DOI: 10.5061/dryad.v57t0
title: Data from: Cascading effects of changes in land use on the invasion of the walnut Juglans regia in forest ecosystems

description: 1. Plant invasions are affected by many factors that must be favourable in
 order for invasions to occur. Factors can be grouped into three major
 categories: propagule pressure, biotic factors and abiotic
 characteristics; all may be moderated by human activity. However, studies
 examining all factors simultaneously are rare, and most are limited to a
 single factor. This hampers our understanding of the mechanisms driving
 invasions. 2. In recent decades, an alien walnut (Juglans regia) has
 become invasive in Central Europe due to an increase in the populations of
 native dispersers, rooks (Corvus frugilegus), and political changes that
 have resulted in the abandonment of farmland. Here, we test whether
 increased propagule pressure resulting from the presence of seed-bearing
 walnuts in abandoned fields interacts with native forest-seed dispersing
 animals (biotic factors), landscape structure and management, and passive
 secondary dispersal by gravity (abiotic factors) to facilitate the further
 invasion of walnuts into forests. 3. Seed-dispersing animals were observed
 visiting walnuts growing in abandoned fields and in human settlements.
 Walnuts and seed dispersers were surveyed in 102 forest patches. Forest
 characteristics were examined, and field experiments examining passive
 dispersal were conducted. 4. Walnut seeds were carried into forests by
 native birds, jays (Garrulus glandarius). Jays harvested seeds from both
 wild walnuts growing in abandoned fields and walnuts planted in human
 settlements. The density of walnuts in forest patches was correlated with
 jay density. Forest occupancy by walnuts and walnut densities were
 correlated with the cover of abandoned fields and human settlements
 containing seed-bearing walnuts. Secondary seed dispersal also affected
 forest colonization. Walnut seeds hidden by native rooks in sloping,
 arable fields may be transported passively to forest edges. Synthesis Our
 results show that this invasion is a multifaceted process in which
 human-related alterations to propagule pressure and biotic and abiotic
 factors have led to the spread of alien walnuts from human settlements and
 abandoned fields into forest ecosystems. Thus, politically related land
 use changes can create an invasion debt that causes unexpected linkages
 among the invasive plant, native dispersers, land management and
 topography that together can cause cascading changes in ecosystems.
description: Data on investigated
 forest patchesFile contains data of
 walnut (Juglans regia), jay (Garrulus glandarius) and red squirrel
 (Sciurus vulgaris) occurrences and abundance in studied forest patches in
 southern Poland. Environmental variables influencing species occurrences
 and densities are described inside the file.Data_subimitted_forests.xlsWalnut seed harvesting rate by jaysData on the jay visitation rate and the walnut seed harvesting rate for walnut trees planted in human settlements and on walnuts those growing in abandoned agricultural fieldsWalnutHarvestingRate.xlsxPassive dispersal of walnut seedsData on distances, net displacement and reaching a forest border by walnut seeds in sloping and horizontal fields. Detailed description of variables is inside the file (sheet "Explanations).Passive_dispersal.xlsWalnut seed sizeData on the size (length and width) of walnut seeds used in the experimentWalnut_seed_size.xlsWalnut changes in abandoned fields of southern PolandData on characteristics linked with the propagule pressure of walnuts in abandoned fields of southern Poland. File contains data on number of walnut trees, number of fruiting trees and number of walnut seeds over 10-year period in ten landscape agricultural plots in southern Poland. Description of variables is inside the file (sheet "Explanations)Walnut_changes_in_abandoned_fields.xls