Soil microorganisms as controllers of atmospheric trace gases (H2, CO, CH4, OCS, N2O, and NO).

The complexity of a forest ecosystem makes difficult any attempt to synthesize knowledge about forest dynamics or to perceive the implications of information and assumptions regarding forest growth. Although digital computer simulation seems to offer a potential for creating a complete model of forest growth, little progress has been reported. Computer simulation has been carried out for the growth of trees in even-aged stands of a single species (Mitchell 1969), and for meteorological energy exchange in a forest canopy (Waggoner & Reifsnyder 1968). A specific simulation built directly from Hubbard Brook data has been reported (Siccama et al. 1969). Successional change in northern hardwood forests has been predicted from observed birth and death rates (Leak 1970). A conceptual model has been created for the growth of individual tree seedlings from rates of photosynthesis and the distribution of photosynthates (Ledig 1969). Computer simulation has been carried out for some aspects in a few other terrestrial ecosystems, such as productivity in a corn crop (Duncan et al. 1967); but apparently no one has successfully reproduced the major characteristics of a mixed-species, mixed-aged forest from a conceptual basis. A computer simulation of forest growth is now developed that successfully reproduces the population dynamics of the trees in a mixed-species forest of north-east North America. The simulator is designed to be used in the Hubbard Brook Ecosystem Study and to provide output in the same form as the original vegetation survey of that study (Bormann et al. 1970). However, the underlying concepts of the simulation are general. The properties of each species are derived from its entire geographic range and in theory any non-hydrophytic species whose relevant characteristics are known can be entered into the simulation. In the present version of the program, the description of the environment is restricted to those features that have been recorded for the Hubbard Brook Forest, but the relative importance attached to each environmental factor has been influenced by the environmental characteristics of the north-eastern United States. It is hoped that a wide dissemination of this simulator will encourage others to test this version with their data and hence lead to later versions of wider usefulness and applicability. The basic goal was to produce a dynamic model of forest growth, a model in which changes in the state of the forest are a function of the present state and random components. This approach has two advantages over the curve-fitting approach to forest growth: first, the simulator can be regarded as a repository for an integrated knowledge of the ecosystem; second, additional hypotheses can be formulated and tested using Monte Carlo samples of simulator runs and comparing the results with observed data. For

Some ecological consequences of a computer model of forest growth

The adaptive significance of leaf life spans has been examined from several different points of view. Evergreenness has been explained in terms of nutrient conservation (86), improving carbon balance (90), and as a general adaptation to environmental stress (47). In this review, we consider these theories and attempt to synthesize divergent viewpoints. We consider the question "How is the length of a leaf's life span related to environmental factors?" In particular, what are the comparative advantages of the evergreen and deciduous habits and how can adaptive differences be related to distributional patterns and climatic gradients?

The Ecology of Leaf Life Spans

Primary productivity of the biosphere

The predominant emphasis on harmful effects of environmental stresses on growth of woody plants has obscured some very beneficial effects of such stresses. Slowly increasing stresses may induce physiological adjustment that protects plants from the growth inhibition and/or injury that follow when environmental stresses are abruptly imposed. In addition, short exposures of woody plants to extreme environmental conditions at critical times in their development often improve growth. Furthermore, maintaining harvested seedlings and plant products at very low temperatures extends their longevity. Drought tolerance: Seedlings previously exposed to water stress often undergo less inhibition of growth and other processes following transplanting than do seedlings not previously exposed to such stress. Controlled wetting and drying cycles often promote early budset, dormancy, and drought tolerance. In many species increased drought tolerance following such cycles is associated with osmotic adjustment that ...

Acclimation and adaptive responses of woody plants to environmental stresses

Early dormancy involves a number of phenomena: cessation of active growth, formation of terminal buds, formation of abscission layers in leaves, development of cold resistance, development of winter rest (a chilling requirement), and leaf fall. The buds of some tree species (or some races of a species) have a true rest or winter chilling requirement; usually the roots do not; perhaps the cambium does not. There is a wide range of genetic variation within and between species in response to photoperiod, winter chilling, water nutrients, and other environmental factors which affect the dormancy condition of plants. During the period of winter rest there are many metabolic and developmental processes going on in the buds and twigs. These processes include respiration, photosynthesis, some cell division, enzyme synthesis, production of growth stimulators, and dissipation of growth inhibitors. Dormancy of trees can be divided arbitrarily into three phases: early rest, winter rest, and after-rest. Each of these phases is marked by a distinct set of physiological processes. The transition between the three phases is gradual. Some processes of after-rest can proceed even though the winter chilling requirement (winter rest) of a tree has not been completed fully. Evidence from grafting experiments and chemical treatments to break winter rest, and studies of genetic variability indicate that the processes and phenomena of dormancy are at least partially independent of each other. Different buds and branches and other parts of the same plant may initiate dormancy, break dormancy, and renew vegetative growth independently. Initiation and cessation of dormancy can be triggered by a number of environmental variables: photoperiod, temperature, nutrition, water, an array of chemicals, and shock treatments. Dormancy regulation must either involve a variety of receptors or involve receptors that are responsive to a variety of stimuli. Unless dormancy is defined in a highly restricted sense (that is, possession of chilling requirement), it is hard to conceive of a single receptor or regulator that controls all of the phenomena of dormancy. A large number of genes are definitely involved and hence a large number of enzymes. The kinds of enzymes, their numbers, and their concentrations can be regulated by manipulating the environment.

https://science.sciencemag.org/content/sci/171/3966/29.full.pdf

Dormancy of trees in winter.

The source of N2O produced in soil is often uncertain because denitrification and nitrification can occur simultaneously in the same soil aggregate. A technique which exploits the differential sensitivity of these processes to C2H2 inhibition is proposed for distinguishing among gaseous N losses from soils. Denitrification N2O was estimated from 24-h laboratory incubations in which nitrification was inhibited by 10-Pa C2H2. Nitrification N2O was estimated from the difference between N2O production under no C2H2 and that determined for denitrification. Denitrification N2 was estimated from the difference between N2O production under 10-kPa C2H2 and that under 10 Pa. Laboratory estimates of N2O production were significantly correlated with in situ N2O diffusion measurements made during a 10-month period in two forested watersheds. Nitrous oxide production from nitrification was most important on well-drained sites of a disturbed watershed where ambient NO3− was high. In contrast, denitrification N2O was most important on poorly drained sites near the stream of the same watershed. Distinction between N2O production from nitrification and denitrification was corroborated by correlations between denitrification N2O and water-filled pore space and between nitrification N2O and ambient NO3−. This technique permits qualitative study of environmental parameters that regulate gaseous N losses via denitrification and nitrification.

Distinguishing between Nitrification and Denitrification as Sources of Gaseous Nitrogen Production in Soil.

Most tree species from the temperate climate must be exposed for a minimulll nu1.lmber of hours to low temperatures (for chilling treatment) before they can break their winter dormancy in a normal fashion. Unless peaches, apiiles, b)lueberries, and similar crops are exposed for a minimum number of days to temperatures below a inaximum of 30-40?F1 the plants will not break dormancy or fruit properly. This chilling requirement is a factor that prevents successful cultivation of nmany decirluous species and varieties in the warm cliirate of the southern U. S. Plan t breeders have striven to modify the wvinter chilling re(qlirements of favorite 1)peah varieties by introdlucinggenes of low chilling requirement from tropical and subtropical varieties. These attempts are meeting with some success (W\\einberger 1.950, Lesley 1944). Witlh the expansion of tree planting p)rograms tile po-siilits if genetic variation in winter chilling requilrements is o0 economic importance and theoretical signliicaic. I his paper describess anl investigation of the patterln of genietie vari-atioii ini the chilling requirenient of red liile, 01 l t rIIibci L. Wiiiter chilling requiremelits arc lr-Con,-licd( Ior norcst trees as vell as for horticultural pecics ( )lste( 1 1' P1 I imvesti-ate(d the plhotoperiod anid chiliii requic mc iits of progenies of sugar mnaple trees aild fonii (l Sone sarliatioli in the chilling requirement of (9)litn r n lls )e(lr'ii Miclix. OlISon et al. ( 1959 ) condoeted similar ilivestig(atiOlls oil the chilling reqniriliu-C(lits (ot efesterll heilli(wt el, I'Sitg(i cmiiocisis, (1. ) Carr. '[lie literatiti-c revic\\\\s Iy D.)(ormillms ( 1 l33d ), Salllis 1 (9 4 ), \\ cg i s ( 1955 i \\\\arein, 1( 1956) ,\\ciiic1r-cr (1950) and the wvorks oI I lmaord-(i 1910, 10150l, 19)1 b) give a good co average ()t tile ,(cileti s, tiir plhilvoi(, aii(i tle variety o( siiccies tihat I) c)siss clilliiiI Cqlll iiriits [lie authorS w\\ ish to tiai iii tiios, w\\ ho ii(ie nir (d cillectioiis for tins studs -\\ 9 J ('amiihlc I (I i M. 1 lolsti-()mitariii 1)riit . ()I L ils iI o -l Irt ). 1. I '). i ir1k (CoIISir 5 ition 1 ) pIMi tior. it; \\\\ ( do Mi lid l /) Zak ---L. S. I orst Sricsmr N Jolim t sr\\ie 5 IC

Thomas O. Perry

Papers

Dormancy of trees in winter.

Distinguishing between Nitrification and Denitrification as Sources of Gaseous Nitrogen Production in Soil.

Genetic Variation in the Winter Chilling Requirement for Date of Dormacy Break for Acer Rubrum

Winter-Season Photosynthesis and Respiration by Twigs and Seedlings of Deciduous and Evergreen Trees

Net Assimilation Rate and Growth in Loblolly Pine Seedlings