SURVIVAL OF SEVERE DROUGHT BY A NON-SPROUTING CHAPARRAL SHRUB

A 2-yr drought (1975-1977) in much of California provided unusually severe water stress on many native plant species. In this paper we evaluate the effects of this drought on Arctostaphylos viscida Parry, a common non-sprouting chaparral shrub in the foothills of the southern Sierra Nevada. At the peak of the drought, water potentials as low as –74 bars were measured. While only three of 90 shrubs sampled were killed by the drought, all but one showed signs of drought-induced dieback. First and second year post-drought twig growth was significantly greater on shrubs with 90% or greater branch mortality than on those suffering lesser dieback. In all cases, new growth occurred only on surviving branches. Midday water stress measurements showed little difference for shrubs exhibiting high and low levels of dieback. These results suggest that drought survival in Arctostaphylos viscida may depend on a mechanism where certain branches are sacrificed with the surviving ones preferentially receiving available resources. The possibility that this may involve the dieback of below ground roots and/or lack of production of new roots to compensate for lack of root turnover is supported by water stress measurements of adjacent fire damaged shrubs. Implications of possible specialized drought survival mechanisms in non-sprouting shrubs are explored.     

Variation in woody plant mortality and dieback from severe drought among soils, plant groups, and species within a northern Arizona ecotone

Vegetation change from drought-induced mortality can alter ecosystem community structure, biodiversity, and services. Although drought-induced mortality of woody plants has increased globally with recent warming, influences of soil type, tree and shrub groups, and species are poorly understood. Following the severe 2002 drought in northern Arizona, we surveyed woody plant mortality and canopy dieback of live trees and shrubs at the forest–woodland ecotone on soils derived from three soil parent materials (cinder, flow basalt, sedimentary) that differed in texture and rockiness. Our first of three major findings was that soil parent material had little effect on mortality of both trees and shrubs, yet canopy dieback of trees was influenced by parent material; dieback was highest on the cinder for pinyon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma). Ponderosa pine (Pinus ponderosa) dieback was not sensitive to parent material. Second, shrubs had similar mortality, but greater canopy dieback, than trees. Third, pinyon and ponderosa pines had greater mortality than juniper, yet juniper had greater dieback, reflecting different hydraulic characteristics among these tree species. Our results show that impacts of severe drought on woody plants differed among tree species and tree and shrub groups, and such impacts were widespread over different soils in the southwestern U.S. Increasing frequency of severe drought with climate warming will likely cause similar mortality to trees and shrubs over major soil types at the forest–woodland ecotone in this region, but due to greater mortality of other tree species, tree cover will shift from a mixture of species to dominance by junipers and shrubs. Surviving junipers and shrubs will also likely have diminished leaf area due to canopy dieback.     

Patch dieback of Colophospermum mopane in a dysfunctional semi‐arid African savanna

Abstract Patch dieback occurred in an almost monospecific Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard woodland in the Northern Province, South Africa, following severe droughts in 1988–1989 and 1991–1992. Discrete patches of dieback and adjacent paired areas of ‘healthy’ vegetation lost an average of 87 and 13% of basal area to mortality, respectively. Whole trees mostly died on ‘dead’ plots, while single‐stem mortality prevailed on ‘live’ plots. Tree mortality decreased with increasing stem number per tree. Patch dieback did not occur on sandy soils. On fine‐textured soils, variation in soil type, topography or slope did not affect dieback. Dieback was influenced by vegetation structure, soil surface condition and soil chemistry. Intense intertree competition, shown by self‐thinning occurring prior to dieback, was a precondition for dieback. Intertree competition had heightened during the 30 years prior to dieback because of an increase in woody cover. Dieback patches had changed from functioning as sinks of water and sediment to sources of these as a result of loss of perennial herbaceous cover, decreased water retention on bared surfaces, and accelerated erosion during 50 years of livestock ranching. Vegetation had thus become increasingly drought‐prone, exacerbated in places by soils with a high sodium concentration. Dieback had occurred because the water requirements of C. mopane could no longer be met during drought years on the dysfunctional patches.     

Shifts in functional traits elevate risk of fire‐driven tree dieback in tropical savanna and forest biomes

Numerous predictions indicate rising CO₂ will accelerate the expansion of forests into savannas. Although encroaching forests can sequester carbon over the short term, increased fires and drought‐fire interactions could offset carbon gains, which may be amplified by the shift toward forest plant communities more susceptible to fire‐driven dieback. We quantify how bark thickness determines the ability of individual tree species to tolerate fire and subsequently determine the fire sensitivity of ecosystem carbon across 180 plots in savannas and forests throughout the 2.2‐million km² Cerrado region in Brazil. We find that not accounting for variation in bark thickness across tree species underestimated carbon losses in forests by ~50%, totaling 0.22 PgC across the Cerrado region. The lower bark thicknesses of plant species in forests decreased fire tolerance to such an extent that a third of carbon gains during forest encroachment may be at risk of dieback if burned. These results illustrate that consideration of trait‐based differences in fire tolerance is critical for determining the climate‐carbon‐fire feedback in tropical savanna and forest biomes.     

Direct and indirect interactions among plants explain counterintuitive positive drought effects on an eastern Mediterranean shrub species

Coexistence of woody and herbaceous plants may be governed by a complex set of direct and indirect interactions, whose relative importance have been rarely assessed. We experimentally studied woody species establishment in a mixed plant community by disentangling the potential role of such biotic interactions and the effect of environmental variations on them. Seedling establishment of the common eastern Mediterranean shrub species Sarcopoterium spinosum was investigated under different rainfall and light conditions, combined with the effect of the presence of adult shrubs and annual neighbors. We predicted that seedlings will be directly affected by competition with annuals with increasing water availability, while direct effects of adult shrubs will be positive via amelioration of water stress. Indirect effects were expected beneath shrub canopies due to reduced water stressed and light availability for both annuals and shrub seedlings, which may intensify competition between annuals and shrub seedlings. To test these predictions we performed field and garden experiments in which we combined manipulation of shrub and annual presence with manipulations of water availability and light conditions to simulate the effect of shrub canopy. In contrast to our prediction, shrub seedling establishment was not facilitated but inhibited by adult shrubs because of light limitation. As expected, annuals had direct negative effects on shrub seedlings under wet conditions, which shifted to neutral or positive effects under dry conditions. Thus, interactions among shrubs and annuals, and in particular the release from competition during drought years, leads to a counterintuitive positive effect of drought on shrub seedling establishment. Our findings point to the importance of experimentally studying multidimensional interactions for coexistence of different life forms and to the underestimated role of light for success in water‐limited ecosystems.     

The main factors that drive plant dieback under extreme drought differ among Mediterranean shrubland plant biotypes

Questions

Knowledge of how extreme drought events induce plant dieback and, eventually, plant mortality, may improve our forecasting of ecosystem change according to future climate projections, especially in Mediterranean drylands. In them, shrublands are the main vegetation communities in transition areas from a subhumid to semi-arid climate. This study analyzed differences in plant dieback after an unusual drought in 2014 and identified their main underlying factors in relation to three groups of explanatory variables: water availability, soil properties and vegetation structure attributes.

Location

Four Mediterranean shrublands along a climatic gradient in SE Spain.

Methods

At each experimental field site, we sampled a pool of vegetation structure characteristics, soil depth and soil surface properties, and we also determined water availability by continuously monitoring soil moisture and the microclimate conditions.

Results

The climatic analysis showed that there was an extreme drought event in 2014, which was below the first percentile of the driest years. Under such conditions, vegetation dieback occurred at all the study sites. However, plant dieback differed between sites and plant biotypes. Subshrubs were the main affected biotype, with diebacks close to 60% at the driest sites, and up to 40% dieback for shrubs depending on their vertical development. Relative extractable water and bare soil surface cover were the best explanatory variables of plant community dieback but changed between plant biotypes. Vegetation structure variables related to plant vertical development (leaf area index [LAI], plant height, phytovolume) were significant explanatory variables of plant dieback in shrubs, subshrubs and grasses. Consecutive dry days fitted the best model to explain subshrub dieback.

Conclusions

We found that rainfall pattern rather than total annual rainfall was the climatic factor that best determined water availability for plants in Mediterranean drylands. These results also pointed out the relevance of plant structure and soil properties for explaining ecosystem responses to extreme drought.     

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

The stress gradient hypothesis (SGH) predicts that the importance or intensity of competition and facilitation will change inversely along abiotic stress gradients. It was originally postulated that increasing environmental stress can induce a monotonic increase in facilitation. However, more recent models predicted that the relationship between severity and interaction exhibits a hump‐shaped pattern, in which positive interactions prevail under moderate stress but decline at the extreme ends of stress gradients. In the present study, we conducted a field experiment along a temporal rainfall gradient for five consecutive years, in order to investigate interactions in a shrub‐herbaceous plant community at the southern edge of the Badain Jaran Desert, and, more specifically, investigated the effects of Calligonum mongolicum, a dominant shrub species, on both abiotic environmental variables and the performance of sub‐canopy plant species. We found that shrubs can improve sub‐canopy water regimes, soil properties, plant biomass, density, cover, and richness and, more importantly, that the positive effect of shrubs on sub‐canopy soil moisture during the summer diminishes as rainfall decreases, a pattern that partly explains the collapse of the positive interaction between shrubs and their understory plants. These results provide empirical evidence that the positive effect of shrubs on understory plant communities in extreme arid environments may decline and become neutral with increasing drought stress.     

Impact of Prosopis invasion on a keystone tree species in the Kalahari Desert

Several Prosopis species were introduced into South Africa in the last century. Since then two species, Prosopis glandulosa var. torreyana and Prosopis velutina have invaded large parts of arid southern Africa. Here, we examine the extent to which increased mortality of Acacia erioloba, a keystone species in the Kalahari Desert, can be attributed to competition for water with Prosopis. We do this for A. erioloba and Prosopis sp. at invaded, as well as cleared sites through a determination of species abundance, canopy vitality, plant water stress and plant water source. Our stable isotope results show that in the riparian zone both A. erioloba and Prosopis are using the same water source. Our results also show that there is a 50 % increase in canopy dieback of A. erioloba in the invaded river plots relative to the cleared river plots. This dieback cannot be related to changes in rainfall and temperature as there were no adverse fluctuations (drought) in the weather in the 10 years preceding our study. We speculate that because A. erioloba is more water stressed in the invaded river plot this increase in mortality and dieback is related to plant moisture stress that is not related to climate but to competition for water with Prosopis. Our study gives strong support for the eradication of Prosopis from rivers in arid parts of Southern Africa.     

Projected drought effects on the demography of Ashe juniper populations inferred from remote measurements of tree canopies

Tree mortality from drought is anticipated to increase as climate change promotes more frequent or severe water limitation. Ecosystem impacts of woody mortality depend on both the number and sizes of trees that die, but a limited capacity to predict mortality risk for individual trees hinders the capacity to forecast drought effects on tree population demography and ecosystem processes. We remotely measured leaf area of living Ashe juniper trees at three savanna sites in central Texas, USA to characterize the frequency-size distribution (FSD) of juniper populations and evaluate mortality risk from drought as a function of tree size. Mortality risk of individuals was assessed from the deviation in leaf area per tree from that of a similarly sized individual with near maximal leaf area using correlations among leaf area, growth rate, and mortality measured during a prior drought. We found that the FSD of juniper trees is bell-shaped at each site. Mortality risk from drought exceeded 25% of emergent (>?4 m height) trees in savanna juniper populations, but was highest for largest trees. Mortality risk was greatest at a grazed savanna, exceeding 50% of trees with projected canopy area >?20 m². Results imply that severe drought could kill a large fraction (18–85%) of intermediate- to large-sized Ashe juniper trees in central Texas savannas. Our analysis demonstrates a novel use of remote measurements of canopy foliation to link mortality risk from drought to the demography of Ashe juniper populations through properties of individual trees.     

Responses of riparian trees to interannual variation in ground water depth in a semi-arid river basin

We investigated the physiological and growth responses of native (Populus fremontii S. Wats. and Salix gooddingii Ball) and exotic (Tamarix chinensis Lour.) riparian trees to ground water availability at the free‐flowing Hassayampa River, Arizona, during dry (1997) and wet (1998) years. In the drier year, all species experienced considerable water stress, as evidenced by low shoot water potentials, low leaf gas exchange rates and large amounts of canopy dieback. These parameters were significantly related to depth of ground water (DGW) in the native species, but not in T. chinensis, in 1997. Canopy dieback was greater in the native species than in T. chinensis when ground water was deep in 1997, and dieback increased rapidly at DGW > 2·5–3·0 m for the native species. Analysis of combined data from wet and dry years for T. chinensis tentatively suggests a similar physiological sensitivity to water availability and a similar DGW threshold for canopy dieback. In 1998, shoot water potential and leaf gas exchange rates were higher and canopy dieback was lower for all species because of increased water availability. However, T. chinensis showed a much larger increase in leaf gas exchange rates in the wet year than the native species. High leaf gas exchange rates, growth when water is abundant, drought tolerance and the maintenance of a viable canopy under dry conditions are characteristics that help explain the ability of T. chinensis to thrive in riparian ecosystems in the south‐western United States.     

Tree-shrub interactions in a subtropical savanna parkland: competition or facilitation?

Abstract. Prosopis glandulosa var. glandulosa has played a central role in the encroachment of woody plants in southern Texas, grasslands and savannas by acting as a nurse plant for various shrubs that establish in its understory. To test for continued facilitation of established understory shrubs by Prosopis and to determine if established shrubs compete with the Prosopis nucleus, selective removal experiments were conducted and monitored over a 2–5 yr period. Short-term (1–3 days) and long-term (2 yr) growth and physiological activities (midday net photosynthesis and leaf/shoot water potential) of two common understory shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing with Prosopis, were generally comparable to those of individuals occurring in clusters where Prosopis was removed. Shrubs growing with an intact Prosopis occasionally showed significantly higher leaf-[N] and pre-dawn water potentials than those in clusters lacking a live Prosopis, especially under drought conditions; however, these differences did not translate into greater midday leaf gas exchange or shoot growth. By comparison, removal of understory shrubs elicited large increases in Prosopis net photosynthesis, annual trunk growth in each of the 5 yr monitored, and seed pod production in three of the four years monitored. Seven of 26 Prosopis plants in experimental clusters with an intact understory died over a 5-yr period, compared to only two of the 26 plants in clusters with the cleared understory. Results indicate that (1) the founding overstory Prosopis plant may continue to facilitate understory shrubs following their establishment, but these beneficial effects appear to be small and transitory, and (2) the understory shrubs have a pronounced negative effect on Prosopis, such that competition between overstory and understory woody plants is strongly asymmetrical. These findings suggest that understory shrubs will likely persist despite changes in microclimate and soils (potentially) that occur after the Prosopis plant, which facilitated their ingress or establishment, has died. Soil resource depletion by shallow-rooted understory shrubs appears to be a primary factor contributing to the demise of the deeply rooted, overstory Prosopis plants, especially on upland sites with duplex soils where below-ground competition is accentuated.     

Floristic composition,plant species abundance,and soil properties of montane savannas in the Gran Sabana,Venezuela

Floristic composition, species abundance, and soil properties were studied in slope, flat and disturbed savannas in the northern part of the Gran Sabana, Venezuela. All savannas presented shallow soils (<30 cm depth) with high content of sand and low content of clay. In general, the soils were poor in nutrients and strongly acidified. The major difference between the soils was the content of the stony fraction, which was significantly higher (P<0.05) in the slope savannas than in the flat savannas. A total of 57 dicot, 42 monocot, and 7 fern species were recorded in all studied savannas. In the flat and slope savannas predominated the monocot species, while in the disturbed savanna predominated the dicots. The families with the largest number of species were Poaceae (19.8%), Cyperaceae (13.2%), Asteraceae (10.4%) and Melastomataceae (8.5%). The number of species in the flat savannas was higher than that of the slope savannas. The lowest plant species richness was associated to slope savannas and their high content of stony fraction of the soils. The highest floristic similarity was found between slope savannas, and the lowest between disturbed savanna and slope savannas. The most abundant life forms in the studied savannas were perennial (42.4%) and annual (24.5%) herbs, followed by suffruticoses (16.0%) and shrubs (12.3%), and the less frequent was lianas (4.7%). The disturbed savanna showed the higher richness and diversity index. Trachypogon plumosum (Poaceae) was the most abundant species in all studied savanna.     

土壤水分变化对黄土丘陵区撂荒群落共存种竞争的影响

群落共存种对限制性资源的竞争响应是群落组建、维持与演替的重要影响因素。以黄土丘陵区摞荒群落共存种为研究对象,研究不同水分条件下的竞争响应差异,以期揭示生境变化时群落演替的植物竞争机理。试验设土壤水分与混植密度两个处理,其中混植密度采用CDS （Community density series）法,含3个密度水平（衡水试验为3、12、21株/m²,变水试验为5、20、35株/m²）;土壤水分设衡水（高（75% FC）、中（60% FC）、低（45% FC）3个水平）与变水（由高水（75% FC）经50 d持续干旱至低水（45% FC））两种处理。根据共存种株高的阶段性生长与其在群落中的相对生物量等生长表现,在CDS法基础上扩展并测算了生长变化量、相对生物量和相对生长率等指标。据此,研究竞争效应和水分胁迫效应的相对影响,共存种相对竞争能力和竞争等级对土壤水分变化的响应,及高水期因竞争引起的个体大小不对称性与低水期死亡率的关系。结果表明：1）不同水分多种混植共存条件下,竞争效应对多数共存种在群落中的生长表现较水分胁迫效应影响更大;以相对生物量为指标的竞争的群落效应（4.30±0.49）较水分胁迫的群落效应（-23.56±20.38）大,且二者累加效应（4.76±0.4）大于二者单项效应之和（-19.26±20.48）,即竞争+水分胁迫对多种共存群落具有协同效应。2）当水分条件发生变化时,群落共存种的竞争等级并不完全相同（P > 0.05）,相应的会发生变化,且水分差别越大,共存种竞争等级的差别也越大;高水条件下群落共存种竞争所引起的群落效应最大（4.84±0.23）,部分支持最大生长率理论,即限制性资源水平越高,群落共存种竞争越激烈。3）变水试验中,10种群落共存种在高水期因竞争引起的个体大小差异与低水期的死亡率呈正相关关系（P=0.015,r²=0.801）,根据资源动态两阶段假说,说明试验条件下群落共存种竞争是群落组建的重要因素。     

Factors influencing carbon fixation and water use by mediterranean sclerophyll shrubs during summer drought

Summary Mediterranean sclerophyll shrubs respond to seasonal drought by adjusting the amount of leaf area exposed and by reducing gas exchange via stomatal closure mechanisms. The degree to which each of these modifications can influence plant carbon and water balances under typical mediterranean-type climate conditions is examined. Leaf area changes are assessed in the context of a canopy structure and light microclimate model. Shifts in physiological response are examined with a mechanistically-based model of C₃ leaf gas exchange that simulates progressive reduction of maximum photosynthesis and transpiration rates and increasingly strong midday stomatal closure over the course of drought. The results demonstrate that midday stomatal closure may effectively contribute to drought avoidance, increase water use efficiency, and strongly alter physiological efficiency in the conversion of intercepted light energy to photoproducts. Physiological adjustments lead to larger reductions in water use than occur when comparing leaf area index 3.5 to 1.5, extremes found for natural stands of sclerophyll shrubs in the California chaparral. Reductions in leaf area have the strongest effect on resource capture and use during non-water-stressed periods and the least effect under extreme drought conditions, while shifts in physiological response lead to large savings of water and efficient water use under extreme stress. An important model parameter termed GFAC (proportionality factor expressing the relation of conductance [g] to net photosynthesis rate) is utilized, which changes in response to the integrated water stress experimence of shrubs and alters the degree to which stomata may open for a given rate of carbon fixation. We attempt to interpret this parameter in terms of physiological mechanisms known to modify control of leaf gas exchange during drought. The analysis helps visualize means by which canopy gas exchange behavior may be coupled to physiological changes occurring in the root environment during soil drying.     

Future vulnerability mapping based on response to extreme climate events: Dieback thresholds in an endemic California oak

Aim

This study presents a bioclimate modelling approach, using responses to extreme climate events, rather than historical distributional associations, to project future species vulnerability and refugia. We aim to illustrate the compounding effects of groundwater loss and climate on species vulnerability.

Location

California, USA.

Methods

As a case study, we used the 2012–2015 California drought and resulting extensive dieback of blue oak (Quercus douglasii). We used aerial dieback surveys, downscaled climate data and subsurface water change data to develop boosted regression tree models identifying key thresholds associated with dieback throughout the blue oak distribution. We (1) combined observed dieback–climatic threshold relationships with climate futures to anticipate future areas of vulnerability and (2) used satellite‐derived measurements of subsurface water loss in drought/dieback modelling to capture the mediating effect of groundwater on species response to climatic drought.

Results

A model including climate, climate anomalies and subsurface water change explained 46% of the variability in dieback. Precipitation in 2015 and subsurface water change accounted for 62.6% of the modelled probability of dieback. We found an interaction between precipitation and subsurface water in which dieback probability increased with low precipitation and subsurface water loss. The relationship between precipitation and dieback was nonlinear, with 99% of dieback occurring in areas that received <363 mm precipitation. Based on a MIROC_rcp85 future climate scenario, relative to historical conditions, 13% of the blue oak distribution is predicted to experience more frequent years below this precipitation threshold by mid‐century and 81% by end of century.

Main conclusions

As ongoing climate change and extreme events impact ecological processes, the identification of thresholds associated with observed dieback may be combined with climate futures to help identify vulnerable populations and refugia and prioritize climate change‐related conservation efforts.     

Stress responses of native and exotic grasses in a Neotropical savanna predict impacts of global change on invasion spread

Communities subject to stress, including those with low invasibility, may be dominated by exotic generalist species. African grasses are aggressive invasive species in Neotropical savannas, where their response to abiotic stress remains unknown. We assessed the role of waterlogging and canopy closure on the presence, abundance and reproductive tillering of African and native grasses in a Neotropical savanna in southeastern Brazil. We obtained abundance and reproductive tillering data of exotic (Melinis minutiflora, Melinis repens and Urochloa decumbens) and common native grasses in 20 sites. We also determined the groundwater depth, soil surface water potential and canopy cover at these sites. The grass species generally had a low frequency and performed poorly where soil remained waterlogged throughout the year, except for two native species. Most native species were exclusive to either well‐drained savannas or better drained wet grasslands. However, two species (Loudetiopsis chrysothrix and Trachypogon spicatus) occurred in both vegetation types. Two exotic species (M. minutiflora and M. repens) were less common but demonstrated reasonable performance in wet grasslands, possibly due to their root system plasticity. Furthermore, U. decumbens had a lower occurrence, density and reproductive tillering at these sites, but was successful at sites where the groundwater level was slightly deeper. Although the favourable water regime in the savannas increases their invasibility in general, resistance to invasion by African grasses may be greater at microsites with high canopy closure, where these species showed lower performance and did not affect the abundance of co‐occurring native grasses. In summary, the Brazilian savanna becomes more susceptible to the spread of African grasses when disturbances decrease canopy closure or lower rainfall associated with climate change reduces the average groundwater depth and consequently releases invasive species from soil waterlogging in grasslands.     

Exotic deer diminish post-fire resilience of native shrub communities on Santa Catalina Island, southern California

Browsing by exotic mule deer on Santa Catalina Island (SCI) off the coast of southern California may diminish the post-fire resilience of native shrublands. To assess this, deer exclosures were established following a wildfire to monitor post-fire recovery of three dominant, native shrub species (Heteromeles arbutifolia, Rhus integrifolia, and Rhamnus pirifolia). Post-fire resprout growth, mortality, and tissue water status as well as pre- and post-fire shrub density and cover were measured inside and outside of deer exclosures. We found that deer browsing significantly limited post-fire resprout growth and led to increased mortality of resprouting H. arbutifolia shrubs (88 % mortality outside compared to 11 % inside exclosures). Post-fire resprouts maintained favorable water status during the study despite drought conditions, indicating that water stress was not a proximate cause of resprout mortality. Deer browsing resulted in a >93 % reduction in canopy coverage of dominant shrub species. The dramatic reduction of native shrubs at this site may create opportunities for displacement by exotic species, resulting in eventual vegetation-type conversion. The observed link between intense browsing and post-fire shrub mortality provides much needed information concerning the environmental impact of exotic deer on SCI and illustrates the interaction between exotic herbivores and fire on an island system.     

Change in water loss regulation after canopy clearcut of a dominant shrub in Sahelian agrosystems, Guiera senegalensis J. F. Gmel

This paper analyzes the effect of the canopy age of Guiera senegalensis J.F. Gmel on water regulation processes and adaptative strategy to drought over a period of 2 years. The species is widespread in the agricultural Sahel. Before sowing, farmers cut back the shrubs to limit competition with crops. The stumps resprout after the millet harvest. Leaf water potential and stomatal conductance were measured in two fallows and in the two adjacent cultivated fields. Leaf transpiration rate and soil-to-leaf hydraulic conductance were deduced. The decrease in both stomatal and plant hydraulic conductance caused by seasonal drought was greater in mature shrubs than in current year resprouts. The decrease in predawn and midday leaf water potentials in response to seasonal drought was isohydrodynamic, and it was greater in mature shrubs, suggesting that current year resprouts are under less stress. In resprouts, the leaf transpiration rate stopped increasing beyond a hydraulic conductance threshold of 0.05 mol. m^?2 s^?1 MPa^?1. Vulnerability to cavitation was determined on segments of stems in the laboratory. The leaf water potential value at which stomatal closure occurred was ?2.99 ± 0.68 MPa, which corresponded to a 30 % loss in xylem conductivity. Thanks to its positive safety margin of 0.6 MPa, G. senegalensis can survive above this value. The observed strategy places G. senegalensis among the non-extreme xeric plants, leading us to suppose that this species will be vulnerable to the expected increase in regional drought.     

Analysis of the relationship between the spectral characteristics of maize canopy and leaf area index under drought stress

Maize is one of the most widespread grain crops in the world; however, more than 70% of corn in China suffers some degree of drought disaster every year. Leaf area index (LAI) is an important biophysical parameter of the vegetation canopy and has important significance for crop yield estimation. Using the data of canopy spectral reflectance and leaf area index (LAI) for maize plants experiencing different levels of soil moisture from 2011 to 2012, the characteristics of the canopy reflective spectrum and its first derivative, and their relationships to leaf area index, were analyzed. Soil moisture of the control group was about 75% while that of the drought stress treatment was about 45%. In addition, LAI retrieval models for maize were established using vegetation indices (VIs) and principal component analysis (PCA) and the models were tested using independent datasets representing different soil water contents and different developmental stages of maize. The results showed that canopy spectral reflectances were in accordance with the characteristics of green plants, under both drought stress and at different developmental stages. In the visible band, canopy reflectance for both healthy and damaged vegetation had a green-wavelength peak and a red-wavelength valley; reflectance under drought stress, especially in the green peak (about 550 nm) and the red valley (about 676 nm) was higher than in the control group. In the near-infrared band, the canopy spectral reflectance decreased substantially between 780 and 1350 nm under drought stress. Moreover, the red edge of the spectrum was shifted toward blue wavelengths. The first derivative spectrum showed a double peak phenomenon at the edge of the red band at different developmental stages: the main peak appeared between 728 and 732 nm and the minor peak at about 718 nm. The double peaks become more obvious through the growth and development of the maize, with the most notable effect during the silking and milk stages, after which it gradually decreased. During maize growth, the LAI of all plants, regardless of soil moisture conditions, increased, and the largest LAI also occurred during the silking and milk stages. During those stages, the LAI of plants under different drought stress levels was significantly lower (by 20% or more) than in normal plants with sufficient water supplies. The LAI was highly significantly correlated with canopy spectral reflectance in the bands from 350 nm to 510 nm, from 571 nm to 716 nm, and from 1450 nm to 1575 nm. Also, the LAI was significantly correlated with red edge parameters and several VIs. The Perpendicular Vegetation Index (PVI) had the best correlation with LAI, with a coefficient of determination (R²) of 0.726 for the exponential correlation. Using dependent data, a LAI monitoring model for the maize canopy was constructed using PCA and VI methods. The test results showed that both the VI and PCA methods of monitoring maize LAI could provide robust estimates, with the predicted values of LAI being significantly correlated with the measured values. The model based on PVI showed higher precision under the drought stresses, with a correlation coefficient of 0.893 (n = 27), while the model based on PCA was more precise under conditions of adequate soil moisture, with a correlation coefficient of 0.877 (n = 32). Therefore, a synthesis of the models based on both VI and PCA could be more reliable for precisely predicting LAI under different levels of drought stresses in maize.     

Differential growth patterns and fitness may explain contrasted performances of the invasive <Emphasis Type="Italic">Prunus serotina</Emphasis> in its exotic range

This research investigates why the invasive American black cherry tends to dominate the forest canopy on well-drained, nutrient-poor soils, but usually hardly establishes on both waterlogged and calcareous soils in its exotic range. Prunus serotina was sampled from four soil types and two light conditions, to measure (1) radial growth; (2) height growth compared to the main native competitor, Fagus sylvatica; (3) leaf traits; (4) seed production; and (5) rate of fungal attack. We found that P. serotina invested a significant amount of energy in height growth and seed production on well-drained, nutrient-poor soils. These characteristics enabled it to rapidly capture canopy gaps and subsequently exert a mass effect on neighbouring stands. On moist soils, we found irregular growth patterns and high rates of fungal attack, while on calcareous soils, leaf traits suggested a low nitrogen assimilation rate, limiting the production of N-containing compounds. We conclude that P. serotina fails on waterlogged and calcareous soils because it is unable to allocate sufficient energy to fruiting and/or height growth. Conversely, it succeeds on well-drained, nutrient-poor soils because of high fitness which increases its invasiveness.