Paul A. Harcombe

Woody Seedling Dynamics in an East Texas Floodplain Forest

We monitored woody plant seed deposition, seedling emergence, and the survival and growth of seedlings (i.e., plants ≤0.5 m tall regardless of age) in an East Texas river floodplain forest from 1979 through 1984. In addition, we estimated the relative importance of flooding, drought, fungal attack, herbivory, proximity to a conspecific adult, and shade in causing seedling mortality. Tree species fell into two major groups on the basis of their demographic characteristics and responses to unfavorable conditions. The first group was composed of heavy—seeded species, of which water oak (Quercus nigra) was the primary example. They produced few seeds, but had high seedling survival. Seedlings of these species emerged late in the summer, thereby avoiding peak periods of flooding and damping—off mortality. Seedling survival was little affected by drought, herbivory, or proximity to a conspecific adult. The second group included most of the common tree species (e.g., ironwood, Carpinus caroliniana; sweetgum, Liq...

DISTURBANCE, SUCCESSION, AND MAINTENANCE OF SPECIES DIVERSITY IN AN EAST TEXAS FOREST'

Vegetation change following selective logging and major natural disturbance was in- vestigated through the use of stand history reconstruction and population monitoring in an east Texas pine-hardwood forest. High frequencies of release in beech (Fagus grandifolia) and white oak (Quercus alba) occurred after apparently natural disturbances in 1800-1810, 1860-1870, and 1960-1970, and after selective logging beginning 1910. Pulses of recruitment occurred after all disturbances except the most recent one. Age data suggest that recruitment after the 1910 disturbance was asynchronous among species, with loblolly pine (Pinus taeda) regenerating first, followed sequentially by white oak, red maple (Acer rubrum), and beech. At present, only magnolia (Magnolia grandiflora) is showing significant new recruitment. After the 1910 disturbance, regenerating pines grew rapidly and became dominant in the re-forming overstory. Most hardwoods originating after the 19 10 disturbance did not reach the overstory. White oak initially outgrew maple and beech and is presently larger. Over the last 20 yr beech growth has been greater than white oak growth, resulting in a decline in the average size difference between individuals of these species. Presently, mortality rates of understory populations are high for sweetgum, blackgum, and the oaks, and are low for beech and magnolia. These latter two species are also the faster growing. Ifthese trends continue, beech and magnolia will become increasingly predominant in the forest understory and eventually in the overstory as well. Map data show that saplings and small trees are abundant under pine and oak, but not under beech and magnolia. Future disturbance is therefore likely to accelerate succession to more shade-tolerant species in parts of the forest now dominated by pine and oak, but is most likely to re-initiate new regeneration, including pine and oak, in areas now dominated by beech and magnolia. These results suggest a pattern of cyclical replacement driven by disturbance, a pattern which may help preserve species diversity in southern mesic forests.

Forest Vegetation of the Big Thicket, Southeast Texas

Tree and shrub species composition and environmental characteristics were measured in 56 stands representing the range of natural vegetation types of the lower coastal plain of southeast Texas, and vegetation characteristics were related to the physical environment. Most of the stand—to—stand variation in species composition was expressed by the first axis of a reciprocal averaging ordination, and stand position along the first axis in turn was most highly correlated with percent sand in the surface soil (0—15 cm) (r = —.81, P <.05). The first ordination axis is interpreted as a soil moisture gradient. A more weakly expressed second axis of variation is interpreted as a soil aeration—parent material gradient (from high organic matter, low extractable Ca stands to low organic matter, high Ca stands). Overstory basal area, woody plant species richness of overstory and understory, and relative basal area of broad—leaved evergreens in both strata all varied in a regular way along the first ordination axis, ha...

Initiation of a new woodland type on the Texas Coastal Prairie by the Chinese tallow tree (Sapium sebiferum (L.) Roxb.).

BRUCE, K. B., G. N. CAMERON (Department of Biology, University of Houston, Houston, Texas 77204) AND P. A. HARCOMBE (Department of Ecology and Evolutionary Biology, Rice University, Houston, Texas 77251). Initiation of a new woodland type on the Texas Coastal Prairie by the Chinese tallow tree (Sapium sebiferum (L.) Roxb.). Bull. Torrey Bot. Club 122:215-225. 1995.-The chronosequence method (stand ages 0-20 years) was used to study the invasion of the Upper Coastal Prairie of Texas by the Chinese tallow tree and to learn whether this exotic plant would be replaced by native woodland species. This invasion marked a dramatic transformation of community structure. Dominance of life forms shifted rapidly (< 10 years to canopy closure) as graminoids and forbs were replaced by trees and shrubs during succession. Size-frequency distributions of the trees showed that stands were not even-aged. Many stands in the oldest three age classes exhibited descending monotonic tree size distributions. Because Chinese tallow produces seeds soon after establishment, it seems likely that the rapid increase in density was the result of initial trees acting as seed sources. The most common native trees that appeared in the survey were generally small-seeded inhabitants of local riparian areas, e.g., hackberry (Celtis laevigata), elm (Ulmus americana), green ash (Fraxinus pensylvanica), and yaupon (Ilex vomitoria). Low densities of these species indicated slow invasion rates compared to that of tallow. Currently, stands are virtually monospecific. However, the non-tallow species showed a significant increase in density with stand age which suggests that these woodlands may become more diverse in the future.

Effects of the December 1983 tornado on forest vegetation of the Big Thicket, southeast Texas, U.S.A.

Abstract On December 10, 1983, a large tornado touched down in southwestern Tyler County in southeastern Texas, U.S.A., causing major damage to two areas of forest in the Big Thicket National Preserve (BTNP). The sizes of the disturbed areas were 31.1 ha in the Hickory Creek Unit and 10.4 ha in the Turkey Creek Unit of the BTNP. Canopy cover at Hickory Creek was reduced by 83%, live basal area declined by 66.7%, and live density declined by 33.6%. At Turkey Creek, canopy cover was reduced by 46%, live basal area declined by 37.8% and live density declined by 14.4%. Large percentages of surviving trees at both sites were badly damaged: 31.3% at Hickory Creek and 48.9% at Turkey Creek. Tornado mortality was strongly concentrated in the large size classes and, for this reason, the primary short-term effect on forest composition at both sites was to select strongly against the previously dominant canopy species.

Five years of tree death in a Fagus-Magnolia forest, southeast Texas (USA)

SummaryDeath of trees (>4.5 cm dbh) and saplings (1.4 m tall–4.5 cm dbh) was monitored over 5 years in a Southern Mixed Hardwood Forest to determine causes of death, death rates, and to assess the nature and direction of forest change. Most trees died standing (77%), presumably the result of pathogen attack or adverse physiological condition; knockdown by other trees (10%) and wind breakage (11%) were other common causes of death. Frequency of wind break and knockdown varied with size, but standing death did not. Most trees (64%) and some saplings (26%) died outright (complete death of the genetic individual); the others had sprouts or living residual parts. For some species, survival of residual parts was high, and so resprouting or persistence may be important in population recruitment. For others, low survival of residual parts means that sprouting does not effectively prolong the life of an individual. Most populations were stable or declining slightly. Shade-intolerant species showed higher death rates in small size classes than did the more tolerant species, so in the absence of disturbance one might expect some change in species composition toward the putative climax dominants.

Flooding alters apparent position of floodplain saplings on a light gradient

It has been hypothesized that flooding, interacting with light availability, can determine spatial patterns of tree regeneration by causing differential germination or survival among seedlings in river floodplains. We conducted a canonical correspondence analysis (CCA, a form of direct gradient analysis) of sapling and environmental data collected from 1980 to 1990 to investigate this hypothesis. The CCA recovered two significant axes, and of the eight environmental variables included, only elevation and percentage sky explained significant portions of the variability in species location. Axis 1 was correlated with elevation, while Axis 2 was correlated with percentage sky. Comparison of first axis rankings with species flood tolerance rankings obtained from the literature showed substantial agreement, indicating that elevation was a surrogate for flood tolerance. Comparison of second axis rankings with species shade tolerance rankings obtained from the literature showed little correspondence between position on the light gradient and species shade tolerance rankings; several of the most shade-tolerant species were placed on the high-light end of the light gradient. These results suggest that sapling distributions are responding primarily to flooding and light. The failure of the species to assort along the light gradient in accordance with their shade tolerance suggests that flooding interacts with life history traits of individual species, especially shade tolerance, resulting in complex species-specific responses. When we attempt to tease apart the interaction between light and flooding, we find two components: flood tolerance may allow persistence of some species under lower light conditions than normal, and flood intolerance may limit some normally shade-tolerant species to high-light conditions, where growth is fast enough to allow escape from the flooding hazard.

IDENTIFYING FUNCTIONAL GROUPS OF TREES IN WEST GULF COAST FORESTS (USA): A TREE-RING APPROACH

A dendroclimatic study of important tree species in the west Gulf Coastal Plain region, USA, was conducted to evaluate how climate affects tree radial growth in this southeasternmost section of the Eastern Deciduous Forest/Southern Evergreen Forest. We established an east–west transect from western Louisiana to central Texas that crossed the western range limits of each of 16 species and developed a network of 104 annual tree ring-width chronologies from 38 sites. Of the 104 chronologies, 99 series from the genera Pinus, Quercus, and Fagus were analyzed using rotated principal components analysis (RPCA). The RPCA revealed the presence of three robust phylogenetic signals in the tree-ring patterns, which partitioned the data into the Pinus species (PISP), the oak species in the black oak subgenus Erythrobalanus (QUBO), and the oak species in the white oak subgenus Leucobalanus (QUWO). The Fagus chronologies (FAGR) also loaded most highly with the QUWO series, resulting in a combined QUWO/FAGR factor. This partitioning occurred even though tree species within each phylogenetic group came from contrasting xeric and mesic sites and, in the case of the QUWO/FAGR factor, from different genera. Only in the xeric western range limits of the transect did site location begin to override the phylogenetic groupings. Consequently, responses to climate based on genetics appeared to be more important than ecological and site characteristics in determining the tree-ring patterns of the sampled species overall. We tested this hypothesis by independently modeling the dendroclimate signals in the tree-ring chronologies using monthly precipitation and maximum temperature data. The resulting climate correlation functions were subjected to RPCA as before. As we did so, the same phylogenetic groups emerged. All of the chronologies were drought sensitive. However, the phylogenetic differences in climate response were related to differences in the timing of the peak monthly responses to climate and to the differing patterns of climate response in the months prior to the current growing season. The findings of this study indicate that there is an underlying organizing principle based on genetics that determines how certain phylogenetic groups of trees respond to climate in a way that is largely independent of the site environment. At a coarse level, these phylogenetic distinctions persist even at the most stressed sites near tree range limits, though distinctions within genera start to break down. These findings therefore suggest functional groupings of tree species, which can be used in vegetation/climate models that attempt to predict realistically how such forests will respond to future climate changes.

Sapling growth and survivorship as a function of light in a mesic forest of southeast Texas, USA

Abstract. For seven species in a mature mesic forest in southeast Texas, we estimated species-specific parameters representing radial growth in high light and low light for tree saplings. Shade-intolerant species had higher asymptotic growth rates and lower low-light growth than tolerant species. Inspection of species positions on graphs of low-light growth versus high-light growth suggested that there was a trade-off between these two processes across species. By linking functions of growth versus light and mortality versus growth, we also found that shade-intolerant species had higher mortality risk at low light and stronger sensitivity of mortality to light than shade-tolerant species. Moreover, we found that low-light survival and high-light growth were negatively correlated across species. In contrast to northern hardwood forests, where sapling survival in low light may be achieved at the expense of growth, our results suggested that shade-tolerant species in this southern mixed forest can grow faster as well as survive better than shade-intolerant species in low light. We conclude that both sapling growth and survival are important components of shade tolerance and their relationships may be system-specific.

Stand development in a 130-year-old spruce-hemlock forest based on age structure and 50 years of mortality data

Abstract Influences on forest stand development were assessed using tree age and long-term mortality data in ten 0.4 ha permanent plots established in 1935. The stand originated following a major fire in the 1840s with a period of rapid invasion (1851–1870). This was followed by slower filling to full stand closure (1871–1900), recruitment suppression (1901–1930), reinitiation of hemlock recruitment (1931–1950), and continued slow recruitment (1951-present). Recruitment re-initiation was apparently associated with natural stand thinning and not with major disturbance. Large blowdown patches occurred in two plots in the 1960s, but the primary effect was release of hemlock advance regeneration rather than recruitment of new individuals. From 1935 to 1983 annual mortality (calculated from 5–10-years remeasurements) was 0.5–1.2% for both species. At this rate, continued thinning will likely allow a gradual transition to an all-aged forest, as population turnover time is short relative to frequency of intermediate or major disturbance.