Tracheid production phenology of Picea mariana and its relationship with climatic fluctuations and bud development using multivariate analysis.

Research on cambium phenology in trees and its limiting factors in natural conditions is still at an early stage of development, restricting our capacity to precisely evaluate the effect of growing season length and climate fluctuations on tracheid production. The first objective of this paper was to describe cambial tracheid production phenology of black spruce (Picea mariana (Mills.) BSP). Repeated tree ring sampling was performed from 2002 to 2006 on four sites (48 degrees 13.78' N, 71 degrees 15.18' W; 48 degrees 51.92' N, 70 degrees 20.57' W; 49 degrees 43.92' N, 71 degrees 56.88' W; and 50 degrees 41.78' N, 72 degrees 11.03' W) representative of closed black spruce forest in Quebec, Canada. The timing of cambial initiation and cambial cessation in black spruce differs from year to year, the first occurring on 4 June on average, whereas the second occurs on 15 August. During a single year, these events do not vary significantly in space within the study area. The duration of cambial tracheid production does not vary significantly in either time or space. The second objective of this study was to identify the climatic factors that explain variations in initiation and cessation. Air temperature and humidity, soil temperature and water content, rain precipitations, snow cover as well as photosynthetically active radiation were monitored at each studied site. These were then used to create sets of candidate regressors to explain timing of phenological events. Timing of cambial initiation is primarily dependent on mean temperature between mid-March and initiation itself. Vapor pressure during this period is also important but in a negative way. A significant effect of the previous year's August soil and air temperature conditions suggests a link with spring bud activity resumption, an interpretation that is supported by an analysis significantly linking measured timing of bud break to cambial initiation. Cessation of cambial tracheid production is influenced by factors linked to photosynthesis during the period from mid-July to cessation. Those related to water status, namely saturation vapor pressure, soil water content and vapor pressure are particularly influential, but light intensity and soil temperature also have an effect. Also, because mid-July corresponds to the timing of bud set and because the previous late summer's soil temperature has a significant effect, a clear link is established with apical cessation.