Summary Plant interactions can be defi ned as the ways plants act upon the growth, fi tness, survival and reproduction of other plants, largely by modifying their environment. These interactions can be positive (facilitation) or negative (competition or exploitation). During plantation establishment or natural forest regeneration after a disturbance, high light levels and, sometimes, increased availability of water and nutrients favour the development of opportunistic, fast-growing herbaceous and woody species which capture resources at the expense of crop trees. As a consequence, the growth and survival of crop trees can be dramatically reduced. Although the effects of this competition are well documented, the physical and physiological mechanisms of competition are not. Moreover, the competition process is never constant in time or space. We present a conceptual competition model based on plant growth forms common in global forests, i.e. graminoids, forbs, small shrubs, large shrubs and mid-storey trees, and main-storey trees. Their competitive attributes and successional dynamics are examined. An overview is presented on the way forest vegetation management (FVM) treatments infl uence these components and outcomes regarding crop tree performance and diversity conservation. Finally, a synthesis of literature yields FVM guidelines for effi ciently optimizing crop tree performance and safeguarding diversity. Future research needs to further sustainable FVM are presented.

/pdf/designing-forest-vegetation-management-strategies-based-on-2roqlwr8e0.pdf

Designing forest vegetation management strategies based on the mechanisms and dynamics of crop tree competition by neighbouring vegetation

Competition between mature trees and regeneration has been well studied in closed-canopy forests, but less so in open-canopy woodlands. We examined competition in a longleaf pine (Pinus palustris Mill.) woodland to understand effects of overstory abundance on resource availability and seedling responses to resource levels. We examined resources and planted seedling performance across a gradient from intact canopies to the centers of large gaps. Growth of seedlings increased slowly across a wide range of decreasing overstory basal area, but increased substantially only in the center of gaps. Seedling survival was unrelated to overstory abundance. Overstory basal area was negatively related to light and nitrogen. Light increased from 40% of full sunlight in intact forest to 80% in gaps. Nitrogen only increased in the center of gaps, suggesting that below-ground and above-ground gaps are spatially distinct. Correlations between seedling growth and light and nitrogen were positive and curvilinear upward. The data suggest that increases in both light and nitrogen may be required for seedling growth increases. We use our results to propose a silvicultural approach for green tree retention that minimizes inhibition of regeneration through clumping of residuals to maximize gap size, even while holding remaining basal area constant. Resume : La competition entre les arbres murs et la regeneration a ete bien etudiee dans les forets a voute fermee, moins par contre dans les forets claires a voute ouverte. La competition dans une foret claire de pin des marais ( Pinus palustris Mill.) a ete examinee afin de comprendre les effets de l'abondance de l'etage dominant sur la disponibilite des ressources et la reaction des semis par rapport au niveau de ces dernieres. Les ressources et la performance des semis plantes ont ete examinees le long d'un gradient allant des voutes intactes jusqu'au centre des larges trouees. La croissance des semis augmentait lentement a mesure que diminuait la surface terriere de l'etage dominant, mais elle augmentait considerablement seulement au centre des trouees. La survie des semis n'etait pas reliee a l'abondance de l'etage dominant. Il y avait, par contre, une relation negative entre la surface terriere de l'etage dominant et la lumiere ainsi que l'azote. La lumiere est passee de 40% de luminosite totale dans les forets intactes a 80% dans les trouees. L'azote a augmente uniquement au centre des trouees, suggerant que les trouees hypogees et epigees ne correspondent pas dans l'espace. Les correlations entre la croissance des semis et la lumiere ainsi que l'azote etaient positives et curvilignes ascendantes. Les donnees suggerent que l'accroissement de la lumiere, comme de l'azote, pourrait etre requis pour augmenter la croissance des semis. Les resultats de cette etude sont utilises pour proposer une methode sylvicole de conservation des arbres vivants qui minimise l'inhibition de la regeneration par les arbres residuels. Elle consiste a garder ces derniers par groupes afin de maximiser la taille des trouees, tout en maintenant la surface terriere residuelle constante. (Traduit par la Redaction)

Effects of canopy structure on resource availability and seedling responses in a longleaf pine ecosystem

Recent models suggest that a trade—off in plants between tolerance of water limitation vs. tolerance of light limitation results in changes in dominant species over productivity gradients of increasing soil moisture and decreasing forest—floor light. With increasing elevation (1568—2296 m) in the Chiricahua Mountains in southeastern Arizona, soil moisture and plant cover increased and, as a result, mean forest—floor light levels decreased, in accordance with the models. The light—moisture trade—off hypothesis predicts that, over this gradient, (1) shade tolerance and drought resistance should be negatively correlated, (2) decreasing light and lack of shade tolerance (i.e., tolerance of light competition) should control upper elevational limits of species distributions, and (3) low soil moisture availability and lack of drought resistance should control lower elevational limits. With increasing elevation, however, fire frequency and litter depth also increased and soil temperature decreased. I tested the trade—off hypothesis and the role of these additional factors in controlling upper elevational limits of three pine species distributed along this gradient. Consistent with the trade—off hypothesis, results suggested that water stress controlled lower elevational limits of all three species. Seeds of each species germinated with the summer rains in experimental plots below their respective lower elevational limits, but all seedlings died by the end of the following May—June drought, apparently from water stress. In contrast, seedlings were still alive in experimental plots within each species' range after 2 yr. Furthermore, with decreasing elevation, seedlings of the three species increasingly occurred in microsites with relatively low light, low soil temperature, and deep litter, all reflecting high soil moisture compared to random microsites. From the middle to the lower portion of each species' range, recruitment, seedling survival, and seedling abundance decreased but height growth increased. Thus, dry season water stress appeared to control lower elevational limits by causing high mortality of young seedlings, rather than by curtailing seed germination or the performance of older seedlings. Inconsistent with the trade—off hypothesis, upper elevational limits were not controlled uniformly across species by light limitation. In Pinus leiophylla, the middle elevation species, low light and deep litter appeared to control the upper elevational limits. In a field experiment, P. leiophylla emergence and survival were significantly lower above its upper elevational limit than in plots within its range, removal of litter increased emergence, and removal of canopy increased seedling survival. In a greenhouse experiment, P. leiophylla was significantly less shade tolerant than higher elevation pine species. In contrast, P. discolor, the low elevation species, low light, deep litter, and low soil temperature appeared not to influence distribution. Emergence and survival were actually higher at high than middle elevations in the field experiment. Litter removal and canopy removal did not increase P. discolor emergence and survival, respectively, even at high elevation. In the highest elevation plots, P. discolor seedlings occurred in microsites slightly lower in light, higher in litter depth, and equivalent in soil temperature to random microsites, contrary to expectations if these variables were limiting. Finally, in greenhouse experiments, P. discolor was more shade tolerant than higher elevation species, including P. leiophylla. Two tests supported the hypothesis that the upper elevational limits of P. discolor were controlled by the high fire frequency found at higher elevation. First, P. discolor exhibited slow juvenile growth rates, thin bark, and other traits suggesting a lack of fire resistance compared with the two higher elevation pine species. Second, in two wild fires, survival of P. discolor stems was significantly lower than that for the other two species. This conclusion is corroborated by the observation that juvenile P. discolor occurred commonly at much higher elevations than did adults, into plots with very low light and soil temperature levels and very deep litter, a pattern likely resulting from fire suppression. Results for a third species, P. engelmannii, were equivocal, showing weak support for control of upper elevational limits by light. The lack of a light—soil moisture trade—off in these species may result from P. discolor's strategy of exploiting nurse tree sites at low elevation and the apparent fire—associated regeneration of the other two species. Nevertheless, control of P. discolor upper elevational limits by fire may, in part, be a result of constraints imposed by drought resistance on maximum growth rate and height. These results suggest that fire, or other agents of selective mortality correlated with soil resource gradients, can exert strong control over plant distribution and community composition, and should be incorporated into the proposed general models relating plant strategies to community structure.

Factors Controlling Plant Distributions: Drought, Competition, and Fire in Montane Pines in Arizona

Classical water relations theory predicts that predawn plant water potential should be in equilibrium with soil water potential (soil Ψw) around roots, and many interpretations of plant water status in natural populations are based on this expectation. We examined this expectation for two salt-tolerant, cold-desert shrub species in glasshouse experiments where frequent watering assured homogeneity in soil Ψw and soil-root hydraulic continuity and where NaCl controlled soil Ψw. Plant water potentials were measured with a pressure chamber (xylem Ψp) and thermocouple psychrometers (leaf Ψw). Soil Ψw was measured with in situ thermocouple psychrometers. Predawn leaf Ψw and xylem Ψp were significantly more negative than soil Ψw, for many treatments, indicating large predawn soil-plant Ψw disequilibria: up to 1.2 MPa for Chrysothamnus nauseosus (0 and 100 mm NaCl) and 1.8 MPa for Sarcobatus vermiculatus (0, 100, 300, and 600 mm NaCl). Significant nighttime canopy water loss was one mechanism contributing to predawn disequilibrium, assessed by comparison of xylem Ψp for bagged (to minimize transpiration) and unbagged canopies, and by gas exchange measurements. However, nighttime transpiration accounted for only part of the predawn disequilibrium. Other mechanisms that could act with nighttime transpiration to generate large predawn disequilibria are described and include a model of how leaf apoplastic solutes could contribute to the phenomenon. This study is among the first to conclusively document such large departures from the expectation of predawn soil-plant equilibrium for C3 shrubs, and provides a general framework for considering relative contributions of nighttime transpiration and other plant-related mechanisms to predawn disequilibrium.

Predawn disequilibrium between plant and soil water potentials in two cold-desert shrubs.

Summary

1
Differences between growth forms in the spatial heterogeneity of associated soil resources, such as water, are well-documented. We tested for differences in the temporal heterogeneity of soil moisture between natural grassland, shrubland and Populus tremuloides forest at the northern edge of the Great Plains.

2
Weekly measurements of soil moisture over a year, and daily measurements during a growing season, both showed significant interactions between habitat and time. Soils under grassland and shrubs were wettest at the start of the growing season but driest at the end.

3
The coefficient of variation of soil moisture content over time during the growing season was significantly higher in grassland than in forest. Similar results were found for whole-year measurements, and at two depths beneath the soil surface (10 and 30 cm).

4
We found little interception of rainfall in any vegetation type. The net effect of vegetation on soil moisture during a drying period, however, was significantly greater in grassland than in forest, suggesting that differences in the temporal heterogeneity of soil moisture content are related to resource uptake.

Temporal heterogeneity of soil moisture in grassland and forest

The effects of different plant life—forms, including a bunch grass species, Andropogon virginicus L. (broomsedge), and a sprouting deciduous tree species, Liquidambar styraciflua L. (sweetgum), on soil moisture and competitive responses of a transplanted coniferous tree seedling, Pinus taeda L. (loblolly pine), were investigated. Addition of the bunch grass and/or hardwood sprouts either had no effect or increased soil moisture in the surface soil (0—14 cm) depending on the time, while addition of sweetgum and/or broomsedge (greatest density alone) decreased soil moisture in deeper portions of the solum during the summer months. Soil moisture available to pine seedlings at various points in time was assessed by measuring predawn xylem pressure potential. Temporal variation in predawn xylem pressure potential was accounted for through a water stress integral approach. More than half of the variation in pine size after one growing season could be accounted for by the water stress integral. See full-text art...

https://www.jstor.org/stable/info/1941799

Spatial and Temporal Variation in Competitive Effects on Soil Moisture and Pine Response

The effects of different plant life-forms, including a bunch grass species, Andropogon virginicus L. (broomsedge), and a sprouting deciduous tree species, Liquid- ambar styraciflua L. (sweetgum), on soil moisture and competitive responses of a trans- planted coniferous tree seedling, Pinus taeda L. (loblolly pine), were investigated. Addition of the bunch grass and/or hardwood sprouts either had no effect or increased soil moisture in the surface soil (0-14 cm) depending on the time, while addition of sweetgum and/or broomsedge (greatest density alone) decreased soil moisture in deeper portions of the solum during the summer months. Soil moisture available to pine seedlings at various points in time was assessed by measuring predawn xylem pressure potential. Temporal variation in predawn xylem pressure potential was accounted for through a water stress integral ap- proach. More than half of the variation in pine size after one growing season could be accounted for by the water stress integral.

T. H. Green

Papers

Spatial and Temporal Variation in Competitive Effects on Soil Moisture and Pine Response

Effects on soil moisture and pine response