Henry L. Gholz

Modeling and measuring the effects of disturbance history and climate on carbon and water budgets in evergreen needleleaf forests

The effects of disturbance history, climate, and changes in atmospheric carbon dioxide (CO2) concentration and nitrogen deposition (Ndep) on carbon and water fluxes in seven North American evergreen forests are assessed using a coupled water–carbon–nitrogen model, canopy-scale flux observations, and descriptions of the vegetation type, management practices, and disturbance histories at each site. The effects of interannual climate variability, disturbance history, and vegetation ecophysiology on carbon and water fluxes and storage are integrated by the ecosystem process model Biome-BGC, with results compared to site biometric analyses and eddy covariance observations aggregated by month and year. Model results suggest that variation between sites in net ecosystem carbon exchange (NEE) is largely a function of disturbance history, with important secondary effects from site climate, vegetation ecophysiology, and changing atmospheric CO2 and Ndep. The timing and magnitude of fluxes following disturbance depend on disturbance type and intensity, and on post-harvest management treatments such as burning, fertilization and replanting. The modeled effects of increasing atmospheric CO 2 on NEE are generally limited by N availability, but are greatly increased following disturbance due to increased N mineralization and reduced plant N demand. Modeled rates of carbon sequestration over the past 200 years are driven by the rate of change in CO2 concentration for old sites experiencing low rates of N dep. The model produced good estimates of between-site variation in leaf area index, with mixed performance for between- and within-site variation in evapotranspiration. There is a model bias

Soil CO2 efflux and its spatial variation in a Florida slash pine plantation.

The efflux of CO2 from the soil surface can vary markedly in magnitude both in time and space and its correct determination is crucial in many ecological studies. In this paper, we report results of field measurements, using an open-top dynamic chamber, of soil CO2 efflux in a mature Florida slash pine (Pinus elliottii Engelm. var.elliottii) plantation. The daily average efflux was 0.217 mg CO2 m-2s-1 in the autumn and 0.087 mg CO2 m-2s-1 in the winter. Soil temperature, which accounts for most of the temporal variability in CO2 efflux, is by far the most influential factor controlling soil respiration rate and its temporal variation. The CO2 efflux in the slash pine plantation is highly spatially variable and effluxes from the soil under palmetto is significantly higher than that from the open floor. The CO2 efflux generally increases with increase in soil fine root biomass, litter and humus amount on the forest floor but is inversely related to the amount of organic matter in the mineral soil. The spatial variation in CO2 efflux can be well characterised by a simple multiple regression model incorporating live and dead biomass and soil total porosity as predictor variables. Understorey plants, mostly Serenoa repens, are an important component of the C cycle and the major contributor to the spatial heterogeneity of soil CO2 efflux. The influence of understorey plants on soil respiration is probably via two approaches: increasing litterfall and root metabolism, both consequently stimulating microbial activity in the mineral soil.

Woody tissue maintenance respiration of four conifers in contrasting climates

We estimate maintenance respiration for boles of four temperate conifers (ponderosa pine, western hemlock, red pine, and slash pine) from CO2 efflux measurements in autumn, when construction respiration is low or negligible. Maintenance respiration of stems was linearly related to sapwood volume for all species; at 10°C, respiration per unit sapwood volume ranged from 4.8 to 8.3 μmol CO2 m−3 s−1. For all sites combined, respiration increased exponentially with temperature (Q10=1.7, r2=0.78). We estimate that maintenance respiration of aboveground woody tissues of these conifers consumes 52–162 g C m−2 y−1, or 5–13% of net daytime carbon assimilation annually. The fraction of annual net daytime carbon fixation used for stem maintenance respiration increased linearly with the average annual temperature of the site.

DYNAMICS OF CANOPY STRUCTURE AND LIGHT INTERCEPTION IN PINUS ELLIOTTII STANDS, NORTH FLORIDA'

In order to develop a model of the carbon cycle for mature slash pine (Pinus elliottii) stands in north Florida, we studied seasonal variation in leaf area index (LAI, all- sided), aboveground biomass increment and litterfall, and light penetration through the forest canopy, over a 3-yr period. The primary approach to establishing monthly LAI included annual destructive analyses and monthly measurements of needle fall and elon- gation. Imagery from the Landsat Thematic Mapper (TM) and patterns of light penetration were also used in attempts to derive less arduous estimates; the TM imagery was most promising. LAIs ranged from 3.0 to 6.5 on control plots over the 3 yr, with repeated fertilization increasing maximum LAI by >40%. Seasonal variation was high (40%), as was variation from year to year. An average of 3 1% of the incident photosynthetically active radiation (PAR) penetrated the canopies annually, ranging from 18 to 42% seasonally. Seasonal light penetration could not be described using a simple application of the Beer-Lambert law, perhaps due to the highly aggregated nature of the canopies. Models incorporating more information on canopy structure are necessary to predict light penetration through slash pine stands accurately. A model of needle litterfall was derived that could account for much of the seasonal and annual variation using stand basal area and climate conditions from the spring of the previous year; this model may be useful for developing climate-driven predictions of LAI. Efficiencies of use of incoming and intercepted PAR were low compared to other forest types. Low light interception and high nutrient-use efficiencies (demonstrated in earlier studies) are important adaptive characteristics of slash pine stands to these relatively warm and nutrient-poor sites.

Effects of Land-Use Change on Soil Nutrient Dynamics in Amazônia

Over the past several decades, the conversion of native forest to agricultural land uses has accelerated across the Amazon Basin. Despite a growing body of research on nutrient dynamics in Amazonian primary forest and forest-derived land uses, the effects of widespread land-use change on nutrient contents and cycles in soil and vegetation are not well understood. We reviewed over 100 studies conducted in Amazônia over the past 40 years on nutrient dynamics in natural forests and forest-derived land uses (pasture, shifting cultivation, and tree plantations). Our objectives were to compare soil data from land uses across Amazônia and identify any gaps in our present knowledge that might offer direction for future research. Specifically, by analyzing data we tested the following five widely cited hypotheses concerning the effects of land-use change on soil properties compiled from 39 studies in multifactorial ANOVA models; (a) soil pH, effective cation exchange capacity (ECEC), and exchangeable calcium (Ca) concentrations rise and remain elevated following the slash-and-burn conversion of forest to pasture or crop fields; (b) soil contents of total carbon (C), nitrogen (N), and inorganic readily extractable (that is, Bray, Mehlich I, or resin) phosphorus (Pi) decline following forest-to-pasture conversion; (c) soil concentrations of total C, N, and Pi increase in secondary forests with time since abandonment of agricultural activities; (d) soil nutrient conditions under all tree-dominated land-use systems (natural or not) remain the same; and (e) higher efficiencies of nutrient utilization occur where soil nutrient pools are lower. Following the conversion of Amazonian forest to pasture or slash-and-burn agriculture, we found a significant and lasting effect on soil pH, bulk density, and exchangeable Ca concentrations. Unlike the other three land uses studied, concentrations of extractable soil Pi were equally low in both forest and pastures of all age classes, which demonstrates that postburning pulses in soil Pi concentration following a slash-and-burn decrease rapidly after forest-to-pasture conversion, perhaps due to accumulation in organic P fractions. Neither the concentrations nor the contents of total C and N appeared to change greatly on a regionwide basis as a result of forest-to-pasture conversion, but surface soil C:N ratios in 5-year-old pastures were significantly higher than those in older pastures, suggesting changes in the soil concentrations of at least one of these elements with time after pasture creation. Pasture soils did have higher total C and N concentrations than land uses such as annual cropping and secondary forest fallow, indicating that soil C and N maintenance and/or accumulation following forest conversion may be greater in pastures than in these other two land uses. The low concentrations of C and N in shifting cultivation soils appear to persist for many years in secondary forests regenerating from abandoned crop fields, suggesting that the recuperation of soil losses of C and N resulting during no-input annual cropping is slower than previously thought. Soil C, N and P concentrations were strongly related to clay content. Across all land uses, efficiencies of N, P, and Ca use (estimated as the inverse of litterfall N, P, and Ca contents) were not related to the sizes of their soil pools. More work is needed to test and standardize P extraction procedures that more accurately reflect plant availability. Few studies have been conducted to determine the role of organic P fractions and dissolved organic N (DON) in the elemental cycles of both natural and managed systems in this region. In general, we recommend further study of annual and perennial cropping systems, as well as more detailed examination of managed pastures and fallows, and secondary forests originating from various disturbances, since the intensity of previous land use likely determines the degree of soil degradation and the rate of subsequent secondary regrowth.

Seasonal LAI in slash pine estimated with LANDSAT TM

The leaf area index (LAI, total area of leaves per unit area of ground) of most forest canopies varies throughout the year, yet for logistical reasons it is difficult to estimate anything more detailed than an annual average LAI. To determine if remotely sensed data can be used to estimate LAI at times throughout the year (herein termed seasonal LAI), field measurements of LAI were compared to normalized difference vegetation index (NDVI) values, derived using Landsat Thematic Mapper (TM) data, for 16 fertilized and control slash pine plots on three dates. Linear relationships existed between NDVI and LAI with R2 values of 0.35, 0.75, and 0.86 for February 1988, September 1988and March 1989, respectively. Predictive relationships based on data from eight of the plots were used to estimate the LAI of the other eight plots with a root-mean-square error of 0.74 LAI, which is 15.6% of the mean LAI. This demonstrates the potential use of Landsat TM data for studying seasonal dynamics in forest canopies.

CARBON DYNAMICS ALONG A CHRONOSEQUENCE OF SLASH PINE PLANTATIONS IN NORTH FLORIDA

To determine factors controlling the carbon dynamics of an intensively man- aged landscape, we measured net CO2 exchange with the atmosphere using eddy covariance and soil CO2 fluxes using static chambers along a chronosequence of slash pine (Pinus elliottii var. elliottii) plantations consisting of a recent clearcut, a mid-rotation (10-yr-old) stand, and a rotation-aged (24-yr-old) stand. Daytime net ecosystem exchange of CO2 (NEEday) at the clearcut was not significantly different than zero during the growing season of the first year following harvest and reached levels that were -40% of those at the older stands during the second growing season. NEEday was similar at the mid-rotation and ro- tation-aged sites, reflecting the similar leaf areas of these stands. Nighttime net ecosystem exchange of CO2 (NEEnight) was an exponential function of air or soil temperature at all sites. However, low decomposition rates of litter and flooding of the site following harvest likely constrained NEEnight at the clearcut, and drought affected rates at the mid-rotation site. Annual net ecosystem exchange of CO2 (NEEyr) was estimated at -1269 and -882 g C.m-2-yr-1 at the clearcut, and 576 and 603 g C.m-2.yr-1 at the mid-rotation stand in 1998 and 1999, respectively. For comparison, NEEyr was 741 and 610 g C-m-2.yr-~ at the rotation- aged stand in 1996 and 1997, respectively. In contrast, annual ecosystem respiration (Reco) was similar in magnitude at all sites during all years. Although Reco is similar in magnitude, NEEyr is highly dynamic across this intensively managed landscape, with a maximum range of -2000 g C.m-2.yr-1. This range exceeds that across all the sites in both the Ameriflux and Euroflux networks and illustrates the need to include the range of stand ages and disturbance histories in landscape- to regional-scale flux estimates.

ENVIRONMENTAL CONTROLS OVER NET EXCHANGES OF CARBON DIOXIDE FROM CONTRASTING FLORIDA ECOSYSTEMS

Net CO2 exchange estimated using eddy covariance and relaxed eddy accumulation indicated that evergreen pine upland and deciduous cypress wetland ecosystems in north-central Florida had similar apparent light compensation points during the growing season (125 vs. 150 μmol PPFD·m−2·s−1), but that maximum rates at 1800 μmol PPFD·m−2·s−1 at the cypress ecosystem were only 59% of those at the pine ecosystem (8.9 vs. 15.2 μmol CO2·m−2·s−1). During both the summer and winter months at the pine ecosystem, net CO2 exchange in the daytime was a curvilinear function of PPFD, with no significant seasonal differences in slope or intercept. In contrast, net CO2 exchange at the cypress ecosystem was minimal during the daytime in the winter. Net CO2 exchange during the nighttime was an exponential function of air temperature at both sites, with Q10 values of 2.0 and 1.9 for the pine and cypress ecosystems, respectively. Lower nighttime fluxes of CO2 occurred at the cypress ecosystem across the entire temperature range. Both of these relatively sparse canopies stored CO2 during stable atmospheric conditions. Mean maximum net CO2 exchange during the daytime and mean nighttime net CO2 exchange for these ecosystems were highly contrasting, and together resulted in a relatively low rate of annual carbon accumulation in the wetland when compared to the aggrading pine ecosystem. However, values reported here are within the ranges of values for other boreal, temperate, and tropical forest ecosystems.

Energy exchange across a chronosequence of slash pine forests in Florida

We measured net atmospheric exchanges of energy and water vapor using eddy covariance along a chronosequence ofPinus elliottii plantations in north Florida: a recent clear-cut, a mid-rotation stand, and a 24-year-old, rotation-aged stand. Reflected energy averaged 0.26 of incoming solar radiation at the clear-cut and 0.18 at the closed-canopy stands. The sum of sensible (S), latent (LE) and soil heat fluxes accounted for 89 and 85% of net radiation ( Rnet) at the clear-cut and mid-rotation age sites. Both S and LE were linearly related to Rnet at all sites. Seasonal differences occurred in the proportion of Rnet attributable to S and LE. S was a much smaller proportion of Rnet when the clear-cut and the mid-rotation age stands were flooded in the summer. LE was a greater proportion of Rnet during the summer/fall at all sites when LAI was greatest. Bowen ratios (S/LE) were 0.34, 0.50 and 0.59 in the summer/fall and 0.71, 0.77 and 1.00 in the winter/spring at the clear-cut, mid-rotation and rotation-aged stands, respectively. Maximum rates of evapotranspiration (ET) in the summer were 0.6 mm h −1 at all sites. Mean daily rates averaged 3.3 mm per day in the summer/fall and 2.0 mm per day in the winter/spring. Although, changes in LAI and canopy structure were large, annual ET estimates were similar and averaged 959, 951 and 1110 mm per year along the chronosequence. Results suggest that energy partitioning and annual ET in these pine forests are more sensitive to environmental fluctuations than to management activities.

Water and Forest Productivity

Abstract Water availability is a major factor influencing the distribution and productivity of the earths vegetation, but details of the mechanisms by which its effects are felt are not well understood. This is due in large part to the interactions between water and vegetation, such as through interception and change in leaf-area, which affect rates of canopy photosynthesis and transpiration. Physiological differences among species are not always directly translated to differences among stands, emphasizing the importance of climate and microclimate as controls. Leaf-area index ( L ) is a critical integrator of water availability and productivity, and changes in leaf-area, such as occur through thinning and understory control, may have dramatic effects on both. There is increasing evidence that L changes significantly within an annual cycle and from year to year, even in closed-canopy conifer stands. Consequently, the season and year in which a measurement of L is made may explain much of the variability noted before in its response to water availability and effects on productivity. Because carbon, water, and nutrient cycles are so closely coupled, simulation models that represent both direct and indirect relationships are useful tools for understanding and managing forest ecosystems.