Helen M. Poulos

Topographic influences on vegetation mosaics and tree diversity in the Chihuahuan Desert Borderlands

The abundance and distribution of species reflect how the niche requirements of species and the dynamics of populations interact with spatial and temporal variation in the environment. This study investigated the influence of geographical variation in environmental site conditions on tree dominance and diversity patterns in three topographically dissected mountain ranges in west Texas, USA, and northern Mexico. We measured tree abundance and basal area using a systematic sampling design across the forested areas of three mountain ranges and related these data to a suite of environmental parameters derived from field and digital elevation model data. We employed cluster analysis, classification and regression trees (CART), and rarefaction to identify (1) the dominant forest cover types across the three study sites and (2) environmental influences on tree distribution and diversity patterns. Elevation, topographic position, and incident solar radiation were the major influences on tree dominance and diversity. Mesic valley bottoms hosted high-diversity vegetation types, while hotter and drier mid-slopes and ridgetops supported lower tree diversity. Valley bottoms and other topographic positions shared few species, indicating high species turnover at the landscape scale. Mountain ranges with high topographic complexity also had higher species richness, suggesting that geographical variability in environmental conditions was a major influence on tree diversity. This study stressed the importance of landscape- and regional-scale topographic variability as a key factor controlling vegetation pattern and diversity in southwestern North America.

A Review of the Evidence for Pine-Oak Niche Differentiation in the American Southwest

ABSTRACT Pine-oak forests of the Southwestern United States contain a unique mixture of species that are distributed across light, temperature, and moisture resource gradients. Pines and oaks in this region have developed an array of physiological and structural mechanisms that promote their success in hot, dry environments with high incident solar radiation. This article reviews the structural and physiological mechanisms responsible for the current abundance and distribution patterns of pines and oaks in high elevation woodlands of the Southwest. It highlights the niche differentiation of species as functions of their stress tolerance or avoidance strategies. This review reveals that pinyon pines are generally more stress tolerant than oaks, but that oaks are more tolerant of extreme growing conditions than other pines of the Southwest. While limited information exists that describes the physiological ecology of some of the pines and oaks in this biogeographical region, there are many opportunities for future research that could contribute to a more holistic understanding of species-environment relationships in Southwestern forests.

Differential stress tolerance of four pines (Pinaceae) across the elevation gradient of the San Bernardino Mountains, Southern California, USA1

Abstract Ecophysiological studies that investigate species sorting across environmental gradients provide insights into the mechanisms underscoring tree distribution patterns. We examined pine functional traits by measuring a suite of physiological parameters including needle water content, spectral reflectance, transpiration decline, and leaf morphology to explore pine stratification patterns across the elevation gradient of the San Bernardino Mountains in southern California. The high and low elevation species demonstrated moisture and solar radiation tolerance. The lowest elevation species, Pinus attenuata Lem., maintained low transpiration rates under drought stress and had high concentrations of photosynthetic pigments indicating high photosynthetic capacity. Pinus coulteri Don was the most stress intolerant pine as demonstrated by high transpiration rates. Although Pinus lambertiana Doug. dominated cooler north-facing slopes, its lower transpiration rates suggested that it was well adapted to moisture stress. Pinus contorta Doug. was a typical high elevation stress tolerant tree growing under high incident solar radiation, heavy snow pack, and severe winds. Our results suggest that water availability and solar radiation intensity are the major factors influencing southern California pine distributions.

Seagrass blue carbon dynamics in the Gulf of Mexico: Stocks, losses from anthropogenic disturbance, and gains through seagrass restoration

Seagrasses comprise a substantive North American and Caribbean Sea blue carbon sink. Yet fine-scale estimates of seagrass carbon stocks, fluxes from anthropogenic disturbances, and potential gains in sedimentary carbon from seagrass restoration are lacking for most of the Western Hemisphere. To begin to fill this knowledge gap in the subtropics and tropics, we quantified organic carbon (Corg) stocks, losses, and gains from restorations at 8 previously-disturbed seagrass sites around the Gulf of Mexico (GoM) (n=128 cores). Mean natural seagrass Corg stocks were 25.7±6.7MgCorgha-1 around the GoM, while mean Corg stocks at adjacent barren sites that had previously hosted seagrass were 17.8MgCorgha-1. Restored seagrass beds contained a mean of 38.7±13.1MgCorgha-1. Mean Corg losses differed by anthropogenic impact type, but averaged 20.98±7.14MgCorgha-1. Corg gains from seagrass restoration averaged 20.96±8.59Mgha-1. These results, when combined with the similarity between natural and restored Corg content, highlight the potential of seagrass restoration for mitigating seagrass Corg losses from prior impact events. Our GoM basin-wide estimates of natural Corg totaled ~36.4Tg for the 947,327ha for the USA-GoM. Including Mexico, the total basin contained an estimated 37.2-37.5Tg Corg. Regional US-GoM losses totaled 21.69Tg Corg. Corg losses differed significantly among anthropogenic impacts. Yet, seagrass restoration appears to be an important climate change mitigation strategy that could be implemented elsewhere throughout the tropics and subtropics.

Gulf of Mexico estuarine blue carbon stock, extent and flux: Mangroves, marshes, and seagrasses: A North American hotspot

The Gulf of Mexico blue carbon habitats (mangroves, seagrass, and salt marshes) form an important North American blue carbon hot spot. These habitats cover 2,161,446 ha and grow profusely in estuaries that occupy 38,000 km2 to store substantial sedimentary organic carbon of 480.48 Tg C. New investigations around GoM for Mexican mangroves, Louisiana salt marshes and seagrasses motivated our integration of buried organic carbon to elucidate a new estimate of GoM blue carbon stocks. Factors creating this include: large GoM watersheds enriching carbon slowly flowing through shallow estuarine habitats with long residence times; fewer SE Mexican hurricanes allowing enhanced carbon storage; mangrove carbon productivity enhanced by warm southern basin winter temperatures; large Preservation reserves amongst high anthropogenic development. The dominant total GoM mangrove blue carbon stock 196.88 Tg from total mangrove extent 650,482 ha is highlighted from new Mexican data. Mexican mangrove organic carbon stock is 112.74 Tg (1st sediment meter) plus USA 84.14 Tg. Mexican mangroves vary greatly in storage, total carbon depositional depths and in sediment age (to 3500 y). We report Mexican mangroves conservative storage fraction for the normally-compared top meter, whereas the full storage depth estimates ranging above 366.78 Tg (high productivity in very deep sediment along the central Veracruz/Tabasco coast) are not reflected in our reported estimates. Seagrasses stock of 184.1 Tg C organic is derived from 972,327 ha areal extent (in 1st meter). The Louisiana marshes form the heart of GoM salt marsh carbon storage 99.5 Tg (in 1st meter), followed by lesser stocks in Florida, Texas, finally Mexico derived from salt marsh extent totaling 650,482 ha. Constraints on the partial estuarine fluxes given for this new data are discussed as well as widespread anthropogenic destruction of the GoM blue carbon. A new North American comparison of our GoM blue carbon stocks versus Atlantic coastal blue carbon stock estimates is presented.