Distribution Of Vegetation Zones Research Articles

Long-term succession of closed boreal forests at their range limit in eastern North America shows resilience to fire and climate disturbances

The distribution of vegetation zones in northeastern North America forms a gradual transition from closed to open forests, and to tundra. Closed forests near the open forest/tundra boundary generally correspond to residual patches of a once larger forest community. Whether these forest patches have survived as untouched, fire-free communities or as resilient communities during the Holocene remains unknown. To answer to this question, we reconstructed the successional history of two black spruce closed-crown forests and one jack pine forest near the northern limit of closed forest ecosystems. Stand-scale postfire succession was analysed based on charcoal macrofossils. Extensive 14C dating and botanical identification of charcoal fragments located at the organic soil surface and buried in the mineral podzolic solum were used to reconstruct the successional pathways of the sites. The forest sites developed in a context of continuous fire disturbance, except for a short interval of several centuries. The fire history of the three sites is similar with a fire frequency of 34–37 fires over the last 5000–5800 years, which corresponds to a mean fire interval of 150–180 years. Based on parametric and non-parametric estimators, the regional fire regime of the Laforge area (combined fire data of two sites located about 7 km from each other) was most likely characterized by 38 fires over the last 5000–5800 years, i.e., at an interval of about 150–160 years. Birch (Betula papyrifera) was present in the spruce sites between 5800 and 3000 cal. years BP. Jack pine arrived at the pine site about 3000 cal. years BP, at the time when birch was extirpated from the spruce forests. The closed-crown forests near the open forest/tundra boundary are resilient postfire ecosystems which regenerated successfully since mid-Holocene. Compositional change of these forests, which led to the decline of birch but maintained the dominance of conifers, appears related to cooler and wetter conditions of late Holocene. Climatic conditions also slowed the eastward migration of jack pine, which explains the late arrival of this species at its northeastern range. Our data suggest that fire disturbance in these sites of the boreal biome has been a continuous and positive regenerative process since mid-Holocene despite the influence of climatic change on the tree flora.

Fossil floras suggest stable, not drifting, continents

The distribution of forests from the Carboniferous to the early Cretaceous suggests that the continents probably have been stable, not drifting as paleomagnetic evidence has indicated. (1) During this interval of geologic time the vegetation-climatic zones display a symmetrical arrangement from northern to southern hemispheres consistent with continental stability. (2) Sequences of successively younger floras at the same latitude show similarity in composition, not the changes that would be expected with continents drifting across many degrees of latitude, and hence of climate. (3) The distribution of vegetation zones on the continents does not agree with the positions which are postulated by studies of paleomagnetism: in these, temperate forests occur in the deep tropics, tropical forests in polar regions, and dryclimate scrub in the moist tropics. (4) There are gradients of change in plant structures (annual rings, leaf size) and in general composition across latitudes which, in terms of the evolutionary (adaptive) relations of plants, appear to require stable continents.