Shaded Microsites Research Articles

Drought-induced photosynthetic decline and recruitment losses are mediated by light microenvironment in Rocky Mountain subalpine forest tree seedlings

Rates of tree mortality continue to rise with climate change, particularly in high elevation subalpine forests. In these systems, which depend on a narrow range of microsite conditions for successful seedling establishment, growing drought frequency and intensity may impose regeneration bottlenecks that further exacerbate population declines. However, it is unclear how regeneration processes tied to species-specific stress tolerances and microsite preferences are affected by varying magnitudes and durations of drought. To simulate moderate (50%) and severe (100% precipitation exclusion) growing-season drought, we installed precipitation exclusion shelters in shade and canopy gap light microenvironments over naturally-regenerating seedlings across a range of size-delimited establishment cohorts spanning first year emergents to large and well-established seedlings of two dominant subalpine forest species – Engelmann spruce (Picea engelmannii Parry ex Engelmann) and subalpine fir (Abies lasiocarpa (Hook) Nutt.). Seedling survival and gas exchange physiology was assessed to test the interacting effects of drought and light microenvironment on seedling performance, with contrasts to each species’ historically preferred microsite under ambient precipitation conditions. Shade partially ameliorated drought effects when precipitation reductions were moderate as illustrated by rates of seedling mortality that were statistically indistinguishable from reference rates for both species, whereas moderate drought in canopy gaps significantly or nearly-significantly increased seedling mortality in several cohorts of both spruce and fir (1.9–2.9 and 2.3–7.5 times greater mortality risk in spruce and fir, respectively). When drought was severe, however, shade induced prolonged net-negative photosynthetic carbon assimilation as low as a −1.5 μmol CO2·m−2·s−1 (growing-season average), which occurred alongside significantly increased mortality in spruce (3.2–9.4 times greater mortality risk). In contrast, mortality was significantly higher in canopy gaps under severe drought for fir (5.5–23.1 times greater mortality risk). Larger seedling size was strongly associated with lower mortality risk and more conservative stomatal behavior (e.g., predominantly <0.1 mol·H2O·m−2·s−1 growing-season averages), illustrating the sensitivity of new seedlings to microsite environments. However, persistent declines in photosynthetic carbon uptake observed across all seedling sizes in shaded microsites suggests the potential for lagged mortality and greater susceptibility to recurring drought even for larger and seemingly well-established seedlings. These results demonstrate how acute, intense droughts may alter historical patterns of spruce-fir seedling regeneration and highlight the importance of maintaining suitable microsites when implementing adaptive management strategies to avoid regeneration bottlenecks in forests that are already vulnerable to accelerating drought-induced mortality.

Long-term microclimate study of a peatland in Central Europe to understand microrefugia

Peatlands perform many important ecosystem functions at both the local and global scale, including hydrologic and climatic regulation. Although peatlands often act as climatic microrefugia, they have rarely been the subject of long-term microclimatic studies. In this study, we aimed to compare the local climatic conditions of a mid-forest mire to that of an open area and examine the differences in microclimates within the mire based on plant community diversity, shading, and water table depths. The peatland studied in this work was significantly cooler than the reference site, mainly due to a higher decline in nighttime air temperatures. However, the daily maximum air temperature near the ground was often higher. We also noticed that microclimates significantly differed within the studied peatland. Wet and shaded microsites were cooler than the sites having a lower water level and receiving higher amounts of solar radiation. The results of the study suggest that peatlands have locally cooler climates, and thus can serve as climate change refugia. These findings can help us interpret reconstructed data from the peat archive, and, when combined with experiments, identify tipping points for peatland ecosystems.

Interactive effects of shading and disturbance on plant invasion in an arid shrubland: Assembly processes and CSR‐strategies

Abstract Disturbance by small mammals and shrub canopies are ecological factors typical of arid ecosystems that may influence plant invasion through environmental and community changes. Whereas disturbance beneath shrub canopies may promote invasion by removing dominant species, disturbance in open areas may hinder plant invasion by increasing environmental harshness. However, we are unaware of studies explicitly addressing the interactive effects of disturbance by mammals and shading by shrubs on community assembly processes to understand plant invasion. In an arid shrubland, disturbance and shading were caused by the fossorial rodent Spalacopus cyanus and the shrub Flourensia thurifera, respectively. We used functional dispersion data (trait convergence vs. divergence) and Grime´s theory (competitive, stress‐tolerant and ruderal strategies, CSR) to gain insights into the underlying assembly processes. We compared environmental conditions, richness and abundance of native and exotic species, as well as functional dispersion and prevalence of CSR‐strategies across four microsites (60 × 60 cm plots): undisturbed/open, undisturbed/shaded, disturbed/open and disturbed/shaded. We tested for functional differences between native and exotic species. Species richness was similar among microsites for both native and exotic species. Shading ameliorated environmental conditions, promoted trait divergence and increased prevalence of C‐ and R‐strategies. Disturbance increased the abundance of exotic species and removed dominant species in shaded microsites. Exotic and native species were functionally different: exotics showed shorter life span, lower height, thinner stems and smaller leaves than natives. Synthesis. Disturbance, by removing plant biomass, favoured exotic species in shaded microsites, where shrub canopy ameliorated environmental conditions and—by relaxing habitat filtering—promoted niche partitioning processes as well as C‐ and R‐strategies. We illustrate the value of linking trait‐based ecology approaches to micro‐environmental conditions, as it may provide insights into the underlying community processes when studying plant invasions at the local and microsite scales. Disturbed microsites beneath shrub canopies could be ‘hotspots’ where to concentrate efforts to manage plant invasions in arid ecosystems.

Sublethal fitness costs of chronic exposure to hot weather vary between sexes in a threatened desert lark

ABSTRACT Behavioural trade-offs between foraging and thermoregulation result in negative relationships between diurnal body mass (M b) gain and daily maximum air temperature (T max) in arid-zone birds. However, it remains unclear how these trade-offs are affected by habitat type or sexually dimorphic behaviour. Using the South African arid-zone endemic Red Lark (Calendulauda burra) as a model taxon and focusing solely on non-breeding adults, we evaluated predictions that a) time-activity budgets are more strongly affected by air temperature (T a) than is the case for species occurring in arid savanna habitats with greater access to shaded microsites, and b) the effects of T max on diurnal M b gain differ between males and females. Behavioural observations revealed an effect of T a on activity, and sex differences in the temperature-dependence of resting and wing-spreading. Moreover, M b data obtained from habituated larks trained to perch on an electronic balance revealed that in males, but not females, diurnal M b gain was negatively related to T max, with diurnal M b gain equivalent to average overnight M b loss when T max = 33.5°C and zero diurnal M b gain (i.e. net 24-hr M b loss of ~ 5 %) when T max = 40.0°C. Under recent climate conditions, male Red Larks rarely, if ever, experienced consecutive days with T max > 40.0°C but, under an unmitigated climate change scenario, will experience 10–15 consecutive days per summer by the end of the 21st century. Our findings suggest that this threatened lark species is unlikely to persist across much of its current range by 2100.

Spatial genet dynamics of a dwarf bamboo: Clonal expansion into shaded forest understory contributes to regeneration after an episodic die‐off

AbstractThe ability of clonal plants to spread horizontally and to share resources within genets has long been considered advantageous in spatially heterogeneous environments, yet our understanding of how such traits relate to its widespread success and dominance is still limited. Using a dwarf bamboo, Sasa kurilensis, that often dominates cool‐temperate forest understorys, we investigated how population recovery over 20 years after an episodic die‐off may be augmented by clonal expansion via rhizomes. Previous analyses on genet demography using 9‐m2 plots showed that more productive genets were more likely to survive, spread laterally, and replace less productive ones. In this study, we examined whether the recovery of biomass in lower light microsites, where biomass recovery was initially slower, was supported by the spread of productive genets at larger scales, from surrounding higher‐light microsites. We found that the biomass recovery in lower‐light plots was more supported by genets that spread clonally into the plots. Such genets that spread from outside plots produced larger culms than those that had originally germinated there. Whereas genets that contributed much to the biomass of the low‐light plots spread extensively from higher‐light microsites, the spatial extent of genets that originally germinated in these plots was quite limited, so that the patterns of clonal expansion appeared to be unidirectional along the light gradient. Our findings suggest that clonal expansion of productive genets from higher‐light into shaded microsites may be important for S. kurilensis to proliferate across heterogeneous light environments.

Chronic, sublethal effects of high temperatures will cause severe declines in southern African arid-zone birds during the 21st century

Birds inhabiting hot, arid regions are among the terrestrial organisms most vulnerable to climate change. The potential for increasingly frequent and intense heat waves to cause lethal dehydration and hyperthermia is well documented, but the consequences of sublethal fitness costs associated with chronic exposure to sustained hot weather remain unclear. Using data for species occurring in southern Africa's Kalahari Desert, we mapped exposure to acute lethal risks and chronic sublethal fitness costs under past, present, and future climates. For inactive birds in shaded microsites, the risks of lethal dehydration and hyperthermia will remain low during the 21st century. In contrast, exposure to conditions associated with chronic, sublethal costs related to progressive body mass loss, reduced nestling growth rates, or increased breeding failure will expand dramatically. For example, by the 2080s the region will experience 10-20 consecutive days per year on which Southern Pied Babblers (Turdoides bicolor) will lose ∼4% of body mass per day, conditions under which this species' persistence will be extremely unlikely. Similarly, exposure to air temperature maxima associated with delayed fledging, reduced fledgling size, and breeding failure will increase several-fold in Southern Yellow-billed Hornbills (Tockus leucomelas) and Southern Fiscals (Lanius collaris). Our analysis reveals that sublethal costs of chronic heat exposure are likely to drive large declines in avian diversity in the southern African arid zone by the end of the century.

Early establishment and survival of the neotropical dry deciduous forest treeLysiloma microphyllumfrom mountainous Bajio Queretano, Mexico

Abstract:The tropical dry deciduous forest (TDF) of the semi-arid region of Bajio in Mexico is dominated by successional communities, in whichLysiloma microphyllumis irregularly distributed, however the environmental factors influencing its establishment have been poorly studied. We conducted field experiments to isolate the impact of three factors on the early life-history stages of this tree. Over a 2-y period, we monitored the effects of (1) predation, (2) understorey light and (3) aspect on germination, seed-to-seedling transition (establishment) and seedling survival, and we found that open space negatively affected germination (14.9 ± 1.5% vs 96 ± 1.5% in shade; mean ± SE), while shade increased seedling establishment and survival (36.9 ± 8.2% vs 4.7 ± 3.8% in open sites). Predation decreased survival and was strongest in open conditions (0 vs 62.7 ± 9.4% in shaded exclosures). In contrast, aspect had little influence on germination and survival. The results suggest that shaded microsites positively affect the early life stages ofL. microphyllum, a key process in TDF recovery.

Plasticity in Pinus pinaster populations of diverse origins: Comparative seedling responses to light and Nitrogen availability

The effects of light and N availability, and population on seedling morphology and biomass allocation were assessed in eleven months seedlings from ten Pinus pinaster populations. We conducted a garden experiment using a light gradient (full sunlight, HL= 337.1 μmol m2 s-1; medium light, ML= 120.6 μmol m2 s-1 and low light, LL= 30 μmol m2 s-1) and two contrasting soil fertility regimes: low-N and high-N availability. Light availability was an important factor controlling phenotypic plasticity. However, differences between productivity environments were observed, especially in full sunlight and medium light conditions, which suggest that soil fertility may be very important at early successional stages at open and medium shaded microsites, but would exert low influence in deep shade conditions. Population-specific responses were observed for many traits, which suggest a genetic control of morphological plasticity. All populations produced more total dry weight with HL and high-N. When light and/or N richness varied, growth depended upon which resource was more limiting. Results indicated ontogenetic drift but when seedlings were compared at the same size, results fitted with optimal partitioning theory. Allocation to needles was higher in HL and ML/high-N conditions, while allocation to root was higher in low-N seedlings in HL and ML conditions. With shade (ML and LL) all populations allocated more dry weight to stem, regardless N availability. A significant increment in stem height was observed with shade increase.

Low clouds and cloud immersion enhance photosynthesis in understory species of a southern Appalachian spruce–fir forest (USA)

High-altitude forests of the southern Appalachian Mountains (USA) are frequently immersed in clouds, as are many mountain forests. They may be particularly sensitive to predicted increases in cloud base altitude with global warming. However, few studies have addressed the impacts of immersion on incident sunlight and photosynthesis. Understory sunlight (photosynthetically active radiation, PAR) was measured during clear, low cloud, and cloud-immersed conditions at Mount Mitchell and Roan Mountain, NC (USA) along with accompanying photosynthesis in four representative understory species. Understory PAR was substantially less variable on immersed vs. clear days. Photosynthesis became light-saturated between ∼100 and 400 μmol · m(-2) · s(-1) PAR for all species measured, corresponding closely to the sunlight environment measured during immersion. Estimated daily carbon gain was 26% greater on clear days at a more open canopy site but was 22% greater on immersed/cloudy days at a more closed canopy site. F(v)/F(m) (maximum photosystem II efficiency) in Abies fraseri seedlings exposed to 2.5 min full sunlight was significantly reduced (10%), indicating potential reductions in photosynthesis on clear days. In addition, photosynthesis in microsites with canopy cover was nearly 3-fold greater under immersed (2.6 mmol · m(-2) · h(-1)) vs. clear conditions (0.9 mmol · m(-2) · h(-1)). Thus, cloud immersion provided more constant PAR regimes that enhanced photosynthesis, especially in shaded microsites. Future studies are needed to predict the survival of these refugial forests under potential changes in cloud regimes.

Facilitation of Seedling Microsites by Rhododendron Caucasicum Extends the Betula Litwinowii Alpine Treeline, Caucasus Mountains, Republic of Georgia

ABSTRACTIn the Central Greater Caucasus Mountains, Georgia, Betula litwinowii (birch) occurs on north-facing slopes of east-west ridgelines that extend upward to high mountain peaks, forms the alpine timberline at higher elevation, and reaches its highest treeline limit only when associated with the broadleaf evergreen shrub, Rhododendron caucasicum. This association might generate an ecological facilitation of either temperatures or sky exposure, both of which have been related to the altitudes at which timberlines/treelines occur. At the lowest site (2072 m) the greatest abundance of birch seedlings (up to 2.3 seedlings/m2) occurred at shaded microsites beneath the B. litwinowii overstory and along shaded north-facing walls of polyhedral soil depressions just beyond this treeline. These seedling microsites also had substantially colder air and soil temperature regimes than more sun-exposed microsites. Similarly, at the highest elevation site (2512 m) the second greatest seedling abundance (0.73 seedlings/m2) occurred in the shaded understory beneath R. caucasicum. Moreover, these microsites had the coldest minimum air and soil temperatures (−1.3°C at 5 cm depths), along with the greatest number of days (40) with minimum soil temperatures <5°C recorded for the measurement period (11 July to 25 October 2003). In addition to the lowest number of seedlings, the more sun-exposed microsites at all sites also had the greatest percent (28–32%) of red leaves per plant, indicative of high concentrations of photoprotective anthocyanins. Thus, reduced sky exposure, and not cold temperature effects, was associated with greater seedling abundance and fewer red leaves per seedling, despite colder temperature regimes. Thus, facilitation of B. litwinowii seedling establishment by the R. caucasicum overstory appeared to extend the maximum altitude of the Betula treeline via reductions in sunlight exposure, despite lower temperatures.

Differences in growth and light requirement of two sympatric congeneric tree species in an Indonesian floodplain forest

A prominent hypothesis to account for the maintenance of the woody species diversity in tropical forests is niche differentiation with respect to resources (Ashton 1969, Connell 1978). According to the hypothesis, species can coexist when each species is specialized for a specific set of growth conditions. Spatial and temporal variations in the forest understorey light conditions can be an important niche gradient. If species are adapted to a certain light condition for optimizing growth and share different optimal light conditions with each other, they can coexist by reducing direct competition; i.e. the regeneration niche differentiation sensu Grubb (1977). Species difference in rate of exclusion of trees in suboptimal light conditions due to mortality also contributes to species coexistence. Many scientists have emphasized the importance of the spatial and temporal variations in understorey light conditions on species coexistence in tropical forests (Clark & Clark 1992, Kohyama & Hotta 1990, Poorter & Arets 2003). Allometry and architecture of a tree reflect strategies of space occupation in a forest (King 1990, Kohyama & Hotta 1990, Poorter & Werger 1999). Thus, tree allometry and architecture provide information on species coexistence. Yamada et al. (2005) analysed interspecific differences in allometry and architecture of two sympatric congeneric species, Pterospermum diversifolium and P. javanicum, in a Bornean floodplain forest and concluded that the sapling growth strategy of P. diversifolium appears to be dynamic, emphasizing the opportunistic use of light following a disturbance, whereas that of P. javanicum appears to be static, emphasizing better leaf display for current light conditions. Based on these results, they proposed equilibrium coexistence mechanisms between the two species in a community. The advantage of these strategies depends on context, and thus the two species may coexist by adopting different regeneration niches based on differing understorey light conditions; P. diversifolium was hypothesized to outperform P. javanicum in bright microsites, and on the other hand, P. javanicum is hypothesized to outperform P. diversifolium in shaded microsites.

Effect of shrubs and seasonal variability of rainfall on the establishment of Aspidosperma quebracho‐blanco in two edaphically contrasting environments

Abstract Three experiments were conducted to verify if an increase in environmental stress level would affect the interactions between two species of nurse shrubs and seedlings of Aspidosperma quebracho‐blanco. This is a mesic species with a generalist distribution over an extensive environments gradient. The relationship between Larrea divaricata and seedlings of A. quebracho‐blanco was studied in two contrasting soils, a silty loam soil with higher surface clay content and a sandy loam soil. The effect of seasonal variability of rainfall on the initial establishment of seedlings under the shade of L. divaricata was evaluated in three consecutive years. The effect of nurse plant shade was tested comparing two shrub species with different types of leaf life span (sclerophyllous‐evergreen and leguminous‐deciduous). The natural establishment of A. quebracho‐blanco depended on shaded microsites, but not on the type of shade provided by different nurse shrubs. Emergence and initial establishment depended on interactions of soil type and seasonal rainfall variation with nurse plants. The importance of facilitation increased with clay soil (CS). Sandy soil was ‘less humid’ than CS under shrub shade. However, establishment success depends on opportune even rainfall distribution in interaction with nurse plant presence.

Light-dependent seedling survival and growth of four tree species in Costa Rican second-growth rain forests

The dependence of tree seedling survival and growth on light availability was evaluated over 15 mo in three second-growth, wet forest stands (15–18 y old) in north-eastern Costa Rica. Seedlings of four canopy tree species (Dipteryx panamensis, Hyeronima alchorneoides, Virola koschnyi and Vochysia guatemalensis) were planted into four canopy treatments in three replicated stands and in a pasture site, spanning a nearly complete gradient of light availability. Survival and growth of all species increased in response to increasing light availability, but species differed in survival in shaded microsites (6–20% light transmittance) and in growth increments per light increase. Hyeronima showed the highest mortality at low light levels and the highest relative height growth increase per light increase. In contrast, Virola showed high survival at low and moderate light, but showed the lowest leaf area and above-ground biomass per light increase among all species. Dipteryx and Vochysia maintained relatively high rates of survival and growth across the entire light gradient. Hyeronima and Virola showed trade-offs between growth and survival responses to light, unlike Dipteryx and Vochysia. Differences among species in seedling survival at low light may be determined by a variety of physiological and morphological traits that may or may not be mechanistically linked to growth responses.

Sex expression, skewed sex ratios, and microhabitat distribution in the dioecious desert moss Syntrichia caninervis (Pottiaceae)

The moss Syntrichia caninervis is the dominant soil bryophyte in a blackbrush (Coleogyne ramosissima) community in the southern Nevada Mojave Desert, with a mean cover of 6.3%. A survey of the 10-ha study site revealed an expressed ramet sex ratio of 14♀ : 1♂ (N = 890), with 85% of ramets not expressing sex over their life span, and an expressed population sex ratio of 40♀ : 2♂ : 1♀♂ (female : male : mixed-sex, N = 89), with 52% of populations not expressing sex. A greater incidence of sex expression was associated with shaded microsites, higher soil moisture content, and taller ramets. Shaded microsites had higher surface soil moisture levels than exposed microsites. In the exposed microhabitat, surface soil moisture was positively correlated with ramet height but not with sex expression. Male ramets and populations were restricted to shaded microhabitats, whereas female ramets and populations were found in both shaded and exposed microhabitats, suggesting gender specialization. The rarity of mature sporophytes, found in 0% of the ramets sampled and in only 3% of the populations, is probably due to the rarity of mixed-sex populations. We hypothesize that mixed-sex populations are rare because of factors relating to male rarity and that the differential cost of sex expression reduces the clonal growth capacity of male individuals.

REGENERATION OF THREE SYMPATRIC BIRCH SPECIES ON EXPERIMENTAL HURRICANE BLOWDOWN MICROSITES

Tip-up mounds, pits, and other microsites created by hurricanes may promote diversity in many forests by providing opportunities for different species to regenerate. To see if we could detect differences in microsite preference among closely related species, we studied the regeneration of three sympatric Betula species on five types of microsites on experimental mound–pit complexes. Microsites were created by pulling down canopy trees to simulate damage from past hurricanes in southern New England. Seeds were collected in litter traps and experimentally released over mounds and pits to determine effects of microtopography on fine-scale dispersal patterns. The fate of naturally germinating seedlings was monitored on the disturbed site, and seedlings were also transplanted onto microsites to examine growth patterns, causes of mortality, and leaf-level physiology. Seed rain onto the disturbed site was abundant and spatially heterogeneous because of scattered residual canopy trees and surviving uprooted trees. Seeds tended to disperse away from vertically oriented surfaces of mounds and to accumulate in pits. Most seedlings germinated on scarified level areas rather than on mounds or in pits, but mounds became more favorable for germination the second year following disturbance. Two fundamentally different types of mortality were observed in transplanted seedlings. Extrinsic factors such as frost heaving, burial by soil and litter, and browsing were dominant on some microsites. Mortality due to these factors occurred primarily during the winter and was unrelated to seedling size. On other microsites, resource limitation (low light levels and lack of water or nutrients) was the major cause of death. Small seedlings were most susceptible to mortality on these microsites, and most deaths occurred during the growing season. White birch (Betula papyrifera Marsh.) exhibited the fastest growth and most flexible photosynthetic response to changing light levels but suffered greatest mortality on shaded microsites. Black birch (B. lenta L.) showed increased leaf area ratio in shaded conditions. Yellow birch (B. alleghaniensis Britt.) was least flexible and grew more slowly than the other species but was best able to survive on shaded microsites. All species attained maximum growth on tip-up mounds. After three growing seasons, the tallest seedlings reached nearly 3 m above the forest floor, enabling us to predict which individuals would ultimately reach the canopy to complete the regeneration process.

Thermoregulation in a Lacertid Lizard: The Relative Contributions of Distinct Behavioral Mechanisms

The lacertid lizard Podarcis hispanica atrata maintained remarkably constant body temperatures (Tb) throughout its daily activity period in early autumn, despite significant variability in available operative temperatures (Te). Lizard Tbs were substantially closer to the species' selected temperature range (the target Tbs for thermoregulation, estimated by the central 80% of the temperatures selected in a laboratory photothermal gradient) than were a random sample of Tes, indicating that this population thermoregulates both accurately and effectively. We compared observed behaviors and Tbs to the predictions of two hypotheses. The no thermoregulation null hypothesis predicted that lizard activity and microsite use would be unaffected by thermal constraints. The only thermoregulation" hypothesis predicted that lizards would be active only at those times and in those places where Tes fell within the selected temperature range. To evaluate the contributions of various behavioral adjustments (activity times, use of microhabitats and sun—shade patches, basking, and shuttling) to temperature regulation, we combined results from direct behavioral observations with information on the Tes available at different times of day and in different microsites (combinations of structural microhabitats and sun—shade patches). Lizard activity patterns did not match the predictions of either hypothesis, but the observed pattern of activity produced Tbs that were, on average, 1.0°C closer to the selected temperature range than were Tbs predicted by the no thermoregulation" hypothesis. The lizards' use of microsites (combinations of structural microhabitats and sun—shade patches) was more similar to predictions of the no thermoregulation hypothesis than to those of the only thermoregulation hypothesis. Nevertheless, the lizards' use of specific microsites produced Tbs that were, on average, 1.9°C closer to the selected temperature range than were randomly available Tes. The relative contributions of different behavioral mechanisms varied with time of day. In the early morning, low movement rates, selection of sunny microsites, and most notably, adoption of a basking posture contributed to temperature regulation. In the late morning and early afternoon, no thermoregulatory behaviors were necessary, as indicated by randomly positioned models that achieved mean Tes within the selected temperature range. At midday, frequent movements between sunny and shaded microsites probably reduced heat loads on lizards, preventing Tbs from exceeding the selected temperature range. In the late afternoon, nonrandom use of microsites and basking behavior appeared to contribute equally to the elevation of Tbs above randomly sampled Tes.

The Influence of Light Intensity and Temperature on Microhabitat Selection in Two Anolis Lizards

Although basking behaviour has been widely cited as a thermoregulatory mechanism in reptiles, few previous researchers have studied how lizards select basking sites in nature. 2. We conducted field and laboratory studies to determine whether two species of Anolis lizards in Puerto Rico use light intensity as a proximal cue for recognizing the thermal properties of microhabitat patches. 3. Under natural conditions Anolis cristatellus, a behavioural thermoregulator that we sampled in an open woodland, frequently occupied microsites that were more brightly illuminated than randomly sampled microsites. In contrast, A. gundlachi, a behavioural thermoconformer that we sampled in a shaded forest, occupied perch sites randomly with respect to light intensity. Operative temperatures were significantly correlated with light intensity at all times of day in the A. cristatellus habitat; in the A. gundlachi habitat, however, operative temperatures and light mtensity were significantly correlated only at the higher and more variable light intensities prevalent at midday. 4. We therefore hypothesized that A. cristatellus, using light intensity as a proximal cue, would perch in brightly lighted microsites to raise its body temperature and more shaded microsites to lower its body temperature; we also hypothesized that A. gundlachi would not use light intensity as a cue for selecting thermally appropriate microsites. 5. We tested these hypotheses in a laboratory experiment in which light intensity and operative temperature were uncoupled. Lizards were offered a gradient of light intensities under three constant temperature conditions: 10$^\circ$C below acclimation temperature, the acclimation temperature (set equal to the mean field-active body temperature) and 4$^\circ$C above the acclimation temperature. 6. Microhabitat selection in the light gradient confirmed our hypotheses: light intensity at microsites selected by A. cristatellus varied inversely with experimental temperature but light intensity at microsites selected by A. gundlachi did not vary with experimental temperature.

Shade as a Cause of the Association Between the Cactus Neobuxbaumia Tetetzo and the Nurse Plant Mimosa Luisana in the Tehuacan Valley, Mexico

SUMMARY (1) The establishment phase of Neobuxbumia tetetzo, a giant columnar cactus dominant in the Tehuacan Valley in central-southern Mexico, occurs mostly beneath the canopies of trees and shrubs which act as nurse plants. This pattern cannot be attributed to preferential seed dispersion, as Neobuxbaumia fruits open while still on the plant, dropping c. 1000 seeds fruit-1 randomly around the parent plant. (2) An experimental field analysis of germination and survivorship of cohorts under different environmental conditions was performed in order to determine the underlying mechanisms associated to the establishment of N. tetetzo beneath the canopies of Mimosa luisana, a legume which is the most abundant shrub in the community. The experimental treatments followed a factorial design, with three levels of radiation (open space, artificial homogeneous shade and the shade produced by Mimosa luisana), two levels of predation (exclusion and non-exclusion of predators), and four topographic aspects (north, east, south and west). (3) Seed germination was lowest in open spaces. In all treatments, exclusion from predators significantly increased seedling survival. Only the shaded treatments had live individuals at the end of the experiment, 2 years later. The results suggest that the nurse-plant effect between Neobuxbaumia tetetzo and Mimosa luisana is chiefly the result of differential survival in shaded microsites with less direct solar radiation, and consequently with lower daytime temperatures and lower evaporative demand. (4) In order to determine the generality of these results, field samplings were conducted in two Mexican deserts located outside the tropical belt: the Vizcaino Desert in Baja California and the Gran Desierto de Altar in Sonora. In these deserts direct solar radiation has a southern azimuth all year round. Five of six succulent species analysed showed a significant pattern of greater establishment on the shaded north sides of nurse plants. (5) The amelioration of the physical environment produced by the nurse plants is important in the patch-structurated population dynamics of many communities of desert plants.

Photosynthesis, water relations and mortality in Abies lasiocarpa seedlings during natural establishment.

Few studies have investigated the ecophysiology of natural seedling establishment in forest trees not associated with anthropogenic disturbance. Photosynthesis and water relations measurements were made on one- through four-year-old seedlings of Abies lasiocarpa Nutt. (subalpine fir) establishing naturally in an understory environment. First (current)-year seedlings generally had only cotyledons, whereas most second-year seedlings had both cotyledons and primary leaves. Mortality was high (> 60%) in first-year seedlings with the greatest mortality (> 90%) measured at the more open, sun-exposed sites within the understory. Seedling mortality was negligible after the first year of growth at shaded microsites and after the second year of growth at sunny microsites. Photosynthetic CO(2) uptake at light saturation was considerably lower in first-year than in fourth-year seedlings (0.6 micromol m(-2) s(-1) versus 1.7 micromol m(-2) s(-1)) and occurred at lower solar irradiance (240 micromol m(-2) s(-1) versus 600 micromol m(-2) s(-1) of photosynthetically active radiation). Differences in photosynthetic capacity were due to differences in both stomatal and non-stomatal limitations to CO(2) uptake. Carbon dioxide assimilation in first- and second-year seedlings was 28 and 29%, respectively, of the mean value measured for fourth-year seedlings. Although first-year seedlings had low transpiration rates, their water use efficiency (photosynthesis/transpiration) was less than half that of fourth-year seedlings and their water potentials were lower than those of all other age classes (0.46 mol CO(2) mmol(-1) H(2)O and -3.0 MPa, respectively). The stomatal limitation to CO(2) uptake was approximately 21% in first- and second-year seedlings, and increased to 39% in fourth-year seedlings. Intercellular CO(2) concentrations were greater in first- and second-year seedlings (255 and 250 microl l(-1), respectively) than in third- and fourth-year seedlings (203 and 186 microl l(-1), respectively). Thus, abrupt increases in water status and photosynthetic capacity after the first or second year of growth appear crucial for survival to maturity. Moreover, differences in temperature and water relations according to microsite may be major factors determining seedling establishment and, thus, the distributional and successional patterns observed for adult trees of Abies lasiocarpa.

Effects of light quality and quantity on growth of the clonal plant Eichhornia crassipes.

Plant canopy shade reduces photosynthetic photon flux density (PPFD) and ratio of red to far-red light (z). Both effects can cause plants to increase potential for light acquisition through vertical growth and leaf area expansion. Clonal plants such as Eichhornia crassipes might alternatively increase light interception via horizontal growth of stolons or rhizomes and placement of new ramets in less shaded microsites. Effect of simulated canopy shade and component effects of PPFD and z were tested by filtering or adding light uniformly, to a whole group of connected ramets, or locally, to individual ramets within a group. In uniform treatments, low PPFD reduced total growth but low z did not. Low PPFD and low z independently reduced stolon and ramet production and caused etiolation of petioles; effect of low PPFD plus low z on ramet production was greater than that of either factor alone. Lateral clonal growth thus did not seem to be a response to uniform shading; instead, uniformly low PPFD or low z increased partitioning to established ramets. Low z changed partitioning without changing total growth. In local treatments, reduction of growth of individual ramets due to low PPFD and inhibition of new ramet production attributable to spectral composition of light were mitigated when connected ramets were unshaded; plants may respond differently to patchy than to uniform shade.