A phytogeoclimatic study of the high subarctic region of Canada between Hudson Bay and the Cordillera at the northern Yukon-Mackenzie border was undertaken to provide a verifiable and quantitative synthesis of forest-tundra vegetation ecology. Three field seasons of vegetation and terrain studies provided ground truth for a grid of 1314 black-and-white air photos that cover Ca. 24% of the forest-tundra and adjacent low Subarctic and low Arctic. Air photos were analyzed for percentage cover of nine vegetation-terrain types, bedrock and parent materials, landforms, and elevations. The forest-tundra, as bounded by the 1 O : l and 1:lOOO tree:upland tundra cover isolines, spans an average 145 f 72 km (median 131 km) and increases in width from northwest to southeast. The transition from 101 to 1:lO treexpland tundra cover occupies one-fourth to one-half the area of the forest-tundra. Regional slope of the land probably accounts for much of the variation in width of the forest-tundra. Southern outliers of forest-tundra in the northwest are found mainly in areas of high elevation. Across much of the northwest, steep vegetation gradients occur near the northern limit of trees. North of Great Slave Lake, steep vegetation gradients shift from the northern to the southern half of the forest-tundra and maintain this position eastward to Hudson Bay. The forest-tundra of the northwest receives roughly three-fourths the mean annual net radiation available to the southeast and central districts.

Twenty-four specimens of six boreal tree species were excavated: eight Pinusbanksiana Lamb., five Populustremuloides Michx., four Piceaglauca (Moench) Voss, three Piceamariana (Mill.) B.S.P., two Abiesbalsamea (L.) Mills., and two Lara laricina (Du Roi) K. Koch. Root systems were described, and percent biomass determined for above- and below-ground components for half these specimens. Tree ages ranged from 3 to 120 years. Horizontally spreading lateral roots dominated the root systems of all species and occurred within 3– 15 cm of the ground surface. Nutrient and moisture absorption appear to be the principal functions of lateral roots. Maximum vertical root growth occurred near the tree stump as: tap-, heart, sinker, and oblique lateral roots. Vertical root morphology is influenced by site conditions. In addition to providing mechanical support, sinker and taproots may be important adaptations for deep-water utilization on xeric sites. Our data suggest that roots may be important in boreal succession processes through two related mechanisms: (i) nutrient and water deprivation occurring through preemptive growth of tree roots from climax species over roots of serai species, and through interception by mosses; and (ii) niche partitioning occurring below ground between serai and climax, and among climax tree species, by vertical separation of the root systems.

Ross, M. S., L. B. FLANAGAN, and G. H. LA ROI. 1986. Seasonal and successional changes in light quality and quantity in the understory of boreal forest ecosystems. Can. J. Bot. 64: 2792-2799. Surveys of the spectral photon distribution of light in the understory of eight boreal forest stands in northern Alberta were made during 1984. Two seasonal patterns of change in the red:far-red (R:FR) ratio and in the amount of photosynthetically active radiation (expressed as a percentage of full sun, %PAR) were observed. In stands dominated by broadleaf deciduous trees, the R:FR ratio and %PAR values declined rapidly in May and June during the time of leaf expansion and then increased in association with leaf senescence in September. In stands dominated by mature evergreen conifers, both the R:FR ratio and %PAR values declined continuously from an early season high to a late season low. Several young evergreen stands exhibited no seasonal change in light characteristics. During mid-July, the R:FR ratios and %PAR values were lower in older stands than in younger stands of the same successional sequence. The variability in light characteristics within mature forests was higher in midduly than in September; the difference was attributed to the lower solar angle in September. The differences in light regime in early and late successional forests may have important implications for phytochrome-controlled development and succession in understory plant species. Ross, M. S., L. B. FLANAGAN et G. H. LA ROI. 1986. Seasonal and successional changes in light quality and quantity in the understory of boreal forest ecosystems. Can. J. Bot. 64: 2792-2799. Des Ctudes sur la distribution des photons dans le sous-bois de huit peuplements en for8ts boreales au nord de 1'Alberta furent r6alisCes en 1984. Deux patrons saisonniers de changements dans le ratio rouge: rouge lointain (R:Fr) et dans la quantit6 de radiations actives en photosynthkse (exprimies en pourcentage de la pleine intensit6 lumineuse, %PAR) furent observCs.Chez les peuplements dominks par les espkces dicidues B feuilles larges, le ratio R:Fr et les valeurs de %PAR ont dCclinC rapidement en mai et en juin durant le stade d'expansion foliaire et puis augment6 en association avec la ~Cnescence, en septembre. Chez les peuplements dominks par des coniferes matures, le ratio R:Fr et les valeurs %PAR ont dCclinC continuellement B partir d'un maximum en dCbut de saison, jusqu'a un minimum, B la fin. Plusieurs peuplements de coniferes n'ont pas montrC de changement saisonnier dans les caractCristiques de la lumikre. A la mi-juillet, les ratios R:Fr et les valeurs %PAR Ctaient plus basses dans les peuplements plus dgCs que plus jeunes d'une mCme sCquence de succession. La variabilitk des caract6ristiques de la lumibre dans les forgts parvenues a maturit6 6tait plus grande B la mi-juillet qu'en septembre; la diff6-rence Ctant attribuie a l'angle solaire plus bas en septembre. Les diffirences du rCgime de lumibre en successions forestikres peuvent avoir des implications importantes a c...

No abstract

Abstract. Spatial changes in tree and upland tundra cover in response to a complex environmental gradient and to landscape factors were investigated in the high subarctic forest‐tundra of NW Canada. Vegetation and terrain studies provided ground truth for a grid of 1314 air photos which covered 24 % of the Canadian high subarctic and some of the adjacent low subarctic and low arctic. Across the high subarctic, gradual spatial change in % cover of tree and upland tundra vegetation is typical at both high and low cover values, with more rapid change occurring at intermediate cover. Cover gradients of zonal tree and tundra vegetation in the forest‐tundra region in general follow a sigmoid pattern. Tundra and tree patch sizes increase in area and variability with higher tundra and tree cover, respectively.