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Questions: How does the grass layer affect seedlings across large environmental gradients in savannas? Location: Savanna sites in Argentina, Tanzania, and South Africa. Methods: We carried out a joint analysis of three grass removal experiments in which seedlings of various Fabaceae species were transplanted into plots with native grass and companion plots where grass had been removed. First, we estimated the effect of grasses on tree seedling mortality and seedling growth rate at each site. Then, we used the resulting coefficient estimates from site‐level models to examine the impact of two climate (monthly precipitation and aridity index) and two soil (soil organic carbon content and clay content) variables on the direction and magnitude of the grass effects. Results: Grasses increased the risk of mortality, but there was no evidence for a global effect of grasses on tree seedling rate of height growth. The best model fit indicated a high mortality risk of tree seedlings in response to grasses at intermediate aridity index values. No other climate or soil variable influenced tree seedling survival or growth (monthly precipitation, soil organic carbon content and clay content). Conclusions: Our results support the notion that the grass layer consistently creates a bottleneck to tree seedling establishment in African and South American savannas beyond climate and soil conditions, mainly by affecting tree seedling survival. The negative effect of grasses on seedling survival was lower in dry conditions compared to intermediate aridity levels. These results suggest that grass–seedling interaction is less intense in drier conditions, possibly due to reduced total grass biomass, which leads to decreased site evapotranspiration and improved soil water retention capacity. [ABSTRACT FROM AUTHOR]

Root‐based functional traits are relatively overlooked as drivers of savanna plant community dynamics, an important gap in water‐limited ecosystems. Recent work has shed light on patterns of trait coordination in roots, but less is known about the relationship between root functional traits, water acquisition, and plant demographic rates. Here, we investigated how fine‐root vascular and morphological traits are related in two dominant PFTs (C3 trees and C4 grasses from the savanna biome), whether root traits can predict plant relative growth rate (RGR), and whether root trait multivariate relationships differ in trees and grasses. We used root data from 21 tree and 18 grass species grown under greenhouse conditions, and quantified a suite of vascular and morphological root traits. We used a principal components analysis (PCA) to identify common axes of trait variation, compared trait correlation matrices between the two PFTs, and investigated the relationship between PCA axes and individual traits and RGR. We found that there was no clear single axis integrating vascular and morphological traits, but found that vascular anatomy predicted RGR in both trees and grasses. Trait correlation matrices differed in trees and grasses, suggesting potentially divergent patterns of trait coordination between the two functional types. Our results suggested that, despite differences in trait relationships between trees and grasses, root conductivity may constrain maximum growth rate in both PFTs, highlighting the critical role that water relations play in savanna vegetation dynamics and suggesting that root water transport capacity is an important predictor of plant performance in the savanna biome. [ABSTRACT FROM AUTHOR]

Mixed oak stands in the Ozark Highlands of southern Missouri were revisited eight years after a severe episode of red oak decline to determine which predictor variables, collected in 2003, best predicted subsequent tree growth and mortality patterns. Between 2002 and 2009, the mortality rate was 5% (0.625% annual mortality rate), generally below previously reported background rates. Generalized linear mixed models indicated that dieback (an estimate of branch mortality), age, relative height, and the interaction between the last two were most effective at predicting tree mortality. By contrast, tree vigor index (TVI), a composite variable derived from basic measurements of crown and stem architecture, was unequivocally the best predictor of basal area growth trend from one long-term period to the next. Basal area growth increases linearly with TVI, reinforcing the notion that even in ring-porous oaks (which must build new earlywood vessels each year), sustained growth is a low priority for carbon allocation in chronically stressed trees. The findings validate TVI as a useful metric for predicting growth rates of scarlet oak (Quercus coccinea Munchh.) and black oak (Quercus velutina Lam.). Key words: oak decline, dieback, crown condition, cohort senescence. Des peuplements melanges de chenes de la region des Ozark Highlands dans le sud du Missouri ont ete revisites huit ans apres un episode severe de deperissement du chene rouge pour determiner quelles variables de prediction collectees en 2003 ont le mieux predit les patrons de croissance et de mortalite des arbres. Entre 2002 et 2009, le taux de mortalite a atteint 5 % (taux annuel de mortalite de 0,625 %), soit approximativement le double du taux de base rapporte precedemment. Des modeles lineaires mixtes generalises ont indique que le deperissement (une estimation de la mortalite des branches), l'age, la hauteur relative et l'interaction entre les deux dernieres variables etaient particulierement efficaces pour predire la mortalite des arbres. Par contre, l'indice de vigueur des arbres (IVA), une variable composee derivee de mesures de base de l'architecture de la tige et de la cime, etait sans equivoque le meilleur predicteur de la tendance de l'accroissement en surface terriere a long terme d'une periode a l'autre. L'accroissement en surface terriere augmente lineairement avec l'IVA, renforcant l'idee que meme chez les chenes a zone poreuse (qui doivent reformer de nouveaux vaisseaux de bois initial chaque annee) la croissance soutenue a un faible degre de priorite pour l'allocation du carbone chez les arbres soumis a un stress chronique. Les resultats confirment que l'IVA est une metrique utile pour predire le taux de croissance des chenes ecarlate (Quercus coccinea Munchh.) et des teinturiers (Quercus velutina Lam.). [Traduit par la Redaction] Mots-cles: deperissement du chene, mort en cime, etat de la cime, senescence de cohorte., Introduction Recent episodes of forest decline have claimed prominence in the ecological literature because of the possibility that climate change, especially in areas where increasing drought stress is predicted, will [...]