Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still... Show moreTree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-conti- nental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1–100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks. Show less
Increases in drought-induced tree mortality are being observedin tropical rain forests worldwide and are also likely to affectthe geographical distribution of tropical vegetation. However,the... Show moreIncreases in drought-induced tree mortality are being observedin tropical rain forests worldwide and are also likely to affectthe geographical distribution of tropical vegetation. However,the mechanisms underlying the drought vulnerability and environmental distribution of tropical species have been little studied. We measured vulnerability to xylem embolism (P50) of 13woody species endemic to New Caledonia and with differentxylem conduit morphologies. We examined the relation be-tween P50, along with other leaf and xylem functional traits,and a range of habitat variables. Selected species had P50values ranging between 4.03 and 2.00 MPa with most species falling in a narrow range of resistance to embolism above 2.7 MPa. Embolism vulnerability was significantly correlatedwith elevation, mean annual temperature and percentage ofspecies occurrences located in rain forest habitats. Xylem conduit type did not explain variation in P50. Commonly used functional traits such as wood density and leaf traits were notrelated to embolism vulnerability. Xylem embolism vulnerability stands out among other commonly used functional traits as amajor driver of species environmental distribution. Drought-induced xylem embolism vulnerability behaves as a physiological trait closely associated with the habitat occupation of rainforest woody species. Show less
Li, S.; Lens, F.P.; Espino, S.; Karimi, Z.; Klepsch, M.; Schenk, H.J.; ... ; Jansen, S. 2016
Pit membranes in bordered pits between neighbouring vessels play a major role inthe entry of air-water menisci from an embolised vessel into a water-filled vessel(i.e., air-seeding). Here, we... Show morePit membranes in bordered pits between neighbouring vessels play a major role inthe entry of air-water menisci from an embolised vessel into a water-filled vessel(i.e., air-seeding). Here, we investigate intervessel pit membrane thickness (TPM)and embolism resistance (P50, i.e., the water potential corresponding to 50% lossof hydraulic conductivity) across a broad range of woody angiosperm species.Data on TPM and double intervessel wall thickness (TVW) were compiled based onelectron and light microscopy. Fresh material that was directly fixated for transmission electron microscopy (TEM) was investigated for 71 species, while non-freshsamples were frozen, stored in alcohol, or air dried prior to TEM preparation for anadditional 60 species. TPM and P50 were based on novel observations and literature.A strong correlation between TPM and P50 was found for measurements based onfreshly fixated material (r = 0.78, P <0.01, n = 37), and between TPM and TVW(r = 0.79, P < 0.01, n = 59), while a slightly weaker relationship occurred betweenTVW and P50 (r = 0.40, P <0.01, n = 34). However, non-fresh samples showed nocorrelation between TPM and P50, and between TPM and TVW. Intervessel pit membranes in non-fresh samples were c.28% thinner and more electron dense thanfresh samples. Our findings demonstrate that TPM measured on freshly fixatedmaterial provides one of the strongest wood anatomical correlates of drought-induced embolism resistance in angiosperms. Assuming that cellulose micro fibrilsshow an equal spatial density, TPM is suggested to affect the length and the shapeof intervessel pit membrane pores, but not the actual pore size. Moreover, theshrinking effect observed for TPM after dehydration and frost is associated withan increase in micro bril density and porosity, which may provide a functionalexplanation for embolism fatigue. Show less
