grazing. Limited data on soil leaching indicated a possibility of strong increases in nitrate leaching with grazing for an aspen vegetation type at one exclosure. Although we found little effect of grazing on soil N supply, we note that N fertilization doubled the production of grasses and shrubs; if grazing eventually led to changes in soil N supply, species composition and growth would likely change.

Aspen structure and variability in Rocky Mountain National Park, Colorado, USA

Elk, fire and climate have influenced aspen populations in the Rocky Mountains, but mostly subjective studies have characterized these factors. A broad-scale perspective may shed new light on the status of aspen in the region. We collected field measurements of aspen (Populus tremuloides Michx.) patches encountered within 36 randomly located belt transects in 340 km2 of Rocky Mountain National Park, Colorado, to quantify the aspen population. Aspen covered 5.6% of the area in the transects, much more than expected based on previously collected remotely sensed data. The distribution and structure of aspen patches were highly heterogeneous throughout the study area. Of the 123 aspen patches encountered in the 238 ha surveyed, all but one showed signs of elk browsing or had conifer species mixed with the aspen stems. No significant difference occurred in aspen basal area, density, regeneration, browsing of regeneration and patch size, between areas of concentrated elk use (elk winter range) and areas of dispersed elk use (elk summer range). Two-thirds of the aspen patches were mixed with conifer species. We concluded that the population of aspen in our study area is highly variable in structure and that, at a landscape-scale, evidence of elk browsing is widespread but evidence of aspen decline is not.

Interspecific variation in growth responses to climate and competition of five eastern tree species

Climate and competition are often presented from two opposing views of the dominant driver of individual tree growth and species distribution in temperate forests, such as those in the eastern United States. Previous studies have provided abundant evidence indicating that both factors influence tree growth, and we argue that these effects are not independent of one another and rather that interactions between climate, competition, and size best describe tree growth. To illustrate this point, we describe the growth responses of five common eastern tree species to interacting effects of temperature, precipitation, competition, and individual size using maximum likelihood estimation. Models that explicitly include interactions among these four factors explained over half of the variance in annual growth for four out of five species using annual climate. Expanding temperature and precipitation analyses to include seasonal interactions resulted in slightly improved models with a mean R2 of 0.61 (SD 0.10). Growth responses to individual factors as well their interactions varied greatly among species. For example, growth sensitivity to temperature for Quercus rubra increased with maximum annual precipitation, but other species showed no change in sensitivity or slightly reduced annual growth. Our results also indicate that three-way interactions among individual stem size, competition, and temperature may determine which of the five co-occurring species in our study could have the highest growth rate in a given year. Continued consideration and quantification of interactions among climate, competition, and individual-based characteristics are likely to increase understanding of key biological processes such as tree growth. Greater parameterization of interactions between traditionally segregated factors such as climate and competition may also help build a framework to reconcile drivers of individual-based processes such as growth with larger-scale patterns of species distribution.

Community assembly responses to warming and increased precipitation in an early successional forest

Experimental climate manipulations provide the opportunity to link predicted changes in climate to the process of community assembly. We studied plant community assembly of a recently harvested forest exposed to three years of experimental 2°C warming and 20% increased precipitation. By the end of the experiment, trees were the only functional group that shifted composition in response to warming and precipitation treatments (p = 0.03), while the composition of the grass, forbs, and shrub/small tree/vine functional groups were unresponsive. Individual species within groups were associated with specific treatments, but did not result in a predictable community composition shift. Temporal dynamics of functional group cover were more sensitive to treatment effects than single, static measures of plant community responses such as biomass. Both static and dynamic plant analyses revealed interactive effects of warming and increased precipitation on cover and biomass of grass and all plants together (grass cover p < 0.01, grass biomass p = 0.02, total cover p < 0.01, total biomass p = 0.05). Short forb cover was negatively affected by increased precipitation throughout our experiment (p = 0.03). Grass, tree, and shrub/small tree/vine functional groups showed independent year effects on cover that can be attributed to successional development of the forest community (all p ≤ 0.01). Random forest modeling indicated that cover of other plant functional groups and static plot-level variables such as plot location and components of soil texture were often the most important predictors of cover for a given functional group, while temperature and moisture availability measures were the least important. Importance of predictors of functional group cover varied greatly among random forest models from different treatments, suggesting that diverse environmental factors constrain functional group cover and may provide resilience of community assembly to climate change.