Stand Dynamics of a Unique Pygmy Forest, El Malpais National Monument, New Mexico, U.S.A

The pygmy forest is a rare ecosystem found on the 3000-year-old McCarty’s basalt flow in El Malpais National Monument, New Mexico. The forest is dominated by contorted, shortstatured forms of ponderosa pine (Pinus ponderosa Douglas ex C. Lawson) and piñyon pine (Pinus edulis Engelm.). The purpose of this research was to investigate the history of this unique forest and attempt to isolate reasons why the trees grow in such peculiar forms. We first used tree-ring patterns from 32 ponderosa pines growing on a sandstone kipuka located within the McCarty’s flow as a proxy for past precipitation, and used this to identify linkages between climate and tree establishment. We collected 286 cores in seven plots in the pygmy forest to analyze species composition, age distribution, and establishment dates, and also tallied seedlings and saplings to project the future forest composition. Climate response analyses revealed that tree growth responds positively to precipitation during the pre-monsoonal months of January through May. The pines are sensitive to drought years, and we found three major drought episodes since the early 1800s. We found that the oldest trees in the pygmy forest were junipers aged at least 400 years, and that no forest likely existed until approximately A.D. 1600. The forest is regenerating mostly with Gambel oak (Quercus gambelii Nutt.) and piñyon pine. We found only 10 seedlings of ponderosa pine. Disturbance features observed on the trees, such as lightning scars, dead leaders, and “stagheaded” shapes in the crown, indicated that the peculiar growth forms of the pygmy trees are likely the result of dieback from one or more lightning strikes and new apical dominance from branches of the tree. Lightning storms are known to occur more often over the dark basalt than over the surrounding sandstone areas, and the McCarty’s flow may have properties that enhance convectional uplift and increase storm activity.