Search

Microbial communities respond to changes in environmental conditions; however, how compositional shifts affect ecosystem processes is still not well-understood and it is often assumed that different microbial communities will function equally under similar environmental conditions. We evaluated this assumption of functional redundancy using biological soil crusts (BSCs) from two arid ecosystems in Mexico with contrasting climate conditions (hot and cold deserts) following an experimental approach both in the field (reciprocal transplants) and in laboratory conditions (common garden), focusing on the community’s composition and potential for nitrogen fixation. Potential of nitrogen fixation was assessed through the acetylene reduction assay. Community composition and diversity was determined with T-RFLPs of nifH gene, high throughput sequencing of 16S rRNA gene amplicons and metagenomic libraries. BSCs tended to show higher potential nitrogen fixation rates when experiencing temperatures more similar to their native environment. Moreover, changes in potential nitrogen fixation, taxonomic and functional community composition, and diversity often depended on an interactive effect of origin of the communities and the environment they experienced. We interpret our results as legacy effects that result from ecological specialization of the BSC communities to their native environment. Overall, we present evidence of nonfunctional redundancy of BSCs in terms of nitrogen fixation.

In urban Mediterranean ecosystems, invasive alien plants should have characteristics that enable faster resource acquisition and utilization than native species. We tested this hypothesis by comparing stem and leaf functional traits from five abundant native woody shrubs and five of the most abundant alien species within an urban coastal scrub community in Ensenada, Baja California, Mexico. Twelve anatomical and ecophysiological traits were studied: light-saturated CO2 assimilation rate, stomatal conductance, stem and leaf water potentials during transpiration, % stem allocation to bark, xylem and pith, stem xylem vessel diameter and density, leaf size, and % allocation to spongy and palisade mesophyll. Alien species varied more in these traits, and their trait combinations reflected less investment in long-lived tissues; they had higher CO2 assimilation and stomatal conductance rates, higher values of leaf and stem water potential, and higher allocation to pith, larger stem xylem vessels, and larger leaf areas. In contrast, native woody shrub species were relatively convergent in leaf and stem traits reflecting more conservative resource use. Within disturbed urban environments, alien species may outcompete the native stress-tolerant species through rapid resource acquisition, enhanced by a broader set of functional trait combinations across species.

Owing to periodic inundation, wetland plants may have adapted to minimize leaf gas exchange impairments. Increasing the leaf cuticle surface roughness might contribute to buffer flooding stress, since it might increase both leaf water repellency (i.e. leaf hydrophobicity) and the presence of gas films. Thus, a higher leaf hydrophobicity, as well as an increased cuticle roughness, can be expected in species subject to continuous water-logging. We tested this hypothesis by: 1) measuring the contact angle in the liquid-solid interface on leaves of 52 plant species from three coastal wetlands with different diversity and flooding regime in Baja California, Mexico, and 2) by performing scanning electron microscopic analyses in the most hydrophilic and hydrophobic species. Contrary to our expectations, the highest values of leaf water repellency were found in species from non-flooded areas adjacent to the wetlands, whereas neutral leaf water repellency values were found in species subject to flooding, as indicated by the contact angle measurements (77 to 100°). The most water repellent species presented a striking layer of wax on top of the leaf cuticles, while the lest water repellent species were mostly glabrous, which suggest that the augmentation of wax covering in the cuticle might be responding to other environmental factors, such as irradiation.

Variation in the occurrence and the intensity of pre-dispersal seed predation can help understand ecological and evolutionary dynamics of plant populations. Pre-dispersal seed predation can vary in intensity, resulting in costs that differ in magnitude among populations. We examined variation in the cost of pre-dispersal seed predation across nine Yucca schidigera populations, by quantifying yucca moth infestation, damage on fertile seeds and seed mass in 650 fruits sampled from 73 Yucca plants. We recorded Tegeticula mojavella larvae on 100% of the Y. schidigera plants. Across populations, mean infestation rates ranged from 2.9 to 6.11 larvae per fruit. Despite slight variation in the level of fruit infestation, predation on mature seeds differed among the nine populations, ranging from 10 to 26%. Mean seed mass and germination rate differed among populations, but the effect of larvae infestation on these traits was not significant. Altogether, our results demonstrate that infestation and pre-dispersal seed predation by yucca moths differ significantly across populations of Y. schidigera. Variable ratios of seed predation result in different costs across plant populations, despite experiencing similar infestation ratios. It seems that, regardless of its spatial distribution, the interaction between Y. schidigera and T. mojavella is dependent on local ecological factors that regulate oviposition and seed predation.

BackgroundIn arid and semiarid shrublands, water availability directly influences ecosystem properties. However, few empirical tests have determined the association between particular soil and hydrology traits with biodiversity and ecosystem biomass at the local scale.MethodsWe tested if plant species richness (S) and aboveground biomass (AGB) were associated with soil and topographic properties on 36 plots (ca. 12.5 m2) in 17 hectares of chaparral in the Mediterranean-climate of Valle de Guadalupe, Baja California, México. We used close-to-the-ground aerial photography to quantify sky-view cover per species, including all growth forms. We derived an elevation model (5 cm) from other aerial imagery. We estimated six soil properties (soil water potential, organic matter content, water content, pH, total dissolved solids concentration, and texture) and four landscape metrics (slope, aspect, elevation, and topographic index) for the 36 plots. We quantified the biomass of stems, leaves, and reproductive structures, per species.Results86% of AGB was in stems, while non-woody species represented 0.7% of AGB but comprised 38% ofS(29 species). Aboveground biomass and species richness were unrelated across the landscape.Swas correlated with aspect and elevation (R= 0.53, aspectP= 0.035, elevationP= 0.05), while AGB (0.006–9.17 Kg m-2) increased with soil water potential and clay content (R= 0.51,P= 0.02, andP= 0.04). Only three species (11% of totalS) occupied 65% of the total plant cover, and the remaining 26 represented only 35%. Cover was negatively correlated withS(R= -0.38,P= 0.02). 75% of AGB was concentrated in 30% of the 36 plots, and 96% of AGB corresponded to only 20% of 29 species.DiscussionAt the scale of small plots in our studied Mediterranean-climate shrubland in Baja California, AGB was most affected by soil water storage. AGB and cover were dominated by a few species, and only cover was negatively related toS.Swas comprised mostly by uncommon species and tended to increase as plant cover decreased.

Se realizó una comparación multitemporal de cuatro índices de la vegetación en 50 sitios en una región árida subtropical con costa. Los índices de verdor (NDVI, SAVI y TSAVI) y de humedad (NDII) fueron evaluados. NDVI y SAVI estuvieron muy correlacionados mientras que TSAVI fluctuó menos y NDII mostró fuertes variaciones estacionales. La corrección topográfica (superficie iluminada) de los datos crudos causó un incremento en el valor de pendiente de la línea del suelo >20%. Los índices, excepto NDII, se usaron para estimar el Índice de área foliar, y el análisis señal-a-ruido evidenció que SAVI está muy cercano a NDVI, pero TSAVI tuvo mucha mayor señal en los meses secos.

We describe the design and implementation of a low-cost electronic modular data-acquisition (DAQ) system in the context of the eddy-covariance technique. The system was fully tested under laboratory conditions and later installed on two eddy-covariance towers (ECTs) sited in natural terrestrial and marine environments, respectively. The system was divided in four parts: signal conditioning, data-acquisition, data-transfer and data-processing. A data-acquisition module (DAM) based on the ADuC848 microcontroller was designed in order to acquire these data. This DAM could transfer data directly to a computer or embedded system. By configuring a RS-485 network, a DAQ system could be expanded up to 8 DAMs working simultaneously. Due to its modular design, different parts of the equipment could be easily replaced without affecting the operation of the ECT. The system registered high-frequency (20 Hz) measurements of CO 2 , water vapor and wind velocity in the free atmosphere as well as additional ancillary biometeorological variables (1/60 Hz) such as air temperature, solar radiation, soil heat flux, precipitation, and others. Both towers were installed in remote sites using solar cells providing a continuous and autonomous source of energy. This DAQ system proved to be reliable and useful for long-term deployments (> 1 year). Due to its modularity and flexibility the system can be used for any other application involving data recording.

Vegetation greenness (normalized difference vegetation index, NDVI) showed significant temporal and spatial correlations with precipitation and topography-derived features within the context of slope aspect (South- (SFS) and North-facing slopes (NFS) and an intermountain valley (IMV)) in a semi-arid Mediterranean-climate watershed in northwestern Baja California, Mexico. Rank correlation with annual precipitation (1986–2016) showed a strong positive relationship with wet season NDVI at SFS (Rs = 0.82), IMV (Rs = 0.79), and NFS (Rs = 0.65) but moderate relation and only on hillslopes in the dry season (SFS, Rs = 0.47; NFS, Rs = 0.39). Thus, the vegetation on the more xeric SFS sites was more responsive to intra-annual and inter-annual precipitation than on either IMV or NFS. The correlation of NDVI with six topography-derived environmental attributes (elevation, slope gradient, curvature, drainage density, topographic wetness index, solar radiation) was weak to moderate, varied in degree and significance between years with exceptionally high or low NDVI, and often differed in sign between SFS, NFS, and IMV. Results showed that precipitation controlled vegetation greenness, under the three aspect conditions, more closely than did the other topography-derived features, and the sparse deciduous vegetation of SFS showed stronger associations with precipitation than IMV or NFS. The measurement of these relationships should be continued and complemented by other studies to improve the overall model, because they are important to modeling ecohydrology and productivity, and may be of use for projecting and hindcasting vegetation dynamics.

Soil respiration (Rs) has been usually measured during daylight hours using manual chambers. This approach assumes that measurements made during a typical time interval (e.g., 9 to 11 am) represent the mean daily value; locally, this may not always be correct and could result in systematic bias of daily and annual Rs budgets. We propose a simple method, based on the temporal stability concept, to determine the most appropriate time of the day for manual measurements to capture a representative mean daily Rs value. We introduce a correction factor to adjust for biases due to non-optimally timed sampling. This approach was tested in a semiarid shrubland using 24 hr campaigns using two treatments: trenched plots and plots with shrubs. In general, we found optimum times were at night and potential biases ranged from −29 to + 40% in relation to the 24 hr mean of Rs, especially in trenched plots. The degree of bias varied between treatments and seasons, having a greater influence during the wet season when efflux was high than during the dry season when efflux was low. This study proposes a framework for improving local Rs estimates that informs how to decrease temporal uncertainties in upscaling to the annual total.

Soil CO2 efflux is the primary source of CO2 emissions from terrestrial ecosystems to the atmosphere. The rates of this flux vary in time and space producing hot moments (sudden temporal high fluxes) and hot spots (spatially defined high fluxes), but these high reaction rates are rarely studied in conjunction with each other. We studied temporal and spatial variation of soil CO2 efflux in a water-limited Mediterranean ecosystem in Baja California, Mexico. Soil CO2 efflux increased 522% during a hot moment after rewetting of soils following dry summer months. Monthly precipitation was the primary driver of the seasonal trend of soil CO2 efflux (including the hot moment) and through changes in soil volumetric water content (VWC) it influenced the relationship between CO2 efflux and soil temperature. Geostatistical analyses showed that the spatial dependence of soil CO2 efflux changed between two contrasting seasons (dry and wet). During the dry season high soil VWC was associated with high soil CO2 efflux, and during the wet season the emergence of a hot spot of soil CO2 efflux was associated with higher root biomass and leaf area index. These results suggest that sampling designs should accommodate for changes in spatial dependence of measured variables. The spatio-temporal relationships identified in this study are arguably different from temperate ecosystems where the majority of soil CO2 efflux research has been done. This study provides evidence of the complexity of the mechanisms controlling the spatio-temporal variability of soil CO2 efflux in water-limited ecosystems.

In northwestern Mexico in an annual cycle comprised months 2008-2009, we estimated the effects of cumulative rainfall and previous vegetation status on current vegetative greenness adjusted for soil reflectance (via the Transformed Soil Adjusted Vegetation Index (TSAVI)) and canopy water content (via the Normalized Difference Infrared Index (NDII)). Sample kernels (540 of

Approximately four-fifths of the land area of Baja California (BC) in Mexico are occupied by arid and semiarid soils, the mycobiota of which is virtually uncharacterized. In the first culture-independent study of the mycobiota of BC, we collected soil from five different locations in the region and constructed a Dikarya-specific gene library for the ITS region of nuclear ribosomal DNA. Clones were analyzed by RFLP, were sequenced for phylogenetic analyses, and diversity and similarity indices were calculated. The ascomycete Penicillium dipodomyicola was the most frequent fungus found in soil at the most arid location studied, and the basidiomycete Coprinellus radians was the most frequent at the location receiving the highest rainfall. Other frequent members of the soil mycobiota were identified as Alternaria spp., Ceratobasidium sp., Coniozyma leucospermi, Nematoctonus robustus, Penicillium griseofulvum, Tulostoma kotlabae and uncultured members of the Dikarya. Several sequences were identified as those of uncultured fungi, one of which was previously reported from other hot deserts. Arid soils and the transitional zones between arid and semiarid soils had the most similar fungal diversity, with the former soils having a community from which basidiomycetes were absent, and the soil receiving the highest precipitation having a community dominated by basidiomycetes.

Vegetation and small features of ground cover are known to affect erosion, but the areal extent of small features is rarely studied because they are difficult to estimate for large areas. We studied the variation in components of cover, with 67 sites representing the regional variation of vegetation and topography, in northwestern Mexico. Surface-level cover, mostly by litter and gravel, was skewed to higher values than canopy cover, and only about one third of inter-plant points had bare soil. However, litter mass was not closely related to the spatial measure of ground cover. Drip height for intercepted precipitation was always

Keywords: borderlands carbon emissions ecosystem carbon balance industrial ecology solid waste flux urban development Summary We estimated vertical and lateral fluxes of carbon for the isolated coastal city of Ensenada (Baja California, MIn 2005, the city had a resident population of about 261,000, with tourism adding about 1.5%; it occupied an area of roughly 68 square kilometers (km 2 ). Carbon (C) export was esti- mated at 400 gigagrams of carbon per year (Gg C/yr); no- table sources to the atmosphere were combustion engines (42%), cement production (38%), water heating and cook- ing (7%), and human respiration (6%). Solid waste (6%) was exported for burial, but efflux to the bay was minor (about 0.1 Gg C/yr). Local deposition was limited to sewage sludge (about 2 Gg C/yr), asphalt, and extremely low primary pro- duction. Remote fluxes driven by local demand could be es- timated only for electricity (61 Gg C/yr), but local flux from cement and other industrial production might be attributed largely to external demand. The urban system output to the atmosphere was about 6.4 kilograms of carbon per square meter per year (kg C/m 2 /yr), or roughly 23.6 kg/m 2 /yr in CO2 equivalence. By comparison, net ecosystem productivity in the surrounding watershed has been estimated at 0.04 kg C/m 2 /yr, so the city's atmospheric output of C might be balanced by productivityoverabout11,000km 2 ofthesurroundingecosys- tems. Between 2000 and 2005, C output increased faster than population growth, particularly from engine fuels.

This work describes a genetic programming (GP) approach that creates vegetation indices (VI’s) to automatically detect the sum of healthy, dry, and dead vegetation. Nowadays, it is acknowledged that VI’s are the most popular method for extracting vegetation information from satellite imagery. In particular, erosion models like the “Revised Universal Soil Loss Equation” (RUSLE) can use VI’s as input to measure the effects of the RUSLE soil cover factor (C). However, the results are generally incomplete, because most indices recognize only healthy vegetation. The aim of this study is to devise a novel approach for designing new VI’s that are better - correlated with C, using field and satellite information. Our approach consists on stating the problem in terms of optimization through GP learning, building novel indices by iteratively recombining a set of numerical operators and spectral channels until the best composite operator is found. Experimental results illustrate the efficiency and reliability of our approach in contrast with traditional indices like those of the NDVI and SAVI family. This study provides evidence that similar problems related to soil erosion assessment could be analyzed with our proposed methodology.

The presence of species disjunct from more northern distributions and quantitative aspects of species associations were studied on the Sierra La Asamblea, an isolated range reaching 1,661 m elevation at about 29.3°N on the Baja California Peninsula. The flora above 800 m is composed of ≥299 species of tracheophytes, notably including Pinus monophylla and Adenostoma fasciculatum. Eighty-seven species are considered disjunct from more northern distributions, of which only 19 have affinity limited to the California Floristic Province. Data from 10 transects and adjacent flora show few species that are either broadly distributed or locally common, and both composition and structure differ from studies at lower elevations. Disjuncts were most common in association with P. monophylla. Range extensions (38) are reported, including three new records for the peninsula, and a floristic list is provided.

In topographically complex terrains, downslope movement of soil organic carbon (OC) can influence local carbon balance. The primary purpose of the present analysis is to compare the magnitude of OC displacement by erosion with ecosystem metabolism in such a complex terrain. Does erosion matter in this ecosystem carbon balance? We have used the Revised Universal Soil Loss Equation (RUSLE) erosion model to estimate lateral fluxes of OC in a watershed in northwestern Mexico. The watershed (4900 km2) has an average slope of 10 degrees +/- 9 degrees (mean +/- SD); 45% is10 degrees, and 3% is30 degrees. Land cover is primarily shrublands (69%) and agricultural lands (22%). Estimated bulk soil erosion averages 1350 Mg x km(-2) x yr(-1). We estimate that there is insignificant erosion on slopes2 degrees and that 20% of the area can be considered depositional. Estimated OC erosion rates are 10 Mg x km(-2) x yr(-1) for areas steeper than 2 degrees. Over the entire area, erosion is approximately 50% higher on shrublands than on agricultural lands, but within slope classes, erosion rates are more rapid on agricultural areas. For the whole system, estimated OC erosion is approximately 2% of net primary production (NPP), increasing in high-slope areas to approximately 3% of NPP. Deposition of eroded OC in low-slope areas is approximately 10% of low-slope NPP. Soil OC movement from erosional slopes to alluvial fans alters the mosaic of OC metabolism and storage across the landscape.

Aim of study: To evaluate the degree of trunk sway in relation to wind velocity, with varying temporal integration and to compare this relation among seasons.Area of study: Sierra de Juárez, Baja California, MéxicoMaterials and Methods: Displacements of a 19 m tall Jeffrey pine tree were recorded at 6 m from a three dimensional digital compass during one year, at c. 4 Hz. Adjacent wind speed at 6 m was recorded at 1 Hz.Main results: Sway was essentially unaffected by wind in the same second but increasing dependence of cumulative displacement on average sustained wind speed was found for intervals of 1 to 60 minutes (r2 up to 0.89). The relation is generally log-linear but apparently differs in parameters between seasons.Research highlights: Wind-sway relations are clear from integration of several-to-many minutes. However, to estimate cumulative stress, sub-second data on sway are essential. Sub-second, precision measurements of sway can be registered from small, inexpensive sensors.Keywords: biomechanics; Pinus jeffreyi; seasonality; stress accumulation; time series; tree bending.

Coastal succulent scrub of Baja California is being fragmented and replaced by agriculture, residential, recreational, commercial, and industrial land uses. This study evaluated fragmentation effects of a residential-recreational development. The site was a mosaic of native coastal succulent scrub (CSS) in a matrix of introduced grasslands and tourism infrastructure. Comparisons were made among extensive areas of mature or old CSS and smaller patches, which were either recently fragmented in a golf course or fragmented more than 20 years ago in a residential area. Total plant species richness was 108 species. New fragments exhibited the highest floristic richness due to invasion by opportunistic species. The number of CSS species increased with patch size. There were differences in flora among seasons due to spring annuals. In contrast, cover by native perennials was similar among all conditions and MANOVA results showed that only the variables related to origin (natives and nonnatives) were sign...

We evaluated the effect of stigma-anther separation (SAS) in flowers of the self-incompatible tropical tree Ipomoea wolcottiana (Convolvulaceae). We measured the deposition of self pollen on the stigma and the growth of pollen tubes. We also evaluated phenotypic selection on female fertility using univariate regressions of SAS with fruit production. SAS varied among individuals in two years, with a range of 0–6 mm. There was a negative correlation between SAS and the number of self pollen grains on the stigma; trees with SAS of 0–2 mm received averages of ca 80–130 self grains. The effect of loading the stigma with self-pollen was tested by allowing isolated flowers to self-pollinate, then cross-pollinating by hand: there was a negative correlation between the pollen load and the proportion of grains forming pollen tubes. The highest proportion of grains forming tubes was found with loads of 50 grains. The results suggest that more self pollen deposition leads to fewer tubes from outcross pollen. There was a significant effect of SAS on fruit production in both years. The analyses suggested stabilizing selection; very small SAS is disadvantageous due to the high load of self pollen, while very large SAS probably led to damage to the style during pollinator visits. We suggest that herkogamy may be a key character for determining the success of fruit production in self-incompatible hermaphroditic species.

Seed bank dynamics are a critical consideration for management of arid and semi-arid shrublands. In the threatened “coastal succulent scrub” vegetation (at Punta Banda, Baja California), we studied the seed bank's composition and size, as well as its dynamics as shown by field emergence and as inferred from literature reports on germination. We also analyzed sampling effort for both seeds and seedlings. The seed bank was sampled just prior to the rainy season, with 130 soil units of 7-cm diameter ×1-cm depth distributed systematically over 1.66 ha. The field samples were mixed together then laboratory samples (17 of 14 cm3) were taken for manual extraction of seeds. We found 25 species, mostly of short-lived herbs. Some obvious species of the local flora were not encountered. Overall density was approximately 11,807 seeds m−2, but densities differed by more than 100-fold among species. Analysis of sampling effort suggested that only 8–12 laboratory samples were needed to approach estimated asympt...

Aim Our purpose was to discern long-term large-scale patterns of survivorship and recruitment of two dominant, charismatic and useful desert plants, cirio [Fouquieriaceae; Fouquieria columnaris (Kell.) Kell. ex Curran] and cardon [Cactaceae; Pachycereus pringlei (S.Watson) Britton & Rose], and to evaluate the effects of physical and human factors. Location The study included 77 sites distributed over c. 13,200 km2 in the northern Vizcaino Region of the Sonoran Desert, state of Baja California, Mexico. Method Censuses used identified individuals (n = 3780 cirio and 2246 cardon) and were based on repeat photography. Time intervals between censuses ranged from 29 to 96 years, ending in 1996–2002. Earlier repetition (1963) of old photographs at 14 sites allowed analysis of temporal variation. The effect of elapsed time was modelled with Weibull functions for survivorship and linear functions for recruitment. To explain the residual variations, we tested categorical and continuous variables related to substrate, geomorphology, climate and geography. Human impacts were weighed with historical and quantitative analysis. Results Elapsed time accounted for 45% of the variation of survivorship in cirio but only 35% in cardon. The fitted Weibull curves were used to estimate longevity: all-size cohorts of 1000 individuals would die out in 388 (cirio) and 723 years (cardon). Recruitment in cirio was linearly related to time (r2 = 0.71) and averaged 0.005 year−1, was linearly related to time (r2 = 0.66) and was less than mortality at only 33% of the sites. Vital rates were not strongly correlated between mid- and late-century but were mostly similar to the long-term regional rates. Removing the factor of elapsed time, the residual variations showed that survivorship was greater on sites protected from winds for cirio and was less on very coarse soils for cardon. Recruitment increased with latitude in cirio and was greater on southern exposures for both species. The residual variations were not clearly affected, at a regional scale, by other factors thought to be important, including elevation, distance to the Pacific Ocean, geology, slope gradient, soil stability, older vs. young alluvial soils, and soil Ca : Mg and Na : K ratios. Human impacts have been sporadic and heterogeneous but locally strong; our quantitative indices of accessibility did not show regionally significant effects. Blowdown by hurricanes is a sporadic cause of major mortality for cirio but not necessarily for cardon. Main conclusions At our scales, effects of time outweigh those of substrate, and human impact was scant. Large patterns were pronounced: cirio experienced a prolonged, widespread decline in the 1900s, while cardon fluctuated in different directions and degree among local populations. Cirio was found to be inherently much shorter-lived than cardon. We also suggest that recruitment in cirio was low due to long periods with relatively dry winters that affected the entire region, while spatial heterogeneity of cardon recruitment seemed more related to the variation in summer rains. Resumen Metas Nuestro proposito fue discernir, en escalas de largo plazo y geograficamente amplias, patrones de sobrevivencia y reclutamiento de dos plantas carismaticas, utiles y dominantes del desierto: el cirio [Fouquieriaceae; Fouquieria columnaris (Kell.) Kell. ex Curran] y el cardon [Cactaceae; Pachycereus pringlei (S.Watson) Britton & Rose]. A su vez, evaluar los efectos de factores fisicos y humanos sobre estos patrones. Localizacion El estudio incluyo 77 sitios distribuidos en c. 13,200 km2 en el norte de la Region Vizcaino del Desierto Sonorense, en Baja California, Mexico. Metodo Con censos basados en la repeticion de fotografias se identificaron los individuos (n = 3780 cirios y 2246 cardones). Los intervalos de tiempo entre los censos variaron entre 29 y 96 anos, terminando en 1996–2002. La repeticion en 1963 de viejas fotografias en 14 sitios, permitio el analisis de la variacion temporal. El efecto del tiempo transcurrido fue modelado con funciones Weibull para la sobrevivencia y con funciones lineares en el caso del reclutamiento. Para explicar las variaciones residuales, se pusieron a prueba variables categoricas y continuas relacionadas a substrato, geomorfologia, clima y geografia. Los impactos humanos se consideraron mediante analisis historico y cuantitativo. Resultados El tiempo transcurrido explico el 45% de la variacion en la sobrevivencia del cirio pero solamente el 35% en la del cardon. El ajuste a la funcion Weibull permitio estimar la longevidad: las cohortes de todos los tamanos de 1000 individuos moririan en 388 (para el cirio) y 723 anos (para el cardon). El reclutamiento del cirio estuvo linearmente relacionado al tiempo (r2 = 0.71) y promedio 0.005 ano−1, estuvo linearmente relacionado al tiempo (r2 = 0.66) y fue menor que la mortalidad en solamente el 33% de los sitios. Las tasas vitales no estuvieron fuertemente correlacionadas entre mediados y finales del siglo pero fueron principalmente similares a las tasas regionales de largo plazo. Removiendo el factor del tiempo transcurrido, las variaciones residuales mostraron que la sobrevivencia fue mayor en sitios protegidos del viento para el cirio y fue menor en suelos muy gruesos para el cardon. El reclutamiento incremento con la latitud para el cirio y fue mayor en las exposiciones sur para ambas especies. Las variaciones residuales no fueron claramente afectadas, a una escala regional, por otros factores considerados como importantes, incluyendo la elevacion, la distancia al Oceano Pacifico, la geologia, el gradiente de pendiente, la estabilidad del suelo, la caracterizacion de los suelos aluviales en jovenes vs. viejos o las proporciones de Ca:Mg o Na:K en el suelo. Los impactos humanos han sido esporadicos y heterogeneos pero localmente muy fuertes; nuestros indices cuantitativos de accesibilidad no mostraron efectos regionalmente significativos. El derribamiento por huracanes fue una causa esporadica de mayor mortalidad en el cirio pero no necesariamente en el cardon. Principales conclusiones En nuestras escalas, los efectos del tiempo pesan mas que los del substrato y el impacto humano es escaso. Los grandes patrones son pronunciados: el cirio experimento una prolongada y amplia disminucion en los 1900s, mientras el cardon fluctuo en diferentes direcciones y grados entre poblaciones locales. Se encontro que el cirio es inherentemente una planta de mucha mas corta vida que el cardon. Tambien sugerimos que el reclutamiento en el cirio fue bajo debido a largos periodos con inviernos relativamente secos que afectaron la region entera, mientras que la heterogeneidad espacial en el reclutamiento del cardon parecio corresponder a la variacion en la lluvias de verano.

We modify a method of estimating size–age relations from a minimal set of individual increment data, recognizing that growth depends not only on size but also varies greatly among individuals and is consistent within an individual for several to many time intervals. The method is exemplified with data from a long-lived desert plant and a range of autocorrelation factors encompassing field-measured values. The results suggest that age estimates based on size and growth rates with only moderate autocorrelation are subject to large variation, which raises major problems for prediction or hindcasting for ecological analysis or management.

Precipitation patterns are characterized for an area of the Sonoran Desert in Baja California, with emphasis on temporal and spatial variability, based on nine stations between 30·03° and 28·58°, ca. 1953–1998. The interior stations formed a coherent region, while some distinct tendencies were shown for the Pacific and Gulf coasts. Precipitation was predominately in the cool season (61–91% in November–April), and was non-normal in both seasonal and annual totals. The duration of moist conditions was examined as the probability of some number of consecutive months above a threshold; for the cool season, the distributions approximated the Poisson with means dependent on the threshold. Spatial coherence was examined as the between-station correlation of inter-annual variation of seasonal totals. This also emphasized the cool-moist regime: the correlations of histories were all significant and exceeded 0·69 between interior stations, but most were weak or insignificant in the warm season. The full range of cool-season precipitation, showed a linear correlation with a seasonalized Southern Oscillation Index (−0·64) in the decades of record. Exceptionally, wet cool seasons were related to other proxy data; an historical reconstruction indicated very wet cool seasons were not evenly distributed in the last 150 years.

Path analysis was used to evaluate a group of variables related to fruit production in the tropical tree Ipomoea wolcottiana (Convolvulaceae). The variables were tree size (trunk diameter), available phosphorus in the soil, floral phenotypic sex expression (i.e. gynoecium biomass allocation), flower production, fruits initiated, and length of the corolla and diameter of its opening. The path model explained 79.9% of the total variation, and the production of mature fruits was related in large part by the number of fruits initiated and by phenotypic sex expression. In turn, fruits initiated were related to corolla tube diameter and to a lesser extent to corolla tube length. The model could explain a low percentage of the variation (18.5%) of the number of fruits initiated, but once a fruit was initiated it had a high probability of maturing. Phenotypic sex expression related negatively to the number of flowers produced and positively, to almost the same degree, to available phosphorus in the soil. Tree size had a positive effect on flower production but a negligible effect on fruits initiated and matured. Soil phosphorus affected positively, although weakly all variables. The relationships between tree size, number of flowers and phenotypic sex expression suggested that abundant flowering in this species functions more for pollen donation than its reception.

Factors affecting temporal and spatial variations of growth were studied in cirio, a succulent often surpassing 10 m in height. Measurements were obtained for intervals of 3–93 years at 14 sites in Baja California. Height tended to be asymptotic but total plant length (i.e. including all succulent stems) increased in linear relation to trunk diameter. Growth in length showed an overall average of 4·6 cm year −1 but ranged from 0·3–16·7 cm year −1 (annualized from data for intervals >34 year). Most of the variation was among individuals, but some differences between sites were also significant. Growth was clearly and strongly related to size only for individuals less than 1 m in height. Repeated measures on the same individuals showed that the autocorrelation of growth was about 0·8 between intervals of 3 or more years. Differences among individuals were related to physiographic position, the presence of caliche and the density of neighboring plants, which together explained 43% of the variation. We discuss the implications of these results for age estimates, indicators of site quality and studies of similar plants.

For 26 tree species in very dry tropical forest in Mexico, the developmental trends of relationships among trunk diameter, tree height, and crown diameter were inferred from a one-time measurement of dispersed individuals across the size range from saplings to large, mature trees. On hillside sites in this high diversity forest, maximum dimensions were usually

Intra-population patterns of phenotypic variation in biomass allocation to flower parts are analyzed for the treeIpomoea wolcottiana, and the effects of this variation on fruit production are evaluated in two consecutive years. Significant variation among individuals was found for biomass of both sexual and attractive structures. Repeatability values showed that variation in these structures could be heritable. Analyses of phenotypic natural selection indicated a positive directional selection for both, gynoecium and petals mass in both years. These results suggest that floral resource allocation can be modified by phenotypic natural selection. To our knowledge this is the first paper that reports phenotypic natural selection in floral characteristics of a tropical tree species.

In southwestern Mexico, the number of new cladodes on Opuntia excelsa was a skewed bell-shaped function of size (“size” being the number of cladodes persisting from previous years). Fruit number was a sigmoidal function of size, but the number of seeds per fruit was independent of number of fruits per plant. The curves for production of new cladodes and of fruits both showed maximum increase for plants with 150–350 cladodes, when the crowns were reaching or in the canopy. New cladode production declined at about the size where reproduction plateaued; we suggest this was due to limitation of crown dimensions by mechanical and physiological factors.

To explain very low fruit production in the tropical tree Ipomoea wolcottiana (Convolvulaceae), experiments and observations are presented regarding hypotheses that fecundity is limited by foliar herbivory, low soil phosphorus, inadequate cross-pollination, low rates of pollinator visitors, and parasitism of flower buds. A 2 × 2 factorial experiment was undertaken in two consecutive years which included control of herbivores with contact and systemic insecticides and addition of phosphorus. Phosphorus application in the second year increased the number of inflorescences and flowers by 200%, and the initiation of fruits by 400%. No response was observed for the number of mature fruits. Herbivore control and the addition of phosphorus increased seed weight by 22% in the first year, but could not be measured in the second year. Hand-pollination experiments increased the fruit set by 58% in the first year and by 75% in the second. Observations on visits to the flowers showed a six-fold difference between years in rates of pollinator visits. Parasitism of flower buds was 18% in the first year and 33% in the second. Several of the differences between years probably resulted from lesser and desynchronized flowering, due to heavy unseasonal rains in the second year. The differences in floral visitation and predation of floral buds were reflected in open fruit production: 16% in the first year and 6% in the second. All the factors investigated have important effects on the fecundity of I. wolcottiana. From the evolutionary point of view, the results suggest a complex interaction among selective forces which interact with the reproductive system. The theories of sexual selection and bet-hedging are the most plausible alternatives for explaining high levels of floral abortion in this species.

Seasonal drought may limit primary productivity in most of the tropics, but the determinants of tree growth are not well known. A 10-year study of the deciduous trees Cochlospermum vitifolium (Willd.) Spreng. (Cochlospermaceae) and Cnidoscolus spinosus Lundell (Euphorbiaceae) in southwestern Mexico showed radial growth to be highly correlated (both r>0.85) only with precipitation during an interval of 0.85) was a linear combination of mid-summer precipitation (positive coefficient) and total precipitation over the previous 2 years (negative coefficient). Comparison with other species showed heterogeneous responses of wood production to climate variation, and suggests that the range of functional types of dry forest trees is still unknown.

Prolonged seasonal drought affects most of the tropics, including vast areas presently or recently dominated by 'dry forests'. These forests have received scant attention, despite the fact that humans have used and changed them more than rain forests. This volume reviews the available information, often making contrasts with wetter forests. The world's dry forest heterogeneity of structure and function is shown regionally. In the neotropics, biogeographic patterns differ from those of wet forests, as does the spectrum of plant life-forms in terms of structure, physiology, phenology and reproduction. Biomass distribution, nutrient cycling, below-ground dynamics and nitrogen gas emission are also reviewed. Exploitation schemes are surveyed, and examples are given of non-timber product economies. It is hoped that this review will stimulate research leading to more conservative and productive management of dry forests.

Remote sensing has become a powerful tool to derive biophysical properties of plants. One of the most popular methods for extracting vegetation information from remote sensing data is through vegetation indices. Models to predict soil erosion like the "Revised Universal Soil Loss Equation" (RUSLE) can use vegetation indices as input to measure the effects of soil cover. Several studies correlate vegetation indices with RUSLE's cover factor to get a linear mapping that describes a broad area. The results are considered as incomplete because most indices only detect healthy vegetation. The aim of this study is to devise a genetic programming approach to synthetically create vegetation indices that detect healthy, dry, and dead vegetation. In this work, the problem is posed as a search problem where the objective is to find the best indices that maximize the correlation of field data with Landsat5-TM imagery. Thus, the algorithm builds new indices by iteratively recombining primitive-operators until the best indices are found. This article outlines a GP methodology that was able to design new vegetation indices that are better correlated than traditional man-made indices. Experimental results demonstrate through a real world example using a survey at "Todos Santos" Watershed, that it is viable to design novel indices that achieve a much better performance than common indices such as NDVI, EVI, and SAVI.

Today the most popular method for the extraction of vegetation information from remote sensing data is through vegetation indices. In particular, erosion models are based on vegetation indices that are used to estimate the "cover factor" (C) defined by healthy, dry, or dead vegetation in a popular soil erosion model named RUSLE, ("Revised Universal Soil Loss Equation"). Several works correlate vegetation indices with C in order to characterize a broad area. However, the results are in general not suitable because most indices focus only on healthy vegetation. The aim of this study is to devise a new approach that automatically creates vegetation indices that include dry and dead plants besides healthy vegetation. For this task we propose a novel methodology based on Genetic Programming (GP) as summarized below. First, the problem is posed as a search problem where the objective is to find the index that correlates best with on field C factor data. Then, new indices are built using GP working on a set of numerical operators and bands until the best composite index is found. In this way, GP was able to develop several new indices that are better correlated compared to traditional indices such as NDVI and SAVI family. It is concluded with a real world example that it is viable to synthesize indices that are optimally correlated with the C factor using this methodology. This gives us confidence that the method could be applied in soil erosion assessment.

Las flores de Crescentia alata son probablemente polinizadas por murciélagos (Glossophaga soricina) y son parasitadas por dos especies de abejas sociales: Partamona bilineata y Trigona fulviventris. Mientras que P. bilineata colecta néctar y polen, T. fulviventris colecta solamente polen. En cuatro árboles en los que se midió el robo de polen se encontró que la intensidad de éste era mayor en el árbol en el que la dehiscencia de las anteras ocurría más temprano que en aquéllos en los que ocurría más tarde. Las abejas remueven aproximadamente 40% de la producción total de néctar antes de la llegada de los murciélagos. Las abejas actúan como parásitos de las plantas y mantienen una relación amensalista con los murciélagos. Sugerimos que las plantas polinizadas por murciélagos son especialmente susceptibles al robo de polen debido a que tienen flores en las que el polen está expuesto. Además, este polen parece ser especialmente atractivo para las abejas. Las características de las plantas polinizadas por murciélagos son probablemente el resultado de la interacción no solamente con polinizadores sino que también con las abejas que parasitan el polen y néctar.

In this study we surveyed branches of vines and trees to compare the ability of their wood to transport water by measuring their specific hydraulic conductivities; that is, we quantified the volume of water passing a given xylem transverse area per time for a known pressure gradient across excised twigs of vines and trees

Prolonged seasonal drought affects most of the tropics, including vast areas presently or recently dominated by 'dry forests'. These forests have received scant attention, despite the fact that humans have used and changed them more than rain forests. This volume reviews the available information, often making contrasts with wetter forests. The world's dry forest heterogeneity of structure and function is shown regionally. In the neotropics, biogeographic patterns differ from those of wet forests, as does the spectrum of plant life-forms in terms of structure, physiology, phenology and reproduction. Biomass distribution, nutrient cycling, below-ground dynamics and nitrogen gas emission are also reviewed. Exploitation schemes are surveyed, and examples are given of non-timber product economies. It is hoped that this review will stimulate research leading to more conservative and productive management of dry forests.

Anemophily may be frequent in neotropical dioecious trees, of diverse taxonomic affinity, as shown here for 19 species in 13 families at one locality, on the basis of floral morphology and pollen presentation. Flower-visiting bees often are not pollinators but commensal or parasitic. The perceived scarcity of anemophily in tropical trees is brought into question by a reconsideration of the evidence regarding pollen type, nectar, weather, insect visitors, and tree density