Dan Binkley

EXOTIC PLANT SPECIES INVADE HOT SPOTS OF NATIVE PLANT DIVERSITY

Some theories and experimental studies suggest that areas of low plant spe- cies richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m 2 subplots (20 1000-m 2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m 2 subplots (16 1000-m 2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1-m 2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m 2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m 2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m 2 plots was explained by the number of native plant species and the total percentage of soil carbon. At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant in- vasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

The Components of Nitrogen Availability Assessments in Forest Soils

The availability of nitrogen (N) limits production in many forest ecosystems, and many methods are available for estimating N availability (Keeney, 1980; Binkley 1986; Mahendrappa et al., 1986; Binkley and Vitousek, 1989). The concept of soil availability may represent the rate at which N is converted from unavailable to available forms within the rooting zone (Scarsbrook, 1965). Alternatively, it may refer to the extent to which plant production is constrained by a limited supply of available N. These two aspects of N availability were termed N supply rate and N limitation by Chapin et al. (1986). In agroecosystems, N supply rate and N limitation are often closely linked. In forest ecosystems, differences in species composition, stand age, and soil moisture may uncouple low N supply from N limitation (Chapin et al., 1986). In addition, the nonuniform rooting distribution of trees and the presence of forest floors add spatial complexities to forest N cycles that make it more difficult to estimate N availability in forests than in agroecosystems.

Foliage litter quality and annual net N mineralization: comparison across North American forest sites

Abstract The feedback between plant litterfall and nutrient cycling processes plays a major role in the regulation of nutrient availability and net primary production in terrestrial ecosystems. While several studies have examined site-specific feedbacks between litter chemistry and nitrogen (N) availability, little is known about the interaction between climate, litter chemistry, and N availability across different ecosystems. We assembled data from several studies spanning a wide range of vegetation, soils, and climatic regimes to examine the relationship between aboveground litter chemistry and annual net N mineralization. Net N mineralization declined strongly and non-linearly as the litter lignin:N ratio increased in forest ecosystems (r2 = 0.74, P < 0.01). Net N mineralization decreased linearly as litter lignin concentration increased, but the relationship was significant (r2 = 0.63, P < 0.01) only for tree species. Litterfall quantity, N concentration, and N content correlated poorly with net N mineralization across this range of sites (r2 < 0.03, P = 0.17–0.26). The relationship between the litter lignin:N ratio and net N mineralization from forest floor and mineral soil was similar. The litter lignin:N ratio explained more of the variation in net N mineralization than climatic factors over a wide range of forest age classes, suggesting that litter quality (lignin:N ratio) may exert more than a proximal control over net N mineralization by influencing soil organic matter quality throughout the soil profile independent of climate.

AN EXPERIMENTAL TEST OF THE CAUSES OF FOREST GROWTH DECLINE WITH STAND AGE

The decline in aboveground wood production after canopy closure in even-aged forest stands is a common pattern in forests, but clear evidence for the mechanism causing the decline is lacking. The problem is fundamental to forest biology, commercial forestry (the decline sets the rotation age), and to carbon storage in forests. We tested three hypotheses about mechanisms causing the decline in wood growth by quantifying the complete carbon budget of developing stands for over six years (a full rotation) in replicated plantations of Eucalyptus saligna near Pepeekeo, Hawaii. Our first hypothesis was that gross primary production (GPP) does not decline with stand age, and that the decline in wood growth results from a shifft in partitioning from wood production to respiration (as tree biomass accumulates), total belowground carbon allocation (as a result of declining soil nutrient supply), or some combination of these or other sinks. An alternative hypothesis was that GPP declines with stand age and that the decline in aboveground wood production is proportional to the decline in GPP. A decline in GPP could be driven be reduced canopy leaf area and photosynthetic capacity resulting from increasing nutrient limitation, increased abrasion between tree canopies, lower turgor pressure to drive foliar expansion, or hydraulic limitation of water flux as tree height increases. A final hypothesis was a combination of the first two: GPP declines, but the decline in wood production is disproportionately larger because partitioning shifts as well. We measured the entire annual carbon budget (aboveground production and respiration, total belowground carbon allocation [TBCA], and GPP) from 0.5 years after seedling planting through 6 1/2 years (when trees were ~25m tall). The replicated plots included two densities of trees (1111 trees/ha and 10 000 trees/ha) to vary the ratio of canopy leaf mass to wood mass in the individual trees, and three fertilization regimes (minimal, intensive, and minimal followed by intensive after three years) to assess the role of nutrition in shaping the decline in GPP and aboveground wood production. The forest closed its canopy in 1-2 years, with peak aboveground wood production, coinciding with canopy closure, of 1.2-1.8 kg C.m-2yr-1. Aboveground wood production declined from 1.4 kg C.m-2yr-1 at age 2 to 0.60 kg C.m-2yr-1 at age 6. Hypothesis 1 failed: GPP declined from 5.0 kg C.m-2yr-1 at age 2 to 3.2 kg C.m-2yr-1 at age 6. Aboveground woody respiration declined from 0.66 kg C.m-2yr-1 at age 2 to 0.22 kg C.m-2yr-1 at age 6 and TBCA declined from 1.9 kg C.m-2yr-1 at age 2 to 1.4 kg C.m-2yr-1 at age 6. Our data supported hypothesis 3: the decline in aboveground wood production (42% of peak) was proportionally greater than the decline in canopy photosynthesis (64% of peak). The fraction of GPP partitioned to belowground allocation and foliar respiration increased with stand age and contributed to the decline in aboveground wood production. The decline in GPP was not caused by nutrient limitation, a decline in leaf area or in photsynthetic capacity, or (from a related study on the same site) by hydraulic limitation. Nutrition did interact with the decline in GPP and aboveground wood production, because treatments with high nutritient availablity declined more slowly than did our control treatment, which was fertilized only during stand establishment.

Greater Soil Carbon Sequestration under Nitrogen-fixing Trees Compared with Eucalyptus Species

Forests with nitrogen-fixing trees (N–fixers) typically accumulate more carbon (C) in soils than similar forests without N–fixing trees. This difference may develop from fundamentally different processes, with either greater accumulation of recently fixed C or reduced decomposition of older soil C. We compared the soil C pools under N–fixers with Eucalyptus (non–N–fixers) at four tropical sites: two sites on Andisol soils in Hawaii and two sites on Vertisol and Entisol soils in Puerto Rico. Using stable carbon isotope techniques, we tracked the loss of the old soil organic C from the previous C4 land use (SOC4) and the gain of new soil organic C from the C3, N–fixer, and non–N–fixer plantations (SOC3). Soils beneath N–fixing trees sequestered 0.11 ± 0.07 kg m−2 y−1 (mean ± one standard error) of total soil organic carbon (SOCT) compared with no change under Eucalyptus (0.00 ± 0.07 kg m−2 y−1; P = 0.02). About 55% of the greater SOCT sequestration under the N–fixers resulted from greater retention of old SOC4, and 45% resulted from greater accretion of new SOC3. Soil N accretion under the N–fixers explained 62% of the variability of the greater retention of old SOC4 under the N–fixers. The greater retention of older soil C under N–fixing trees is a novel finding and may be important for strategies that use reforestation or afforestation to offset C emissions.

Does atmospheric deposition of nitrogen threaten Swedish forests

Abstract The health and productivity of forests is fundamentally important to many societies, and the culture and economy of Sweden are intimately linked with Swedens forests. Are the health and productivity of Swedens forests at risk from too much nitrogen from acid deposition? We evaluated this question by posing a number of specific questions, and synthesized information from the extensive research in Sweden on N deposition, fertilization and forest growth. We addressed the questions: Have Swedish forest soils acidified in recent decades? Are Swedish forest saturated with N, or do they have an excess of N? Will excessive N lead to forest decline? Will liming or vitalisation fertilization improve forest nutrition and health? We examined the ideas behind these questions, the available evidence, and whether the evidence supported or refuted the ideas. Several studies have documented reductions in soil pH (measured in water) in Swedish forests over periods of several decades. This acidification has been accompanied by increases in ionic strength of soil solutions, reduced base saturation and increased soil organic matter. The importance of each of these acidifying processes has not been investigated directly, but evidence supports a substantial role for each mechanism in at least some cases. Current expectations about rates of mineral weathering are not consistent with the evidence; rates of weathering appear to be greater than expected, and to differ depending on tree species. Swedish forests are not saturated with N; only a few stands near the southwest coast (with the highest deposition rates) show leaching losses of N that rival N deposition rates. Across all regions of Sweden, inadequate supplies of N limit forest growth. Within each region, some stands fail to respond to N fertilization (or respond with a decrease in growth), which is common for other forest types in other countries. Stands that have received heavy fertilization with N may become responsive to further fertilization with phosphorus (P) or base cations, and fertilization with trace amounts of boron (B) may be important on some soils. No evidence supports any widespread responsiveness of the forests to fertilization with other elements unless N is also added. Vitalisation fertilization (with non-N nutrients) has not demonstrated substantial improvements in tree growth, although most experiments have focused on N-limited sites rather than N-excess sites. Liming studies from Sweden and around Scandinavia indicate that forest health typically suffers after liming, including growth losses of 5 to 10% lasting one or more decades. The forests of Sweden averaged about 30% greater growth per hectare in the 1990s than in the 1950s, and extensive forest inventories show no indication of abnormal forest declines within this overall picture of improving growth. Hypotheses of declining forest growth as a function of the ratio of base cations to aluminum in soil solution can be tested with N fertilization experiments. The cation ratios uniformly declined with N fertilization, but growth typically increased, refuting the idea that cation ratios can represent changes in forest productivity. We conclude that no evidence supports the hypothesis of past or current deposition of N has reduced the health or productivity of Swedens forests; the opposite may have occurred. We also stress that several important questions cannot be addressed satisfactorily with current information. In particular, we recommend additional research on sites where N leaching rivals N deposition, elucidating mechanisms controlling N leaching and responses of trees to excessive levels of soil N.

Age-related Decline in Forest Ecosystem Growth: An Individual-Tree, Stand-Structure Hypothesis

Forest growth is important both economically (yielding billions of dollars of annual revenues) and ecologically (with respect to ecosystem health and global carbon budgets). The growth of all forests follows a predictable general trend with age. In young forests, it accelerates as canopies develop; it then declines substantially soon after full canopy leaf area is reached. The classic explanation for the decline in growth invoked the increasing respiration costs required to sustain the larger masses of wood characteristic of older forests. Direct measurements of respiration have largely refuted this hypothesis, and recent work has focused on stand-level rates of resource supply, resource use, and growth. We developed and tested a hypothesis at the scale of individual trees (in relation to stand structure) to explain this declining stand-level rate of stem growth. According to our hypothesis, changes in stand structure allow dominant trees to sustain high rates of growth by increasing their acquisition of resources and using these resources efficiently (defined as stem growth per unit of resource used); smaller, nondominant trees grow more slowly as a result of their more limited acquisition of resources and a reduced rate of growth per unit of resource acquired. In combination, these two trends reduce overall stand growth. We tested this hypothesis by comparing growth, growth per unit of leaf area, and variation among trees within plots in two series of plantations of Eucalyptus in Brazil and by estimating individual-tree rates of growth and use of light, water, and nutrients in a plantation of Eucalyptus saligna in Hawaii. Our results supported the individual-tree hypothesis. We conclude that part of the universal age-related decline in forest growth derives from competition-related changes in stand structure and the resource-use efficiencies of individual trees.

Fifty-year biogeochemical effects of green ash, white pine, and Norway spruce in a replicated experiment

Binkley, D. and Valentine, D., 1991. Fifty-year biogeochemical effects of green ash, white pine, and Norway spruce in a replicated experiment. For. Ecol. Manage., 40:13-25. Few long-term, replicated experiments are available to provide information on the effects of tree species on soil chemistry and ecosystem biogeochemistry. We examined replicated, 50-year-old plots of green ash (Fraxinus pennsylvanica Marsh), white pine (Pinus strobus L.), and Norway spruce [ Picea abies ( L. ) Karst. ] that had been planted in an abandoned agricultural field. The pHwater of the 0-5-cm soil layer under green ash was 4.6, compared with 4.2 under white pine and 3.8 under Norway spruce. The Norway spruce soil was substantially less-well buffered against further acidification than the soil under green ash, and contained less than half the quantity of exchangeable Ca 2 + + Mg 2 + + K + in the 0-15-cm depth. The decline in these cations under Norway spruce was accompanied by higher concentrations of exchangeable A13 +. The most important factor in the lower pH under Norway spruce was the greater acid strength of the soil organic matter, with a secondary role played by the higher saturation of the exchange complex with aluminum. Nitrogen mineralization in resin cores averaged 40 kg/ha under green ash, 84 kg/ha under white pine, and 56 kg/ha under Norway spruce. The only significant difference in litterfall biomass and chemistry was a greater content of aluminum and lower content of magnesium in litterfall in the Norway spruce plots relative to green ash plots. These major biogeochemical differences between tree species demonstrate the need for replicated experiments for assessing the mechanisms that drive long-term changes in ecosystems relative to differing species, management regimes, and atmospheric deposition.

Soil nutrient availability

Many methods have been developed for assessing the availability of soil nutrients, but for a variety of reasons none are universally applicable. In this chapter, we discuss the conceptual basis for measuring nutrient availability and describe the strengths and limitations of some of the methods for assessing nonagricultural soils. We also discuss methods for characterizing soil acidity, salinity and redox potential because they often control nutrient cycling and availability.

CHANGES IN SOIL CARBON FOLLOWING AFFORESTATION IN HAWAII

Afforestation in the tropics may sequester soil C and has been proposed as a management tool to aid in controlling rising levels of atmospheric CO2. We measured changes in soil C following afforestation of sugarcane fields with fast-growing Eucalyptus saligna (Sm.) plantations in Hawaii. Using stable C isotopes, we estimated the contributions to changes in total soil C that were due to the loss of C from the prior cane cultivation, and to the gain of C from the new Eucalyptus plantations. Total soil C 10–13 yr after afforestation was 114 and 113 Mg/ha, respectively, in the Eucalyptus and cane plantation. Eucalyptus increased total soil C in the 0–10 cm layer by 11.5 Mg/ha, but that was offset by a loss of 10.1 Mg/ha of cane-derived C from the 10–55 cm layer. The net effect on soil C of afforestation of cultivated lands depends not only on new C gained, but also on C lost from the previous management.