Climate and disturbance influence self‐sustaining stand dynamics of aspen (Populus tremuloides) near its range margin

Abstract

Species that are primarily seral may form stable (self‐sustaining) communities under certain disturbance regimes or environmental conditions, yet such populations may also be particularly vulnerable to ecological change. Aspen (Populus spp.) are generally considered seral throughout the Northern Hemisphere, including P. tremuloides, the most widely distributed tree species in North America. Recent declines in aspen populations have occurred, especially along drought‐sensitive margins of its range and where fire exclusion and herbivory have promoted community transition. However, aspen also forms stable stands, and examination of the mechanisms that influence persistence can offer conservation insights, especially where populations are vulnerable to changing climate or altered disturbance dynamics. We sampled tree age and stand characteristics of isolated aspen forests in the arid Great Basin (USA) to determine if (1) aspen communities are more fire‐dependent and seral or fire‐independent and stable; (2) ungulate browsing inhibits aspen stability; and (3) temporal patterns of vegetative reproduction (i.e., ramet establishment or “suckering”) are correlated with climate. Aspen size and age class densities strongly fit negative exponential distributions, whether grouped geographically or by functional type, suggesting landscape‐scale persistence. Continuous age distributions and high proportions of recruitment‐sized to overstory trees suggest stability at stand scales, with exceptions including stands with higher browsing pressure. Few stands had evidence of fire, and relationships between dead tree size and variability in live tree size suggest a lack of fire dependency. Several 5‐yr averaged climate variables and one sea surface temperature index were correlated with aspen ramet establishment densities over time, with strongest relationships occurring ~5 yr prior to establishment year, often followed by inverse relationships ~1 yr after. Indeed, aspen establishment density for a recent 41‐yr period was reliably reconstructed using antecedent climate conditions derived from a single drought index. Temporally synchronized aspen ramet establishment across the study region may be due to climate‐driven storage of nonstructural carbohydrate reserves in clonal root systems later used for regeneration. Complex regeneration dynamics of these self‐sustaining aspen stands, especially sensitivity to climate variability, suggest they may serve as harbingers of ecological change in the arid Great Basin and in other aspen populations near their range margin.

Ungulate herbivores as drivers of aspen recruitment and understory composition throughout arid montane landscapes

“…Notably, Kay (2001b) reported little evidence of aspen persistence outside of fenced sites, whereas we found that recruit stem densities were not significantly different in fenced versus unfenced stands (Figure 2c). Our findings are in closer agreement with those of Shinneman and McIlroy (2019), who observed both aspen persistence and decline across a large array of study plots in the northern Great Basin.…”

“…Aspen recruitment was not affected by livestock fencing but was associated with browsing intensity on juvenile aspen, as well as adult tree density and fire history. To the best of our knowledge, this was the first region‐wide study to assess the effectiveness of livestock fencing in the Great Basin and southern Cascades and one of only a few studies to evaluate the importance of multiple drivers of aspen recruitment and persistence in these regions (see Margolis & Farris, 2014; Shinneman & McIlroy, 2019). Our findings will aid land managers in selecting appropriate management practices to promote aspen persistence across this relatively understudied region and will inform conservation efforts for a number of tree species for which recruitment is limited by ungulate herbivory.…”

“…Browsing on juvenile aspen removes both foliage and apical meristems, often preventing these trees from attaining sufficient height to escape ungulate herbivory and recruit into the forest overstory (Baker et al, 1997; Smith et al, 1972). Failure of juvenile aspen to recruit into the overstory can alter the population dynamics of stands, resulting in missing age/size classes and even the replacement of aspen with other tree species (Brown et al, 2006; Shinneman & McIlroy, 2019). In addition, extensive browsing of aspen by ungulates has the capacity to alter the vegetation composition of the stand understory (Kay, 2001a; Kay & Bartos, 2000).…”

Pando’s Pulse: Vital Signs Signal Course Correction at World-Renowned Aspen Forest

“…If regeneration success in protected areas progresses at Pando, we would expect signs of burgeoning recruitment. While long-term aspen recruitment may be episodic based on disturbance, climate, or other human factors [32,33], the decadal dearth of recruitment at Pando suggests recent management is driving successes and failures [14]. A deeper examination was required.…”

Pando's pulse: Vital signs signal need for course correction at world‐renowned aspen forest

“…If regeneration success in protected areas progresses at Pando, we would expect signs of burgeoning recruitment. While long‐term aspen recruitment may be episodic based on disturbance, climate, or other human factors (Kurzel et al, 2007; Shinneman & McIlroy, 2019), the decadal dearth of recruitment at Pando suggests recent management is driving successes and failures (Rogers & McAvoy, 2018). A deeper examination was required.…”