Does salvage logging erase a key physical legacy of a tornado blowdown? A case study of tree tip-up mounds

While large-scale wind disturbances are rare, they are nonetheless powerful drivers of plant community reassembly in temperate forests worldwide. These disturbances cause the formation of tree tip-up mounds that serve as regeneration niches, but the time scale at which novel plant communities develop on mounds is unknown. Moreover, salvage logging can cause mounds to “tip back down” and could therefore erase these microsites. Here, we test three hypotheses with a replicated field experiment: (1) novel plant communities rapidly form on tip-up mounds; (2) salvaging erases these microsites; and (3) “tipped-down” tip-up mounds are novel intermediate microsites. We salvaged a random half of four 3–6 ha blowdowns created by an F1 tornado, measured 249 mounds, and censused the vegetation on 48 mounds and 48 reference plots. Plant communities on mounds had two to three fewer species, 50% less cover, and lower diversity than reference communities. However, salvaging caused modest increases in species richness and diversity on mounds and caused 40% of mounds to tip back down. The physical characteristics and vegetation of these tipped-down “inclined mounds” were more similar to vertical mounds than to reference plots. Our results suggest that salvaging may increase microsite heterogeneity across the landscape by creating novel intermediate mounds.

Long-term influence of disturbance-generated microsites on forest structural and compositional development

Wind disturbance generates heterogeneous microsite structures, including downed logs, windthrow mounds, and pits. While these structures can provide opportunities for regeneration of certain tree species, the long-term influence of microsites and microsite heterogeneity on forest development has not been quantified. We used long-term measurements of a formerly old-growth Tsuga canadensis – Pinus strobus forest severely damaged by a category 3 hurricane in 1938 to quantify the impact of microsite conditions on overstory composition and structure. We asked (i) “What are the patterns in live-tree size, growth, and mortality five and seven decades after disturbance?” and (ii) “What roles do microsite heterogeneity and the presence of disturbance-generated microsites play in long-term forest development following disturbance?” We compared live-tree (>2 cm DBH) development and survival to microsite heterogeneity at the 100 m2 scale. Microsite diversity was positively related to overstory species diversity and stem density and negatively related to average tree size. We propose that plots with higher microsite diversity may have experienced more severe local disturbance, which allowed more species and individuals to establish and created varied niches that allowed these individuals to coexist and generate greater stand-level diversity. These persistent relationships highlight how microsite conditions affect forest development after severe disturbances.

Genetic variation in the diameter–height relationship in Scots pine (Pinus sylvestris)

Genetic parameters were estimated for the diameter–height (d-h) relationship and three other tree stem-form characteristics (total height, breast height diameter, and total tree volume) for data from 10 diallel progeny trials of Scots pine ( Pinus sylvestris L.), at about 30 years of age in Sweden. Linear mixed models were fit to the data, where adjustments for intertree competition and microsite heterogeneity were made by means of covariates in a nearest-neighbour analysis. The d-h relationship was analyzed with a covariate (tree height) adjusted model of diameter. Average estimates of the additive coefficient of variation and narrow-sense heritability for the d-h relationship were 7.4% and 0.22, respectively. Estimates of dominance were comparatively small (average dominance: phenotypic variance ratio of 0.04). The results indicate that there is scope to modify the d-h relationship by selection and breeding. Additive genetic correlations between the d-h relationship and height were negative, with a mean of –0.62. Selection for height would thus result in stems that are more slender than average, suggesting that tall trees allocate relatively more resources to height growth than to diameter growth. Selection based on height alone will negatively affect volume gain.

Short-Term Effects of Silvicultural Treatments on Microsite Heterogeneity and Plant Diversity in Mature Tennessee Oak-Hickory Forests

Growing emphasis on sustainability has increased the demand for information on effects of forest management on species diversity. We investigated the hypothesis that plant diversity is a function of microsite heterogeneity by documenting plant diversity and heterogeneity in canopy cover, light, and soil moisture produced by four silvicultural treatments during the first growing season following treatment: prescribed burning, wildlife retention cut with prescribed burning, wildlife retention cut, and shelterwood cutting. Treatments and controls were randomly assigned within four relatively undisturbed, 70–90-year-old oak-hickory stands. Heterogeneity in canopy cover and photosynthetically active radiation was greatest after shelterwood cutting, whereas the wildlife retention cut resulted in less removal of canopy trees and a smaller increase in heterogeneity of these factors. The addition of prescribed burning enhanced the effects of the wildlife retention cut. Prescribed burning alone had the least impact on heterogeneity of these factors. Soil moisture variability appeared to be independent of treatments. Shelterwood cutting increased first-year herbaceous plant diversity, and this increase was likely due, in part, to increased heterogeneity in canopy cover, light, and seedbed condition. These first-year results partially support the hypothesis that plant diversity is a function of microsite diversity in these forests. Long-term monitoring is underway.

Seedling performance of four sympatric Entandrophragma species (Meliaceae) under simulated fertility and moisture regimes of a Central African rain forest

Relative densities of mahogany species vary across tropical Africa and correspond with changes in soil fertility and moisture status. Seedling growth of four co-occurring African mahoganies (Entandrophragma spp.) was studied in relation to soil nutrient and moisture status in a shade-house experiment. On naturally occurring forest soils, E. cylindricum and E. utile exhibited increased relative growth rate (RGR) and decreased root mass ratio (RMR) with an increase in soil fertility while E. angolense and E. candollei did not. Changes in leaf morphology with fertility did not correspond to changes in species performance. On moist, fertile soils, E. angolense outperformed congeners but E. candollei performed equally well on moist infertile soils. Entandrophragma cylindricum performed as well as E. angolense and E. candollei on two of three soil moisture stress treatments but E. utile consistently performed poorly. Comparative seedling performance fitted well with limited available data on the distribution of Entandrophragma spp. in relation to soil fertility and moisture gradients and suggests that within-forest microsite heterogeneity may help explain the distribution of Entandrophragma species within the Dzanga-Sangha Dense Forest Reserve, Central African Republic.

Genetic variation and productivity of Populustrichocarpa and its hybrids. VII. Two-year survival and growth of native black cottonwood clones from four river valleys in Washington

A common-garden study of Populustrichocarpa Torr. & Gray was initiated in 1985 when clonal material from 128 trees was collected from sites distributed along two mesic (Hoh and Nisqually) and two xeric (Dungeness and Yakima) river valleys. This material was grown for 1 year at Puyallup, Wash. In spring 1986, cuttings from this material were used to establish two replicate plantations, one at Puyallup and one at Wenatchee, Wash. Over the 2 year period, trees were assessed for survival, damage, and growth. Two-year survival was 86% at Puyallup and 59% at Wenatchee; survival was higher for mesic-origin trees at both sites. At Wenatchee, mortality was mainly due to a droughty soil and hot, dry climate in the first year, and damage was due to the tarnished plant bug (Lygus spp.), field voles (Microtus spp.), and grasshoppers (Family Acrididae). At both locations, Melampsoraoccidentalis Jacks. leaf rust was found mainly on trees originating from Yakima. Mean 2-year height and diameter at Puyallup were 457 cm and 40 mm, respectively; corresponding values at Wenatchee were 320 cm and 29 mm. At Puyallup, 2-year stem volume (diameter2 × height) decreased significantly in this order: Nisqually (13 500 cm3) = Hoh > Dungeness > Yakima (4700 cm3). Within the Nisqually trees, clones from lower, milder climate elevations grew more than those from the upper elevations. The reverse was true for the Yakima trees, presumably because the lower elevation trees are adapted to an arid climate and are very susceptible to Melampsora rust. At Wenatchee, high microsite heterogeneity masked much of the genetic variation. The genetic variance component (among rivers, sites, and clones) for stem volume at Puyallup increased from 27 to 51% over the second year. In analyses of each river valley, genetic components (elevational group and clone) were also high at 27–63%; however, for the Yakima trees, the elevational group variance (40%) was much larger than for the other valleys (0–7%) and suggests a steep selection gradient midway along the river transect.

Growth patterns of sugar maple seedlings and mature trees in healthy and in declining hardwood stands

Openings created in the forest canopy as a result of the decline of sugar maple (Acersaccharum Marsh.) may increase microsite heterogeneity and favor the growth of tree seedlings on the forest floor and possibly neighboring healthy trees because of resource release. To corroborate these hypotheses, I studied the growth of sugar maple seedlings and mature trees, and some microsite characteristics, in healthy and in declining hardwood stands. Sampling was carried out in 400-m2 quadrats in four stands of similar composition. In two of the stands, the trees showed no apparent symptoms of decline (healthy stands), but in the other two (declining stands), dieback had caused tree cover to be reduced by ≈25 to 30%. Photosynthetically active radiation below the canopy was significantly lower and less variable in the healthy than in the declining stands, under both cloudy and sunny conditions. In one of the declining stands, soil pH was higher and soil organic matter content was lower than in both healthy stands. Stem elongation of sugar maple seedlings did not differ among the stands prior to 1984, but following that date it was significantly higher and more variable among seedlings in the declining stands. Ring width of apparently healthy trees decreased markedly in the early 1980s and increased somewhat during the 1985–1987 period on the declining sites. Microsite heterogeneity and growth of tree seedlings on the forest floor were thus greater in the declining than in the healthy stands. Neighboring healthy trees did not necessarily respond to the opening of the canopy (as a result of dieback) by increased ring width; this possibly resulted from the hierarchical position within the canopy, the differential time of reaction, and the age and (or) the health status of each individual.

The variety of soil microsites created by tree falls

The uprooting of forest trees leads to the formation of microsites on the forest floor, contributing to fine-scale heterogeneity in soil properties. We found the types of microsites formed depended on the way the tree fall occurred. Tree falls were classified as either hinge or rotational types. Hinge tree falls formed when the root mat of a tree and the surrounding soil were uplifted vertically, leaving an adjacent pit in the soil. Hinge tree falls varied as to thickness of the root mat and angle of uplift. Rotational tree falls were usually a result of a ball and socket motion of the root mat and soil, which positioned the tree bole over the newly created pit. The tree falls disrupted and redistributed surface soil organic matter and subsoil. In rotational tree falls, the surface material remained intact, covering some of the pit and the adjacent side of the mound. In hinge tree falls, the surface organic matter was deposited on the throw side of the mound, leaving subsoil on the other side and in the pit. With time, however, hinge-type pits accumulated litter and eventually had more organic matter than mounds. Old mounds from both hinge and rotational tree falls had lower concentrations of calcium and magnesium, lower pH, and less moisture than pits. The tree fall process creates long-term soil patterns and maintains microsite heterogeneity in forest communities.