Douglas G. Sprugel

Disturbance, equilibrium, and environmental variability: What is ‘Natural’ vegetation in a changing environment?

Abstract To most early ecologists, the ‘natural’ ecosystem was the community that would be reached after a long period without large-scale disturbance (fire, windstrom, etc.). More recently, it has been realized that in most areas some type of large-scale disturbance is indigenous, and must be included in any realistic definition of ‘naturalness’. In some areas an equilibrium may exist in which patchy disturbance is balanced by regrowth, but in others equilibrium may be impossible because (1) individual disturbances are too large or infrequent; (2) ephemeral events have long-lasting disruptive effects; and/or (3) climate changes interrupt any movement toward equilibrium that does occur. Examples of non-equilibrium ecosystems include the African savannas, the Big Woods of Minnesota, the lodgepole pine forests of Yellowstone National Park, and possibly the old-growth Douglas-fir forests of the Pacific Northwest. Where an equilibrium does not exist, defining the ‘natural’ vegetation becomes much more challenging, because the vegetation in any given area would not be stable over long periods of time even without mans influence. In many areas it may be unrealistic to try to define the natural vegetation for a site; one must recognize that there are often several communities that could be the ‘natural’ vegetation for any given site at any given time.

A MODEL OF COMPETITION INCORPORATING PLASTICITY THROUGH MODULAR FOLIAGE AND CROWN DEVELOPMENT

The model of competition for light presented here uses modular autonomy to incorporate plasticity in plant growth under competition. Once plants are characterized as composed of modules, then model structure for competition changes in a fundamental way. Interactions between the plant module and its local resource environment must be modeled rather than the traditionally viewed interactions between whole plants and their neighbors. We assume that a plant module interacts with its local resource environment regardless of whether this environment was altered by a neighbor or by the same plant. Two spatial processes are considered: resource acquisition and growth. The spatial pattern of resource acquisition by a module determines a growth and allocation pattern, e.g., the elongation of branches into a gap. The spatial structure of a module and its connection to the whole tree then determines the pattern of resource distribution and resource acquisition of the next time step.Plasticity of plant growth is incorporated by variation in both the efficiency of resource capture of modules and patterns of resource allocation for individuals of different canopy positions and results in individuals in the community having different spatial structures. The model simulates the three—dimensional development of tree crown structure over time. It is applied to the 30—yr development of a dense, spatially aggregated stand of Abies amabilis beginning with an initial pattern of seedlings. The importance of incorporation of plasticity is apparent when the model output is compared to observed height distribution and crown structure data. Simulations indicate that asymmetrical crown development, one form of plasticity, is advantageous to stand productivity and becomes more advantageous as the degree of spatial aggregation in the initial spacing of trees increases.

Respiration from the Organ Level to the Stand

Publisher Summary Respiration is a major factor in plant, stand, or ecosystem energy budgets, estimated to consume anywhere from 30–70% of total carbon fixed. Respiration has been an area of particular interest and concern recently because of the possibility that C02-induced global warming might lead to substantial increases in respiration in temperate and boreal ecosystems that could decrease net primary productivity This chapter focuses on respiration. It describes dark respiration as a process by which glucose is enzymatically combined with oxygen to liberate chemical energy and CO2. Most respiration in trees is through the normal cytochrome-mediated pathway, but there is an alternative. Cyanide-resistant or salicylhydroxamic acid-sensitive respiration is a nonphosphorylating respiration pathway that generates only 40–50% as much chemical energy per glucose oxidized. When the respiration costs of producing plant tissue are estimated from tissue analyses, any ATP required for synthesis is produced by cytochromemediated respiration. Photorespiration is a by-product of photosynthesis in C3 plants in which ribulose bisphosphate carboxylase/oxygenase binds to O2 instead of CO2. It manifests as a reduction in the rate of photosynthesis. Considerable effort has been put into developing techniques for scaling individual measurements up to stand level. This chapter focuses on these scaling techniques and how they deal with known sources of variation in respiration.

Reconstructing fire regimes with charcoal from small-hollow sediments: a calibration with tree-ring records of fire

Interpretations of charcoal records from small hollows lack a strong theoretical and empirical foundation, and thus their potential for providing useful fire-history records is unclear. To evaluate this potential, we examined charcoal records in 210Pb-dated cores from 12 small hollows and looked for evidence of 20 local fires reconstructed with tree-ring records from the surrounding forest. Using all charcoal > 0.15 mm wide we established an optimum threshold that identified charcoal peaks corresponding to known fires while minimizing charcoal peaks identified that were not associated with known fires (i.e., false positives). This threshold detected four of four high-severity fires, five of 10 moderate-severity fires, and three of six low-severity fires. Analysis of larger charcoal alone (> 0.50 mm wide) yielded nearly identical temporal patterns and detection rates, but four false positives were identified, twice as many as identified using all charcoal >0.15 mm wide. Charcoal peak magnitude was highly variable within severity classes: although half of the low-and moderate-severity fires left no detectable peaks, others left peaks larger than some high-severity fires. Our results suggest that fire detection depends strongly on fire severity and that fine-scale spatial patterns of lower-severity burns play an important role in determining the charcoal signature of these events. High detection rates for high-severity fires and low false-positive rates indicate that charcoal records from small hollows will be most useful in systems where fires are large, severe and infrequent.

Components of woody-tissue respiration in young Abies amabilis (Dougl.) Forbes trees.

SummaryWoody-tissue respiration was measured on five different dates at three to five locations on each of 12 30-year-old Abies amabilis trees. On any given date, temperature-corrected respiration per unit surface area varied 10 to 40-fold between sampling locations. In stems, the two major components of respiration were growth respiration and sapwood maintenance respiration, which were of roughly equal importance during the growing season. There was no evidence of significant cambial maintenance respiration, suggesting that a stand with high bole surface area would not automatically have high respiration. Respiration in branches was much greater than in boles of comparable volume and growth rates, and was significantly correlated with branch height. Branch respiration may include an another significant component in addition to the two seen in bole respiration, possibly associated with carbohydrate mobilization and transport or with CO2 efflux from the transpiration stream.

The effects of light acclimation during and after foliage expansion on photosynthesis ofAbies amabilis foliage within the canopy

Variation in the photosynthetic function ofAbies amabilis foliage within a canopy was examined and related to three different processes that affect foliage function: foliage aging, sun-shade acclimation that occurred while foliage was expanding, and reacclimation after expansion was complete. Foliage produced in the sun had higher photosynthesis at light saturation (Amax, μmol·m-2·s-1), dark respiration (μmol·m-2·s-1), nitrogen content (g·m-2), chlorophyll content (g·m-2), and chlorophylla:b ratio, and a lower chlorophyll to nitrogen ratio (chl:N), than foliage produced in the shade. As sun foliage becomes shaded, it becomes physiologically similar to shade foliage, even though it still retains a sun morphology. Shaded sun foliage exhibited lowerAmax, dark respiration, nitrogen content, and chlorophylla:b ratio, and a higher chl:N ratio than sun foliage of the same age remaining in the open. However, shaded sun foliage had a higher chlorophyll content than sun foliage remaining in the open, even though true shade foliage had a lower chlorophyll content than sun foliage. This anomaly arises because as sun foliage becomes shaded, it retains a higher nitrogen content than shade foliage in a similar light environment, but the two forms have similar chl:N ratios. Within the canopy, most physiological indicators were more strongly correlated with the current light environment than with foliage age or leaf thickness, with the exception of chlorophyll content.Amax decreased significantly with both decreasing current light environment of the foliage and increasing foliage age. The same trend with current light and age was found for the chlorophylla:b ratio. Foliage nitrogen content also decreased with a decrease in current light environment, but no distinct pattern was found with foliage age. Leaf thickness was also important for predicting leaf nitrogen content: thicker leaves had more nitrogen than thinner leaves regardless of light environment or age. The chl:N ratio had a strong negative correlation with the current light environment, and, as with nitrogen content, no distinct pattern was found with foliage age. Chlorophyll content of the foliage was not well correlated with any of the three predictor variables: current light environment, foliage age or leaf thickness. On the other hand, chlorophyll content was positively correlated with the amount of nitrogen in a leaf, and once nitrogen was considered, the current light environment was also highly significant in explaining the variation in chlorophyll content. It has been suggested that the redistribution of nitrogen both within and between leaves is a mechanism for photosynthetic acclimation to the current light environment. Within theseA. amabilis canopies, both leaf nitrogen and the chl:N ratio were strongly correlated with the current light environment, but only weakly with leaf age, supporting the idea that changing light is the driving force for the redistribution of nitrogen both within and between leaves. Thus, our results support previous theories on nitrogen distribution and partitioning. However,Amax was significantly affected by both foliage age and the current light environment, indicating that changes in light alone are not enough to explain changes inAmax with time.

Acclimation responses of mature Abies amabilis sun foliage to shading

This paper addresses two main questions. First, can evergreen foliage that has been structurally determined as sun foliage acclimate physiologically when it is shaded? Second, is this acclimation independent of the foliage ageing process and source-sink relations? To investigate these questions, a shading and debudding experiment was established using paired branches on opengrown Abies amabilis trees. For each tree, one branch was either shaded, debudded, or both, from before budbreak until the end of summer, while the other branch functioned as a control. Foliage samples were measured both prior to and during treatment for photosynthesis at light saturation (Amax), dark respiration, nitrogen content, chlorophyll content, chlorophyll-to-nitrogen ratio and chlorophyll a:b ratio. All age classes of foliage responded similarly during the treatment, although pre-treatment values differed between age classes. Within 1 month after the treatment began, Amax was lower in shaded foliage and remained lower throughout the treatment period. For debudded branches, Amax was lower than the controls only during active shoot elongation. At the end of the treatments in September, Amax in shade-treated sun foliage matched the rates in the true shade-formed foliage, but nitrogen remained significantly higher. By 1.5 months after treatment, chlorophyll content in shaded foliage was higher than in controls, and the chlorophyll a:b ratio was lower for the shaded foliage. On debudded branches, chlorophyll content and chlorophyll a:b ratio were similar to the values in control samples. Shading lowered the rate of nitrogen accumulation within a branch, while removing debudding decreased the amount of sequestered N that was exported from the older foliage to supply new growth. By September, chlorophyll content in shade-treated foliage was higher than that in the control sun foliage or in true shade foliage. The chlorophyll increase as a result of shading was unexpected. However, the chlorophyll-to-nitrogen ratio was identical for the shade-treated sun foliage and the true shade foliage while being significantly lower than the control sun foliage. It appears that acclimation to shading in mature foliage involves a reallocation of nitrogen within the leaf into thylakoid proteins. A redistribution of resources (nitrogen) among leaves is secondary and appears to function on a slower time scale than reallocation within the leaf. Thus, A. amabilis foliage that is structurally determined as sun foliage can acclimate to shade within a few months; this process is most likely independent of ageing and is only slightly affected by source-sink relations within a branch.

Methodological Considerations in Measuring Biomass, Production, Respiration and Nutrient Resorption for Tree Roots in Natural Ecosystems

Field monitoring of roots presents different problems from those encountered in laboratory or greenhouse studies, where a distinct individual plant is studied under non-competitive conditions. Examinations of individual plant roots in the field present different problems because (1) roots from many individuals may occupy the same rooting zone, (2) root grafting is common, (3) root distribution is irregular and (4) roots from one individual may extend a considerable distance from the parent plant so their origin is not easily identifiable. This paper discusses field methods for studying carbon and nutrient cycling through the belowground and identifies how sampling schemes need to be adjusted when laboratory-developed techniques are transferred to the field. Specific types of data collection will be examined involving direct field measurements on intact tissues monitored over time (i.e. respiration) and on excised tissues sampled over time (i.e. biomass, production, resorption).

Spatial aspects of tree mortality strongly differ between young and old‐growth forests

Rates and spatial patterns of tree mortality are predicted to change during forest structural development. In young forests, mortality should be primarily density dependent due to competition for light, leading to an increasingly spatially uniform pattern of surviving trees. In contrast, mortality in old-growth forests should be primarily caused by contagious and spatially autocorrelated agents (e.g., insects, wind), causing spatial aggregation of surviving trees to increase through time. We tested these predictions by contrasting a three-decade record of tree mortality from replicated mapped permanent plots located in young (< 60-year-old) and old-growth (> 300-year-old) Abies amabilis forests. Trees in young forests died at a rate of 4.42% per year, whereas trees in old-growth forests died at 0.60% per year. Tree mortality in young forests was significantly aggregated, strongly density dependent, and caused live tree patterns to become more uniform through time. Mortality in old-growth forests was spatially aggregated, but was density independent and did not change the spatial pattern of surviving trees. These results extend current theory by demonstrating that density-dependent competitive mortality leading to increasingly uniform tree spacing in young forests ultimately transitions late in succession to a more diverse tree mortality regime that maintains spatial heterogeneity through time.

Examining age- and altitude-related variation in tree architecture and needle efficiency in Norway spruce using trend surface analysis

Abstract The aim of this study was to examine the variation in tree architecture and needle efficiency in stemwood production in Norway spruce in relation to tree age and altitude of growing site. The data, which were obtained from the literature, described individual Norway spruce ( Picea abies (L.) Karst.) trees from even-aged stands in Switzerland. Second-order trend surface models, with tree age and altitude as independent variables, were used in the analysis. The fitted models for stem, branch and needle dry masses explained 95%, 75% and 64% of the variation, respectively. The model for the estimated mean branch density in the crown (kg m −3 ) explained 64% of the variation and the model for mean needle density in the crown (kg m −3 ) only 28% of the variation. Crown structural characteristics, which showed age- and altitude-related variations, included live crown ratio (59% of variation explained), number of living whorls (43% of variation explained), mean weight of single needle (40% of variation explained) and specific needle area (27% of variation explained). Tree age had a strong effect on needle efficiency in stemwood production, so that needle efficiency increased up to the age of 50–70 years, depending on altitude.