Precipitation magnitude and timing differentially affect species richness and plant density in the sotol grassland of the Chihuahuan Desert

Arid and semi-arid environments are dynamic ecosystems with highly variable precipitation, resulting in diverse plant communities. Changes in the timing and magnitude of precipitation due to global climate change may further alter plant community composition in desert regions. In this study, we assessed changes in species richness and plant density at the community, functional group, and species level in response to variation in the magnitude of natural seasonal precipitation and 25% increases in seasonal precipitation [e.g., supplemental watering in summer, winter, or summer and winter (SW)] over a 5-year period in a sotol grassland in the Chihuahuan Desert. Community species richness was higher with increasing winter precipitation while community plant density increased with greater amounts of winter and summer precipitation, suggesting winter precipitation was important for species recruitment and summer precipitation promoted growth of existing species. Herb and grass density increased with increasing winter and summer precipitation, but only grass density showed a significant response to supplemental watering treatments (SW treatment plots had higher grass density). Shrubs and succulents did not exhibit changes in richness or density in response to natural or supplemental precipitation. In this 5-year study, changes in community species richness and density were driven by responses of herb and grass species that favored more frequent small precipitation events, shorter inter-pulse duration, and higher soil moisture. However, due to the long life spans of the shrub and succulent species within this community, 5 years may be insufficient to accurately evaluate their response to variable timing and magnitude of precipitation in this mid-elevation grassland.     

Plant distribution and the temperature coefficient of metabolism

The spatial distribution of a plant species is limited by the range of climatic conditions to which the species can adapt. Temperature is one of the most significant determinants of plant distribution, but except for the effects of lethal limits, little is known about physiological changes in responses to differences in environmental temperature. In this study, temperature coefficients of non-photosynthetic metabolism have been determined in the normal environmental temperature range for selected annual and perennial plants. Distinct differences were found in the temperature coefficient of metabolism of woody perennial plants from high latitudes and high elevations and closely related low-latitude and low-elevation plants. Low-latitude and low-elevation woody perennials have Arrhenius temperature coefficients for metabolism that are larger than those for congeneric high-latitude and high-elevation plants. The Arrhenius temperature coefficient is not rapidly adapted to new environments. A simple function was developed relating Arrhenius temperature coefficient to latitude and elevation for accessions of three, woody, perennial species complexes of plants collected from a wide geographic range but grown in common gardens. Within these taxa, plants that experience broader ranges of temperature during growth in their native habitat have smaller temperature coefficients. Temperature coefficients also varied with growth stage or season. No similar relationship was found for annuals and herbaceous perennials. For the plants tested, Arrhenius temperature coefficients are high during early spring growth, but shift to lower values later in the season. The shift in Arrhenius temperature coefficients occurs early in the season for southern and low-elevation plants and progressively later for plants from further north or higher elevation. The changes in Arrhenius temperature coefficients result largely from increases in plant metabolic rates at lower temperatures while little change occurs in the rates at higher temperatures. Altering the temperature dependence of the control of metabolic rate is apparently an important means of response to climate change.     

Geographic variation in growth response of Douglas‐fir to interannual climate variability and projected climate change

We used 179 tree ring chronologies of Douglas‐fir [Pseudotsuga menziesii (Mirb.) Franco] from the International Tree‐Ring Data Bank to study radial growth response to historical climate variability. For the coastal variety of Douglas‐fir, we found positive correlations of ring width with summer precipitation and temperature of the preceding winter, indicating that growth of coastal populations was limited by summer dryness and that photosynthesis in winter contributed to growth. For the interior variety, low precipitation and high growing season temperatures limited growth. Based on these relationships, we chose a simple heat moisture index (growing season temperature divided by precipitation of the preceding winter and current growing season) to predict growth response for the interior variety. For 105 tree ring chronologies or 81% of the interior samples, we found significant linear correlations with this heat moisture index, and moving correlation functions showed that the response was stable over time (1901–1980). We proceeded to use those relationships to predict regional growth response under 18 climate change scenarios for the 2020s, 2050s, and 2080s with unexpected results: for comparable changes in heat moisture index, the most southern and outlying populations of Douglas‐fir in Mexico showed the least reduction in productivity. Moderate growth reductions were found in the southern United States, and strongly negative response in the central Rocky Mountains. Growth reductions were further more pronounced for high than for low elevation populations. Based on regional differences in the slope of the growth–climate relationship, we propose that southern populations are better adapted to drought conditions and could therefore contain valuable genotypes for reforestation under climate change. The results support the view that climate change may impact species not just at the trailing edges but throughout their range due to genetic adaptation of populations to local environments.     

The influence of fire on the phenological behaviour of Mediterranean plant species in Bas-Languedoc (southern France)

Fire is an ancient ecological factor influencing the Mediterranean vegetation of southern France. The study was carried out on three areas to determine the phenological behaviour of plants with regard to fire. First we studied the flowering responses of perennials in relation to the time since fire: in a Quercus coccifera garrigue most species flower during the year following burning. In comparing species by species between burned and unburned areas most species did not show major differences in the phenological stages. However, fire did increase the number of inflorescences of grasses. A phenological synthesis showed that differences at the community level existed for the flowering stages between the burned areas and the unburned control sites during the first and second years following fire. The growth of some woody species was also studied; the elongation and growth of the plants were biggest during the first or second year after fire. The lack of differences in phenological response between burned and unburned plants may be an adaptive trait to fire.     

Sporadic rainy events are more critical than increasing of drought intensity for woody species recruitment in a Mediterranean community

The understanding of the impact of extreme climatic events under a global climate change scenario is crucial for the accurate forecast of future plant community dynamics. We have experimentally assessed the effect of drier and wetter summer conditions on the recruitment probabilities and the growth of seedlings from eight woody species representative of the most important functional groups in the community, pioneer shrubs, mid-successional shrubs and trees, across the main habitats in the study area (open habitat, shrubland, and forest). Our hypothesis proposes that wet summer conditions would represent a good opportunity for tree species regeneration, enhancing both forest maintenance and expansion. A drier summer scenario, on the other hand, would limit forest regeneration, and probably hinder the colonization of nearby habitats. We found a habitat effect on the emergence, survival, and final biomass, whereas different climate scenarios affected seedling survival and biomass. A wet summer boosted growth and survival, whereas greater drought reduced survival only in some cases. These results were modulated by the habitat type. Overall, shrub species presented higher survival and growth and were less affected by more severe drought, whereas some tree species proved to be extremely dependent on wet summer conditions. We conclude that the reduction in frequency of wet summers predicted for the coming decades in Mediterranean areas will have greater consequences for species recruitment than will increased drought. The different response of the species from the various functional groups has the potential to alter the composition and dominance of future plant communities.     

Monosoonal precipitation responses of shrubs in a cold desert community on the Colorado Plateau

South-eastern Utah forms a northern border for the region currently influenced by the Arizona monosoonal system, which feeds moisture and summer precipitation into western North America. One major consequence predicted by global climate change scenarios is an intensification of monosoonal (summer) precipitation in the aridland areas of the western United States. We examined the capacity of dominant perennial shrubs in a Colorado Plateau cold desert ecosystem of southern Utah, United States, to use summer moisture inputs. We simulated increases of 25 and 50 mm summer rain events on Atriplex canescens, Artemisia filifolia, Chrysothamnus nauseosus, Coleogyne ramosissima, and Vanclevea stylosa, in July and September with an isotopically enriched water (enriched in deuterium but not ¹⁸O). The uptake of this artificial water source was estimated by analyzing hydrogen and oxygen isotope ratios of stem water. The predawn and midday xylem water potentials and foliar carbon isotope discrimination were measured to estimate changes in water status and water-use efficiency. At. canescens and Ch. nauseosus showed little if any uptake of summer rains in either July or September. The predawn and midday xylem water potentials for control and treatment plants of these two species were not significantly different from each other. For A. filifolia and V. stylosa, up to 50% of xylem water was from the simulated summer rain, but the predawn and midday xylem water potentials were not significantly affected by the additional summer moisture input. In contrast, C. ramosissima showed significant uptake of the simulated summer rain (>50% of xylem water was from the artificial summer rain) and an increase in both predawn and midday water potentials. The percent uptake of simulated summer rain was greater when those rains were applied in September than in July, implying that high soil temperature in midsummer may in some way inhibit water uptake. Foliar carbon isotope discrimination increased significantly in the three shrubs taking up simulated summer rain, but pre-treatment differences in the absolute discrimination values were maintained among species. The ecological implications of our results are discussed in terms of the dynamics of this desert community in response to changes in the frequency and dependability of summer rains that might be associated with a northward shift in the Arizona monsoon boundary.     

Elevated precipitation alters the community structure of spring ephemerals by changing dominant species density in Central Asia

Global climate change is one of the most pressing conservation challenges; in particular, changes in precipitation regimes have already substantially influenced terrestrial ecosystems. However, the mechanisms influencing precipitation changes on individual plants and the plant communities in desert grasslands have yet to be fully elucidated. We therefore examine the influence of increased precipitation on plant community compositions in the Gurbantunggut Desert, Xinjiang, northwestern China, from 2005 to 2009. We found that growth of all plant species and the community productivities increased markedly with enhanced water input. Cover of ephemeral synusia also significantly increased due to increased precipitation, implying that the role of the ephemeral community for stabilization of sand dunes was strengthened by increased precipitation. The response of plant community compositions to increased precipitation was primarily reflected as changes in plant density, while increased precipitation did not affect plant species richness and the diversity index. Dominant species drove the response of plant density to increasing precipitation during the five‐year study period. However, the relative responses of rare species were stronger than those of the dominant species, thereby potentially driving species turnover with long‐term increased precipitation. This finding improved our understanding of how increased precipitation drives the changes in plant community composition in desert grasslands and will help to better predict changes in the community composition of ephemerals under future global climate change scenarios.     

Growth delay by winter precipitation could hinder Juniperus sabina persistence under increasing summer drought

Plants in Mediterranean mountains are particularly vulnerable to climatic change. In these environments, low temperature is combined with water shortage during summer, and as a result, the positive effect of global warming theoretically expanding the growing season length may be counterbalanced by rising drought stress. These circumstances may be exacerbated in the rear edge of species distribution, where warmer conditions occur. Here, we examined the climate-growth relationships of Juniperus sabina, a major prostrate shrub above the treeline in Mediterranean mountains, to investigate climate sensitivity and long-term signals stability in four rear-edge populations from southern Spain. We demonstrate that, over recent decades, local climatic conditions have modulated the response of J. sabina secondary growth to the ongoing climate change. We observed a negative effect of winter-spring moisture on secondary growth that suggests a limitation for earlier growth activity at higher elevation, potentially hindering the ability of J. sabina to compensate forthcoming increases in summer drought. At the driest site, we also detected a positive effect of October precipitation, suggesting a second growth pulse by early autumn. Our results provide an example of how local climatic conditions may limit plastic responses of secondary growth to climatic variability. The implications of growth limitation in J. sabina exceed the species-scale level, since these prostrate shrubs play a critical role as nurse plants and local biodiversity foci in Mediterranean mountains.     

Vegetation development on deposit soils starting at different seasons

Permanent plots were created in different seasons (autumn and spring) and filled with two substrates: nutrient-rich topsoil and nutrient-poor ruderal soil (n = 5 for each treatment). My objectives were to assess the influence of starting season on initial species composition, whether differences at the start cause divergent or convergent pathways of succession and which mechanisms are operating during vegetation development. Mean species richness (number of species per plot) and mean total cover of herb layer differed significantly between substrates and changed significantly during 10 year succession, but there were no significant differences with respect to starting season. However, seasonal as well as substrate effects were evident for particular dominant species and for the pattern of successional sequences. When succession on topsoil plots started in spring, first summer annuals dominated, then monocarpic and polycarpic perennial herbs, then herbaceous perennials together with woody perennials, and at the end of the decade woody perennials. When succession started in autumn, polycarpic perennial herbs dominated from the beginning, and then were replaced by woody perennials in the second half of the decade. On ruderal soil, there was a less rapid but continuous increase of polycarpic perennial herbs and woody species, both on spring and on autumn plots, whereas short-lived plants were more abundant in the first years and then decreased. Species turnover was very high from the first to the second year for all treatments (except topsoil plots starting in autumn), but slowed down during succession. Priority effects due to starting season caused high dissimilarity at the start on the nutrient-rich substrate, but convergent succession towards the end of the first decade. The main mechanisms during early succession on the nutrient-rich topsoil were tolerance based on different life-history traits and inhibition due to reduced light availability. There was no evidence for obligate facilitation. However, an indirect facilitative effect by annuals, which slowed the development of herbaceous perennials down, and thus facilitated growth of woody species, could be seen on topsoil when succession started in spring.     

The dynamics of species richness in an experimentally restored calcareous grassland

Abstract. This paper concerns the hypothesis that shoot (light) competition is the main interaction determining the community change during succession from a relatively species-poor deciduous forest (an overgrown former grassland) to a species-rich grassland, while root (nutrient) competition is of little importance. In a 4-yr restoration experiment, clearcutting, mowing and root trenching were used as treatments. The results did not reject the hypothesis. However, the significance of separating two kinds of shoot competition - ‘coarse-scale (between different growth forms) and ‘fine-scale’ (between similar growth forms) became evident. Release from the ‘coarse-scale’ shoot competition (between different growth forms) increased species richness at the beginning of the experiment. This change was interpreted as the replacement of one species pool (shade-tolerant herbaceous perennials) by another (light-demanding herbaceous perennials), the second pool containing considerably more species. The importance of ‘fine-scale’ shoot competition increased gradually - the levelling of competition by mowing resulted in a more pronounced increase in species richness during successive years. The elimination of ‘coarse-scale’ root competition seemed to be important to some extent only in combination with another treatment - mowing. Initial colonization of the cleared area by individual species was a stochastic process which had little relevance to life-history traits. True grassland species were able to colonize quickly. On the community scale, the developing community still remained relatively poor in species. In all plots which were cleared but not mown, succession already started to reverse towards woody vegetation in the third year.     

Importance of Seasonality for the Response of a Mesic Temperate Grassland to Increased Precipitation Variability and Warming

Timing of precipitation events within the growing season and the non-uniformity of warming might be decisive for alterations in productivity and community composition, with consequences for ecosystem functioning. The responses of aboveground production, community composition, functional group and species evenness to altered intra-annual precipitation variability and their interactions with winter or summer warming were examined in European, mesic temperate grassland. Increased precipitation variability with an induced spring drought resulted in a 17% reduction in ANPP, and late drought reduced ANPP by 18% compared to regular rainfall patterns throughout the entire growing season. Winter warming increased ANPP by 12%, whereas summer warming showed no significant effect on biomass but decreased species richness. The effects of increased precipitation variability and warming on ANPP were independent of each other. Forbs benefited from high precipitation variability with spring drought events, likely due to reduced competitive pressure by decreasing, water stressed grasses. Increased precipitation variability coinciding with higher summer temperatures led to reduced species evenness and likely promoted the establishment of specialists and drought-tolerant species. Seasonality of climatic factors, here early versus late drought events in the high precipitation variability treatments, was important in driving shifts in community composition but not for decreases in ANPP. Non-uniform warming, here winter versus summer, affected the direction of response of both community composition and ANPP. Variability of resources is affecting ecosystem processes and species interactions. Recognition of seasonality and non-uniformity of climatic factors will improve predictions of plant performance and biotic interactions in response to climate change.     

Modulation of relative growth rate and its components by water stress in Mediterranean species with different growth forms

Effects of water availability on seedling growth were analysed in eight Mediterranean species naturally occurring in the Balearic Islands. Seedlings were grown outdoors during summer under two irrigation treatments: field capacity and 35% of field capacity. The relative growth rate (RGR) strongly depended on the growth form, from highest values in herbs to lowest in woody perennials. The main component associated with interspecific variation in RGR was the specific leaf area (SLA), and a quantitative grouping of the different growth forms appeared along the regression line between both parameters. The slow-growing species, i.e. woody perennial shrubs, had the lowest SLA and the fast-growing perennial herbs, the highest, while woody semi-deciduous shrubs appeared intermediate. Decreases in RGR due to water stress were analysed in terms of the relative contribution of the leaf mass ratio (LMR), SLA and the net assimilation rate (NAR). Pooling all species, the decrease in RGR caused by water deficit was mainly explained by decreases in SLA. However, this general pattern was strongly dependent of growth form. Thus, in the woody perennial plants, the decrease in RGR was accompanied by a three-fold decrease in NAR which, however, increased in perennial herbs. SLA increased with decreasing water supply in woody perennial plants, and decreased in woody semi-deciduous shrubs and perennial herbs. Finally, decreases in LMR partly explained decreases in RGR in perennial herbs and woody perennial shrubs. This different response of the different growth forms may reflect differences in seedling adaptation and surviving strategies to drought periods.     

Drivers of secondary succession rates across temperate latitudes of the Eastern USA: climate,soils, and species pools

Climate change is widely expected to induce large shifts in the geographic distribution of plant communities, but early successional ecosystems may be less sensitive to broad-scale climatic trends because they are driven by interactions between species that are only indirectly related to temperature and rainfall. Building on a biogeographic analysis of secondary succession rates across the Eastern Deciduous Forest (EDF) of North America, we describe an experimental study designed to quantify the relative extent to which climate, soil properties, and geographic species pools drive variation in woody colonization rates of old fields across the EDF. Using a network of five sites of varying soil fertility spanning a latitudinal gradient from central New York to northern Florida, we added seeds of nine woody pioneer species to recently tilled old fields and monitored first-year growth and survivorship. Results suggest seedlings of southern woody pioneer species are better able to quickly establish in fields after abandonment, regardless of climate regime. Sites of lower soil fertility also exhibited faster rates of seedling growth, likely due to the slower development of the successional herbaceous community. We suggest that climate plays a relatively minor role in community dynamics at the onset of secondary succession, and that site edaphic conditions are a stronger determinant of the rate at which ecosystems develop to a woody-dominated state. More experimental research is necessary to determine the nature of the herbaceous–woody competitive interface and its sensitivity to environmental conditions.     

Community structure and species composition along a disturbance gradient in a communally managed South African savanna

The woody and herbaceous vegetation was investigated around four rural settlements in the savanna area of the eastern Transvaal Lowveld, South Africa. At each settlement three transects radiating out from the settlement, representing a gradient from high to low disturbance, were sampled for community structure and species composition. In general, attributes of woody community structure (density, height, biomass, basal area and diversity) were negatively related to increasing disturbance. Herbaceous cover responded positively. Although individual woody species exhibited a range of responses to disturbance, overall species compositional changes were not related directly to the intensity of disturbance. Individual woody species were classified into behavioural species response groups according to their response along the disturbance gradient. Local wood harvesters demonstrated marked selection for particular species and size classes, which should have disproportional impacts on community structure. However, proportional size class distributions were little altered along the gradient.     

Phylogenetic conservatism and trait correlates of spring phenological responses to climate change in northeast China

Climate change has resulted in major changes in plant phenology across the globe that includes leaf‐out date and flowering time. The ability of species to respond to climate change, in part, depends on their response to climate as a phenological cue in general. Species that are not phenologically responsive may suffer in the face of continued climate change. Comparative studies of phenology have found phylogeny to be a reliable predictor of mean leaf‐out date and flowering time at both the local and global scales. This is less true for flowering time response (i.e., the correlation between phenological timing and climate factors), while no study to date has explored whether the response of leaf‐out date to climate factors exhibits phylogenetic signal. We used a 52‐year observational phenological dataset for 52 woody species from the Forest Botanical Garden of Heilongjiang Province, China, to test phylogenetic signal in leaf‐out date and flowering time, as well as, the response of these two phenological traits to both temperature and winter precipitation. Leaf‐out date and flowering time were significantly responsive to temperature for most species, advancing, on average, 3.11 and 2.87 day/°C, respectively. Both leaf‐out and flowering, and their responses to temperature exhibited significant phylogenetic signals. The response of leaf‐out date to precipitation exhibited no phylogenetic signal, while flowering time response to precipitation did. Native species tended to have a weaker flowering response to temperature than non‐native species. Earlier leaf‐out species tended to have a greater response to winter precipitation. This study is the first to assess phylogenetic signal of leaf‐out response to climate change, which suggests, that climate change has the potential to shape the plant communities, not only through flowering sensitivity, but also through leaf‐out sensitivity.     

Species richness,vegetation structure,and floristic composition of woody plants along the elevation gradient of Mt. Meru,Tanzania

Understanding the change in vegetation composition along elevational gradients is critical for species conservation in a changing world. We studied the species richness, tree height, and floristic composition of woody plants along an elevation gradient of protected habitats on the eastern slope of Mount Meru and analyzed how these vegetation variables are influenced by the interplay of temperature and precipitation. Vegetation data were collected on 44 plots systematically placed along five transects spanning an elevational gradient of 1600 to 3400 m a.s.l. We used ordinary linear models and multivariate analyses to test the effect of mean annual temperature and precipitation on woody plant species richness, tree height, and floristic composition. We found that species richness, mean tree height, and maximum tree height declined monotonically with elevation. Models that included only mean annual temperature as an explanatory variable were generally best supported to predict changes in species richness and tree height along the elevation gradient. We found significant changes in woody plant floristic composition with elevation, which were shaped by an interaction of mean annual temperature and precipitation. While plant communities consistently changed with temperature along the elevation gradient, levels of precipitation were more important for plant communities at lower than for those at higher elevations. Our study suggests that changes in temperature and precipitation regimes in the course of climate change will reshape elevational gradients of diversity, tree height, and correlated carbon storage in ecosystems, and the sequence of tree communities on East African mountains.     

Woody plants and the prediction of climate-change impacts on bird diversity

Current methods of assessing climate-induced shifts of species distributions rarely account for species interactions and usually ignore potential differences in response times of interacting taxa to climate change. Here, we used species-richness data from 1005 breeding bird and 1417 woody plant species in Kenya and employed model-averaged coefficients from regression models and median climatic forecasts assembled across 15 climate-change scenarios to predict bird species richness under climate change. Forecasts assuming an instantaneous response of woody plants and birds to climate change suggested increases in future bird species richness across most of Kenya whereas forecasts assuming strongly lagged woody plant responses to climate change indicated a reversed trend, i.e. reduced bird species richness. Uncertainties in predictions of future bird species richness were geographically structured, mainly owing to uncertainties in projected precipitation changes. We conclude that assessments of future species responses to climate change are very sensitive to current uncertainties in regional climate-change projections, and to the inclusion or not of time-lagged interacting taxa. We expect even stronger effects for more specialized plant–animal associations. Given the slow response time of woody plant distributions to climate change, current estimates of future biodiversity of many animal taxa may be both biased and too optimistic.     

Temporal and spatial variation of litter production in Sonoran Desert communities

The seasonal pattern of litter production was analyzed in three contiguous desert communities near the southern boundaries of the Sonoran Desert. There was a large spatial variation in annual litter production mainly caused by differences in the composition and structure of vegetation. In the most productive site (Arroyos) annual litterfall was 357 g m^-2yr^-1, a figure higher than some tropical deciduous forests. Litter production was only 60g m^-2yr^-1in the open desert in the plains (Plains) and 157 g m^-2yr^-1 in the thornscrub on the slopes (Hillsides). Topographic and hydrologic features influence the composition, structure and function of the vegetation, modifying the general relationship between rainfall and productivity described for desert ecosystems. The temporal pattern of litter production showed marked seasonality with two main periods of heavy litterfall: one after the summer rains from September to November (autumn litter production) and another after the winter rains from March to May (spring litter production). In the open desert areas, spring litter production was significantly higher than the autumn pulse, while in the slopes, the autumn production was the most important. The Arroyos site produced similar litterfall amounts during the two dry seasons. The species composition defined the season of maximum leaf-fall. In the Plains, the vigorous winter growth of ephemeral and perennial plants made up most of the litter production, while in the Hillsides, most perennials remained dormant throughout the winter-spring period and a significant peak of litterfall occurred only after the summer growth. This difference in growth between seasons was less pronounced in the Arroyos. The timing of maximum production of reproductive and woody litter also differed from site to site.     

Climate change effects on plant biomass alter dominance patterns and community evenness in an experimental old‐field ecosystem

Atmospheric and climatic change can alter plant biomass production and plant community composition. However, we know little about how climate change‐induced alterations in biomass production affect plant species composition. To better understand how climate change will alter both individual plant species and community biomass, we manipulated atmospheric [CO₂], air temperature, and precipitation in a constructed old‐field ecosystem. Specifically, we compared the responses of dominant and subdominant species to our climatic treatments, and explored how changes in plant dominance patterns alter community evenness over 2 years. Our study resulted in four major findings: (1) all treatments, elevated [CO₂], warming, and increased precipitation increased plant community biomass and the effects were additive rather than interactive, (2) plant species differed in their response to the treatments, resulting in shifts in the proportional biomass of individual species, which altered the plant community composition; however, the plant community response was largely driven by the positive precipitation response of Lespedeza, the most dominant species in the community, (3) precipitation explained most of the variation in plant community composition among treatments, and (4) changes in precipitation caused a shift in the dominant species proportional biomass that resulted in lower community evenness in the wet relative to dry treatments. Interestingly, compositional and evenness responses of the subdominant community to the treatments did not always follow the responses of the whole plant community. Our data suggest that changes in plant dominance patterns and community evenness are an important part of community responses to climatic change, and generally, that such compositional shifts can alter ecosystem biomass production and nutrient inputs.     

Forest tree growth response to hydroclimate variability in the southern Appalachians

Climate change will affect tree species growth and distribution; however, under the same climatic conditions species may differ in their response according to site conditions. We evaluated the climate‐driven patterns of growth for six dominant deciduous tree species in the southern Appalachians. We categorized species into two functional groups based on their stomatal regulation and xylem architecture: isohydric, diffuse porous and anisohydric, ring porous. We hypothesized that within the same climatic regime: (i) species‐specific differences in growth will be conditional on topographically mediated soil moisture availability; (ii) in extreme drought years, functional groups will have markedly different growth responses; and (iii) multiple hydroclimate variables will have direct and indirect effects on growth for each functional group. We used standardized tree‐ring chronologies to examine growth of diffuse‐porous (Acer, Liriodendron, and Betula) and ring‐porous (Quercus) species vs. on‐site climatic data from 1935 to 2003. Quercus species growing on upslope sites had higher basal area increment (BAI) than Quercus species growing on mesic, cove sites; whereas, Acer and Liriodendron had lower BAI on upslope compared to cove sites. Diffuse‐porous species were more sensitive to climate than ring porous, especially during extreme drought years. Across functional groups, radial growth was more sensitive to precipitation distribution, such as small storms and dry spell length (DSL), rather than the total amount of precipitation. Based on structural equation modeling, diffuse‐porous species on upslope sites were the most sensitive to multiple hydroclimate variables (r² = 0.46), while ring‐porous species on upslope sites were the least sensitive (r² = 0.32). Spring precipitation, vapor pressure deficit, and summer storms had direct effects on summer AET/P, and summer AET/P, growing season small storms and DSL partially explained growth. Decreasing numbers of small storms and extending the days between rainfall events will result in significant growth reduction, even in regions with relatively high total annual rainfall.