Xeric Sites Research Articles

Analyzing ingrowth using zero-inflated negative binomial models

Ingrowth is an important element of stand dynamics in several silvicultural systems, especially in continuous cover forestry. Earlier predictive models for ingrowth in Finnish forests are few and not based on up-to-date statistical methods. Ingrowth is here defined as the number of trees over 1.3 m entering a plot. This study developed new ingrowth models for Scots pine ( L.), (Picea abies (L.) H. Karst.) and birch ( Roth and Ehrh.) using data from the permanent sample plots of the Finnish national forest inventory. The data were over-dispersed compared to a Poisson process and had many zeros. Therefore, a zero-inflated negative binomial model was used. The total and species-specific stand basal areas, temperature sum and fertility class were used as predictors in the ingrowth models. Both fixed-effects and mixed-effects models were fitted. The mixed-effects model versions included random plot effects. The mixed-effects models had larger likelihoods but provided biased predictions. Also censored prediction was considered where only a certain maximum number of ingrowth trees were accepted for a plot. The models predicted most pine ingrowth in pine-dominated stands on sub-xeric and xeric sites where stand basal area was low. The predicted amount of spruce ingrowth was maximized when the basal area of spruce was 13 m ha. Increasing temperature sum increased spruce ingrowth. Predicted birch ingrowth decreased with increasing stand basal area and towards low fertility classes. An admixture of pine increased the predicted amount of spruce ingrowth.Pinus sylvestrisNorway spruceBetula pendulaB. pubescens2–1

Physiological responses of germinant Pinus palustris and P. taeda seedlings to water stress and the significance of the grass-stage

Loblolly pine (Pinus taeda L.) has gradually replaced longleaf pine (P. palustris Mill.) across much of the coastal southeastern United States. Given recent interest in the regeneration of longleaf pine ecosystems, understanding how these two species will respond to drier conditions associated with climate change will be critically important. We conducted a progressive water-stress experiment on germinants of P. palustris and P. taeda over the course of 9 weeks, measuring gas exchange, allometry, water relations, and tracheid anatomy. P. palustris developed more overall biomass, had wider tracheids, allocated more of its biomass into leaf tissue, and maintained a narrower safety margin against hydraulic failure than P. taeda. Both species reduced leaf gas exchange as water stress increased, though P. palustris experienced earlier and greater declines in photosynthetic rates than P. taeda. The greater aboveground growth and narrower hydraulic safety margin observed in P. palustris represents a trade-off between growth and hydraulic safety and is likely related to the species’ defensive strategy against fire. We suggest that this trade-off may limit P. palustris germinant regeneration on sandier, xeric sites as climate change intensifies.

Evidence for trait‐based community assembly patterns in hardwood hammock forests

AbstractThe hardwood forests of south Florida, commonly referred to as hammocks, persist as well‐drained patches of broadleaf forest, embedded in a matrix of brackish water swamp, freshwater marsh, or pineland. Little is known about the patterns of community assembly of these subtropical forest communities in the landscape mosaic of south Florida. We used a functional trait approach to understand the composition of these communities and their responses to environmental variation across four areas of south Florida: the pine rocklands and freshwater marshes in Everglades National Park, and the lower and upper Florida Keys, where the hammocks are surrounded by halophytic swamp communities. These sites represent an environmental gradient from less‐productive, more xeric sites in the lower Florida Keys, to more‐productive, wetter sites in the Everglades marshes. We examined the patterns of trait variation at three levels (individual, population, and community) to examine the underlying processes driving assembly in these hammock communities. To understand processes governing community composition in each site, we used methods that partition variance in six traits (maximum tree height, specific leaf area, wood specific gravity, leaf nitrogen, leaf phosphorus, and leaf stable carbon isotope ratios) into internal and external filtering components. Community‐weighted mean trait values for three traits (specific leaf area, height, and leaf phosphorus) increased significantly from dry, less‐productive coastal sites in the Florida Keys to the moist, more‐productive areas on the mainland, while wood specific gravity and leaf δ13C showed the opposite pattern. For one or more traits per site, standardized effect sizes differed significantly from null expectation. Processes such as competition for resources (e.g., water, nutrients, light) and species sorting across microhabitats (i.e., within site) operate to increase local functional trait variation within communities and among species across sites. External filtering on individuals for height and leaf phosphorus differed significantly from null expectations across sites, while external filtering on species was only observed for specific leaf area, maximum tree height, and leaf phosphorus. These results are consistent with strong environmental filtering across the region, among local communities differing in freshwater accessibility or that occupy different positions along strong edaphic gradients. Our results confirm the importance of intraspecific variation among species and reflect a high degree of trait plasticity across the environmental gradient.

Tough as old boots: amphibians from drier habitats are more resistant to desiccation, but less flexible at exploiting wet conditions

Intra-specific variation in the ability of individuals to tolerate environmental perturbations is often neglected when considering the impacts of climate change. Yet this information is potentially crucial for mitigating any deleterious effects of climate change on threatened species. Here we assessed patterns of intra-specific variation in desiccation tolerance in the frog Pseudophryne guentheri, a terrestrial-breeding species experiencing a drying climate. Adult frogs were collected from six populations across a rainfall gradient and their dehydration and rehydration rates were assessed. We also compared desiccation tolerance of embryos and hatchlings originating from within-population parental crosses from four of the six populations, where selection on desiccation tolerance should be especially strong given that embryos cannot move to escape unfavourable microclimates. Embryos were reared on soil at three soil-water potentials, ranging from wet to dry (ψ = −10, −100 & −400 kPa), and their desiccation tolerance was assessed across a range of traits including survival, time to hatch after inundation, wet mass at hatching, hatchling malformations and swimming performance. We found significant and strong patterns of intra-specific variation in almost all traits, both in adults and first generation offspring. Adult frogs exhibited clinal variation in their water balance responses, with populations from drier sites both dehydrating and rehydrating more slowly compared to frogs from more mesic sites. Similarly, desiccation tolerance of embryos and hatchlings was significantly greater in populations from xeric sites. Taken together, our findings suggest that populations within this species will respond differently to the regional reduction in rainfall predicted by climate change models. We emphasise the importance of considering geographic variation in phenotypic plasticity when predicting how species will respond to climate change.

Long-term nutrient imbalances linked to drought-triggered forest dieback

Drought-induced forest dieback is causing reductions in productivity, increasing tree mortality and impairing terrestrial carbon uptake worldwide. However, the role played by long-term nutrient imbalances during drought-induced dieback is still unknown. To improve our knowledge on the relationships between dieback and nutrient imbalances, we analysed wood anatomical traits (tree-ring width and wood density), soil properties and long-term chemical information in tree-ring wood (1900–2010) by non-destructive Micro X-ray fluorescence (μXRF) and destructive (ICP-OES) techniques. We studied two major European conifers with ongoing drought-induced dieback in mesic (Abies alba, silver fir) and xeric (Pinus sylvestris, Scots pine) sites. In each site we compared coexisting declining (D) and non-declining (ND) trees. We used dendrochronology and generalized additive and linear mixed models to analyse trends in tree-ring nutrients and their relationships with wood traits. The D trees presented lower growth and higher minimum wood density than ND trees, corresponding to a smaller lumen area of earlywood tracheids and thus a lower theoretical hydraulic conductivity. These differences in growth and wood-anatomy were more marked in silver fir than in Scots pine. Moreover, most of the chemical elements showed higher concentrations in D than in ND trees during the last two-five decades (e.g., Mn, K and Mg), while Ca and Na increased in the sapwood of ND trees. The Mn concentrations, and related ratios (Ca:Mn, Mn:Al and P:Mn) showed the highest differences between D and ND trees for both tree species. These findings suggest that a reduced hydraulic conductivity, consistent with hydraulic impairment, is affecting the use of P in D trees, making them more prone to drought-induced damage. The retrospective quantifications of Mn ratios may be used as early-warning signals of impending dieback.

Seed depredation negates the benefits of midstory hardwood removal on longleaf pine seedling establishment

Midstory hardwoods are traditionally removed to restore longleaf pine on fire‐excluded savannas. However, recent evidence demonstrating midstory hardwood facilitation on longleaf pine seedling survival has brought this practice into question on xeric sites. Also, midstory hardwoods could facilitate longleaf pine seedling establishment, as hardwood litter may conceal seeds from seed predators or improve micro‐environmental conditions for seedling establishment. However, little is known about these potential mechanisms. In this study, we tracked longleaf pine seed depredation and germination in artificially seeded plots (11 seeds/m2) in a factorial design fully crossing hardwood retention or removal with vertebrate seed predator access or exclusion in the Sandhills Ecoregion of North Carolina, U.S.A. Seed depredation averaged 78% across treatments and was greatest in unexcluded plots. Hardwood retention did not affect seed depredation. Longleaf pine averaged 3.6 germinants/4 m2 across treatments, and was six times more abundant where vertebrates had been excluded. Hardwood removal had a strong positive effect on seedling germination, likely due to the removal of litter, but only when vertebrates were excluded. Our results indicated midstory hardwoods are not facilitating longleaf pine seedling establishment. Nevertheless, our results indicated that hardwood removal may not increase longleaf pine seedling establishment, as seed depredation diminished the effectiveness of hardwood removal under mast seed availability. Collectively, these results demonstrate the underlying complexity of the longleaf pine ecosystem, and suggest that planting may need to be part of the restoration strategy on sites where seed depredation limits longleaf pine natural regeneration.

The role of understory phenology and productivity in the carbon dynamics of longleaf pine savannas

AbstractSavanna ecosystems contribute ~30% of global net primary production (NPP), but they vary substantially in composition and function, specifically in the understory, which can result in complex responses to environmental fluctuations. We tested how understory phenology and its contribution to ecosystem productivity within a longleaf pine ecosystem varied at two ends of a soil moisture gradient (mesic and xeric). We used the Normalized Difference Vegetation Index (NDVI) of the understory and ecosystem productivity estimates from eddy covariance systems to understand how variation in the understory affected overall ecosystem recovery from disturbances (drought and fire). We found that the mesic site recovered more rapidly from the disturbance of fire, compared to the xeric site, indicated by a faster increase in NDVI. During drought, understory NDVI at the xeric site decreased less compared to the mesic site, suggesting adaptation to lower soil moisture conditions. Our results also show large variation within savanna ecosystems in the contribution of the understory to ecosystem productivity and recovery, highlighting the critical need to further subcategorize global savanna ecosystems by their structural features, to accurately predict their contribution to global estimates of NPP.

Variations in the intrinsic water-use efficiency of north Patagonian forests under a present climate change scenario: tree age, site conditions and long-term environmental effects.

The carbon isotope composition (δ13C) in tree rings were used to derive the intrinsic water-use efficiency (iWUE) of Araucaria araucana trees of northern Patagonia along a strong precipitation gradient. It is well known that climatic and ontogenetic factors affect growth performance of this species but little is known about their influence in the physiological responses, as iWUE. Thus, the main objective of this study was to assess the physiological reactions of young and adult trees from two open xeric and two moderately dense mesic A. araucana forests to the increases in atmospheric CO2 (Ca) and air temperature during the 20th century, and to relate these responses with radial tree growth. The results indicated that the iWUE and the intercellular CO2 concentration (Ci) increased 33% and 32% in average during the last century, respectively, but carbon isotope discrimination (∆13C) was more variable between sites and age classes. Trees from xeric sites presented greater iWUE and lower ∆13C and Ci values than those from mesic sites. In general, iWUE was strongly related with Ca and was significantly affected by mean summer maximum temperature. ∆13C from mesic sites seemed to be mainly affected by summer maximum temperature, while trees from xeric conditions did not show any influence. Tree age also presented a significant effect on iWUE. Adult trees showed higher iWUE values than young trees, indicating an incidence of the tree age and/or height, mainly in closed mesic forests. Moreover, some trees presented positive relationships between iWUE and radial tree growth, while others presented negative or no relationships, indicating that other factors may negatively influence tree growth. Broadly, the results demonstrate the incidence of climatic, environmental and ontogenetic variability in the tree responses; however, more studies are needed to better understand which forests will be more affected by actual and future climate changes.

Natural regeneration after gap cutting in Scots pine stands in northern Finland

ABSTRACTThe objective of the study was to examine the success of regeneration in gaps of variable size in pine-dominated stands in terms of seedling density. It was based on an experiment in central Lapland containing circular clear-cut gaps of 20, 40 and 80 m in diameter on typical sub-xeric and xeric pine sites with site preparation (patch scarification). The observation period covered the first five years after cutting. The average number of seedlings that had emerged after gap cutting was about 22 000 ha−1 for pine and 7 000 ha−1 for birch. The proportion of regeneration sample plots without any pine seedlings was less than 10%. The seedling density diminished constantly with greater distance from the edge stand but indicating sufficient density up to the largest gap size in the study (diameter 80 m, area ca. 0.5 ha). Site preparation promoted regeneration remarkably, and we conclude that successful regeneration would be achieved in most cases with soil scarification exposing just 10–20% of the soil surface.

Response of soil microbial communities to altered precipitation: A global synthesis

AbstractAimClimate change intensifies the hydrological cycle and consequently alters precipitation regimes. Accurately assessing future carbon (C) budgets depends on understanding the influence of altered precipitation on both aboveground C cycling and belowground processes. Our goal was to explore generalities and mechanisms of responses of soil microbial communities to altered precipitation and implications for C cycling in terrestrial ecosystems.LocationGlobal.Time period2001–2017.Major taxa studiedSoil microbes.MethodsWe used the meta‐analytical technique to synthesize data of 41 increased (IPPT) and 53 decreased precipitation (DPPT) studies from 65 publications worldwide. The data covered broad variations in climate, percentage of precipitation change, experimental duration and soil properties.ResultsThe fungi to bacteria ratio did not show a water‐tolerant shift, but the community compositions within the bacteria did. Microbial biomass showed a higher response to moderate IPPT than moderate DPPT, whereas it was more sensitive to extreme DPPT than extreme IPPT, suggesting that the responses of microbial biomass to altered precipitation are double asymmetric. However, such asymmetric responses of microbial biomass varied with climate humidity and soil texture: microbial biomass was more sensitive to IPPT at xeric sites than at mesic sites, whereas it was more responsive to DPPT in humid areas; microbial biomass in coarse‐textured soils was more sensitive to altered precipitation than that in fine‐textured soils. In addition, microbial response was positively correlated with the responses of aboveground/belowground plant biomass, soil respiration and organic C content.Main conclusionsOur meta‐analysis provides the first evidence that the asymmetric response of microbial biomass to altered precipitation varies with climate humidity and soil texture. Given the coordinated responses in the plant–soil–microorganism C continuum, our synthesis extends the double asymmetric model and provides a framework for understanding and modelling responses of ecosystem C cycling to global precipitation change.

Effects of Seedling Quality and Family on Performance of Northern Red Oak Seedlings on a Xeric Upland Site

Cultural practices to develop larger, more robust oak seedlings have been developed, however, the potential improvement conferred by these larger seedlings has received limited testing in the Northeast. We evaluated the effect of seedling size and pedigree on the survival, growth, and competitive ability of northern red oak (Quercus rubra L.) seedlings planted on a xeric site in northeastern Pennsylvania. We planted seedlings from a state tree nursery that represented locally available seedling stock, as well as high-quality seedlings from seven half-sibling families grown following improved nursery protocol. Half-sibling families were split into three size classes based on their root collar diameter and height; large, average, and poor. Eleven years after planting, survival across seedling treatments ranged from 45 percent for locally available seedlings, to 96 percent for one half-sibling family. Two families showed superior growth, survival, and competitive ability compared with the others. Seedling size class conferred moderate height and diameter advantage in four and three of the families, respectively. Initial seedling size was an important variable in models predicting survival, diameter, and dominance (competitive ability). Over time, the relationship between initial diameter and height diminished.

Limits to Ponderosa Pine Regeneration following Large High-Severity Forest Fires in the United States Southwest

High-severity fires in dry conifer forests of the United States Southwest have created large (>1000 ha) treeless areas that are unprecedented in the regional historical record. These fires have reset extensive portions of Southwestern ponderosa pine (Pinus ponderosa Lawson & C. Lawson var. scopulorum Engelm.) forest landscapes. At least two recovery options following high-severity fire are emerging. One option is for post-fire successional pathways to move toward a return to the pre-fire forest type. Alternatively, an area may transition to persistent non-forested ecosystems. We studied regeneration patterns of ponderosa pine following eight fires in Arizona and New Mexico, USA, that burned in dry conifer forests dominated by ponderosa pine during a recent 18-year regional drought period, 1996 to 2013. Our a priori hypotheses were: 1) the most xeric areas within these severely burned dry conifer forests are least likely to regenerate to the pre-fire forest type due to persistent post-fire moisture stress; and 2) areas farther away from conifer seed sources have a lower likelihood of regeneration, even if these areas are climatically favorable for post-fire ponderosa pine establishment. We evaluated our hypotheses using empirical data and generalized linear mixed-effects models. We found that low-elevation, xeric sites are more limiting to conifer regeneration than higher-elevation mesic sites. Areas >150 m from a seed source are much less likely to have ponderosa pine regeneration. Spatial interpolations of modeled post-fire regeneration of ponderosa pine across the study landscapes indicate expansive areas with low likelihood of pine regeneration following high-severity fire. We discuss multiple post-fire successional pathways following high-severity fire, including potentially stable transitions to non-forest vegetation types that may represent long-term type conversions. These findings regarding landscape changes in Southwest forests in response to fires and post-fire regeneration patterns during early-stage climate warming contribute to the development of better-informed ecosystem management strategies for forest adaptation or mitigation under projected hotter droughts in this region.

Coexisting oak species, including rear-edge populations, buffer climate stress through xylem adjustments.

The ability of trees to cope with climate change is a pivotal feature of forest ecosystems, especially for rear-edge populations facing warm and dry conditions. To evaluate current and future forests threats, a multi-proxy focus on the growth, anatomical and physiological responses to climate change is needed. We examined the long-term xylem adjustments to climate variability of the temperate Quercus robur L. at its rear edge and the sub-Mediterranean Quercus pyrenaica Willd. Both species coexist at a mesic (ME, humid and warmer) and a xeric (XE, dry and cooler) site in northern Spain, the latter experiencing increasing temperatures in recent decades. We compared xylem traits at each site and assessed their trends, relationships and responses to climate (1960-2008). Traits included basal area increment, earlywood vessel hydraulic diameter, density and theoretical-specific hydraulic conductivity together with latewood oxygen (δ18O) stable isotopes and δ13C-derived water-use efficiency (iWUE). Quercus robur showed the highest growth at ME, likely through enhanced cambial activity. Quercus pyrenaica had higher iWUE at XE compared with ME, but limited plasticity of anatomical xylem traits was found for the two oak species. Similar physiological performance was found for both species. The iWUE augmented in recent years especially at XE, likely explained by stomatal closure given the increasing δ18O signal in response to drier and sunnier growing seasons. Overall, traits were more correlated at XE than at ME. The iWUE improvements were linked to higher growth up to a threshold (~85 μmol mol-1) after which reduced growth was found at XE. Our results are consistent with Q. pyrenaica and Q. robur coexisting at the central and dry edge of the climatic species distribution, respectively, showing similar responses to buffer warmer conditions. In fact, the observed adjustments found for Q. robur point towards growth stability of similar rear-edge oak populations under warmer climate conditions.

Facilitation by leguminous shrubs increases along a precipitation gradient

Abstract Combining nutrient dynamics (plant nutrient uptake and soil fertility) can help uncover mechanisms of shrub–grass interactions and assess the validity of the stress‐gradient hypothesis, which predicts that facilitation between plants increases in stressful environments. However, how facilitation via shrub‐mediated nutrient increases varies with precipitation is poorly resolved. We first synthesized a global dataset from 66 studies and evaluated how shrubs affected soil organic carbon (C) and nitrogen (N) in grasslands along a precipitation gradient. We then made new measurements in a single‐grassland type encroached by leguminous shrubs from the same genus (Caragana) to constrain the variations caused by combining different grassland types and shrubs traits in the meta‐analysis. Specifically, we investigated how shrubs mediated N dynamics and how shrub–grass interactions varied along a precipitation gradient (147–342 mm) in a temperate steppe of China. At the global scale, leguminous shrub‐mediated effects on soil nutrients increased with precipitation, while no relationship was found for non‐leguminous shrub. For the field experiment, greater N and lower δ15N in Caragana compared to non‐leguminous shrub (reference shrub, Salsola collina) suggested active N‐fixation in Caragana. We found that Caragana enhanced N concentration and leaf quality (low C:N ratio) in neighbouring plants more on mesic sites than on xeric sites. Thus, facilitation increased via higher soil N and with decreasing environmental stress, at least along this relatively arid precipitation gradient. Our results highlight the importance of precipitation in determining the nutritional facilitation to neighbouring grasses from encroaching leguminous shrubs. Conceptual frameworks for plant facilitation may therefore need to include shrub characteristics (N‐fixers vs. non‐fixers) and positive effects of higher precipitation on this type of facilitation to characterize plant interactions along stress gradients. A plain language summary is available for this article.

How can prescribed burning and harvesting restore shortleaf pine-oak woodland at the landscape scale in central United States? Modeling joint effects of harvest and fire regimes

Abstract Historical fire regimes in the central United States maintained open-canopy shortleaf pine-oak woodlands on xeric sites. Following large-scale harvest and fire suppression, those woodlands grew denser with more continuous canopy cover, and they gained mesic species at the expense of shortleaf pine. There is high interest in restoring shortleaf pine-oak woodlands; most have been converted to other forest types but those that remain are valued for high stand-scale and landscape-scale diversity. Prior stand-scale studies suggest that prescribed burning and harvesting could be effective for restoring pine-oak woodlands. However, previous short-term, stand-scale studies provided little insight into long-term, landscape-scale outcomes. To estimate outcomes of alternative restoration treatments on future species composition and forest structure, we employed an integrated field and modeling approach to simulate effects of prescribed burning and harvesting on the restoration of shortleaf pine-oak woodland composition and structure in the Mark Twain National Forest for a 100-year period. Six scenarios were modeled: no management, burn only, harvest only, and a combination of harvest with burns treatments followed by fire-free intervals of differing starting times or durations to facilitate regeneration recruitment. Both no management and prescribed burn only scenarios cannot restore current forest to historical woodland condition (i.e., 40–80% percent canopy cover or less than 55% stocking); however, scenarios including harvest can restore current forest to woodland condition in late 2020s. Under a no management scenario, total basal area would increase to a maximum around 31 m 2 ha −1 , and white oak group remained the most dominant species group throughout the simulation. Under the burn only treatment, total basal area was not reduced substantially as compared to that under no management scenario, however, there were small increases in the basal area and density of shortleaf pine. All of the treatments that included a combination of burning and harvesting reduced total basal area, which fluctuated around 13 m 2 ha −1 throughout the simulation than those under no management and prescribed only scenarios. The simulations suggested that shortleaf pine would become the most dominant group, followed by white, red oak groups, and other species with combined prescribed burning and harvesting. When coupled with harvest, the prescribed burning regime affected species composition: increasing the number of burns increased the basal area and density of shortleaf pine and decreased the basal area and density of white oak group species.

Summer activity patterns for mosses and lichens in Maritime Antarctica

AbstractWithin Antarctica there are large gradients both in climate and in vegetation which offer opportunities to investigate links between the two. The activity (% total time active) of lichens and bryophytes in hydric and xeric environments was monitored at Livingston Island (62°39'S). This adds a northern site with a maritime, cloudy climate to previous studies in the southern Antarctic Peninsula and the Dry Valleys (78°S). Annual activity increases northwards from less than 1% to nearly 100%. There is a major and consistent difference between hydric sites which, with snow melt, can be 100% active in summer months even in the Dry Valleys, and xeric sites which, depending on precipitation, rarely exceed 80% activity even at Livingston Island. Mosses dominate hydric sites and lichens the xeric sites all along the gradient. Mean temperatures when active are 2–4°C at all sites, as liquid water is required. Light is a potential major stress reaching 880 µmol m-2s-1when active in continental sites. The lack of extremes in temperatures and light combined with high activity levels means that summer at Livingston Island is one of the better sites for lichen and bryophyte growth in the world.

Drought Influence over Radial Growth of Mexican Conifers Inhabiting Mesic and Xeric Sites

Drought is a major constraint of forest productivity and tree growth across diverse habitat types. In this study, we investigated the drought responses of four conifer species growing within two locations of differing elevation and climatic conditions in northern Mexico. Two species were selected at a mesic site (Cupressus lusitanica Mill., Abies durangensis Martínez) and the other two species were sampled at a xeric site (Pinus engelmannii Carr., Pinus cembroides Zucc.). Using a dendrochronological approach, we correlated the radial-growth series of each species and the climatic variables. All study species positively responded to wet-cool conditions during winter and spring. Despite the close proximity of species at a mesic site, A. durangensis had high responsiveness to hydroclimatic variability, but C. lusitanica was not responsive. At the xeric site, P. engelmannii and P. cembroides were very responsive to drought severity, differentiated only by the longer time scale of the response to accumulated drought of P. engelmannii. The responsiveness to hydroclimate and drought of these tree species seems to be modulated by site conditions, or by the functional features of each species that are still little explored. These findings indicate that differentiating between mesic and xeric habitats is a too coarse approach in diverse forests with a high topographic heterogeneity.

Physiographic factors underlie rates of biomass production during succession in Great Lakes forest landscapes

Biomass production in living trees is the basis of numerous forest ecosystem functions and services. However, rates of and controls on biomass production vary widely across temperate forests, particularly over successional timescales of decades and centuries. Biomass production in temperate forests is most often interpreted within the context of biotic or top-down controls, such as species composition or disturbance. However, there is need to investigate how bottom-up physiographic factors, such as landform attributes, drainage, and soil properties mediate biomass production. In order to investigate patterns, controls, and potentials for biomass production across spatial levels ranging from individual ecosystems, to landscapes, to entire regions, we synthesized long-term forest inventory datasets from the United States Great Lakes region, placed them in the context of a hierarchical ecological unit classification, and tested the influence of physiographic factors on biomass production rates and temporal trajectories across ecological levels. Key findings include: 1) At nearly all ecological levels, physiographic controls (e.g., soil texture, drainage class, water table depth) on soil moisture status are significant predictors of variation in biomass production rates, with mesic sites accumulating biomass more rapidly than xeric sites, which, in turn accumulate biomass more rapidly than hydric sites. 2) Aboveground live biomass can apparently continue to accumulate through 2–3 centuries of succession, exceeding 300–400Mgha−1 on mesic sites throughout the region. 3) Stand age distributions indicate that hydric sites are harvested least often, while the high production rates of mesic sites suggest they are most appropriate for frequent harvesting. 4) Median, 1st-quartile, and 3rd-quartile growth rates of individual ecosystems, landscapes, and ecoregional subsections and sections reveal ecological units in which forests may vary in their potential for increases or decreases in biomass production, e.g., due to management interventions, climate change, or disturbances. Specifically, some units have tightly constrained distributions, suggesting little capacity for change in production rates relative to observed medians, while other units have wide variation in biomass production rates, indicating the potential for relatively large increases or decreases in production. Altogether, the results of this analysis show that physiography exerts widespread, bottom-up controls on biomass production across the region of study, and can be used in spatially explicit frameworks to understand ecosystem functioning and inform scientific forest management.

Xylem adjustment of sessile oak at its southern distribution limits.

Drought is a key limiting factor for tree growth in the Mediterranean Basin. However, the variability in acclimation via xylem traits is largely unknown. We studied tree growth and vessel features of Quercus petraea (Matt.) Lieb. in five marginal stands across southern Europe. Tree-ring width (TRW), mean earlywood vessel area (MVA) and number of earlywood vessels (NV) as well as theoretical hydraulic conductivity (Kh) chronologies were developed for the period 1963-2012. Summer drought signals were consistent among TRW chronologies; however, climatic responses of vessel features differed considerably among sites. At the three xeric sites, previous year's summer drought had a negative effect on MVA and a positive effect on NV. In contrast, at the two mesic sites, current year's spring drought negatively affected NV, while exerting a positive influence on MVA. In both cases, Kh was not altered by this xylem adjustment. All variables revealed identical east-west geographical patterns in growth and anatomical features. Sessile oak copes with drought in different ways: at xeric sites and after unfavourable previous summer conditions more but smaller vessels are built, lowering vulnerability to cavitation, whereas at mesic sites, dry springs partly lead to tree-rings with wider but fewer vessels. The variability of vessel-related features displays a similar geographical dipole in the Mediterranean Basin previously described for tree growth by other studies.

Responses of sapwood ray parenchyma and non‐structural carbohydrates of Pinus sylvestris to drought and long‐term irrigation

Summary Non‐structural carbohydrates (NSC) play a crucial role in tree resistance and resilience to drought. Stem sapwood parenchyma is among the largest storage tissue for NSC in mature trees. However, there is a limited mechanistic understanding of how NSC reserves, stem parenchyma abundance and growth rates are interrelated, and how they respond to changing water availability. We quantified NSC, ray parenchyma abundance and ring width along four successive 5‐year radial sapwood segments of the stem of 40 mature Pinus sylvestris trees from a 10‐year irrigation experiment conducted at a xeric site in Switzerland. Percentage of ray volume (PERPAR) varied from 3·75 to 8·94% among trees, but showed low intra‐individual variability. PERPAR responded positively to irrigation with a lag of several years, but was unrelated to %NSC. %NSC was lower in wider rings. However, wider rings still contained a larger NSC pool that was positively related to next year's ring growth. Our results suggest that stem ray parenchyma does not limit NSC storage capacity, but responds to long‐term environmental drivers with years of delay. The observed carbon allocation patterns indicate a prioritization of storage over growth independent of growth conditions, likely as a mechanism to ensure long‐term survival. Furthermore, NSC pool size proved to be a determinant for the inter‐annual autocorrelation in tree‐ring growth. Our study highlights the importance of long‐term multi‐parameter studies to better understand tree responses to environmental variability at different time‐scales. A lay summary is available for this article.