Geomorphic principles of terrain organization and vegetation gradients

Abstract

Abstract. Moisture and nutrient gradients consistently explain much of the variation in plant species composition and abundance, but these gradients are not spatially explicit and only reveal species responses to resource levels. This study links these abstract gradients to quantitative, spatial models of hill‐slope assembly. A gradient analysis in the mixed‐wood boreal forest demonstrates that patterns of upland vegetation distribution are correlated to soil moisture and nutrient gradients. Variation in species abundance with time since the last fire is removed from the gradient analysis in order to avoid confounding the physical environment gradients. The physical‐environment gradients are related to qualitative positions on the hill slope i.e. crest, mid‐slope, bottom‐slope. However, hill‐slope shape can be quantitatively described and compared by fitting allometric equations to the slope profiles. Using these equations, we show that hill‐slope profiles on similar surficial materials have similar parameters, despite coming from widely separated locations. We then quantitatively link the moisture and nutrient gradients to the equations. Moisture and nutrients significantly increase as distance down‐slope from the ridgeline increases. Corresponding vegetation composition changes too. These relationships characterize the general pattern of vegetation change down most hill slopes in the area. Since hill slopes are a universal feature of all landscapes, these principles may characterize landscape scale spatial patterns of vegetation in many environments.

Boreal mixedwood species composition in relationship to topography and white spruce seed dispersal constraint

“…HWD stands are positively correlated with sites that can be interpreted as being drier or poorer (ridges, convex slopes, flat areas), while SW is positively correlated with richer and moister sites (channels, concave slopes), but negatively associated with sites where there could be excessive moisture (flats). These results are consistent with those of Bridge and Johnson (2000) for the boreal mixedwoods of Saskatchewan, where the topographic gradient was described using allometric measures of hillslopes from topographic maps. More importantly, this result is in contrast with the idea of boreal forest landscapes as being totally shifting mosaics, where tree species distribution is continuously re-shuffled by disturbance.…”

“…Also unresolved is whether the species' niches are mostly realized in space over environmental gradients, or in time over forest succession, intended as a directional process of autogenic change in the species composition of the biotic community established after a stand-replacement disturbance. Bridge and Johnson (2000) explain the vegetation patterns of the boreal mixedwoods of Saskatchewan, Canada, in terms of moisture and nutrient availability, while Hély et al (2000) explain the variation in composition of southeastern Canadian boreal mixedwoods as a successional gradient. While the successional explanation is supported by species differences in shade tolerance (Messier et al, 1999), it is also well documented that mixedwood stands can originate immediately after stand-replacing disturbance (Thorpe, 1992;Youngblood, 1995;Hope, 2001;Peters et al, 2002), suggesting that succession only explains part of mixedwood diversity (Gutsell and Johnson, 2002).…”

Dynamics of understory vegetation in boreal forests: experiences from Swedish integrated monitoring sites

“…However, small-scale, i.e. within plot, heterogeneity in topography is likely to affect moisture and thereby the supply of nutrients (Bridge and Johnson, 2000;Økland and Eilertsen, 1993). Hence, a second aim of this study was to quantify the spatial variation within each of the vegetation monitoring plots.…”

Boreal Forests of Kamchatka: Structure and Composition

“…The same transition is seen in the upland plots, with the upper slopes dominated by L. cajanderi (upland plots 1 and 2) and becoming increasingly mixed with declining altitude (upland plot 3). In the Canadian boreal forest similar transitions have been attributed to clines in moisture and nutrient availability, which increase down-slope [36]. Lowland plot 3 is dominated by B. platyphylla, though this is likely to be a relatively young stand formed following inundation [37].…”

“…A debate exists over whether patterns in the distribution of boreal forest types arise from environmental variation, especially topographic [36], or disturbance, or an interaction between the two. Though Betula forms the first canopy in post-fire stands, conifers also regenerate immediately [14,40], with subsequent dynamics driven by mortality, and mixed stands commonly form immediately following disturbance [11,41,42], in line with the initial floristics model of succession [43].…”