Climate Moisture Index Research Articles

Annual-perennial lifespan variation in Chaenactis douglasii suggests a drought escape strategy in warm-arid environments.

Intraspecific variation in drought resistance traits, such as drought escape, appear to be frequent within wild, ruderal forb species. Understanding how these traits are arrayed across the landscape, particularly in association with climate, is critical to developing forbs for wildland restoration programs. Use of forbs is requisite for maintaining biological diversity and ecological services. Using 6074 greenhouse-grown Chaenactis douglasii seedlings from 95 wild, seed-sourced populations across the western United States, we recorded bolting phenology and estimated genome size using flow cytometry. Mixed-effects regression models were used to assess whether climate of seed origin was predictive for bolting phenology and genome size. Variation in bolting, reflecting an annual vs. perennial lifespan in this species, was observed in 8.7% of the plants, with bolting plants disproportionately occurring in locations with warm, arid climates. Populations with increasing heat and aridity were positively correlated with observed bolting (r = 0.61, p < 0.0001). About one-third (22%) of the total (61%) lifespan variation was attributed to seed source climate and annual heat moisture index, a measure of aridity. Genome size had no significant effect on bolting. Projected climate modeling for mid-century (2041-2070) supports an increasing occurrence of annual lifespan. Our analyses support a drought escape, bet-hedging strategy in C. douglasii. Populations exposed to greater aridity exhibited a higher proportion of individuals with an annual lifespan. Drought escape leading to an annual lifespan can affect how seeds are propagated and deployed for climate-informed restoration.

Tree size variation induced by stand age mainly regulates aboveground biomass across three major stands of temperate forests in South Korea

Forest biomass and biodiversity are the most important elements of forest functions and ecosystem services. In this study, we explore the possibilities and ways to enhance ecosystem functions and services related to biomass and biodiversity. Biotic drivers (i.e., species, phylogenetic and functional diversity, stand structural attributes, and community-weighted mean of trait values), abiotic drivers (i.e., topography and climate), and stand age were extracted as independent variables to explain aboveground biomass (AGB). Using South Korea’s 7th National Forest Inventory data, we analyzed 2,070 plots belonging to the natural forests consisting of 394 plots (19.0%) of coniferous stands, 829 plots (40.0%) of broadleaved stands, and 847 plots (40.9%) of mixed stands. Multimodel inference test and model-averaging approaches were conducted to determine the most significant control variables on AGB in each stand type, and piecewise structural equation modeling was conducted to quantify the relationships and directions between the variables. Abiotic drivers, including stand age and climate moisture index, control tree size variation in all stand types, but biotic drivers control AGB through different mechanisms depending on the stand type. Our results show that there were differences in the composition of variables for controlling AGB among stand types. Across all forest types and total stands, we found that increasing the tree size variation is the key driver of increasing AGB as stand age increases. Our study suggests that forest carbon accumulation by stand type can be enhanced if the key drivers of each stand type are properly managed across forest succession, and different forest management plans that consider different regulation factors among stand types are required. Moreover, it is important to adapt resource use patterns for each stand type with considering environmental conditions to maintain healthy and sustainable forests.

Climatic conditions modulate the effect of spruce budworm outbreaks on black spruce growth

Current ecological models predict profound climate change-related effects on the natural disturbance regimes of forests. Spruce budworm (Choristoneura fumiferana) (SBW) is the principal insect defoliator in eastern North America, and SBW outbreaks have a major impact on the structure and function of the Canadian boreal forest, as defoliation leads to decreased tree growth, increased mortality, and lower forest productivity. SBW outbreaks have become more severe over the last century with the changing climate; however, little is known about how climate fluctuations affect the growth of SBW host species during the outbreak period. Here we evaluate how climate and outbreak severity combined to affect black spruce (Picea mariana) growth during the SBW outbreak that occurred between 1968–1988 and 2006–2017. We compiled dendrochronological series (2271 trees), outbreak severity (estimated by observed aerial defoliation), and climate data for 164 sites in Québec, Canada. We used a linear mixed effect model to determine the impacts of climatic parameters, cumulative defoliation (of the previous five years), and their coupled effect on basal area growth. At maximum outbreak severity, basal area growth of black spruce was reduced by 14%–18% over five years. This outbreak growth response was affected by climate: warmer previous summer minimum temperatures and a higher previous summer climate moisture index further decreased growth by 11% and 4%, respectively. In contrast, a preceding year's warmer spring minimum temperatures (9%) and summer maximum temperatures (7%) attenuated the negative SBW effect. This study adds knowledge to our landscape-level understanding of combined insect–climate effects and helps predictions of future SBW-related damage to forest stands to bolster sustainable forest management. We also recommend that projections of boreal forest ecosystems include several classes of SBW defoliation and multiple climatic scenarios in future simulations.

Dendroclimatic Analysis of Sierra Nevada Mixed Conifer Species between Different Diameter Size Classes

Climate change is expected to lead to higher temperatures in the Mediterranean region of northern California in the Sierra Nevada. Dendroclimatic studies typically focus on large, old trees, but there relatively limited understanding on how climatic sensitivity can vary with trees of different size classes. We collected tree increment cores and compared radial growth responses of small (20.32 to ≤40.64 cm), medium (40.64 to ≤60.96 cm), and large (&gt;60.96 cm) diameter mixed conifer species in the Sierra Nevada to different climate variables (temperature, precipitation, and climate moisture index (CMI)). The most common tree species encountered were white fir (Abies concolor (Gord. &amp; Glend.) Lindl.), followed by sugar pine (Pinus lambertiana Dougl.), ponderosa pine (Pinus ponderosa Dougl. Ex P. &amp; C. Laws), and incense cedar (Calocedrus decurrens (Torr.) Florin). One of the most coherent responses from all diameter groups and across all species was the positive response to increasing minimum winter temperatures. All diameter groups and species also responded positively to precipitation and CMI at some point in the analysis period, which is the seasonal window of April of the prior year to October of the current year of ring formation. Perhaps the most notable difference when comparing the three diameter groups to climate was the higher occurrence of negative responses to the temperature of the previous year from the largest diameter group, as well as the higher number of negative responses to temperature in general. These results suggest that larger trees may be more sensitive to future climate projections compared with smaller trees and they may carry those effects into the next year. The use of dendroclimatology to assess how mixed conifer species in the Sierra Nevada responded to past climate is a key resource that can be used to infer how trees may respond to a future changing climate.

Prey resources are equally important as climatic conditions for predicting the distribution of a broad‐ranged apex predator

AbstractAimA current biogeographic paradigm states that climate regulates species distributions at continental scales and that biotic interactions are undetectable at coarse‐grain extents. However, advances in spatial modelling show that incorporating food resource distributions are important for improving model predictions at large distribution scales. This is particularly relevant to understand the factors limiting the distribution of widespread apex predators whose diets are likely to vary across their range.LocationNeotropical Central and South America.MethodsThe harpy eagle (Harpia harpyja) is a large raptor, whose diet is largely comprised of arboreal mammals, all with broad distributions across Neotropical lowland forest. Here, we used a hierarchical modelling approach to determine the relative importance of abiotic factors and prey resource distribution on harpy eagle range limits. Our hierarchical approach consisted of the following modelling sequence of explanatory variables: (a) abiotic covariates, (b) prey resource distributions predicted by an equivalent modelling for each prey, (c) the combination of (a) and (b), and (d) as in (c) but with prey resources considered as a single prediction equivalent to prey species richness.ResultsIncorporating prey distributions improved model predictions but using solely biotic covariates still resulted in a high‐performing model. In the Abiotic model, Climatic Moisture Index (CMI) was the most important predictor, contributing 76% to model prediction. Three‐toed sloth (Bradypus spp.) was the most important prey resource, contributing 64% in a combined Abiotic‐Biotic model, followed by CMI contributing 30%. Harpy eagle distribution had high environmental overlap across all individual prey distributions, with highest coincidence through Central America, eastern Colombia, and across the Guiana Shield into northern Amazonia.Main ConclusionsWith strong reliance on prey distributions across its range, harpy eagle conservation programmes must therefore consider its most important food resources as a key element in the protection of this threatened raptor.

Global climate-related predictors at kilometer resolution for the past and future

Abstract. A multitude of physical and biological processes on which ecosystems and human societies depend are governed by the climate, and understanding how these processes are altered by climate change is central to mitigation efforts. We developed a set of climate-related variables at as yet unprecedented spatiotemporal detail as a basis for environmental and ecological analyses. We downscaled time series of near-surface relative humidity (hurs) and cloud area fraction (clt) under the consideration of orography and wind as well as near-surface wind speed (sfcWind) using the delta-change method. Combining these grids with mechanistically downscaled information on temperature, precipitation, and solar radiation, we then calculated vapor pressure deficit (vpd), surface downwelling shortwave radiation (rsds), potential evapotranspiration (pet), the climate moisture index (cmi), and site water balance (swb) at a monthly temporal and 30 arcsec spatial resolution globally from 1980 until 2018 (time-series variables). At the same spatial resolution, we further estimated climatological normals of frost change frequency (fcf), snow cover days (scd), potential net primary productivity (npp), growing degree days (gdd), and growing season characteristics for the periods 1981–2010, 2011–2040, 2041–2070, and 2071–2100, considering three shared socioeconomic pathways (SSP126, SSP370, SSP585) and five Earth system models (projected variables). Time-series variables showed high accuracy when validated against observations from meteorological stations and when compared to alternative products. Projected variables were also highly correlated with observations, although some variables showed notable biases, e.g., snow cover days. Together, the CHELSA-BIOCLIM+ dataset presented here (https://doi.org/10.16904/envidat.332, Brun et al., 2022) allows improvement to our understanding of patterns and processes that are governed by climate, including the impact of recent and future climate changes on the world's ecosystems and the associated services on societies.

Effects of key forest management practices and climatic factors on the growth of Populus tomentosa plantations in the North China Plain

This study aimed to provide sustainable management recommendations for long-term poplar plantations in the North China Plain (NCP) within the context of climate change. Within the NCP, through primary literature and field surveys, we collected tree growth data of 2515 individual trees from monoculture stands of Populus tomentosa with different stand ages (1–40 years). Using linear mixed effects models, we examined the effects of different management practices (planting density, irrigation, and fertilization) and climatic factors (mean air temperature and climate moisture index) on tree growth at different stand ages. Results showed significant effects of all management practices on tree growth across stand ages. Also, the relative importance of management practices was higher than climatic factors. The effects of planting density and growing season climate moisture index (CMI) on tree growth gradually increased with stand ages. In contrast, no significant effect of growing season mean air temperature on tree growth was observed regardless of stand ages. Irrigation was the most important factor impacting tree growth in young stands. In addition, we found that irrigation and fertilization reduced the effect of CMI on tree growth in young stands. The relative importance of planting density and CMI did not differ for younger trees. However, the relative importance of planting density was higher than CMI in middle-aged and mature stands. We also found that the biologically mature age of unmanaged P. tomentosa stands in the NCP was about 15 years. In conclusion, our study highlights the importance of forest management practices and climatic factors for long-term poplar plantations in the NCP. Especially, planting density and moisture were critical factors for managing poplar plantations. The findings of this study can be relevant for intensively managing fast-growing tree plantations in other water-limited areas.

Identifying Western North American Tree Populations Vulnerable to Drought under Observed and Projected Climate Change

Global climate change has affected forest health and productivity. A highly visible, direct climate impact is dieback caused by drought periods in moisture-limited forest ecosystems. Here, we have used a climate moisture index (CMI), which has been developed in order to map forest–grassland transitions, to investigate the shifts of the zero-CMI isopleths, in order to infer drought vulnerabilities. Our main objective was to identify populations of the 24 most common western North American forest tree species that are most exposed to drought conditions by using a western North American forest inventory database with 55,700 plot locations. We have found that climate change projections primarily increase the water deficits for tree populations that are already in vulnerable positions. In order to test the realism of this vulnerability assessment, we have compared the observed population dieback with changes in index values between the 1961–1990 reference period and a recent 1991–2020 average. The drought impacts that were predicted by negative CMI values largely conformed to the observed dieback in Pinus edulis, Populus tremuloides, and Pinus ponderosa. However, there was one notable counter-example. The observed dieback in the Canadian populations of Populus tremuloides were not associated with directional trends in the drought index values but were instead caused by a rare extreme drought event that was not apparently linked to directional climate change. Nevertheless, a macro-climatic drought index approach appeared to be generally suitable to identify and forecast the drought threats to the tree populations.

Autoregressive models for time series of random sums of positive variables: Application to tree growth as a function of climate and insect outbreak

We present a broad class of semi-parametric models for time series of random sums of positive variables. Our methodology allows the number of terms inside the sum to be time-varying and is therefore well suited to many examples encountered in the natural sciences. We study the stability properties of the models and provide a valid statistical inference procedure to estimate the model parameters. It is shown that the proposed quasi-maximum likelihood estimator is consistent and asymptotically Gaussian distributed. This work is complemented by simulation results and applied to time series representing growth rates of white spruce (Picea glauca) trees from a few dozen sites in Québec (Canada). This time series spans 41 years, including one major spruce budworm (Choristoneura fumiferana) outbreak between 1968 and 1991. We found significant growth reductions related to budworm-induced defoliation up to two years post-outbreak. Our results also revealed the positive effects of maximum summer temperature, precipitation, and the climate moisture index on white spruce growth. We also identified the negative effects of the climate moisture index in the spring and the maximum temperature of the previous summer. However, the model’s performance on this data set was not improved when the interactions between climate and defoliation on growth were considered. This study represents a major advance in our understanding of budworm–climate–tree interactions and provides a useful tool to project the combined effects of climate and insect defoliation on tree growth in a context of greater frequency and severity of outbreaks coupled with the anticipated increases in temperature.

Palaeoclimate dynamics within the Summer Rainfall Zone of South Africa

South Africa has a particularly spatially heterogeneous climate driven by a complex set of ocean-atmospheric factors. This results in three distinct regions of rainfall seasonality: the summer- (SRZ), winter- (WRZ) and year-round (YRZ) rainfall zones. Even within these zones there is considerable spatio-temporal variability in precipitation patterns, especially in the SRZ which covers the largest terrestrial area. Over the past several thousand years the position of the boundaries of the southern extent of the SRZ have shifted due to the influences of major climate systems, as well as changes in the characteristics of the adjacent oceanic currents. Here we use the Modern Analogue Technique on a new synthesis of pollen records from across the SRZ to quantitatively reconstruct four climate variables (Mean Temperature of the Coldest month, Mean Temperature of the Warmest Month, Mean Summer Precipitation and Winter Climatic Moisture Index), over the past 36,000 years. The climate variables and number of analogues used were selected after thorough examination of the modern dataset. The results provide evidence for three key features: first, an early deglaciation following a cold, dry and prolonged Last Glacial Maximum. Second, a clear manifestation of the African Humid Period in the north-eastern part of the zone, supported by reconstructed sea surface temperatures. And third, a short-lived cold period at around 10 cal kyr BP, which may have been obscured by larger statistical errors in past studies. As it is still mostly within the margin of error for our study, this period requires further study. Our results show that the technique can be used successfully on large and diverse compilations of South African records, improving comparability with other regions.

Accounting for atmospheric carbon dioxide variations in pollen-based reconstruction of past hydroclimates

Changes in atmospheric carbon dioxide (CO 2 ) concentration directly influence the ratio of stomatal water loss to carbon uptake. This ratio ( e ) is a fundamental quantity for terrestrial ecosystems, as it defines the water requirement for plant growth. Statistical and analogue-based methods used to reconstruct past hydroclimate variables from fossil pollen assemblages do not take account of the effect of CO 2 variations on e . Here we present a general, globally applicable method to correct for this effect. The method involves solving an equation that relates e to a climatic moisture index (MI, the ratio of mean annual precipitation to mean annual potential evapotranspiration), mean growing-season temperature, and ambient CO 2 . The equation is based on the least-cost optimality hypothesis, which predicts how the ratio (χ) of leaf-internal to ambient CO 2 varies with vapour pressure deficit (vpd), growing-season temperature and atmospheric pressure, combined with experimental evidence on the response of χ to the CO 2 level at which plants have been grown. An empirical relationship based on global climate data is used to relate vpd to MI and growing-season temperature. The solution to the equation allows past MI to be estimated from pollen-reconstructed MI, given past CO 2 and temperature. This MI value can be used to estimate mean annual precipitation, accounting for the effects of orbital variations, temperature and cloud cover (inferred from MI) on potential evapotranspiration. A pollen record from semi-arid Spain that spans the last glacial interval is used to illustrate the method. Low CO 2 leads to estimated MI being larger than reconstructed MI during glacial times. The CO 2 effect on inferred precipitation was partly offset by increased cloud cover; nonetheless, inferred precipitation was greater than present almost throughout the glacial period. This method allows a more robust reconstruction of past hydroclimatic variations than currently available tools. • A model using the least-cost optimality hypothesis to account for physiological effects of CO 2 • The model provides a correction for statistical reconstructions of the moisture index • The model provides a new way to reconstruct mean annual precipitation • Application to glacial pollen data shows climate wetter than inferred statistically

The effects of functional diversity and identity (acquisitive versus conservative strategies) on soil carbon stocks are dependent on environmental contexts

Soil carbon (C) plays an important role in mediating global climate change and sustaining environmental condition. Although experimental studies primarily from grasslands revealed the positive effects of plant diversity on soil C storage, there is uncertainty about the directions and magnitudes of tree diversity and soil C relationships in natural forests. Using Canada’s National Forest Inventory data from temperate to boreal forests, we examined the relationships between soil C stocks and tree functional diversity and identity, and how these relationships varied along environmental gradients (i.e., biomes, mean annual temperature, climate moisture index, and soil drainage). In contrast to the results from grassland experiments, we found a negative relationship between tree diversity and soil C stocks in temperate forests and a nonsignificant relationship in boreal forests. Moreover, soil C stocks increased with functional diversity in moist and poorly drained sites, but decreased in dry and well-drained sites within the temperate forest biome. Within the boreal biome, soil C stocks increased with functional diversity in warm climates, but decreased in cold climates. Our findings at the macro-ecological scale indicated that the positive effects of plant diversity might be the highest at the boreal-temperate forests' ecotone. In addition, within the temperate biome, tree communities with higher leaf nitrogen, phosphorus content and specific leaf area were associated with greater mineral horizon soil C stocks, especially in warm or well-drained sites. Our finding of the abiotic context-dependent relationships between soil C stocks and tree functional diversity and identity can guide forest management across different environmental conditions.

Reliability of Moisture Reference Year (MRY) selection methods for hygrothermal performance analysis of wood-frame walls under historical and future climates

Hygrothermal simulations can be used to evaluate the hygrothermal response of building envelope to the effect of climate change. However, to account for the uncertainty in projected future climates, a large number of simulations are required. To reduce computational time, hygrothermal performance of walls is typically evaluated using Moisture Reference Year (MRY), which can be selected based on climate-based indices. This study investigated the reliability of climate-based indices by comparing to the response-based indices obtained from simulations using two methods. Firstly, the direct correlation between two indices is calculated using coefficient of determination (R2). Second method focuses on evaluating the reliability of climate-based indices in ranking the years in terms of moisture severity compared to that based on simulations. Four climate-based indices: Climatic Index (CI), Moisture Index (MI), Severity Index (Isev) and annual WDR and three response-based indices: Mould Index, Moisture Content and RHT index are investigated. In general, R2 between two indices is weak (range of 0–0.79) with highest being 0.79 (between maximum mould index and MI) for stucco cladding wall facing default orientation under historical climate in Calgary. With ranking analysis, the accuracy using climate-based indices in ranking all years is low with some improvement in ranking the first three worst-years. CI and MI perform well and Isev works for north facing walls for cities like Ottawa and Calgary but fails for Vancouver. To have a reliable assessment of moisture risks, further research is needed to improve existing or develop a new climate-based index specific to Canadian climate.

Impacts of Rainfall and Temperature Changes on Smallholder Agriculture in the Limpopo Province, South Africa

The intensity and frequency in the recurrence of extreme climate events are compounding the vulnerability of smallholder farmers, who have always lacked the resources to adapt. The increasing temperatures and decreasing rainfall are exacerbating water scarcity challenges through drought recurrence. There is an urgent need for pathways that lead towards Sustainable Development Goals, mainly Goals 1 (no poverty) and 2 (zero hunger) in poor rural communities. This study assessed rainfall and temperature trends from 1960 to 2018 and their impacts on crop production in the Mopani and Vhembe Districts of Limpopo Province, South Africa. Trend analysis was used to analyse rainfall patterns, as well as the trends in temperature recorded for the past 58 years. The climate moisture index (CMI) and runoff estimates were used to assess the degree of aridity and water availability, respectively. Geographic Information Systems (GIS) and remotely sensed data were used to assess the changes over time. The total annual rainfall has declined significantly while annual minimum and maximum temperatures have increased significantly during the period under observation. An aridity index of −0.70 calculated for the study areas classifies the districts as dry and water scarce. The results of the analysis also indicate that the districts are climate change hot spots, and are highly vulnerable to the impacts of climate change. The changes are compounding water and food insecurity. Policy and decision-makers should focus on enhancing adaptation and resilience initiatives in the study areas through systematic, transformative, and integrated approaches, such as scenario planning, circular economy, and nexus planning.

Agroclimatic indices across the Canadian Prairies under a changing climate and their implications for agriculture

AbstractWith the objective of trying to understand the adaptability of agriculture across the Canadian Prairies under climate change, simple‐to‐use agroclimatic indices were calculated for the base climate period of 1981 to 2010 and for both the medium (RCP4.5) and high (RCP8.5) emission projections extending to the distant future (2071–2100). The agroclimatic indices included the Effective Growing Degree Days (EGDDs), Growing Season Length (GSL), the Climate Moisture Index (CMI), and the Temperature Humidity Index (THI). For climate change in 30‐year periods, these indices were calculated as multi‐model ensembles of six Global Climate Models recommended under the Coupled Model Intercomparison Project Phase 5 (CMIP5) for the study area. We found that the GSL, EGDDs, CMI, and THI were amplified above the values of the base climate period in the order of 40–50 days, 600–1200 heat units, −100 to −120 mm and 3–4 THI units by the close of the distant future (2071–2100) under the RCP4.5 and RCP8.5, respectively. This amplification has implications on where crop and livestock production could become more suitable or riskier in future. Opportunities include expanding crop and livestock production to more northerly regions which currently have insufficient heat units, a short growing season and unfavourable temperature humidity thresholds for livestock production. Moisture deficits will continue to be the greatest risk during the growing season under climate change scenarios but options exist to implement adaptive measures such as staggering seeding times to take advantage of moisture availability in the spring and autumn seasons and crop substitution. This study has relevance for policy and program formulation and implementation in Canada's agricultural regions and potentially, other areas of the world with similar climate change outcomes.

Differential responses of forest strata species richness to paleoclimate and forest structure

Regional factors, such as historical and contemporary climate conditions, and local factors, such as vegetation structural attributes, can influence current patterns of plant species richness but their relative roles remain unknown, particularly across forest strata. Here, we used a multi-scale survey of temperate forest plots across a large region of Northeast China to explore the relative importance of environmental factors (paleoclimate, contemporary climate, topography and anthropogenic disturbance) and forest structural attributes (stem abundance, stand basal area and tree size variation) on tree, shrub, and herb species richness. Although environmental and forest structural factors all played a role in explaining plant species richness patterns, their relative roles varied in direction and magnitude depending on forest stratum. Tree species richness increased with the magnitude of change in temperature since the Last Glacial Maximum (AnomalyMAT) but declined with increasing magnitude of change in precipitation (AnomalyMAP). By contrast, herb species richness declined with increasing AnomalyMAT but increased with AnomalyMAP, highlighting contrasting processes for tree and herb species richness driven by paleoclimate. Contemporary climate played a lesser role in explaining species richness, but tree species richness increased with diurnal temperature range, shrub species richness increased with the climatic moisture index, and herb species richness increased with both climatic variables. Herb species richness also increased with disturbance intensity, whereas tree and shrub richness declined. Overall, plant species richness increased significantly with all forest structural attributes, except for stem abundance, which had a negative effect on herb species richness, and forest structure mediated the linkages between plant species richness and disturbance or climate. The pronounced influence of paleoclimate on forest plant species richness highlights the potential threat of current climate change for forest diversity. Together, simultaneous consideration of past and current climate as well as forest structural attributes could improve our understanding of the complex mechanisms shaping patterns of plant species richness across forest strata.

Wind-bounded grasslands of North America

Climate may separate grasslands from surrounding forests and shrublands, but an alternative option is that grasslands are suitable for trees except in the presence of frequent fires, which are spread great distances by strong winds. To develop a ruleset that demarcated the central North American grasslands, I modeled grasslands using precipitation and its coefficient of variation, ratio of potential evapotranspiration to precipitation, climatic moisture index, July vapor pressure, and wind speed (i.e., a fire indicator) during 5000 (4900–5100) years before present, 1600–1699, 1800–1849, and 1850–1899. Wind speeds of 4.5 m s−1 at 10 m above ground height clearly bounded grasslands. For all time intervals, the random forests and extreme gradient boosting classifiers had accuracies of 0.96 to 0.97 in classification of reserved samples; wind speed was ranked as the most influential model variable (i.e., value of 100 out of 100 scale, with the next most influential variables at ≤ 25). The simulated wind patterns matched recent wind models from atmospheric reanalysis and the Global Wind Atlas downscaled from an atmospheric reanalysis. Wind boundaries were stable over time, shifting slightly northward during 11,000 years before present to recent decades. Although wind is related to precipitation, I directly mapped and modeled precipitation and derivatives. Therefore, I propose that wind is an index of fire spread in grassland biomes, where conditions are appropriate for both fire and woody vegetation. More research on wind models is required to confirm these results and determine how winds affect grassland formation and maintenance globally.

Contrasting life-history traits of black spruce and jack pine influence their physiological response to drought and growth recovery in northeastern boreal Canada

An increase in frequency, intensity and duration of drought events affects forested ecosystems. Trees react to these changes by adjusting stomatal conductance to maximize the trade-off between carbon gains and water losses. A better understanding of the consequences of these drought-induced physiological adjustments for tree growth could help inferring future productivity potentials of boreal forests. Here, we used samples from a forest inventory network in Canada where a decline in growth rates of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) occurred in 1988–1992, an exceptionally dry period, to verify if this growth decline resulted from physiological adjustments of trees to drought. We measured carbon and oxygen isotope ratios in growth rings of 95 spruces and 49 pines spanning 1985–1993. We used 13C discrimination (Δ13C) and 18O enrichment (Δ18O) as proxies for intrinsic water use efficiency and stomatal conductance, respectively. We studied how inter-annual variability in isotopic signals was linked to climate moisture index, vapor pressure deficit and annual snowfall amount. We found significantly lower Δ13C values over 1988–1990, and significantly higher Δ18O values in 1988–1989 and 1991 compared to the 1985–1993 averages. We also observed that a low climatic water balance and a high vapor pressure deficit were linked with low Δ13C and high Δ18O in the two study species, in parallel with low growth rates. The latter effect persisted into the year following drought for black spruce, but not for jack pine. These findings highlight that small differences in physiological parameters between species could translate into large differences in post-drought recovery. The stronger and longer lasting impact on black spruce compared to jack pine suggests a less efficient carbon use and a lower acclimation potential to future warmer and drier climate conditions.

Synthesising the pollen records for the Drakensberg-Maloti through quantitative modelling

The Drakensberg-Maloti has been a region of great hydrological importance for thousands of years, captured in the archaeological record as a region of refuge under dry conditions, and in recent decades supplying adjacent South Africa with water through the Lesotho Highlands Water Scheme. This is due in large part to its high amounts of orographically induced rainfall. The high amounts of rainfall mean the area lends itself well to the preservation of multiple proxies in the region's many wetlands, a very valuable resource for palaeoecological studies in the generally dry southern Africa. However, there have been few quantitative palaeoclimatic studies taking advantage of this, and none using multiple sites within the Drakensberg-Maloti region. This study represents the first attempt at such a regional synthesis. We utilised all published pollen records from the area to produce a dataset from which the standard Modern Analogue Technique was performed to reconstruct mean temperature of the warmest month, mean annual precipitation, mean winter precipitation and an annual climatic moisture index for the past 8000 years. The moisture index showed a very low amount of variation, most likely due to the low amount of variation in the modern climate data associated with the sites in the modern calibration dataset, being so near the saturation point of this index. We found evidence for a cool, wet period at around 2 cal ka BP, a period that has been identified in records from across South Africa as a Neoglacial, and which is consistent with diatom and sedimentary records from the Drakensberg-Maloti.

Assessing the dual-mycorrhizal status of a widespread tree species as a model for studies on stand biogeochemistry.

Viewing plant species by their mycorrhizal type has explained a range of ecosystem processes. However, mycorrhizal type is confounded with plant phylogeny and the environments in which mycorrhizal partners occur. To circumvent these confounding effects, "dual-mycorrhizal" plant species may be potential models for testing the influence of mycorrhizal type on stand biogeochemistry. To assess their use as models, duality in mycorrhizas within a single host species must be confirmed and factors underlying their variation understood. We surveyed roots, soils, and leaves of mature aspen (Populus tremuloides) across 27 stands in western Canada spanning two biomes: boreal forest and parklands. Aspen roots were mostly ectomycorrhizal with sporadic and rare occurrences of arbuscular mycorrhizas. We further tested whether a climate moisture index predicted abundance of ectomycorrhizal roots (number of ectomycorrhizal root tips m-1 root length) surveyed at two depths (0-20cm and 20-40cm) and found that ectomycorrhizal root abundance in subsoils (20-40cm) was positively related to the index. We subsequently examined the relationships between ectomycorrhizal root abundance, leaf traits, and slow and fast pools of soil organic carbon and nitrogen. The ratio of leaf lignin:N, but not its components, increased along with ectomycorrhizal root abundance in subsoils. Soil carbon and nitrogen pools were independent of ectomycorrhizal root abundance. Our results suggest that (1) categorizing aspen as dual-mycorrhizal may overstate the functional importance of arbuscular mycorrhizas in this species and life stage, (2) water availability influences ectomycorrhizal root abundance, and (3) ectomycorrhizal root abundance coincides with leaf quality.