Influence of Light on Herbaceous Layer Aboveground Productivity along a Forest - Savanna Continuum

The herbaceous layer in forest ecosystems is often ignored because of its small stature and contribution to the overall ecosystem biomass. Unlike forests, the herbaceous layer in savanna ecosystems is more noticeable, however little is known about the factors that control the productivity in this layer, especially the influence of light. The study was conducted at Pushmataha Forest Habitat Research Area in southeastern Oklahoma that have units with different overstory densities due to previous mechanical treatments and sustained differences in fire return interval. The goal of this study was to determine relationship between light availability and intercepted photosynthetically active radiation (IPAR) on herbaceous productivity along a forest-savanna continuum. IPAR by the overstory and herbaceous plants was measured multiple times during the 2013 growing season. Herbaceous aboveground net primary production (ANPP) was measured at the end of the 2013 growing season by clipping and weighing biomass components (grass, forb, legume, woody, sedge, and litter). Overstory and herbaceous IPAR showed two distinct trends over the growing season. Forested treatments had a substantial increase in the beginning of the growing season related to canopy development of the deciduous trees. In savanna treatments, the overstory trend of IPAR was more consistent over the year. Herbaceous IPAR in forested units had a trend that was more consistent, while in savanna treatments there was a substantial increase at the onset of the growing season due to the development of the dense herbaceous layer. In general, all the categories of herbaceous ANPP were positively correlated with the light availability. The total herbaceous ANPP had a positive relationship with PAR available and IPAR by the understory. However IPAR by the understory was a better predictor for herbaceous ANPP (r2=0.65). The ability of plants to use IPAR to produce biomass in the herbaceous layer in forest and savanna ecosystems was similar regardless of overstory density and treatment. These results indicate that the pattern of IPAR by overstory and herbaceous layer are dependent of the species and the density of plants. However the ability of plants to use PAR to produce biomass was consistent across a wide range of conditions.Natural Resources and Ecology Managemen

Integrating plant physiology into simulation of fire behavior and effects

Wildfires are a global crisis, but current fire models fail to capture vegetation response to changing climate. With drought and elevated temperature increasing the importance of vegetation dynamics to fire behavior, and the advent of next generation models capable of capturing increasingly complex physical processes, we provide a renewed focus on representation of woody vegetation in fire models. Currently, the most advanced representations of fire behavior and biophysical fire effects are found in distinct classes of fine-scale models and do not capture variation in live fuel (i.e. living plant) properties. We demonstrate that plant water and carbon dynamics, which influence combustion and heat transfer into the plant and often dictate plant survival, provide the mechanistic linkage between fire behavior and effects. Our conceptual framework linking remotely sensed estimates of plant water and carbon to fine-scale models of fire behavior and effects could be a critical first step toward improving the fidelity of the coarse scale models that are now relied upon for global fire forecasting. This process-based approach will be essential to capturing the influence of physiological responses to drought and warming on live fuel conditions, strengthening the science needed to guide fire managers in an uncertain future

The Physiological Response of Conifers to Fire

One of the grand unknowns of ecosystem science is how fire kills trees. Answering this question is critical to parameterize climate-vegetation models given the observed changes in global fire regimes, the feedbacks between fire and forests in the global carbon cycle, and the potential role of forest management in moderating anthropogenic climate change. In this dissertation I conducted three studies using Pinus ponderosa saplings burned under controlled conditions to improve the understanding how fire effects on tree physiology. First, I assessed the impact of two fire intensities on sapling mortality under two water status pre-fire (well-watered and drought-stressed). The results showed that saplings under drought-stress pre-fire were more vulnerable to mortality when exposed to low fire intensities. However, 100% of mortality was observed regardless of the pre-fire water status when saplings were exposed to high fire intensity. Thus, the data also suggest that there is a fire intensity threshold where the pre-fire water stress can have a significant influence on sapling mortality. Second, we investigated the short (one-day post-fire) and long-term (21-months post-fire) effects of fire on sapling water transport. In the short-term, fire did not have impact on sapling xylem hydraulic conductivity or were more vulnerable to drought-induced embolism. However, in the long-term, saplings were more vulnerable to cavitation. But no damage in the xylem conduits cell walls were observed. Thus, it was hypothesized that the new traumatic xylem formed in the edges of the fire scar and the pre-fire xylem clogging with resin could be responsible for increasing vulnerability to cavitation in these plants. Lastly, I evaluated the impact of a lethal fire intensity on sapling hydraulic conductivity and non-structural carbohydrates periodically for 28-days post-fire. Hydraulic conductivity was not affected any day. This confirmed the results found in the second study. Fire caused a decline in total NSC in burned plants compared with unburned saplings, but it was significantly only 28-days post-fire. The results suggest that tree mortality from fire is likely not due to hydraulic failure but may be related to carbon imbalance.doctoral, Ph.D., Natural Resources -- University of Idaho - College of Graduate Studies, 2020-0

The Survival of <i>Pinus ponderosa</i> Saplings Subjected to Increasing Levels of Fire Behavior and Impacts on Post-Fire Growth

Improved predictions of tree species mortality and growth metrics following fires are important to assess fire impacts on forest succession, and ultimately forest growth and yield. Recent studies have shown that North American conifers exhibit a &#8216;toxicological dose-response&#8217; relationship between fire behavior and the resultant mortality or recovery of the trees. Prior studies have not been conclusive due to potential pseudo-replication in the experimental design and time-limited observations. We explored whether dose-response relationships are observed in ponderosa pine (Pinus ponderosa) saplings exposed to surface fires of increasing fire behavior (as quantified by Fire Radiative Energy&#8212;FRE). We confirmed equivalent dose-response relationships to the prior studies that were focused on other conifer species. The post-fire growth in the saplings that survived the fires decreased with increasing FRE dosages, while the percentage mortality in the sapling dosage groups increased with the amount of FRE applied. Furthermore, as with lodgepole pine (Pinus contorta), a low FRE dosage could be applied that did not yield mortality in any of the replicates (r = 10). These results suggest that land management agencies could use planned burns to reduce fire hazard while still maintaining a crop of young saplings. Incorporation of these results into earth-system models and growth and yield models could help reduce uncertainties associated with the impacts of fire on timber growth, forest resilience, carbon dynamics, and ecosystem economics

Pondberry (Lindera melissifolia, Lauraceae) seed and seedling dispersers and predators

Pondberry (Lindera melissifolia(Walter) Blume) is an endangered dioecious, clonal shrub that grows in periodically flooded forests of the southeastern United States. The probability of survival of dispersed pondberry seeds and new germinants is unknown, but few seedlings are noted in the forest. This study was undertaken to: (1) identify herbivores and predators of pondberry seeds and seedlings, (2) record the fate of pondberry seeds and seedlings after simulated dispersal in areas with lower and higher understory cover, and (3) calculate the probability of seed survival in the two cover types. The study was conducted in or near the Delta National Forest and the Delta Experimental Forest, MS. Pondberry seed and seedling plots were established at sites with high or low cover. Video cameras with infrared illumination were set up to monitor animal visitors to the plots. Image analysis indicated that swamp rabbits (Sylvilagus aquaticus (Bachman)) and wood rats (Neotoma floridana Ord) cut or ate seedlings, while other animals visited the plots without damaging seedlings. Numerous bird species and mammals visited the seed plots and some were filmed eating seeds. Pondberry seeds exposed in open habitats had a significantly higher survival rate than those exposed in habitats with more herbaceous and woody understory cover. The novelty and quality of the temporal data collected via video monitoring indicate the importance of this method in collecting data that are not otherwise available on endangered and rare species