N2 fixation and cycling in Alnus glutinosa, Betula pendula and Fagus sylvatica woodland exposed to free air CO2 enrichment.
Millett J, Godbold D, Smith AR and Grant H (2012). Oecologia. DOI: 10.1007/s00442-011-2197-4
We measured the effect of elevated atmospheric CO2 on atmospheric nitrogen (N2) fixation for the tree species Alnus... more We measured the effect of elevated atmospheric CO2 on atmospheric nitrogen (N2) fixation for the tree species Alnus glutinosa growing in monoculture or in mixture with the non-N2-fixing tree species Betula pendula and Fagus sylvatica. We addressed the hypotheses that 1: N2 fixation in A. glutinosa will increase in response to increased atmospheric CO2 concentrations, when growing in monoculture, 2: the impact of elevated CO2 on N2 fixation in A. glutinosa is the same in mixture and in monoculture and 3: the impacts of elevated CO2 on N cycling will be evident in a decrease in leaf δ15N and in the soil-leaf enrichment factor (EF), and that these impacts will not differ between mixed and single species stands. Trees were grown in a forest plantation on former agricultural fields for 4 growing seasons, after which the trees were on average 3.8 m tall and canopy closure had occurred. Atmospheric CO2 concentrations were maintained at either ambient or elevated (by 200 ppm) concentrations using a free-air CO2 enrichment (FACE) system. Leaf δ15N was measured and used to estimate the amount (Ndfa) and proportion (%Ndfa) of N derived from atmospheric fixation. On average 62% of the N in A. glutinosa leaves was from fixation. %Ndfa and Ndfa for A. glutinosa trees in monoculture did not increase under elevated CO2, despite higher growth rates. However, N2 fixation did increase for trees growing in mixture, despite the absence of significant growth stimulation. There was evidence that fixed N2 was transferred from A. glutinosa to F. sylvatica and B. pendula, but no evidence that this affected their CO2 response. This study shows that N2 fixation in A. glutinosa may be higher in a future elevated CO2 world, but that this effect will only occur where the trees are growing in mixed species stands.
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Seen by:Canopy disturbance patterns in secondary hardwood stands on the Highland Rim of Alabama
by Arvind Bhuta
Co-authored with Justin L. Hart (1st author), Department of Geography, University of Alabama, Tuscaloosa, AL and Rebecca M. Schneider (3rd author), Conservation Management Institute, Blacksburg, VA. I am second author.
Disturbance regimes of many hardwood forests of the eastern United States in the complex stage of development are... more
Disturbance regimes of many hardwood forests of the eastern United States in the complex stage of development are characterized by localized canopy disturbance events
that change fine-scale biophysical conditions. Recently, research has demonstrated the importance of gap-scale disturbance processes in secondary hardwood stands of the southern Appalachian Highlands. However, information on canopy disturbance patterns during early developmental stages is required from the broader geographic region for a comprehensive understanding of stand dynamics. The goal of this study was to reconstruct canopy disturbance history for mixed hardwood stands on the Highland Rim of Alabama to elucidate disturbance patterns during early development. We analyzed radial growth from 46 Quercus individuals to reconstruct canopy disturbance history. The majority (67%) of the trees analyzed exhibited release events. In total, 42 releases were detected and some trees experienced multiple events. Of these releases, 28 (67%) were classed as minor and 14 (33%) were classed as major. Mean release duration was 4.00 years 6 0.21 (SE) and the longest release was sustained for eight years. Based on mean release duration, we speculate that most of the canopy gaps were filled by lateral crown expansion rather than subcanopy height growth. We did not document any forest-wide disturbance events; a pattern that may be a regional phenomenon or may be related to forest age. Canopy disturbances became common after ca. 40 years of development. We propose that the frequency of canopy gaps will decrease and the size and magnitude of individual gaps will increase as the stands mature.
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Seen by:Forest vegetation and development patterns in secondary stands on the Alabama Highland Rim: an examination of the landholding in the region
by Arvind Bhuta
Co-authored with Justin L. Hart (2nd author), Department of Geography, University of Alabama, Tuscaloosa, AL, and Rebecca M. Schneider, Conservation Management Institute, Virginia Tech, Blacksburg, VA.
Redstone Arsenal (RA) is a large federal landholding (15,050 ha) managed by the US Department of the Army located on... more Redstone Arsenal (RA) is a large federal landholding (15,050 ha) managed by the US Department of the Army located on the Highland Rim of north Alabama. This large landholding provided a unique opportunity to document forest vegetation and development patterns in a region with a paucity of quantitative information. In this study, we quantified land cover types, delineated forest stands on the reserve using a GIS, and assessed forest composition and structural measures. Stands were classed into Pinus, hardwood, mixed Pinus-hardwood, and forested wetland types. The majority of land area in stands ≥ 2 ha was hardwood dominated, while the majority of the land area in stands < 2 ha was Pinus dominated. We used a stratified random scheme to establish sampling points (n = 684) within forest stands through the RA. Pinus taeda, Liquidambar styraciflua, and Quercus phellos were the three most important tree species in the RA forest. Biodiversity for the forest was relatively high with a tree species richness of 57 and diversity (H’) of 2.50. Total forest evenness (J) was 0.62, but this value was lowered by the abundance of Pinus in plantation management. Average stand age was 38 years with a range from 7 to 90 years. Basal area was 22.6 m2 ha-1 and we expect this value to increase with forest age. Significant differences were noted between tree age and height across the three surveyed forest types. Our results provide information on forest conditions in an understudied region.
The global distribution of net primary production: resolving the paradox
Huston & Wolverton 2009
The distribution of the diversity and abundance of life on Earth is thought to be shaped by the patterns of plant... more The distribution of the diversity and abundance of life on Earth is thought to be shaped by the patterns of plant growth (net primary production, NPP) in the oceans and on land. The well-known latitudinal gradient of species diversity reaches its maximum in tropical rain forests, which are considered to be the most productive ecosystems on the planet. However, this high tropical productivity on land is the opposite of the well-documented distribution of marine productivity, which is greatest in the high-latitude oceans around the poles. This paradox can be resolved by a reevaluation of the terrestrial productivity gradient. Compilations of direct measurements of forest NPP show that annual NPP in tropical forests is no different than annual NPP in temperate forests, contrary to recent syntheses and to the output of global vegetation models. Other properties of forest ecosystems, such as basal area of trees, wood density, and the ratio of wood to leaf production, as well as animal properties such as body size, population density, and reproductive rates, support the conclusion that ecologically relevant terrestrial productivity is actually highest in the temperate latitudes, reaching a maximum between 30° and 50° before declining toward the poles. This “reversal” of the latitudinal productivity gradient, if substantiated by a systematic global sampling effort, will necessitate a major reevaluation of ecological and evolutionary theory, as well as conservation strategies and international development policies.

