Matthew A. Vadeboncoeur

Long-Term Integrated Studies Show Complex and Surprising Effects of Climate Change in the Northern Hardwood Forest

Evaluations of the local effects of global change are often confounded by the interactions of natural and anthropogenic factors that overshadow the effects of climate changes on ecosystems. Long-term watershed and natural elevation gradient studies at the Hubbard Brook Experimental Forest and in the surrounding region show surprising results demonstrating the effects of climate change on hydrologic variables (e.g., evapotranspiration, streamflow, soil moisture); the importance of changes in phenology on water, carbon, and nitrogen fluxes during critical seasonal transition periods; winter climate change effects on plant and animal community composition and ecosystem services; and the effects of anthropogenic disturbances and land-use history on plant community composition. These studies highlight the value of long-term integrated research for assessments of the subtle effects of changing climate on complex ecosystems.

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

It is widely accepted that N limits primary production in temperate forests, although colimitation by N and P has also been suggested, and on some soils, Ca and base cations are in short supply. I ...

From missing source to missing sink: Long-term changes in the nitrogen budget of a northern hardwood forest

Biogeochemical monitoring for 45 years at the Hubbard Brook Experimental Forest in New Hampshire has revealed multiple surprises, seeming contradictions, and unresolved questions in the long-term record of ecosystem nitrogen dynamics. From 1965 to 1977, more N was accumulating in living biomass than was deposited from the atmosphere; the “missing” N source was attributed to biological fixation. Since 1992, biomass accumulation has been negligible or even negative, and streamwater export of dissolved inorganic N has decreased from ∼4 to ∼1 kg of N ha–1 year–1, despite chronically elevated atmospheric N deposition (∼7 kg of N ha–1 year–1) and predictions of N saturation. Here we show that the ecosystem has shifted to a net N sink, either storing or denitrifying ∼8 kg of N ha–1 year–1. Repeated sampling over 25 years shows that the forest floor is not detectably accumulating N, but the C:N ratio is increasing. Mineral soil N has decreased nonsignificantly in recent decades, but the variability of these measurements prevents detection of a change of <700 kg of N ha–1. Whether the excess N is accumulating in the ecosystem or lost through denitrification will be difficult to determine, but the distinction has important implications for the local ecosystem and global climate.

BAAD: a biomass and allometry database for woody plants

Understanding how plants are constructed—i.e., how key size dimensions and the amount of mass invested in different tissues varies among individuals—is essential for modeling plant growth, carbon stocks, and energy fluxes in the terrestrial biosphere. Allocation patterns can differ through ontogeny, but also among coexisting species and among species adapted to different environments. While a variety of models dealing with biomass allocation exist, we lack a synthetic understanding of the underlying processes. This is partly due to the lack of suitable data sets for validating and parameterizing models. To that end, we present the Biomass And Allometry Database (BAAD) for woody plants. The BAAD contains 259 634 measurements collected in 176 different studies, from 21 084 individuals across 678 species. Most of these data come from existing publications. However, raw data were rarely made public at the time of publication. Thus, the BAAD contains data from different studies, transformed into standard units and variable names. The transformations were achieved using a common workflow for all raw data files. Other features that distinguish the BAAD are: (i) measurements were for individual plants rather than stand averages; (ii) individuals spanning a range of sizes were measured; (iii) plants from 0.01–100 m in height were included; and (iv) biomass was estimated directly, i.e., not indirectly via allometric equations (except in very large trees where biomass was estimated from detailed sub-sampling). We included both wild and artificially grown plants. The data set contains the following size metrics: total leaf area; area of stem cross-section including sapwood, heartwood, and bark; height of plant and crown base, crown area, and surface area; and the dry mass of leaf, stem, branches, sapwood, heartwood, bark, coarse roots, and fine root tissues. We also report other properties of individuals (age, leaf size, leaf mass per area, wood density, nitrogen content of leaves and wood), as well as information about the growing environment (location, light, experimental treatment, vegetation type) where available. It is our hope that making these data available will improve our ability to understand plant growth, ecosystem dynamics, and carbon cycling in the worlds vegetation.

Recovery from disturbance requires resynchronization of ecosystem nutrient cycles.

Nitrogen (N) and phosphorus (P) are tightly cycled in most terrestrial ecosystems, with plant uptake more than 10 times higher than the rate of supply from deposition and weathering. This near-total dependence on recycled nutrients and the stoichiometric constraints on resource use by plants and microbes mean that the two cycles have to be synchronized such that the ratio of N:P in plant uptake, litterfall, and net mineralization are nearly the same. Disturbance can disrupt this synchronization if there is a disproportionate loss of one nutrient relative to the other. We model the resynchronization of N and P cycles following harvest of a northern hardwood forest. In our simulations, nutrient loss in the harvest is small relative to postharvest losses. The low N:P ratio of harvest residue results in a preferential release of P and retention of N. The P release is in excess of plant requirements and P is lost from the active ecosystem cycle through secondary mineral formation and leaching early in succession. Because external P inputs are small, the resynchronization of the N and P cycles later in succession is achieved by a commensurate loss of N. Through succession, the ecosystem undergoes alternating periods of N limitation, then P limitation, and eventually co-limitation as the two cycles resynchronize. However, our simulations indicate that the overall rate and extent of recovery is limited by P unless a mechanism exists either to prevent the P loss early in succession (e.g., P sequestration not stoichiometrically constrained by N) or to increase the P supply to the ecosystem later in succession (e.g., biologically enhanced weathering). Our model provides a heuristic perspective from which to assess the resynchronization among tightly cycled nutrients and the effect of that resynchronization on recovery of ecosystems from disturbance.

Soil nitrogen affects phosphorus recycling: foliar resorption and plant-soil feedbacks in a northern hardwood forest.

Previous studies have attempted to link foliar resorption of nitrogen and phosphorus to their. respective availabilities in soil, with mixed results. Based on resource optimization theory, we hypothesized that the foliar resorption of one element could be driven by the availability of another element. We tested various measures of soil N and P as predictors of N and P resorption in six tree species in 18 plots across six stands at the Bartlett Experimental Forest, New Hampshire, USA. Phosphorus resorption efficiency (P < 0.01) and proficiency (P = 0.01) increased with soil N content. to 30 cm depth, suggesting that trees conserve P based on the availability of soil N. Phosphorus resorption also increased with soil P content, which is difficult to explain basdd on single-element limitation, butfollows from the correlation between soil N and soil P. The expected single-element relationships were evident only in the 0 horizon: P resorption was high where resin-available P was low in the Oe (P < 0.01 for efficiency, P < 0.001 for proficiency) and N resorption was high where potential N mineralization in the Oa was low (P < 0.01 for efficiency and 0.11 for proficiency). Since leaf litter is a principal source of N and P to the 0 horizon, low nutrient availability there could be a result rather than a cause of high resorption. The striking effect of soil N content on foliar P resorption is the first evidence of multiple-element control on nutrient resorption to be reported from an unmanipulated ecosystem.

Assessing growing season beginning and end dates and their relation to climate in Taiwan using satellite data

Due to the close relationship between climate and plant phenology, changes in plant phenological patterns have been used as a surrogate of climate change. We analysed Moderate Resolution Imaging Spectroradiometer (MODIS) images to investigate the onset, offset and length of growing season, as well as spatial and inter-annual patterns of Normalized Difference Vegetation Index (NDVI) across six types of vegetation/land use in Taiwan. Regression models indicate that temperature was moderately to strongly related to NDVI for each of the six vegetation/land-use types (coefficients of determination (R 2)  =  0.45–0.86). There was a 1–2 month lag time between changes in temperature and NDVI in the forests that are distributed in mid- to high-elevation areas, but not in low-elevation unirrigated fields, paddy fields and urban areas. The relationship between precipitation and changes in NDVI was only significant for unirrigated fields and urban areas (R 2  =  0.37–0.43). Growing season ended considerably earlier at low elevations than at high elevations, possibly because of the earlier start and more severe dry period in low-elevation areas, such that the length of the growing season was longer in the forests than in the unirrigated fields, paddy fields and urban areas.

Relating vegetation dynamics to temperature and precipitation at monthly and annual timescales in Taiwan using MODIS vegetation indices

To predict the responses of the timing, duration, and density of photosynthetically active plant cover to a changing climate, it is necessary to first quantitatively describe the relationships between temporal and spatial patterns of vegetation cover and both spatial and inter-annual variation in temperature and precipitation. We examined these relationships at multiple scales in Taiwan using monthly maximum composite values of MODIS-NDVI and MODIS-EVI between 2000 and 2012. The two vegetation indices were highly correlated to each other on a monthly basis for non-forest land-cover types, but correlated poorly in forests, probably due to the saturation of NDVI. However, the two indices were equally sensitive in detecting the onset and offset of growing season for all vegetation types. We found that EVI was positively related to both precipitation and temperature on a monthly timescale, although the relationships were not significant at the annual timescale. The much greater variation in monthly than in annual precipitation and temperature probably explains this difference. At low elevations, precipitation had a positive effect and temperature had a negative effect on EVI; however, at high elevations, which are mostly forested, both were positively related to EVI (although precipitation effects were not significant). We interpret this as evidence of water limitation of photosynthetic cover in the warmer, low-elevation parts of the island, whereas in the higher-elevation areas precipitation was usually adequate to satisfy evapotranspirative demand. This study illustrates the importance of examining the effects of precipitation and temperature on plant growth at a range of spatial and temporal scales. Specifically, finer spatial and temporal scales of analysis may better reveal climatic controls over vegetation growth than broader scales of analysis.

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 coll...

Local-Scale Carbon Budgets and Mitigation Opportunities for the Northeastern United States

Economic and political realities present challenges for implementing an aggressive climate change abatement program in the United States. A high-efficiency approach will be essential. In this synthesis, we compare carbon budgets and evaluate the carbon-mitigation potential for nine counties in the northeastern United States that represent a range of biophysical, demographic, and socioeconomic conditions. Most counties are net sources of carbon dioxide (CO2) to the atmosphere, with the exception of rural forested counties, in which sequestration in vegetation and soils exceed emissions. Protecting forests will ensure that the regions largest CO2 sink does not become a source of emissions. For rural counties, afforestation, sustainable fuelwood harvest for bioenergy, and utility-scale wind power could provide the largest and most cost-effective mitigation opportunities among those evaluated. For urban and suburban counties, energy-efficiency measures and energy-saving technologies would be most cost effective. Through the implementation of locally tailored management and technology options, large reductions in CO2 emissions could be achieved at relatively low costs.