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.     

Asymmetric changes of growth and reproductive investment herald altitudinal and latitudinal range shifts of two woody species

Ongoing changes in global climate are altering ecological conditions for many species. The consequences of such changes are typically most evident at the edge of the geographical distribution of a species, where range expansions or contractions may occur. Current demographical status at geographical range limits can help us to predict population trends and their implications for the future distribution of the species. Thus, understanding the comparability of demographical patterns occurring along both altitudinal and latitudinal gradients would be highly informative. In this study, we analyse the differences in the demography of two woody species through altitudinal gradients at their southernmost distribution limit and the consistency of demographical patterns at the treeline across a latitudinal gradient covering the complete distribution range. We focus on Pinus sylvestris and Juniperus communis, assessing their demographical structure (density, age and mortality rate), growth, reproduction investment and damage from herbivory on 53 populations covering the upper, central and lower altitudes as well as the treeline at central latitude and northernmost and southernmost latitudinal distribution limits. For both species, populations at the lowermost altitude presented older age structure, higher mortality, decreased growth and lower reproduction when compared to the upper limit, indicating higher fitness at the treeline. This trend at the treeline was generally maintained through the latitudinal gradient, but with a decreased growth at the northern edge for both species and lower reproduction for P. sylvestris. However, altitudinal and latitudinal transects are not directly comparable as factors other than climate, including herbivore pressure or human management, must be taken into account if we are to understand how to infer latitudinal processes from altitudinal data.     

Climate change reduces the net sink of CH4 and N2O in a semiarid grassland

Atmospheric concentrations of methane (CH₄) and nitrous oxide (N₂O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO₂ and temperature affected CH₄ and N₂O fluxes in a well‐drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH₄ and N₂O fluxes to elevated CO₂ and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO₂ increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO₂ and warming would have opposing effects on CH₄ and N₂O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell‐shaped relationship was observed with soil moisture with highest CH₄ uptake at intermediate soil moisture. Both N₂O emission and uptake occurred at our site with some years showing cumulative N₂O emission and other years showing cumulative N₂O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO₂ and warming reduced the sink of CH₄ and N₂O expressed in CO₂ equivalents (across 5 years by 7% and 11% for elevated CO₂ and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH₄ and N₂O expressed in CO₂‐equivalents.     

Drought tolerance and growth in populations of a wide‐ranging tree species indicate climate change risks for the boreal north

Choosing drought‐tolerant planting stock in reforestation programs may help adapt forests to climate change. To inform such reforestation strategies, we test lodgepole pine (Pinus contorta Doug. ex Loud. var latifolia Englm.) population response to drought and infer potential benefits of a northward transfer of seeds from drier, southern environments. The objective is addressed by combining dendroecological growth analysis with long‐term genetic field trials. Over 500 trees originating from 23 populations across western North America were destructively sampled in three experimental sites in southern British Columbia, representing a climate warming scenario. Growth after 32 years from provenances transferred southward or northward over long distances was significantly lower than growth of local populations. All populations were affected by a severe natural drought event in 2002. The provenances from the most southern locations showed the highest drought tolerance but low productivity. Local provenances were productive and drought tolerant. Provenances from the boreal north showed lower productivity and less drought tolerance on southern test sites than all other sources, implying that maladaptation to drought may prevent boreal populations from taking full advantage of more favorable growing conditions under projected climate change.     

The combined effects of a long‐term experimental drought and an extreme drought on the use of plant‐water sources in a Mediterranean forest

Vegetation in water‐limited ecosystems relies strongly on access to deep water reserves to withstand dry periods. Most of these ecosystems have shallow soils over deep groundwater reserves. Understanding the functioning and functional plasticity of species‐specific root systems and the patterns of or differences in the use of water sources under more frequent or intense droughts is therefore necessary to properly predict the responses of seasonally dry ecosystems to future climate. We used stable isotopes to investigate the seasonal patterns of water uptake by a sclerophyll forest on sloped terrain with shallow soils. We assessed the effect of a long‐term experimental drought (12 years) and the added impact of an extreme natural drought that produced widespread tree mortality and crown defoliation. The dominant species, Quercus ilex, Arbutus unedo and Phillyrea latifolia, all have dimorphic root systems enabling them to access different water sources in space and time. The plants extracted water mainly from the soil in the cold and wet seasons but increased their use of groundwater during the summer drought. Interestingly, the plants subjected to the long‐term experimental drought shifted water uptake toward deeper (10–35 cm) soil layers during the wet season and reduced groundwater uptake in summer, indicating plasticity in the functional distribution of fine roots that dampened the effect of our experimental drought over the long term. An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, which resulted in greater crown defoliation in the drought‐affected plants. This study suggests that extreme droughts aggravate moderate but persistent drier conditions (simulated by our manipulation) and may lead to the depletion of water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.     

Experimental icing affects growth,mortality, and flowering in a high Arctic dwarf shrub

Effects of climate change are predicted to be greatest at high latitudes, with more pronounced warming in winter than summer. Extreme mid‐winter warm spells and heavy rain‐on‐snow events are already increasing in frequency in the Arctic, with implications for snow‐pack and ground‐ice formation. These may in turn affect key components of Arctic ecosystems. However, the fitness consequences of extreme winter weather events for tundra plants are not well understood, especially in the high Arctic. We simulated an extreme mid‐winter rain‐on‐snow event at a field site in high Arctic Svalbard (78°N) by experimentally encasing tundra vegetation in ice. After the subsequent growing season, we measured the effects of icing on growth and fitness indices in the common tundra plant, Arctic bell‐heather (Cassiope tetragona). The suitability of this species for retrospective growth analysis enabled us to compare shoot growth in pre and postmanipulation years in icing treatment and control plants, as well as shoot survival and flowering. Plants from icing treatment plots had higher shoot mortality and lower flowering success than controls. At the individual sample level, heavily flowering plants invested less in shoot growth than nonflowering plants, while shoot growth was positively related to the degree of shoot mortality. Therefore, contrary to expectation, undamaged shoots showed enhanced growth in ice treatment plants. This suggests that following damage, aboveground resources were allocated to the few remaining undamaged meristems. The enhanced shoot growth measured in our icing treatment plants has implications for climate studies based on retrospective analyses of Cassiope. As shoot growth in this species responds positively to summer warming, it also highlights a potentially complex interaction between summer and winter conditions. By documenting strong effects of icing on growth and reproduction of a widespread tundra plant, our study contributes to an understanding of Arctic plant responses to projected changes in winter climatic conditions.     

Linking El Niño,local rainfall,and migration timing in a tropical migratory species

Current climate models project changes in both temperature and precipitation patterns across the globe in the coming years. Migratory species, which move to take advantage of seasonal climate patterns, are likely to be affected by these changes, and indeed, a number of studies have shown a relationship between changing climate and the migration timing of various species. However, these studies have almost exclusively focused on the effects of temperature change on species that inhabit temperate zones. Here, we explore the relationship between rainfall and migration timing in a tropical species, Gecarcoidea natalis (Christmas Island red crab). We find that the timing of the annual crab breeding migration is closely related to the amount of rain that falls during a ‘migration window’ period prior to potential egg release dates, which is in turn related to the Southern Oscillation Index, an atmospheric El Niño‐ Southern Oscillation Index. As reproduction in this species is conditional on successful migration, major changes in migration patterns could have detrimental consequences for the survival of the species. This study serves to broaden our understanding of the effects of climate change on migratory species and will hopefully inspire future work on rainfall and tropical migrations.     

Trophic mismatch and its effects on the growth of young in an Arctic herbivore

In highly seasonal environments, timing of breeding of organisms is typically set to coincide with the period of highest resource availability. However, breeding phenology may not change at a rate sufficient to keep up with rapid changes in the environment in the wake of climate change. The lack of synchrony between the phenology of consumers and that of their resources can lead to a phenomenon called trophic mismatch, which may have important consequences on the reproductive success of herbivores. We analyzed long‐term data (1991–2010) on climate, plant phenology and the reproduction of a long‐distance Arctic migrant, the greater snow goose (Chen caerulescens atlantica), in order to examine the effects of mismatched reproduction on the growth of young. We found that geese are only partially able to adjust their breeding phenology to compensate for annual changes in the timing of high‐quality food plants, leading to mismatches of up to 20 days between the two. The peak of nitrogen concentration in plants, an index of their nutritive quality for goslings, occurred earlier in warm springs with an early snow melt. Likewise, mismatch between hatch dates of young and date of peak nitrogen was more important in years with early snow melt. Gosling body mass and structural size at fledging was reduced when trophic mismatch was high, particularly when the difference between date of peak nitrogen concentration and hatching was >9 days. Our results support the hypothesis that trophic mismatch can negatively affect the fitness of Arctic herbivores and that this is likely to be exacerbated by rising global temperatures.     

The carbon fertilization effect over a century of anthropogenic CO2 emissions: higher intracellular CO2 and more drought resistance among invasive and native grass species contrasts with increased water use efficiency for woody plants in the US Southwest

From 1890 to 2015, anthropogenic carbon dioxide emissions have increased atmospheric CO₂ concentrations from 270 to 400 mol mol^?1. The effect of increased carbon emissions on plant growth and reproduction has been the subject of study of free‐air CO₂ enrichment (FACE) experiments. These experiments have found (i) an increase in internal CO₂ partial pressure (c_i) alongside acclimation of photosynthetic capacity, (ii) variable decreases in stomatal conductance, and (iii) that increases in yield do not increase commensurate with CO₂ concentrations. Our data set, which includes a 115‐year‐long selection of grasses collected in New Mexico since 1892, is consistent with an increased c_i as a response to historical CO₂ increase in the atmosphere, with invasive species showing the largest increase. Comparison with Palmer Drought Sensitivity Index (PDSI) for New Mexico indicates a moderate correlation with Δ¹³C (r² = 0.32, P < 0.01) before 1950, with no correlation (r² = 0.00, P = 0.91) after 1950. These results indicate that increased c_i may have conferred some drought resistance to these grasses through increased availability of CO₂ in the event of reduced stomatal conductance in response to short‐term water shortage. Comparison with C₃ trees from arid environments (Pinus longaeva and Pinus edulis in the US Southwest) as well as from wetter environments (Bromus and Poa grasses in New Mexico) suggests differing responses based on environment; arid environments in New Mexico see increased intrinsic water use efficiency (WUE) in response to historic elevated CO₂ while wetter environments see increased c_i. This study suggests that (i) the observed increases in c_i in FACE experiments are consistent with historical CO₂ increases and (ii) the CO₂ increase influences plant sensitivity to water shortage, through either increased WUE or c_i in arid and wet environments, respectively.     

Limited evidence for CO2‐related growth enhancement in northern Rocky Mountain lodgepole pine populations across climate gradients

Forests sequester large amounts of carbon annually and are integral in buffering against effects of global change. Increasing atmospheric CO₂ may enhance photosynthesis and/or decrease stomatal conductance (g_s) thereby enhancing intrinsic water‐use efficiency (iWUE), having potential indirect and direct benefits to tree growth. While increasing iWUE has been observed in most trees globally, enhanced growth is not ubiquitous, possibly due to concurrent climatic constraints on growth. To investigate our incomplete understanding of interactions between climate and CO₂ and their impacts on tree physiology and growth, we used an environmental gradient approach. We combined dendrochronology with carbon isotope analysis (δ¹³C) to assess the covariation of basal area increment (BAI) and iWUE over time in lodgepole pine. Trees were sampled at 18 sites spanning two climatically distinct elevation transects on the lee and windward sides of the Continental Divide, encompassing the majority of lodgepole pine's northern Rocky Mountain elevational range. We analyzed BAI and iWUE from 1950 to 2015, and explored correlations with monthly climate variables. As expected, iWUE increased at all sites. However, concurrent growth trends depended on site climatic water deficit (CWD). Significant growth increases occurred only at the driest sites, where increases in iWUE were strongest, while growth decreases were greatest at sites where CWD has been historically lowest. Late summer drought of the previous year negatively affected growth across sites. These results suggest that increasing iWUE, if strong enough, may indirectly benefit growth at drier sites by effectively extending the growing season via reductions in g_s. Strong growth decreases at high elevation windward sites may reflect increasing water stress as a result of decreasing snowpack, which was not offset by greater iWUE. Our results imply that increasing iWUE driven by decreasing g_s may benefit tree growth in limited scenarios, having implications for future carbon uptake potential of semiarid ecosystems.     

Ecological outcomes and livelihood benefits of community‐managed agroforests and second growth forests in Southeast Brazil

The Forest and Landscape Restoration movement has emerged as an approach to reconcile biodiversity conservation, ecosystem services provisioning and human well‐being in degraded landscapes, but little is known so far about the potential of different reforestation methods to achieve these objectives. Based on this gap, we assessed the ecological outcomes and local livelihood benefits of community‐managed agroforests and second growth forests to assist natural regeneration in the coastal Atlantic Forest of Brazil. We investigated and compared agroforests and secondary forests according to their structure and floristic composition in 51 circular plots of 314 m², their role in supporting local livelihoods (45 semi‐structured interviews) and the use and cultural importance of plant species (61 interviews). Agroforests and, more remarkably, managed secondary forests (1) re‐established a well‐developed forest structure, with a higher density of tree‐sized individuals and similar basal area compared to nearby old growth forests; (2) were composed by a rich array of native species, including five threatened species, but had lower species richness than old growth remnants; and (3) improved local livelihoods by supplying market valuable and culturally important plants, including 231 native ethnospecies. Overall, local production systems showed remarkable potential to engage smallholders of developing tropical countries in Forest and Landscape Restoration and contribute to achieve its overall goals. We advocate the promotion of these systems as effective Forest and Landscape Restoration approaches in multi‐scale programs and policies.