Leaf dry mass per unit area (LMA) is a product of leaf thickness (T) and of density (D). Greater T is associated with greater foliar photosynthetic rates per unit area because of accumulation of photosynthetic compounds; greater D results in decreased foliage photosynthetic potentials per unit dry mass because of lower concentrations of assimilative leaf compounds and decreases in intercellular transfer conductance to CO2. To understand the considerable variation in T and D at the global scale, literature data were analyzed for 558 broad-leaved and 39 needle-leaved shrubs and trees from 182 geographical locations distributed over all major earth biomes with woody vegetation. Site climatic data were interpolated from long-term world climatologies (monthly precipitation, surface temperature) or modeled using the Canadian Climate Center Model (monthly global solar radiation). Influences of total annual precipitation (WT), precipitation of the driest month (Wmin), monthly mean precipitation of the three dries...

Global-scale climatic controls of leaf dry mass per area, density, and thickness in trees and shrubs

Plant primary production is ultimately dependent on photosynthetic CO2 uptake. However, photosynthate production also depends on the amount of photosynthetically active tissue constructed by the plant and on many aspects of carbon use (expenses and investments). Over almost a century, physiological ecology of gas exchange has dominated causal explanations of plant growth. As gas exchange studies became easier to do, and thus more common over the last decade, it also became clear that their contribution towards a deeper understanding of plant growth has been greatly overestimated. When I was asked to outline some of the future needs for a better understanding of plant growth, my feeling was that other aspects of plant growth control, aside from gas exchange, have already been highlighted sufficiently by several authors. However, most of the concepts discussed still have penetrated neither textbooks nor the entire spectrum of related research. Thus, the only justification for reconsidering this field of plant biology (at the risk of being repetitive) may be the persistent and widespread ignorance of the multitude of plant attributes, other than leaf photosynthesis, that affect plant growth. Recent conceptual advances resulting from detailed investigations of plant growth underline the great significance of dry matter partitioning, respiration, life-spans and 'costs' of leaves, developmental processes, herbivore-defence and canopy structure as growth determinants. These characteristics often completely mask variation in predictions of growth, based on leaf photosynthetic capacity. A particularly nice example is the study on photoperiodism in grasses by Heide and co-workers (e.g. Heide, Bush & Evans, 1985). They showed that dosis-neutral manipulations of daylength can increase relative growth rate by 40%, despite a decline in unit-leaf-area-based rate of photosynthesis. The growth of lettuce (Lorenz & Wiebe, 1980) and tomato (Smeets & Garretsen, 1986) are two other examples where the unsuitability of photosynthesis as a criterion for growth control has been demonstrated. Mooney & Chiariello (1984) showed that 5 weeks of synchronous growth is sufficient for wild radish to reach twice the total mass of cultivated radish, just because a greater fraction of photosynthate is allocated to new leaf tissue rather than storage tissue. Poorter (1989) has examined and reviewed the reasons for the widespread discrepancy between relative growth rate and net assimilation rate, and concludes that leaf weight ratio and specific leaf area are of paramount significance. Both of these plant characteristics are often controlled by cost/benefit requirements of the plant growing under a particular set of environmental conditions (e.g. water or nitrogen supply, or herbivory). However, there are situations, where none of these mechanistic explanations apply, because developmental constraints as, for example, the number and size of primordia or flowering induction, are determining growth (Korner & Pelaez Menendez-Riedl, 1989). Hence, there is no a priori reason to assume that any of the aforementioned growth determinants is 'the' key parameter that controls the translation of leaf photosynthesis into increments of total plant dry matter. It should suffice here to refer to some particularly instructive texts, published within the last few years, for further reading: Mooney & Chiariello (1984); Bloom, Chapin & Mooney (1985); Bazzaz et al. (1987); Chapin et al. (1987); Lambers (1987); Pearcy et a]. (1987); Schulze & Chapin (1987); Lambers et al. (1989); Williams, Field & Mooney (1989). If we are to understand growth, we are left with the problem that a multitude of variables must be studied simultaneously. When conducting a comprehensive growth analysis there is no way to avoid this complexity. Equal growth may be achieved with various combinations of leaf photosynthetic capacity, leaf area per total plant mass,

Some often overlooked plant characteristics as determinants of plant growth: a reconsideration.

It has been proposed that in the harsh arctic and alpine climate zones, small microtopographic variations that can generate more benign conditions than in the surrounding environment could be perceived as safe sites for seedling recruitment. Cushion plants can modify wind pattern, tem- perature and water availability. Such modifications imply that cushion plants could act as 'nurse plants' facilitating the recruitment of other species in the community. This effect should be more evident under stressful conditions. We tested these hypotheses comparing the number of species that grow inside and outside Bolax gummifera cushions at two eleva- tions (700 and 900 m a.s.l.) in the Patagonian Andes of Chile (50 ∞S). At both elevations, and in equivalent areas, the number of species was registered within and outside cushions. A total of 36 and 27 plant species were recorded either within or outside B. gummifera cushions at 700 and 900 m a.s.l., respec- tively. At 700 m a.s.l., 33 species were recorded growing within cushions and 29 outside them, while at 900 m a.s.l. these numbers were 24 and 13 respectively. At both elevations there were significantly more species growing within than outside cushions, and the proportion of species growing within cushions increased with elevation. Thus there is a nurse effect of cushion plants and it is more evident at higher elevations. Shelter from wind and increased soil water availability seem to be the factors that increase plant recruitment within cushions.

Nurse effect of Bolax gummifera cushion plants in the alpine vegetation of the Chilean Patagonian Andes

We studied the effects of high temperature and drought on the survival, growth and water relations of seedlings of Pinus ponderosa (Dougl.) Lawson, one of few coniferous tree species that can successfully colonize drought-prone sites with high soil surface temperatures. Temperature profiles were measured with 0.07-mm thermocouples in a sparse ponderosa pine forest in northern Idaho. The soil surface and the adjacent 5 mm of air reached maximum temperatures exceeding 75 degrees C, well above the lethal temperature threshold for most plants. Air temperatures 50 mm above the soil surface (seedling needle height) rarely exceeded 45 degrees C. Pinus ponderosa seedlings that survived maintained basal stem temperatures as much as 15 degrees C lower than the surrounding air. The apparent threshold temperature at the seedling stem surface resulting in death was approximately 63 degrees C for less than 1 min. No correlation between seedling mortality and needle temperature was found, although some needles reached temperatures as high as 60 degrees C for periods of </= 1 min. Surviving seedlings had significantly higher stomatal conductance than seedlings that did not survive until fall. Transpiration rates, calculated from measured needle temperatures, stomatal conductance and evaporative demand, were high (up to 16.7 mmol m(-2) s(-1)), indicating that water transport through seedling stems may have acted as a heat transfer mechanism, cooling the stem below the lethal threshold temperature. Heat exchange calculations showed that rapid water flow through seedling stems can absorb sufficient energy to reduce stem temperature by 30 degrees C during peak sunlight hours.

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High temperature and drought stress effects on survival of Pinus ponderosa seedlings

Low growing, compact cushion plants are a common and often dominant life form in temperate and subpolar alpine habitats. The cushion life-form can modify wind patterns, temperature and water availability and thus cushion species could be expected to act as nurse-plants facilitating the establishment of other alpine plant species on their surfaces. It has been suggested that the nurse effect should be most pronounced under more stressful environmental conditions, as found with increasing elevation in the alpine. One of the approaches used to detect the nurses has been the study of spatial associations among species, in which extreme clumping within or beneath one species has been interpreted as evidence of nursing. We characterized microclimatic conditions (soil and air temperature) within and outside cushions of Azorella monantha at two elevations (700 m a.s.l., corresponding to an elevation just above treeline, and 900 m a.s.l., corresponding to the upper limit of the cushion belt zone) on Cerro Diente in the Patagonian alpine of southern South America (50° S) and recorded all plant species growing upon cushions of various sizes and for paired sampling areas of equivalent sizes outside cushions. At 5 cm depth, soil temperature was slightly higher under cushions than under bare ground, but only significantly so at 900 m. Air temperature at ground level was significantly higher in the cushion microhabitat at both 700 m and 900 m, with the difference being more exaggerated at the highest elevation. At 700 m, a total of 27 species were recorded growing within cushions as compared to 29 outside cushions. At 900 m the corresponding numbers were 34 and 18. At the highest elevation, significantly more species grow within cushions than for equal areas outside cushions. Here moreover, 17 (48.6%) species grew preferentially within cushions, with eight of the latter being limited to the cushion microhabitat at this elevation. However, at 700 m there was no significant difference in species richness in the two microhabitats, and only one species (3.1%) grew preferentially on cushions. Considering individual species, nine occurring at both elevations showed non-preferential recruitment on cushions at 700 m, but significantly higher frequencies on cushions at 900 m. Results suggest striking altitudinal variation in the association with Azorella monantha on Cerro Diente, ranging from a very strong at 900 m to near absence at 700 m. Milder air and soil temperatures, shelter from wind, and greater water availability within cushions as opposed to outside cushions are discussed as possible factors favoring strong plant recruitment on cushions at higher elevations in the harsh Patagonian alpine environment.

Positive associations between the cushion plant Azorella monantha (Apiaceae) and alpine plant species in the Chilean Patagonian Andes

The thermal motion of molecules responsible for the property we know as temperature influences essentially every aspect of plant biology. Emphasis in this review is on principles of wide biological applicability, with specific examples being chosen from research on agaves and cacti. Plant temperatures can be predicted using energy budgets incorporating shortwave and longwave radiation, heat conduction and convection, latent heat, and heat storage. Energy budgets are used here to show the effect of plant size and shortwave absorptance on tissue temperature. The importance of air temperature or transcription rate for tissue temperature is calculated quantitatively. The influences of surface appendages, such as apical pubescence and spines, on minimal temperatures near the apical meristem of cacti are predicted, such minimal temperatures influencing the geographical distribution of various species. The Boltzmann energy distribution and Arrhenius plots are presented and used to analyse thermal responses in terms of energy barriers and activation energies. The many ways that temperature can influence CO2 diffusion into a leaf are also considered. Tolerances to low and high temperatures, such as the ability of agaves and cacti to tolerate extremely high tissue temperatures of 70 degrees C, are discussed from both cellular and ecological perspectives. Although the influences of temperature on plants are multitudinous, many can be predicted, or at least analysed, based on well-established physical principles.

Nobel Ps

Papers

Principles underlying the prediction of temperature in plants, with special reference to desert succulents.