Regional vegetation die-off in response to global-change-type drought

Abstract

Future drought is projected to occur under warmer temperature conditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggers and potential extent of drought-induced vegetation die-off remain pivotal uncertainties in assessing climate-change impacts. Of particular concern is regional-scale mortality of overstory trees, which rapidly alters ecosystem type, associated ecosystem properties, and land surface conditions for decades. Here, we quantify regional-scale vegetation die-off across southwestern North American woodlands in 2002-2003 in response to drought and associated bark beetle infestations. At an intensively studied site within the region, we quantified that after 15 months of depleted soil water content, >90% of the dominant, overstory tree species (Pinus edulis, a piñon) died. The die-off was reflected in changes in a remotely sensed index of vegetation greenness (Normalized Difference Vegetation Index), not only at the intensively studied site but also across the region, extending over 12,000 km2 or more; aerial and field surveys confirmed the general extent of the die-off. Notably, the recent drought was warmer than the previous subcontinental drought of the 1950s. The limited, available observations suggest that die-off from the recent drought was more extensive than that from the previous drought, extending into wetter sites within the tree species' distribution. Our results quantify a trigger leading to rapid, drought-induced die-off of overstory woody plants at subcontinental scale and highlight the potential for such die-off to be more severe and extensive for future global-change-type drought under warmer conditions.

Fig. 1.

Southwestern North American climate. (A and B) Annual mean precipitation (mm) and average of maximum and minimum temperatures (°C) for all stations in the four-state region (A) and only stations in or near piñon-juniper woodlands within that region (B) are shown. (C and D) Associated maximum (C) and minimum (D) temperatures for piñon-juniper woodlands (black line: long-term mean; red line: 10th or 90th percentile, differentiating driest or hottest years) are shown. Shaded bands indicate the four consecutive driest years of the 1950s drought (1953-1956) and the recent drought (2000-2003). Compared with the 1950s drought, the recent drought was wetter (P < 0.05) but warmer for maximum (P < 0.05) and especially minimum temperature (P < 0.001).

Fig. 2.

Drought-induced mortality at Mesita del Buey, northern New Mexico. Shown for predrought and drought periods are: minimum (blue) and maximum (red) temperature (°C) (A), precipitation (mm) (B), volumetric soil water content at 20 cm (%) (C), P. edulis mortality (D), weekly NDVI at Mesita del Buey (E), and NDVI for late May-June for Mesita del Buey (F) and the region encompassing P. edulis (G). For A-E, circles indicate means (annual for A-C and late May-June for E-G); horizontal lines indicate predrought and drought means, with the latter being warmer, wetter, or lower in NDVI. For E, gray lines are nondetrended estimates; for F-G, squares are nondetrended and circles are detrended estimates.

Fig. 3.

Regional drought-induced vegetation changes. (A) Change map for NDVI for the region encompassing P. edulis distribution within Arizona, New Mexico, Colorado, and Utah, based on deviation from 2002-2003 relative to the predrought mean (1989-1999) during the period late-May to June. (B) Aerial survey map of piñon-juniper woodlands, delineating areas that experienced noticeable levels of tree mortality (including larger, older trees), conducted by the U.S. Forest Service (19) in four study areas throughout the region, corroborates the NDVI and aerial survey maps and documents standlevel estimates of mortality that range from 40% to 80% of nonseedling trees.