Meta-analysis of fertilization experiments indicates multiple limiting nutrients in northeastern deciduous forests

It is widely accepted that nitrogen limits primary production in temperate forests, although co-limitation by N and P has also been suggested, and on some soils Ca and base cations are in short supply. I conducted a meta-analysis to assess the strength of existing experimental evidence for limitation of primary production by N, P, and Ca in hardwood forests of the northeastern United States and southeastern Canada, using data from 35 fertilization experiments in deciduous forests on glaciated soils across the region. There is strong evidence for N limitation (formal meta-analysis weighted mean response ratio = 1.51, p \u3c 0.01; simple mean = 1.42, p \u3c 0.001). Forest productivity also tends to increase with additions of P (simple mean = 1.15, p = 0.05) and Ca (simple mean = 1.36 p \u3c 0.001). Across all treatments, 85% of response ratios were positive. Multiple-element additions had larger effects than single elements, but factorial experiments showed little evidence of synergistic effects between nutrient additions. Production responses correlated positively with the rate of N fertilization, but this effect was reduced at high rates of ambient N deposition

Mycorrhizal roots in a temperate forest take up organic nitrogen from 13C- and 15N-labeled organic matter

Background and Aims The importance of the uptake of nitrogen in organic form by plants and mycorrhizal fungi has been demonstrated in various ecosystems including temperate forests. However, in previous experiments, isotopically labeled amino acids were often added to soils in concentrations that may be higher than those normally available to roots and mycorrhizal hyphae in situ, and these high concentrations could contribute to exaggerated uptake. Methods We used an experimental approach in which we added 13C-labeled and 15N-labeled whole cells to root-ingrowth cores, allowing proteolytic enzymes to release labeled organic nitrogen at a natural rate, as roots and their associated mycorrhizal fungi grew into the cores. We employed this method in four forest types representing a gradient of soil pH, nitrogen mineralization rate, and mycorrhizal type. Results Intact uptake of organic nitrogen was detected in mycorrhizal roots, and accounted for at least of 1-14% of labeled nitrogen uptake. Forest types did not differ significantly in the importance of organic uptake. Conclusions The estimates of organic N uptake here using 13C-labeled and 15N-labeled whole cells are less than those reported in other temperate forest studies using isotopically labelled amino acids, and likely represent a minimum estimate of organic N-use. The two approaches each have different assumptions, and when used in tandem should complement one another and provide upper and lower bounds of organic N use by plants

Modeled nitrogen loading to Narragansett Bay: 1850 to 2015

Nutrient loading to estuaries with heavily populated watersheds can have profound ecological consequences. In evaluating policy options for managing nitrogen (N), it is helpful to understand current and historic spatial loading patterns to the system. We modeled N inputs to Narragansett Bay from 1850 to 2000, using data on population, human waste disposal, livestock, fertilizer, and atmospheric deposition. We found that total N loading to the bay increased 250% from 1850 to 2000, and 80% from 1900 to 2000. Loading to the upper bay increased far more than that to the lower bay, and the most important source shifted from non-point animal waste to human waste concentrated at sewage treatment facilities. We also modeled future N loads in 2015 under four management scenarios. Planned improvements in sewage treatment would reduce N loads 9% below business-as-usual, to the 1990 loading rate. Greater reductions, to circa 1900 rates of loading, may be possible

Mechanisms of nutrient limitation and nutrient acquisition in managed and unmanaged forest ecosystems

Understanding the interactions between global change, human and natural disturbances, and other factors on biogeochemical processes in forests is necessary to ensure the sustainability of forest management. Here I report the results of several investigations into nutrient acquisition processes in the forests of New Hampshire. I begin with a meta-analysis of fertilization studies showing that phosphorus (P) and calcium (Ca) as well as nitrogen (N) may limit primary production in deciduous forests of the region. Because these limiting nutrients are all removed from the ecosystem when trees are harvested, I compared nutrient budgets under a range of harvesting scenarios with a variety of soil nutrient stocks across a range of forest stands. I found that depletion of even long-term P and Ca soil stocks may occur over only a few rotations if intensive harvesting occurs in inappropriate stands. Key to successfully managing such budgets is a better understanding of the processes by which trees access limiting nutrients in primary minerals such as apatite. I conducted a greenhouse and field study examining the potential for lead isotope ratios and rare earth elements to serve as tracers of apatite weathering by mycorrhizal fungi in soils. In the greenhouse experiment, both of these tracers showed clear effects of biological systems (mycorrhizal and non-mycorrhizal birch seedlings) on the weathering rates of trace minerals including apatite. However, there were no clear trends in these tracers when examined in mycorrhizal sporocarps collected from forest stands that we hypothesized would differ in allocation to mycorrhizal weathering. If weathering can balance harvest losses of P and Ca, and N deposition continues its recent decline, N availability may constrain future productivity. I developed a novel tracer experiment intended to confirm earlier reports of the uptake of organic N compounds in temperate forests, which are richer in inorganic N than systems where this process is clearly demonstrated. I found low (≤16%) but significant contributions of organic compounds to the N nutrition of trees across a range of New Hampshire forest types. This research adds to our understanding of how forest ecosystems will respond to management and global change over the long term

Ice storm effects on the canopy structure of a northern hardwood forest after 8 years

Ice storms can cause severe damage to forest canopies, resulting in differential mortality among tree species and size classes and leading to long-lasting changes in the vertical structure and composition of the forest. An intense ice storm in 1998 damaged large areas of the northern hardwood forest, including much of the Hubbard Brook Experimental Forest, New Hampshire (USA). Following up on detailed poststorm assessments, we measured changes in the vertical structure of the forest canopy 8 years poststorm. We focused on how the presence of disease-induced advance regeneration of American beech (Fagus grandifolia Ehrh.) has affected canopy structure in the recovering forest. We measured foliage-height profiles using a point-quadrat approach and a pole-mounted leaf area index (LAI) sensor. Although the total LAIs of damaged and undamaged areas were similar, areas damaged in 1998 showed an increased proportion of total leaf area between 6 and 10 m above the ground. The foliage at this height is largely (54%) beech. To the extent that this heavily beech-dominated understory layer suppresses regeneration of other species, these findings suggest that rare disturbances of mature northern hardwood forests affected by beech bark disease will increase the importance of damage-prone and economically marginal beech

Validation and refinement of allometric equations for roots of northern hardwoods

The allometric equations developed by Whittaker et al. (1974. Ecol. Monogr. 44: 233–252), at the Hubbard Brook Experimental Forest have been used to estimate biomass and productivity in northern hardwood forest systems for over three decades. Few other species-specific allometric estimates of belowground biomass are available because of the difficulty in collecting the data, and such equations are rarely validated. Using previously unpublished data from Whittaker’s sampling effort, we extended the equations to predict the root crown and lateral root components for the three dominant species of the northern hardwood forest: American beech (Fagus grandifolia Ehrh.), yellow birch (Betula alleghaniensis Britt), and sugar maple (Acer saccharum Marsh.). We also refined the allometric models by eliminating the use of very small trees for which the original data were unreliable. We validated these new models of the relationship of tree diameter to the mass of root crowns and lateral roots using root mass data collected from 12 northern hardwood stands of varying age in central New Hampshire. These models provide accurate estimates of lateral roots (diameter) in northern hardwood stands \u3e20 years old (mean error 24%–32%). For the younger stands that we studied, allometric equations substantially underestimated observed root biomass (mean error \u3e60%), presumably due to remnant mature root systems from harvested trees supporting young root-sprouted trees

Terrestrial gastropod responses to an ecosystem-level calcium manipulation in a northern hardwood forest

The effects of acid deposition on soil calcium (Ca), and in turn on land snail populations, have been of heightened concern for several decades. We compiled a 10 year record (1997–2006) of gastropod abundance on two small watersheds at the Hubbard Brook Experimental Forest, one of which was treated with a Ca addition in 1999. In years 3–7 post Ca addition, snail abundance in the treated watershed was 73% higher than in the reference area (p \u3c 0.001); there was no significant difference in the 3 years prior to treatment, and no significant difference in slug abundance in any year. We analyzed relationships between snail density and microsite spatial variation in leaf-litter Ca concentration, litter-layer thickness, tree species composition, slope, dead wood, and forest-floor light level. We found that snail abundance was significantly correlated with litter Ca concentration (p \u3c 0.001) and negatively correlated with the importance value of American beech (p = 0.05). Isotopic-tracer analysis indicated that, on average, 76% of Ca in snail shells 5 years post treatment was derived from the added Ca. However, interannual variation in snail numbers indicates that other factors beyond available Ca have a strong influence on snail abundance

Scaling from single-point sap velocity measurements to stand transpiration in a multi-species deciduous forest: uncertainty sources, stand structure effect, and future scenarios impacts

ABSTRACT A major challenge in studies estimating stand water use in mixed-species forests is how to effectively scale data from individual trees to the stand. This is the case for forest ecosystems in the northeastern USA where differences in water use among species and across different size classes have not been extensively studied, despite their relevance for a wide range of ecosystem services. Our objectives were to assess the importance of different sources of variability ontranspiration upscaling and explore the potential impacts of future shifts in species composition on forest water budget. We measured sap velocity in five tree species (Fagus grandiflora, Acer rubrum, A. saccharum, Betula alleghaniensis, B. papyrifera) in a mature and young stand in NH (USA). Our results showed that the greatest potential source of error was radial variability and that tree size was more important than species in determining sap velocity. Total sapwood area was demonstrated to exert a strong controlling influence on transpiration, varying depending on tree size and species. We conclude that the effect of potential species shifts on transpirationwill depend on the sap velocity, determined mainly by radial variation and tree size, but also on the sapwood area distribution in the stand

Scaling from single-point sap velocity measurements to stand transpiration in a multi-species deciduous forest: uncertainty sources, stand structure effect, and future scenarios impacts

ABSTRACT A major challenge in studies estimating stand water use in mixed-species forests is how to effectively scale data from individual trees to the stand. This is the case for forest ecosystems in the northeastern USA where differences in water use among species and across different size classes have not been extensively studied, despite their relevance for a wide range of ecosystem services. Our objectives were to assess the importance of different sources of variability ontranspiration upscaling and explore the potential impacts of future shifts in species composition on forest water budget. We measured sap velocity in five tree species (Fagus grandiflora, Acer rubrum, A. saccharum, Betula alleghaniensis, B. papyrifera) in a mature and young stand in NH (USA). Our results showed that the greatest potential source of error was radial variability and that tree size was more important than species in determining sap velocity. Total sapwood area was demonstrated to exert a strong controlling influence on transpiration, varying depending on tree size and species. We conclude that the effect of potential species shifts on transpirationwill depend on the sap velocity, determined mainly by radial variation and tree size, but also on the sapwood area distribution in the stand

A comparison of presettlement and modern forest composition along an elevation gradient in central New Hampshire

Tree species composition is influenced not only by edaphic and climatic factors but also by natural and human-caused disturbances. To understand interactions among these influences, we compared forest species composition data from the time of European settlement with modern data. We derived elevation data for 2529 trees mapped by early land surveys (1770–1850) across a 1000 m elevation gradient in central New Hampshire and compared these with modern data (2004–2009) from the Forest Inventory and Analysis program (123 plots containing 2126 trees) and from permanent plots representing case studies of different land-use histories. Spruce and beech are much less abundant today at all elevations than they were prior to settlement, while maples and birches have increased. Fir, hemlock, pines, and oaks have changed little in distribution, although pines and oaks increased in abundance somewhat. Land-use history (agriculture below 500 m and cutting of various intensities at all elevations) is likely the primary explanation for these shifts, although climate change is also an important factor for some. A clearer understanding of presettlement forest composition improves our ability to separate the relative importance of natural and human-driven influences on the species composition of today’s forests