δ13C as indicator of soil water availability and drought stress in Pinus radiata stands in South Africa

[摘要] ENGLISH ABSTRACT: This study investigated the use of carbon isotopes as a potential measure for wateravailability and drought stress in Pinus radiata in the Western and Southern Cape,South Africa. An understanding of water availability and its variation in space isfundamental to the implementation of increasingly site-specific management regimesthat have the potential to greatly improve productivity across sites in the region.Fifteen plantation compartments situated on water shedding sites were identifiedwhere good weather data existed and a water balance model could be run. Inaddition, late wood samples were analysed from four co-dominant trees in the samestand to determine the δ13C values of five tree rings, each representing a specificgrowth year before first thinning. Detailed water balances were constructed for eachtrial site and drought stress indicators (a) relative canopy conductance (after Granieret al., 2000) and (b) the ratio of actual to potential evapotranspiration (supply /demand ratio), were related to δ13C values in latewood.Maximum available soil water ranged from 52 to 313 mm across trial sites. The waterbalance model used adequately described soil water availability throughout eachgrowing season and indicated that stand stress due to the lack of available soil watermainly occurred during the summer months of the study period (November to April).The supply / demand ratio for this period as well as the relative canopy conductanceproved to be good measures of drought stress. The six-month supply demand ratio(calculated for the period November to April) ranged from 0.04 to nearly 1 (winterrainfall zone) and 0.35 to 1 (all-year rainfall zone) and were strongly related to δ13C values (p < 0.001; r2 = 0.7822).It appears that using δ13C values, it may be possible to classify sites into three wateravailability classes. This classification may assist in the implementation of intensivesilvicultural operations on an increasingly site-specific basis. Where sites areenriched with water from lateral flow or upslope positions, δ13C may be the onlyreliable technique to quantify soil water availability.

[发布机构] Stellenbosch University