BRANCH GROWTH OF RIPARIAN COTTONWOODS - A HYDROLOGICALLY SENSITIVE DENDROCHRONOLOGICAL TOOL

The conservation of riparian (river valley flood plain) forests relies on the provision of instream flows that are sufficient to sustain tre e growth. In the present study, annual branch growth increments were i nvestigated as an indicator of environmental favorability for riparian cottonwoods. Trees of three species, Populus angustifolia, P. balsami fera, and P. deltoides, and their natural interspecific hybrids, were studied at five sites along the Oldman and South Saskatchewan rivers i n Alberta, Canada. Annual branch growth increments for the interval fr om 1983 to 1992 were positively correlated with stream flows (r(2) = 0 .79 at Lethbridge) and slightly negatively correlated with weather var iables that contribute to water demand: evaporation, temperature, wind , and/or sunshine. The combination of January to May stream flow (wate r supply) and June evaporation (water demand) almost entirely accounte d for the branch growth variation across years (r(2) = 0.91 at Lethbri dge). Tree ring increments were also investigated but were less closel y correlated than branch increments across trees or with stream flow. Branch growth increments thus provide an accurate but short duration ( 1 or 2 decades) record of environmental favorability for growth. The c lose correlation between branch growth and stream flow indicates that water is the principal limitation to growth of these riparian cottonwo ods and that these trees obtained their water from a source linked to the stream, the riparian water table. Analyses of branch increments sh ould provide a management tool for (i) determining instream flow needs for riparian cottonwoods and (ii) analyzing impacts of stream flow al terations due to river damming or water diversion.