Water-Use Efficiency of Co-occurring Sky-Island Pine Species in the North American Great Basin

Water-use efficiency (WUE), weighing the balance between plant transpiration and growth, is a key characteristic of ecosystem functioning and a component of tree drought resistance. Seasonal dynamics of tree-level WUE and its connections with drought variability have not been previously explored in sky-island montane forests. We investigated whole-tree transpiration and stem growth of bristlecone (Pinus longaeva) and limber pine (Pinus flexilis) within a high-elevation stand in central-eastern Nevada, United States, using sub-hourly measurements over 5 years (2013–2017). A moderate drought was generally observed early in the growing season, whereas interannual variability of summer rains determined drought levels between years, i.e., reducing drought stress in 2013–2014 while enhancing it in 2015–2017. Transpiration and basal area increment (BAI) of both pines were coupled throughout June–July, resulting in a high but relatively constant early season WUE. In contrast, both pines showed high interannual plasticity in late-season WUE, with a predominant role of stem growth in driving WUE. Overall, bristlecone pine was characterized by a lower WUE compared to limber pine. Dry or wet episodes in the late growing season overrode species differences. Our results suggested thresholds of vapor pressure deficit and soil moisture that would lead to opposite responses of WUE to late-season dry or wet conditions. These findings provide novel insights and clarify potential mechanisms modulating tree-level WUE in sky-island ecosystems of semi-arid regions, thereby helping land managers to design appropriate science-based strategies and reduce uncertainties associated with the impact of future climatic changes.

Truffles in the sky: the impact of stochastic and deterministic drivers on Rhizopogon communities of the Madrean Sky Island Archipelago

The Madrean Sky Islands Archipelago (MSIA) comprises mountain “islands” whose Pine-Oak forests appear in stark contrast to the surrounding “sea” of Sonoran Desert vegetation. Rhizopogon (Boletales) consists of obligate ectomycorrhizal (EcM) symbionts that form truffle sporocarps and associate exclusively with Pinaceae. The objectives of this project were to describe the diversity of species of Rhizopogon across the MSIA and to characterize whether community structure is determined by host diversity, island identity, geographic distance, or some interaction among these factors. We selected nine islands, two sites were sampled per island: one site dominated by Pinus species and the other by Pseudotsuga menziesii var. glauca. Rhizopogon diversity was characterized from sporocarps and from bioassay-based EcM root tips derived from P. muricata, Ps. menziesii var. menziesii, and Ps. menziesii var. glauca seedlings inoculated with soil samples collected along transects established at each site. The ITS rDNA fungal barcode was amplified, and sequences were used in community analyses. Twenty-one 99% OTUs in the genus Rhizopogon were identified across nine sky islands. While differential host association with Pinus and Pseudotsuga is a significant driver of community composition, our results supported a stronger island effect. Furthermore, Rhizopogon communities associated within hosts are characterized by random phylogenetic structures across sky islands and are not structured by geographic distance. These results are consistent with a strong isolation effect involving historical habitat fragmentation of sky islands in response to past climate changes, and that both niche partitioning and stochastic demographic processes function in shaping Rhizopogon communities of the MSIA.

Demographic changes of a tropical understory bird in naturally patchy montane habitats in southern India

The occurrence, density and survival of a species often depend on various aspects of the habitat that it occupies including patch size and disturbance. The demography of most threatened tropical species largely remain unstudied but could provide valuable information about their biology and insights for their conservation. Our study examined the effect of patch size and disturbance on different demographic parameters of an understory, threatened, endemic bird, the white-bellied shortwing in the tropical biodiversity hotspot of the Western Ghats in India. We sampled eight plots on a sky-island using mist-nets for four years in a 'Robust design' mark-recapture framework. Based on model selection using AIC values, the model with survival as a function of disturbance fits the data better than models with abundance or with these parameters modelled as functions of forest patch size. Shortwing density and sex ratio were not different across forest patch sizes or differing disturbance regimes. However, the survival rate of the species significantly decreased with increasing disturbance but was not related to forest patch size. Our study is possibly the first to provide critical baseline information on the demography of a tropical understory species from this region.