Water availability and carbon isotope discrimination in conifers

Abstract

The stable C isotope composition (δC) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and δC. A significant component of the variation in δC was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km altitude, r =0.49, and in foliage, 1.91‰ km, r =0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO partial pressure into the leaf (P -P) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P -P of stemwood (r =0.45) and foliage (r=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on δC probably dominated the effect of water availability. We also examined the relationship between plant drought stress (Ψ) and δC within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha, Ψ and δC were less (more negative) than at 250 stems ha indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater δC of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P and increase δC. δC was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the δC-Ψ relationship varied between thinning treatments. Within treatments, the relationship between δC and Ψ was strong (0.38<r <0.58). We conclude that δC may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.

Relationships Between Tree Height and Carbon Isotope Discrimination

“…We corrected the isotopic results for elevation impacts on CO 2 partial pressure (Hultine and Marshall 2000;Warren et al 2001;Körner 2007;McDowell et al 2010). Though this effect is minor, it is relatively straightforward to correct for when elevation and temperature are known.…”

“…Correlations between D and precipitation are routinely found in regional or global scale studies (e.g. Schulze et al 1998;Warren et al 2001;Bowling et al 2002) due to the dominant effect of water availability on g s and p c . The lack of freezing temperatures in tropical regions may also allow higher g s and p c because xylem vessels can grow larger and have subsequently greater hydraulic conductance .…”

Regional And Watershed-Scale Coherence In the Stable-Oxygen and Carbon Isotope Ratio Time Series In Tree Rings Of Coast Redwood (Sequoia sempervirens)

“…For redwood, Roden et al (2009) speculated that the dip in d 18 O in the middlewood portion of the annual growth ring represents the time when stored carbohydrates from the previous year have been fully exhausted and thus represents the truest spring environmental signal. Although not as consistent in their patterns, d 13 C also showed significant intraannual variation in redwood (Roden et al 2009, see also Loader et al 1995Schleser et al 1999;Warren et al 2001). Thus when sampling longer chronologies, we chose to sample the two portions of the annual ring that produced the greatest intraannual differences (latewood and middlewood).…”

Application of isotope discrimination techniques to evaluate the functional response of Mediterranean coppices to high-forest conversion cut

“…Nelle foreste sottoposte a trattamenti selvicolturali questa risposta può essere mediata o anche "sopraffatta" da un simultaneo incremento dei nutrienti o della disponibilità di radiazione luminosa associato alla perturbazione (Warren et al 2001). Per contro, in termini assoluti il rapporto A/E potrebbe rimanere invariato in seguito al diradamento (Leavitt & Long 1986, McDowell et al 2003, sebbene i due parametri possano variare in funzione dell'incremento della disponibilità di luce o di azoto, che può determinare un aumento di A, e per effetto dell'incremento di disponibilità idrica del suolo, che può determinare un aumento di E.…”

“…L'efficienza d'uso dell'acqua, definita come rapporto delle moli di CO 2 fissata (A) rispetto alle moli di H 2 O traspirata (E) è inversamente correlata con la disponibilità idrica del suolo (Dupouey et al 1993, Ehleringer 1993, Livingston & Spittlehouse 1996, poiché l'incremento di E è maggiore di quello di A in condizioni di migliore disponibilità idrica (Meinzer et al 1993). Nelle foreste sottoposte a trattamenti selvicolturali questa risposta può essere mediata o anche "sopraffatta" da un simultaneo incremento dei nutrienti o della disponibilità di radiazione luminosa associato alla perturbazione (Warren et al 2001). Per contro, in termini assoluti il rapporto A/E potrebbe rimanere invariato in seguito al diradamento (Leavitt & Long 1986, McDowell et al 2003, sebbene i due parametri possano variare in funzione dell'incremento della disponibilità di luce o di azoto, che può determinare un aumento di A, e per effetto dell'incremento di disponibilità idrica del suolo, che può determinare un aumento di E.…”