Light regimes beneath closed canopies and tree-fall gaps are compared for five temperate and tropical forests using fish-eye photography of intact forest canopies and a model for calculating light penetration through idealized gaps. Beneath intact canopies, analyses of canopy photographs indicate that sunflecks potentially contribute 37–68% of seasonal total photosynthetically active radiation. In all of the forests, potential sunfleck duration is brief (4–6 min), but the frequency distributions of potential sunfleck duration vary because of differences in canopy geometry and recent disturbance history. Analysis of the photographs reveals that incidence angles for photosynthetically active radiation beneath closed canopies are not generally vertical for any of the forests, but there was considerable variation both among and within sites in the contribution of overhead versus low-angle lighting. Calculations of light penetration through idealized single-tree gaps in old growth Douglas-fir – hemlock forests indicate that such gaps have little effect on understory light regimes because of the high ratio of canopy height to gap diameter. However, single-tree gaps in the other four forest types produce significant overall increases in understory light levels. There is also significant spatial variation in seasonal total radiation in and around single-tree gaps. Our results demonstrate that there can be significant penetration of light into the understory adjacent to a gap, particularly at high latitudes. As gap size increases, both the mean and the range of light levels within the gap increases, but even in large gaps (ca. 1000 m2) the potential duration of direct sunlight is generally brief (<4 h). The major differences in gap light regimes of the five forests are largely a function of canopy height and latitude. The effects of latitude should also result in differences in gap light regimes across the geographic range of individual forest types.
Long generation times limit species' rapid evolution to changing environments. Trees provide critical global ecosystem services, but are under increasing risk of mortality because of climate change-mediated disturbances, such as insect outbreaks. The extent to which disturbance changes the dynamics and strength of selection is unknown, but has important implications on the evolutionary potential of tree populations. Using a 40-y-old Pinus ponderosa genetic experiment, we provide rare evidence of context-dependent fluctuating selection on growth rates over time in a long-lived species. Fast growth was selected at juvenile stages, whereas slow growth was selected at mature stages under strong herbivory caused by a mountain pine beetle (Dendroctonus ponderosae) outbreak. Such opposing forces led to no net evolutionary response over time, thus providing a mechanism for the maintenance of genetic diversity on growth rates. Greater survival to mountain pine beetle attack in slow-growing families reflected, in part, a host-based life-history trade-off. Contrary to expectations, genetic effects on tree survival were greatest at the peak of the outbreak and pointed to complex defense responses. Our results suggest that selection forces in tree populations may be more relevant than previously thought, and have implications for tree population responses to future environments and for tree breeding programs. fluctuating selection | growth-survival trade-offs | selection response | Pinus ponderosa | Dendroctonus ponderosae U nderstanding the dynamics of selection over time is fundamental for understanding life-history evolution (1) and predicting evolutionary change under climate change (2, 3). To date, such understanding is based almost exclusively on data for relatively short-lived species (4, 5), but virtually nonexistent for long-lived organisms, such as trees (ref. 6; but see ref. 7). Trees and forests provide critical ecological and commercial services, including impacts on global carbon cycles, species diversity, water quality, and climate regulation (8). Recent forest mortality (9, 10) highlights the importance of understanding how climate change and climate change-driven disturbances may impact forests (11, 12). Trees may live for hundreds of years and experience varying selection pressures associated with fluctuating climate (13), disturbance regimes (14), and biotic interactions (15), all of which may be magnified under climate change. The extent to which these events may change the strength and direction of selection and contribute to the maintenance of genetic diversity and evolutionary potential is unknown. Of special relevance are insect outbreaks, a biotic interaction expected to increase with climate change (16) but unaccounted for in models to predict the evolutionary potential of tree populations (17, 18). Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) is a native, irruptive forest insect in western North America that uses numerous Pinus species as hosts. Via pheromone-mediated mass attacks th...
Biodiversity hotspots are conservation priorities. We identify the North American Coastal Plain (NACP) as a global hotspot based on the classic definition, a region with > 1500 endemic plant species and > 70% habitat loss. This region has been bypassed in prior designations due to misconceptions and myths about its ecology and history. These fallacies include: (1) young age of the NACP, climatic instability over time and submergence during high sea-level stands; (2) climatic and environmental homogeneity; (3) closed forest as the climax vegetation; and (4) fire regimes that are mostly anthropogenic. We show that the NACP is older and more climatically stable than usually assumed, spatially heterogeneous and extremely rich in species and endemics for its range of latitude, especially within pine savannas and other mostly herbaceous and firedependent communities. We suspect systematic biases and misconceptions, in addition to missing information, obscure the existence of similarly biologically significant regions world-wide. Potential solutions to this problem include (1) increased field biological surveys and taxonomic determinations, especially within grassy biomes and regions with low soil fertility, which tend to have much overlooked biodiversity; (2) more research on the climatic refugium role of hotspots, given that regions of high endemism often coincide with regions with low velocity of climate change; (3) in low-lying coastal regions, consideration of the heterogeneity in land area generated by historically fluctuating sea levels, which likely enhanced opportunities for evolution of endemic species; and (4) immediate actions to establish new protected areas and implement science-based management to restore evolutionary environmental conditions in newly recognized hotspots.
Frequent, low intensity fire was an important component of the natural disturbance regime of presettlement savannas and woodlands in the southeastern USA dominated by longleaf pine (Pinus palustris), and prescribed burning is now a critical part of the management of these endangered habitats. Fire season, fire frequency, and fire intensity are three potentially important, though still little understood, components of both natural and managed fire regimes. In this long—term (8—yr) study, we experimentally (through the use of prescribed burning) tested for effects of fire season (eight different times throughout the year) and fire frequency (annual vs. biennial burning), on population dynamics (recruitment, growth, mortality, change in density, and change in basal area [the total basal area of all stems in a plot]) and species composition of trees in two quite different types of longleaf—pine—dominated habitats (north Florida sandhills and flatwoods). Limited fire temperature and intensity data were also collected during one year to examine the relationship between fire behavior (temperature and intensity) and tree mortality. Contrary to prior hypotheses, our results showed few systematic or predictable effects of season or frequency of burning on dynamics of longleaf pine. Instead, variability in the population dynamics of this species appeared to be related largely to variation in fire behavior, regardless of the season of burning. Consistent with prior hypotheses, we found that deciduous oak species (Quercus laevis, Q. margaretta, and Q. incana) were least vulnerable to dormant—season burning and most vulnerable to burning early in the growing season. This was shown particularly by seasonal trends in the effect of burning on oak mortality (both topkill and complete kill) and, to a lesser extent, on oak recruitment. Oak densities and basal areas also declined in the spring—burned plots, resulting in a shift away from oaks and towards increased dominance by longleaf pine. Detrimental effects of spring burning on oaks were partly explained by fire behavior, but there appeared also to be an important residual effect of burning season, particularly on complete kill. Though longleaf pine population dynamics did not differ markedly as a result of burning season and frequency, we did find important differences in pine dynamics between the two habitats (i.e., sandhills and flatwoods). In general, populations of longleaf pines in the sandhills appeared to be density regulated, while flatwoods pine populations were declining regardless of the level of intraspecific competition. This suggests that long—term persistence of longleaf pine, and perhaps other fire—adapted species in frequently burned longleaf—pine—dominated communities, may be determined by complex interactions between habitat factors and fire regimes.
Badger disturbances in a tall—grass prairie were used to study colonization patterns and the formation of equilibrium plant species associations in a complex mainland community. Colonization processes were described from field observations over a 4—yr period. A qualitative colonization model was developed to predict noninteractive species equilibria. Predicted colonization rates were based upon relative immigration rates determined by interactions among propagule production rates, dispersal capacities, and the source—site distances of the species involved. The immigration rates both between groups with different life history characteristics (mature prairie, prairie fugitive, and ruderal species) and within groups (K—type, intermediate, and r—type fugitive species) were predicted. Manipulation of important variables enabled different conditions affecting relative immigration rates to be simulated. Species with intermediate life history characteristic (propagule production and dispersal capacity) and located at intermediate distances from the colonization site were predicted to have the highest immigration rates. Thus, prairie fugitive forbs were predicted to have higher rates than either mature prairie or ruderal species. Immigration rates of different fugitive species onto a site were predicted to depend upon the frequency and distribution of previously colonized disturbances in the vicinity of the colonization site. In general, model predictions were consistent with field observations. The model predicted the noninteractive species equilibrium among seedlings of mature prairie, fugitive, and ruderal forbs, but not the interactive specie equilibrium among these groups of forbs. Within the fugitive species, both noninteractive and interactive species equilibria were predicted. The noninteractive colonization model was constructed for prediction of colonization patterns on local disturbances in complex plant communities, but since it utilized general life history characteristics to predict immigration rates it should also be applicable to other colonization processes. Equilibrium plant species associations were studied on badger disturbances in virgin prairie and in a less complex tract of overgrazed prairie. Peak standing crop biomass was not different on and off disturbances in either community, but biomass production occurred earlier in the year on disturbances. Although the species present differed, species diversity, equitability, and the distribution of biomass among species were similar on and off disturbances in virgin prairie and on disturbances in overgrazed prairie. The dominant species comprised 20%—26% of the biomass. In contrast, off disturbances in overgrazed prairie were different. The species diversity and equitability were low, and the biomass was concentrated in the dominant species (60%). Thus, in overgrazed prairie local disturbances depressed dominance and resulted in increased complexity of the plant species association formed. In virgin prairie alternate states of equivalent complexity result...
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