Disentangling the formation of contrasting tree line physiognomies combining model selection and Bayesian parameterization for simulation models.

Alpine tree-line ecotones are characterized by marked changes at small spatial scales that may result in a variety of physiognomies. A set of alternative individual-based models was tested with data from four contrasting Pinus uncinata ecotones in the central Spanish Pyrenees to reveal the minimal subset of processes required for tree-line formation. A Bayesian approach combined with Markov chain Monte Carlo methods was employed to obtain the posterior distribution of model parameters, allowing the use of model selection procedures. The main features of real tree lines emerged only in models considering nonlinear responses in individual rates of growth or mortality with respect to the altitudinal gradient. Variation in tree-line physiognomy reflected mainly changes in the relative importance of these nonlinear responses, while other processes, such as dispersal limitation and facilitation, played a secondary role. Different nonlinear responses also determined the presence or absence of krummholz, in agreement with recent findings highlighting a different response of diffuse and abrupt or krummholz tree lines to climate change. The method presented here can be widely applied in individual-based simulation models and will turn model selection and evaluation in this type of models into a more transparent, effective, and efficient exercise

Seed production and dispersal limit treeline advance in the Pyrenees

Aims Pinus uncinata is the major treeline‐forming species in the Pyrenees. Yet, the role of its reproduction and dispersal as drivers of treeline dynamics remains unknown. Here we quantify seed production, dispersal and germination changes along the elevation gradient to assess whether they may constrain the foreseen treeline advance in the Pyrenees. Location Central Pyrenees, Catalonia, NE Spain. Methods We established four plots along an elevation gradient from the closed subalpine forest to the krummholz zone at five study sites. In each plot, we collected cones from five to six trees, measured their length, and triggered their opening in the laboratory to count the number of empty seeds and the number and weight of full seeds. We used the collected seeds in a germination experiment under controlled conditions in growth chambers. Additionally, we installed seed traps along the forest-alpine grassland transition to measure seed rain for three consecutive years in three of the study sites. Results The number of full seeds per cone decreased along the elevation gradient and was correlated with cone length. However, the proportion of full seeds per cone and their weight did not differ between elevation positions. Seed rain decreased drastically with elevation and no seeds arrived into the alpine grassland traps consistently across study years. Although germination success did not significantly differ between elevation provenances (i.e., elevation position of origin), we found significant differences in germination dynamics between study sites and between elevation provenances within sites. Conclusions Our results indicate that whereas the viability of Pinus uncinata seeds is not limited by elevation, seed production and dispersal are constraining the ongoing rates of treeline advance in the Pyrenee

Contrasting human influences and macro-environmental factors on fire activity inside and outside protected areas in North America

Human activities threaten the effectiveness of protected areas (PAs) in achieving their conservation goals across the globe. In this study, we contrast the influence of human and macro-environmental factors driving fire activity inside and outside PAs. Using area burned between 1984 and 2014 for 11 ecoregions in Canada and the United States, we built and compared statistical models of fire likelihood using the MaxEnt software and a set of 11 key anthropogenic, climatic, and physical variables. Overall, the full model (i.e. including all variables) performed better (adjusted area under the curve ranging from 0.71 to 0.95 for individual ecoregions) than the model that excluded anthropogenic variables. Both model types (with and without anthropogenic variables) generally performed better inside than outside the PAs. Climatic variables were usually of foremost importance in explaining fire activity inside and outside PAs, with anthropogenic variables being the second most important predictors, even inside PAs. While the individual contributions of anthropogenic variables indicate that fire activity decreased as of function of increasing human footprint, the anthropogenic effects were often substantially greater than those of physical features and were comparable to or even greater than climatic effects in some densely developed ecoregions, both outside and within PAs (e.g. Mediterranean California, Eastern Temperate Forest, and Tropical Wet Forests). Together, these results show the pervasive impact of humans on fire regimes inside PAs, as well as outside PAs. Given the increasing attractiveness of PAs, the implications for adaptive fire management beyond the concept of naturalness in the PAs are discussed. Our assessment of human-altered fire activity could serve as an indicator of human pressure in PAs; however, we suggest that further analysis is needed to understand specific interactions among fire, human pressures, and the environmental conditions at the scale of PAs

Avaluació regional de la dinàmica recent de l'ecotò bosc subalpí-prats alpins als Pirineus

[cat] El límit altitudinal del bosc, o ecotò superior bosc-prat alpí, constitueix per definició, la frontera entre l'estatge subalpí i l'estatge alpí separant, per tant, sistemes ecològics adjacents. És una de les fronteres més aparents entre sistemes ecològics que s'origina degut a l'increment gradual de l'adversitat de les condicions climàtiques en altitud i que, finalment, comporta el límit biològic per al creixement dels vegetals en port arbori. La formació de boscos arreu del planeta, en absència de pertorbacions naturals o antròpiques (p.e., tales, incendis, pastures), està limitada per condicions climàtiques excessivament seques, humides o fredes. En el cas de la transició natural bosc subalpí-prat alpí, que s'observa a tots els sistemes muntanyosos del planeta suficientment alts, les causes de la seva formació (llindar tèrmic, balanç de carboni,...) és un tema encara avui de debat on no hi ha un consens generalitzat. Però independentment del factor causant del límit altitudinal del bosc, els factors abiòtics controlen en últim terme els processos ecològics que hi tenen lloc. És per això, que els ecotons han estat motiu d'estudi des de fa molt temps, ja que es consideren paradigmàtics per l'anàlisi de la resposta dels patrons de distribució de la vegetació a les fluctuacions climàtiques. La seva importància com a monitors de canvis climàtics recau en el fet que les respostes dels ecosistemes degut a la variació ambiental són detectats primerament a les zones de frontera. Aquesta tesi ha estat realitzada al límit superior del bosc del sector central i oriental dels Pirineus catalans i d'Andorra. L'espècie dominant a totes les localitats estudiades, i a bona part dels Pirineus, és el pi negre (Pinus uncinata). Els resultats obtinguts mostren que tot i la importància de les condicions locals en la dinàmica del límit superior del bosc, hi ha una sincronització en el creixement radial i, en menor mesura, en el reclutament d'individus entre localitats a escala regional. Aquest fet evidencia la presència d'un factor comú extern, el macroclima, que modula la dinàmica de l'ecotò bosc-prats alpins a escala regional dels Pirineus. S'ha detectat una densificació del límit superior del bosc a escala regional durant la segona meitat del segle XX, que s'ha produït en paral·lel a l'increment de les temperatures (canvi climàtic). Però en base al patró de la transició en l'edat i la mida dels individus, el canvi d'usos del sòl hauria estat el factor determinant de l'augment del caràcter forestal en alguns dels ecotons estudiats. Tot i la densificació regional de la zona de l'ecotò des de 1950s, l'estructura demogràfica recent (1971-2000) i la dinàmica del límit de l'arbre de les diverses localitats estudiades evidencien l'existència de respostes diferencials a curt termini entre localitats. Aquest resultat emfatitza la importància de les condicions locals en la modulació de la dinàmica dels ecotons. L'anàlisi del patró espacial de la regeneració sembla indicar que la distribució dels llocs segurs de reclutament, que estan condicionats per la distribució espacial dels elements responsables dels processos de facilitació a nivell microambiental, determinarien la distribució espacial de la regeneració del pi negre a l'ecotò. Per tant, els processos de regeneració recent a l'ecotò superior de Pinus uncinata no han estat condicionats per limitacions reproductives de l'espècie ni per una manca d'hàbitat favorable per a l'establiment de nous individus. Tot i això, el previsible increment dels fenòmens d'estrés hídric a escala regional al sud d'Europa (IPCC 2007), degut a augments de temperatura sense anar acompanyats d'augments en el règim de precipitació, i la probable disminució de la precipitació hivernal en forma de neu, poden aturar i revertir els processos de densificació de la zona de l'ecotò observats durant el segle XX.[eng] The altitudinal limit of forest growth, the alpine treeline ecotone, constitutes the boundary between the subalpine and alpine belts, and thus separates distinct ecological systems. The treeline ecotone is one of the most striking borders between ecosystems that result from the altitudinal increase of climate harshness, which finally leads to the altitudinal limit of tree-like growth. Irrespective of the ultimate cause of the alpine treeline, where abiotic factors limits population processes, ecotones have been focus of scientific research for long since they are regarded as suitable monitors of the influence of climatic variability on species distribution. This is due to the fact that climatic variability effects are first detected in transition zones between ecosystems. This thesis is focused on the alpine "Pinus uncinata" treeline in the Iberian eastern Pyrenees: Catalan Pyrenees (NE Spain) and Andorra. The demographic and spatial analyses of the alpine ecotone at a regional scale evidence that, in spite of the importance of local conditions on the alpine treeline dynamics, there is an important degree of synchronization in tree growth and, to a lesser extent, recruitment processes among treelines at a regional scale. This evidences the presence of an external factor, the macroclimate, that modulates the dynamics of such populations. The treeline densification observed, at a regional scale, during the second half of the 20th century occurred in parallel to temperature rises (climate warming). But based on the transition pattern in tree age and size along the ecotone, land use changes may have triggered the afforestation processes observed in some of the treelines studied. Our results suggest that recent alpine "Pinus uncinata" treeline recruitment, at a regional scale, was not limited by reproductive constraints or by the lack of suitable habitat for regeneration. However, the predicted increase of drought stress events at southern Europe as a result of increased temperature without precipitation increments, together with the expected reduction of snow precipitation in winter, may distort or even reverse the observed treeline densification trends during the 20th century

Disentangling the formation of contrasting tree line physiognomies combining model selection and Bayesian parameterization for simulation models.

Alpine tree-line ecotones are characterized by marked changes at small spatial scales that may result in a variety of physiognomies. A set of alternative individual-based models was tested with data from four contrasting Pinus uncinata ecotones in the central Spanish Pyrenees to reveal the minimal subset of processes required for tree-line formation. A Bayesian approach combined with Markov chain Monte Carlo methods was employed to obtain the posterior distribution of model parameters, allowing the use of model selection procedures. The main features of real tree lines emerged only in models considering nonlinear responses in individual rates of growth or mortality with respect to the altitudinal gradient. Variation in tree-line physiognomy reflected mainly changes in the relative importance of these nonlinear responses, while other processes, such as dispersal limitation and facilitation, played a secondary role. Different nonlinear responses also determined the presence or absence of krummholz, in agreement with recent findings highlighting a different response of diffuse and abrupt or krummholz tree lines to climate change. The method presented here can be widely applied in individual-based simulation models and will turn model selection and evaluation in this type of models into a more transparent, effective, and efficient exercise