Michael J. Clifford

Multi‐decadal drought and amplified moisture variability drove rapid forest community change in a humid region

Climate variability, particularly the frequency of extreme events, is likely to increase in the coming decades, with poorly understood consequences for terrestrial ecosystems. Hydroclimatic variations of the Medieval Climate Anomaly (MCA) provide a setting for studying ecological responses to recent climate variability at magnitudes and timescales comparable to expectations of coming centuries. We examined forest response to the MCA in the humid western Great Lakes region of North America, using proxy records of vegetation, fire, and hydroclimate. Multi-decadal moisture variability during the MCA was associated with a widespread, episodic decline in Fagus grandifolia (beech) populations. Spatial patterns of drought and forest changes were coherent, with beech declining only in areas where proxy-climate records indicate that severe MCA droughts occurred. The occurrence of widespread, drought-induced ecological changes in the Great Lakes region indicates that ecosystems in humid regions are vulnerable to rapid changes in drought magnitude and frequency.

Late-Holocene drought and fire drove a widespread change in forest community composition in eastern North America

Several regions of the world have recently experienced climate-induced changes in forest composition, highlighting the need to understand the causes, likelihood, and dynamics of abrupt vegetation change. Although few historical examples of climate-induced forest change exist from recent centuries, particularly in humid regions like the northeastern United States, paleoecological records are rich with examples. For example, pollen records from portions of the northeastern United States indicate that eastern hemlock (Tsuga canadensis) and American beech (Fagus grandifolia) abruptly declined in abundance between 500 and 600 yr BP. Concomitant increases in pine (Pinus spp.) and oak (Quercus spp.) occurred. Hypotheses to explain this change have included cooling during the ‘Little Ice Age’ (LIA), Native American activity, drought, and/or fires. To better understand spatiotemporal patterns of forest change and assess potential causes and dynamics, we synthesized regional pollen records and developed two high-resolution, coupled records of vegetation, fire, and drought from bogs in Maine. Results of our synthesis reveal >70% of regional pollen sites recorded shifts in forest composition during this time period. Bog records revealed that forest composition changed a few decades after the onset of drought and regional fires, consistent with increased recruitment of pine and oak during post-disturbance succession. Vegetation changes persisted until European settlement. Our data demonstrate that widespread, long-lasting forest changes were triggered by decadal-to-multidecadal drought and associated fires, highlighting the potential for abrupt, long-lasting forest changes in response to transient climate and disturbance events, particularly when such events occur against the backdrop of more gradual temperature change.

Woodland recovery following drought-induced tree mortality across an environmental stress gradient

Recent droughts and increasing temperatures have resulted in extensive tree mortality across the globe. Understanding the environmental controls on tree regeneration following these drought events will allow for better predictions of how these ecosystems may shift under a warmer, drier climate. Within the widely distributed piñon-juniper woodlands of the southwestern USA, a multiyear drought in 2002-2004 resulted in extensive adult piñon mortality and shifted adult woodland composition to a juniper-dominated, more savannah-type ecosystem. Here, we used pre- (1998-2001) and 10-year post- (2014) drought stand structure data of individually mapped trees at 42 sites to assess the effects of this drought on tree regeneration across a gradient of environmental stress. We found declines in piñon juvenile densities since the multiyear drought due to limited new recruitment and high (>50%) juvenile mortality. This is in contrast to juniper juvenile densities, which increased over this time period. Across the landscape, piñon recruitment was positively associated with live adult piñon densities and soil available water capacity, likely due to their respective effects on seed and water availability. Juvenile piñon survival was strongly facilitated by certain types of nurse trees and shrubs. These nurse plants also moderated the effects of environmental stress on piñon survival: Survival of interspace piñon juveniles was positively associated with soil available water capacity, whereas survival of nursed piñon juveniles was negatively associated with perennial grass cover. Thus, nurse plants had a greater facilitative effect on survival at sites with higher soil available water capacity and perennial grass cover. Notably, mean annual climatic water deficit and elevation were not associated with piñon recruitment or survival across the landscape. Our findings reveal a clear shift in successional trajectories toward a more juniper-dominated woodland and highlight the importance of incorporating biotic interactions and soil properties into species distribution modeling approaches.

Effects of a nonnative, invasive lovegrass on Agave palmeri distribution, abundance, and insect pollinator communities

Nonnative Lehmann lovegrass (Eragrostis lehmanniana) has invaded large areas of the Southwestern United States, and its impact on native plants is not fully understood. Palmer’s agave (Agave palmeri), an important resource for many pollinators, is a key native plant potentially threatened by E. lehmanniana. Understanding potential impacts of E. lehmanniana on A. palmeri is critical for anticipating the future of the desert community where they coexist and for addressing management concerns about associated threatened and endangered species. Our study provides strong indications that E. lehmanniana negatively impacts A. palmeri in several ways. Areas of high E. lehmanniana abundance were associated with significantly lower densities and greater relative frequencies of small A. palmeri, suggesting that E. lehmanniana may exclude A. palmeri. There were no significant differences in species richness, abundance, or community composition when comparing flower associates associated with A. palmeri in areas of high and low E. lehmanniana abundance. However, we did find significantly lower connectedness within the pollination network associated with A. palmeri in areas with high E. lehmanniana abundance. Although E. lehmanniana forms thick stands that would presumably increase fire frequency, there was no significant association between E. lehmanniana and fire frequency. Interestingly, medium to high densities of A. palmeri were associated with areas of greater fire frequency. The complex ramifications of E. lehmanniana invasion for the long-lived A. palmeri and interlinked desert community warrant continued study, as these species are likely to continue to be found in close association due to their similar soil preferences.

Widespread dust deposition on North American peatlands coincident with European land-clearance

Ecosystems around the world are being subjected to numerous human disturbances. Climate change and land degradation are the most obvious of these disturbances and have received much attention. However, easily overlooked, indirect disturbances can also alter ecosystem structure and function. Dust deposition is a prime example of an easily overlooked disturbance process. We hypothesized that historic European settlement and land-clearance in eastern North America led to widespread wind erosion of upland soils and subsequent dust deposition onto otherwise undisturbed peatlands, potentially fertilizing these naturally nutrient-poor ecosystems and causing shifts in plant communities. We tested these hypotheses by analyzing 11 peat profiles collected across a broad region of eastern North America. We documented a strong correlation between the concentrations of Ambrosia pollen grains and microscopic mineral particles, interpreting this as a signal of dust deposition coincident with European settlement and land-clearance. Analysis of Sphagnum macrofossils revealed substantial site-to-site variability in both the degree and the direction of ecological response to dust deposition, but suggested that increasing magnitude of dust deposition increased the likelihood of a decline in the relative abundance of Sphagnum. Results also suggested that raised bogs were more sensitive to dust deposition than kettle peatlands. We conclude that European settlement and land-clearance resulted in widespread dust deposition on peatlands, leading to ecological changes in some of these ecosystems, and leaving behind a coherent dust horizon in the late-Holocene peatland stratigraphy of eastern North America. This easily overlooked indirect disturbance process could be ongoing today in areas of widespread soil disturbance and could potentially further alter dust-receiving ecosystems.

Woodland resilience to regional drought: Dominant controls on tree regeneration following overstorey mortality

Drought events occurring under warmer temperatures (i.e. “hotter droughts”) have resulted in widespread tree mortality across the globe, and may result in biome-level vegetation shifts to alternate vegetation types if there is a failure of trees to regenerate. We investigated how overstorey trees, understorey vegetation, and local climatic and edaphic conditions interact to influence tree regeneration, a key prerequisite for resilience, in a region that has experienced severe overstorey tree mortality due to hotter droughts and beetle infestations. We used detailed field observations from 142 sites that spanned a broad range of environmental conditions to evaluate the effects of climate and recent tree mortality on tree regeneration dynamics in the spatially extensive pinon (Pinus edulis)-juniper (Juniperus osteosperma, Juniperus monosperma) woodland vegetation type of the southwestern USA. We used a structural equation modelling framework to identify how tree mortality and local climatic and edaphic conditions affect pinon and juniper regeneration and electivity analyses to quantify the species-specific associations of tree juveniles with overstorey trees and understorey shrubs. Pinon regeneration appears to be strongly dependent upon advanced regeneration, (i.e. the survival of juvenile trees that established prior to the mortality event), the survival of adult seed-bearing trees (inferred from basal area of surviving trees) and the facilitative effects of overstorey trees for providing favourable microsites for seedling establishment. Model results suggest that local edaphoclimatic conditions directly affected pinon and juniper regeneration, such that stands with hotter, drier local climatic conditions and lower soil available water capacity had limited tree regeneration following large-scale dieback. Synthesis. We identify four indicators of resilience to hotter drought conditions: (1) abundant advance regeneration of tree seedlings; (2) sufficient canopy cover for survival of emergent seedlings and existing regeneration; (3) sufficient seed source from surviving trees with high reproductive output; (4) areas with cooler and wetter local climates and greater soil available water capacity. In the absence of these conditions, there is greater likelihood of woodlands transitioning to more xeric vegetation types following dieback.

Pinyon Pine Mortality Alters Communities of Ground-Dwelling Arthropods

Abstract. We documented the effect of drought-induced mortality of pinyon pine (Pinus edulis Engelm.) on communities of ground-dwelling arthropods. Tree mortality alters microhabitats utilized by ground-dwelling arthropods by increasing solar radiation, dead woody debris, and understory vegetation. Our major objectives were to determine (1) whether there were changes in community composition, species richness, and abundance of ground-dwelling arthropods associated with pinyon mortality and (2) whether specific habitat characteristics and microhabitats accounted for these changes. We predicted shifts in community composition and increases in arthropod diversity and abundance due to the presumed increased complexity of microhabitats from both standing dead and fallen dead trees. We found significant differences in arthropod community composition between high and low pinyon mortality environments, despite no differences in arthropod abundance or richness. Overall, 22% (51 taxa) of the arthropod community were identified as being indicators of either high or low mortality. Our study corroborates other research indicating that arthropods are responsive to even moderate disturbance events leading to changes in the environment. These arthropod responses can be explained in part due to the increase in woody debris and reduced canopy cover created by tree mortality.